go time travel meaning

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

Image of galaxies, taken by the Hubble Space Telescope.

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

Illustration of GPS satellites orbiting around Earth

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

Illustration of a hand holding a phone with a maps application active.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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April 26, 2023

Is Time Travel Possible?

The laws of physics allow time travel. So why haven’t people become chronological hoppers?

By Sarah Scoles

yuanyuan yan/Getty Images

In the movies, time travelers typically step inside a machine and—poof—disappear. They then reappear instantaneously among cowboys, knights or dinosaurs. What these films show is basically time teleportation .

Scientists don’t think this conception is likely in the real world, but they also don’t relegate time travel to the crackpot realm. In fact, the laws of physics might allow chronological hopping, but the devil is in the details.

Time traveling to the near future is easy: you’re doing it right now at a rate of one second per second, and physicists say that rate can change. According to Einstein’s special theory of relativity, time’s flow depends on how fast you’re moving. The quicker you travel, the slower seconds pass. And according to Einstein’s general theory of relativity , gravity also affects clocks: the more forceful the gravity nearby, the slower time goes.

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“Near massive bodies—near the surface of neutron stars or even at the surface of the Earth, although it’s a tiny effect—time runs slower than it does far away,” says Dave Goldberg, a cosmologist at Drexel University.

If a person were to hang out near the edge of a black hole , where gravity is prodigious, Goldberg says, only a few hours might pass for them while 1,000 years went by for someone on Earth. If the person who was near the black hole returned to this planet, they would have effectively traveled to the future. “That is a real effect,” he says. “That is completely uncontroversial.”

Going backward in time gets thorny, though (thornier than getting ripped to shreds inside a black hole). Scientists have come up with a few ways it might be possible, and they have been aware of time travel paradoxes in general relativity for decades. Fabio Costa, a physicist at the Nordic Institute for Theoretical Physics, notes that an early solution with time travel began with a scenario written in the 1920s. That idea involved massive long cylinder that spun fast in the manner of straw rolled between your palms and that twisted spacetime along with it. The understanding that this object could act as a time machine allowing one to travel to the past only happened in the 1970s, a few decades after scientists had discovered a phenomenon called “closed timelike curves.”

“A closed timelike curve describes the trajectory of a hypothetical observer that, while always traveling forward in time from their own perspective, at some point finds themselves at the same place and time where they started, creating a loop,” Costa says. “This is possible in a region of spacetime that, warped by gravity, loops into itself.”

“Einstein read [about closed timelike curves] and was very disturbed by this idea,” he adds. The phenomenon nevertheless spurred later research.

Science began to take time travel seriously in the 1980s. In 1990, for instance, Russian physicist Igor Novikov and American physicist Kip Thorne collaborated on a research paper about closed time-like curves. “They started to study not only how one could try to build a time machine but also how it would work,” Costa says.

Just as importantly, though, they investigated the problems with time travel. What if, for instance, you tossed a billiard ball into a time machine, and it traveled to the past and then collided with its past self in a way that meant its present self could never enter the time machine? “That looks like a paradox,” Costa says.

Since the 1990s, he says, there’s been on-and-off interest in the topic yet no big breakthrough. The field isn’t very active today, in part because every proposed model of a time machine has problems. “It has some attractive features, possibly some potential, but then when one starts to sort of unravel the details, there ends up being some kind of a roadblock,” says Gaurav Khanna of the University of Rhode Island.

For instance, most time travel models require negative mass —and hence negative energy because, as Albert Einstein revealed when he discovered E = mc 2 , mass and energy are one and the same. In theory, at least, just as an electric charge can be positive or negative, so can mass—though no one’s ever found an example of negative mass. Why does time travel depend on such exotic matter? In many cases, it is needed to hold open a wormhole—a tunnel in spacetime predicted by general relativity that connects one point in the cosmos to another.

Without negative mass, gravity would cause this tunnel to collapse. “You can think of it as counteracting the positive mass or energy that wants to traverse the wormhole,” Goldberg says.

Khanna and Goldberg concur that it’s unlikely matter with negative mass even exists, although Khanna notes that some quantum phenomena show promise, for instance, for negative energy on very small scales. But that would be “nowhere close to the scale that would be needed” for a realistic time machine, he says.

These challenges explain why Khanna initially discouraged Caroline Mallary, then his graduate student at the University of Massachusetts Dartmouth, from doing a time travel project. Mallary and Khanna went forward anyway and came up with a theoretical time machine that didn’t require negative mass. In its simplistic form, Mallary’s idea involves two parallel cars, each made of regular matter. If you leave one parked and zoom the other with extreme acceleration, a closed timelike curve will form between them.

Easy, right? But while Mallary’s model gets rid of the need for negative matter, it adds another hurdle: it requires infinite density inside the cars for them to affect spacetime in a way that would be useful for time travel. Infinite density can be found inside a black hole, where gravity is so intense that it squishes matter into a mind-bogglingly small space called a singularity. In the model, each of the cars needs to contain such a singularity. “One of the reasons that there's not a lot of active research on this sort of thing is because of these constraints,” Mallary says.

Other researchers have created models of time travel that involve a wormhole, or a tunnel in spacetime from one point in the cosmos to another. “It's sort of a shortcut through the universe,” Goldberg says. Imagine accelerating one end of the wormhole to near the speed of light and then sending it back to where it came from. “Those two sides are no longer synced,” he says. “One is in the past; one is in the future.” Walk between them, and you’re time traveling.

You could accomplish something similar by moving one end of the wormhole near a big gravitational field—such as a black hole—while keeping the other end near a smaller gravitational force. In that way, time would slow down on the big gravity side, essentially allowing a particle or some other chunk of mass to reside in the past relative to the other side of the wormhole.

Making a wormhole requires pesky negative mass and energy, however. A wormhole created from normal mass would collapse because of gravity. “Most designs tend to have some similar sorts of issues,” Goldberg says. They’re theoretically possible, but there’s currently no feasible way to make them, kind of like a good-tasting pizza with no calories.

And maybe the problem is not just that we don’t know how to make time travel machines but also that it’s not possible to do so except on microscopic scales—a belief held by the late physicist Stephen Hawking. He proposed the chronology protection conjecture: The universe doesn’t allow time travel because it doesn’t allow alterations to the past. “It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians,” Hawking wrote in a 1992 paper in Physical Review D .

Part of his reasoning involved the paradoxes time travel would create such as the aforementioned situation with a billiard ball and its more famous counterpart, the grandfather paradox : If you go back in time and kill your grandfather before he has children, you can’t be born, and therefore you can’t time travel, and therefore you couldn’t have killed your grandfather. And yet there you are.

Those complications are what interests Massachusetts Institute of Technology philosopher Agustin Rayo, however, because the paradoxes don’t just call causality and chronology into question. They also make free will seem suspect. If physics says you can go back in time, then why can’t you kill your grandfather? “What stops you?” he says. Are you not free?

Rayo suspects that time travel is consistent with free will, though. “What’s past is past,” he says. “So if, in fact, my grandfather survived long enough to have children, traveling back in time isn’t going to change that. Why will I fail if I try? I don’t know because I don’t have enough information about the past. What I do know is that I’ll fail somehow.”

If you went to kill your grandfather, in other words, you’d perhaps slip on a banana en route or miss the bus. “It's not like you would find some special force compelling you not to do it,” Costa says. “You would fail to do it for perfectly mundane reasons.”

In 2020 Costa worked with Germain Tobar, then his undergraduate student at the University of Queensland in Australia, on the math that would underlie a similar idea: that time travel is possible without paradoxes and with freedom of choice.

Goldberg agrees with them in a way. “I definitely fall into the category of [thinking that] if there is time travel, it will be constructed in such a way that it produces one self-consistent view of history,” he says. “Because that seems to be the way that all the rest of our physical laws are constructed.”

No one knows what the future of time travel to the past will hold. And so far, no time travelers have come to tell us about it.

Time travel: Is it possible?

Science says time travel is possible, but probably not in the way you're thinking.

time travel graphic illustration of a tunnel with a clock face swirling through the tunnel.

Albert Einstein's theory

General relativity and GPS
Wormhole travel
Alternate theories

Science fiction

Is time travel possible? Short answer: Yes, and you're doing it right now — hurtling into the future at the impressive rate of one second per second.

You're pretty much always moving through time at the same speed, whether you're watching paint dry or wishing you had more hours to visit with a friend from out of town.

But this isn't the kind of time travel that's captivated countless science fiction writers, or spurred a genre so extensive that Wikipedia lists over 400 titles in the category "Movies about Time Travel." In franchises like " Doctor Who ," " Star Trek ," and "Back to the Future" characters climb into some wild vehicle to blast into the past or spin into the future. Once the characters have traveled through time, they grapple with what happens if you change the past or present based on information from the future (which is where time travel stories intersect with the idea of parallel universes or alternate timelines).

Related: The best sci-fi time machines ever

Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science fiction or proposed a method of sending a person through significant periods of time that wouldn't destroy them on the way. And, as physicist Stephen Hawking pointed out in his book " Black Holes and Baby Universes" (Bantam, 1994), "The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."

Science does support some amount of time-bending, though. For example, physicist Albert Einstein 's theory of special relativity proposes that time is an illusion that moves relative to an observer. An observer traveling near the speed of light will experience time, with all its aftereffects (boredom, aging, etc.) much more slowly than an observer at rest. That's why astronaut Scott Kelly aged ever so slightly less over the course of a year in orbit than his twin brother who stayed here on Earth.

Related: Controversially, physicist argues that time is real

There are other scientific theories about time travel, including some weird physics that arise around wormholes , black holes and string theory . For the most part, though, time travel remains the domain of an ever-growing array of science fiction books, movies, television shows, comics, video games and more.

Scott and Mark Kelly sit side by side wearing a blue NASA jacket and jeans

Einstein developed his theory of special relativity in 1905. Along with his later expansion, the theory of general relativity , it has become one of the foundational tenets of modern physics. Special relativity describes the relationship between space and time for objects moving at constant speeds in a straight line.

The short version of the theory is deceptively simple. First, all things are measured in relation to something else — that is to say, there is no "absolute" frame of reference. Second, the speed of light is constant. It stays the same no matter what, and no matter where it's measured from. And third, nothing can go faster than the speed of light.

From those simple tenets unfolds actual, real-life time travel. An observer traveling at high velocity will experience time at a slower rate than an observer who isn't speeding through space.

While we don't accelerate humans to near-light-speed, we do send them swinging around the planet at 17,500 mph (28,160 km/h) aboard the International Space Station . Astronaut Scott Kelly was born after his twin brother, and fellow astronaut, Mark Kelly . Scott Kelly spent 520 days in orbit, while Mark logged 54 days in space. The difference in the speed at which they experienced time over the course of their lifetimes has actually widened the age gap between the two men.

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said in a panel discussion on July 12, 2020, Space.com previously reported . "Now I've got that over his head."

General relativity and GPS time travel

Graphic showing the path of GPS satellites around Earth at the center of the image.

The difference that low earth orbit makes in an astronaut's life span may be negligible — better suited for jokes among siblings than actual life extension or visiting the distant future — but the dilation in time between people on Earth and GPS satellites flying through space does make a difference.

Can wormholes take us back in time?

General relativity might also provide scenarios that could allow travelers to go back in time, according to NASA . But the physical reality of those time-travel methods is no piece of cake.

Wormholes are theoretical "tunnels" through the fabric of space-time that could connect different moments or locations in reality to others. Also known as Einstein-Rosen bridges or white holes, as opposed to black holes, speculation about wormholes abounds. But despite taking up a lot of space (or space-time) in science fiction, no wormholes of any kind have been identified in real life.

Related: Best time travel movies

"The whole thing is very hypothetical at this point," Stephen Hsu, a professor of theoretical physics at the University of Oregon, told Space.com sister site Live Science . "No one thinks we're going to find a wormhole anytime soon."

Primordial wormholes are predicted to be just 10^-34 inches (10^-33 centimeters) at the tunnel's "mouth". Previously, they were expected to be too unstable for anything to be able to travel through them. However, a study claims that this is not the case, Live Science reported .

The theory, which suggests that wormholes could work as viable space-time shortcuts, was described by physicist Pascal Koiran. As part of the study, Koiran used the Eddington-Finkelstein metric, as opposed to the Schwarzschild metric which has been used in the majority of previous analyses.

In the past, the path of a particle could not be traced through a hypothetical wormhole. However, using the Eddington-Finkelstein metric, the physicist was able to achieve just that.

Koiran's paper was described in October 2021, in the preprint database arXiv , before being published in the Journal of Modern Physics D.

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These alternate theories share one major flaw: As far as scientists can tell, there's no way a person could survive the kind of gravitational pulling and pushing that each solution requires.

Infinite cylinder theory

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder ) where one could take matter that is 10 times the sun's mass, then roll it into a very long, but very dense cylinder. The Anderson Institute , a time travel research organization, described the cylinder as "a black hole that has passed through a spaghetti factory."

After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around the cylinder — could travel backward in time on a "closed, time-like curve," according to the Anderson Institute.

The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, endless interstellar pasta is beyond our reach.

Time donuts

Theoretical physicist Amos Ori at the Technion-Israel Institute of Technology in Haifa, Israel, proposed a model for a time machine made out of curved space-time — a donut-shaped vacuum surrounded by a sphere of normal matter.

"The machine is space-time itself," Ori told Live Science . "If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time."

Amos Ori is a theoretical physicist at the Technion-Israel Institute of Technology in Haifa, Israel. His research interests and publications span the fields of general relativity, black holes, gravitational waves and closed time lines.

There are a few caveats to Ori's time machine. First, visitors to the past wouldn't be able to travel to times earlier than the invention and construction of the time donut. Second, and more importantly, the invention and construction of this machine would depend on our ability to manipulate gravitational fields at will — a feat that may be theoretically possible but is certainly beyond our immediate reach.

Graphic illustration of the TARDIS (Time and Relative Dimensions in Space) traveling through space, surrounded by stars.

Time travel has long occupied a significant place in fiction. Since as early as the "Mahabharata," an ancient Sanskrit epic poem compiled around 400 B.C., humans have dreamed of warping time, Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science .

Every work of time-travel fiction creates its own version of space-time, glossing over one or more scientific hurdles and paradoxes to achieve its plot requirements.

Some make a nod to research and physics, like " Interstellar ," a 2014 film directed by Christopher Nolan. In the movie, a character played by Matthew McConaughey spends a few hours on a planet orbiting a supermassive black hole, but because of time dilation, observers on Earth experience those hours as a matter of decades.

Others take a more whimsical approach, like the "Doctor Who" television series. The series features the Doctor, an extraterrestrial "Time Lord" who travels in a spaceship resembling a blue British police box. "People assume," the Doctor explained in the show, "that time is a strict progression from cause to effect, but actually from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff."

Long-standing franchises like the "Star Trek" movies and television series, as well as comic universes like DC and Marvel Comics, revisit the idea of time travel over and over.

Related: Marvel movies in order: chronological & release order

Here is an incomplete (and deeply subjective) list of some influential or notable works of time travel fiction:

Books about time travel:

A sketch from the Christmas Carol shows a cloaked figure on the left and a person kneeling and clutching their head with their hands.

Rip Van Winkle (Cornelius S. Van Winkle, 1819) by Washington Irving
A Christmas Carol (Chapman & Hall, 1843) by Charles Dickens
The Time Machine (William Heinemann, 1895) by H. G. Wells
A Connecticut Yankee in King Arthur's Court (Charles L. Webster and Co., 1889) by Mark Twain
The Restaurant at the End of the Universe (Pan Books, 1980) by Douglas Adams
A Tale of Time City (Methuen, 1987) by Diana Wynn Jones
The Outlander series (Delacorte Press, 1991-present) by Diana Gabaldon
Harry Potter and the Prisoner of Azkaban (Bloomsbury/Scholastic, 1999) by J. K. Rowling
Thief of Time (Doubleday, 2001) by Terry Pratchett
The Time Traveler's Wife (MacAdam/Cage, 2003) by Audrey Niffenegger
All You Need is Kill (Shueisha, 2004) by Hiroshi Sakurazaka

Movies about time travel:

Planet of the Apes (1968)
Superman (1978)
Time Bandits (1981)
The Terminator (1984)
Back to the Future series (1985, 1989, 1990)
Star Trek IV: The Voyage Home (1986)
Bill & Ted's Excellent Adventure (1989)
Groundhog Day (1993)
Galaxy Quest (1999)
The Butterfly Effect (2004)
13 Going on 30 (2004)
The Lake House (2006)
Meet the Robinsons (2007)
Hot Tub Time Machine (2010)
Midnight in Paris (2011)
Looper (2012)
X-Men: Days of Future Past (2014)
Edge of Tomorrow (2014)
Interstellar (2014)
Doctor Strange (2016)
A Wrinkle in Time (2018)
The Last Sharknado: It's About Time (2018)
Avengers: Endgame (2019)
Tenet (2020)
Palm Springs (2020)
Zach Snyder's Justice League (2021)
The Tomorrow War (2021)

Television about time travel:

Image of the Star Trek spaceship USS Enterprise

Doctor Who (1963-present)
The Twilight Zone (1959-1964) (multiple episodes)
Star Trek (multiple series, multiple episodes)
Samurai Jack (2001-2004)
Lost (2004-2010)
Phil of the Future (2004-2006)
Steins;Gate (2011)
Outlander (2014-2023)
Loki (2021-present)

Games about time travel:

Chrono Trigger (1995)
TimeSplitters (2000-2005)
Kingdom Hearts (2002-2019)
Prince of Persia: Sands of Time (2003)
God of War II (2007)
Ratchet and Clank Future: A Crack In Time (2009)
Sly Cooper: Thieves in Time (2013)
Dishonored 2 (2016)
Titanfall 2 (2016)
Outer Wilds (2019)

Additional resources

Explore physicist Peter Millington's thoughts about Stephen Hawking's time travel theories at The Conversation . Check out a kid-friendly explanation of real-world time travel from NASA's Space Place . For an overview of time travel in fiction and the collective consciousness, read " Time Travel: A History " (Pantheon, 2016) by James Gleik.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

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Ailsa is a staff writer for How It Works magazine, where she writes science, technology, space, history and environment features. Based in the U.K., she graduated from the University of Stirling with a BA (Hons) journalism degree. Previously, Ailsa has written for Cardiff Times magazine, Psychology Now and numerous science bookazines.

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Time Travel and Modern Physics

Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this attempt to kill your grandfather. Doesn’t this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics.

1. Paradoxes Lost?

2. topology and constraints, 3. the general possibility of time travel in general relativity, 4. two toy models, 5. slightly more realistic models of time travel, 6. the possibility of time travel redux, 7. even if there are constraints, so what, 8. computational models, 9. quantum mechanics to the rescue, 10. conclusions, other internet resources, related entries.

Supplement: Remarks and Limitations on the Toy Models

Modern physics strips away many aspects of the manifest image of time. Time as it appears in the equations of classical mechanics has no need for a distinguished present moment, for example. Relativity theory leads to even sharper contrasts. It replaces absolute simultaneity, according to which it is possible to unambiguously determine the time order of distant events, with relative simultaneity: extending an “instant of time” throughout space is not unique, but depends on the state of motion of an observer. More dramatically, in general relativity the mathematical properties of time (or better, of spacetime)—its topology and geometry—depend upon how matter is arranged rather than being fixed once and for all. So physics can be, and indeed has to be, formulated without treating time as a universal, fixed background structure. Since general relativity represents gravity through spacetime geometry, the allowed geometries must be as varied as the ways in which matter can be arranged. Alongside geometrical models used to describe the solar system, black holes, and much else, the scope of variation extends to include some exotic structures unlike anything astrophysicists have observed. In particular, there are spacetime geometries with curves that loop back on themselves: closed timelike curves (CTCs), which describe the possible trajectory of an observer who returns exactly back to their earlier state—without any funny business, such as going faster than the speed of light. These geometries satisfy the relevant physical laws, the equations of general relativity, and in that sense time travel is physically possible.

Yet circular time generates paradoxes, familiar from science fiction stories featuring time travel: [ 1 ]

Consistency: Kurt plans to murder his own grandfather Adolph, by traveling along a CTC to an appropriate moment in the past. He is an able marksman, and waits until he has a clear shot at grandpa. Normally he would not miss. Yet if he succeeds, there is no way that he will then exist to plan and carry out the mission. Kurt pulls the trigger: what can happen?
Underdetermination: Suppose that Kurt first travels back in order to give his earlier self a copy of How to Build a Time Machine. This is the same book that allows him to build a time machine, which he then carries with him on his journey to the past. Who wrote the book?
Easy Knowledge: A fan of classical music enhances their computer with a circuit that exploits a CTC. This machine efficiently solves problems at a higher level of computational complexity than conventional computers, leading (among other things) to finding the smallest circuits that can generate Bach’s oeuvre—and to compose new pieces in the same style. Such easy knowledge is at odds with our understanding of our epistemic predicament. (This third paradox has not drawn as much attention.)

The first two paradoxes were once routinely taken to show that solutions with CTCs should be rejected—with charges varying from violating logic, to being “physically unreasonable”, to undermining the notion of free will. Closer analysis of the paradoxes has largely reversed this consensus. Physicists have discovered many solutions with CTCs and have explored their properties in pursuing foundational questions, such as whether physics is compatible with the idea of objective temporal passage (starting with Gödel 1949). Philosophers have also used time travel scenarios to probe questions about, among other things, causation, modality, free will, and identity (see, e.g., Earman 1972 and Lewis’s seminal 1976 paper).

We begin below with Consistency , turning to the other paradoxes in later sections. A standard, stone-walling response is to insist that the past cannot be changed, as a matter of logic, even by a time traveler (e.g., Gödel 1949, Clarke 1977, Horwich 1987). Adolph cannot both die and survive, as a matter of logic, so any scheme to alter the past must fail. In many of the best time travel fictions, the actions of a time traveler are constrained in novel and unexpected ways. Attempts to change the past fail, and they fail, often tragically, in just such a way that they set the stage for the time traveler’s self-defeating journey. The first question is whether there is an analog of the consistent story when it comes to physics in the presence of CTCs. As we will see, there is a remarkable general argument establishing the existence of consistent solutions. Yet a second question persists: why can’t time-traveling Kurt kill his own grandfather? Doesn’t the necessity of failures to change the past put unusual and unexpected constraints on time travelers, or objects that move along CTCs? The same argument shows that there are in fact no constraints imposed by the existence of CTCs, in some cases. After discussing this line of argument, we will turn to the palatability and further implications of such constraints if they are required, and then turn to the implications of quantum mechanics.

Wheeler and Feynman (1949) were the first to claim that the fact that nature is continuous could be used to argue that causal influences from later events to earlier events, as are made possible by time travel, will not lead to paradox without the need for any constraints. Maudlin (1990) showed how to make their argument precise and more general, and argued that nonetheless it was not completely general.

Imagine the following set-up. We start off having a camera with a black and white film ready to take a picture of whatever comes out of the time machine. An object, in fact a developed film, comes out of the time machine. We photograph it, and develop the film. The developed film is subsequently put in the time machine, and set to come out of the time machine at the time the picture is taken. This surely will create a paradox: the developed film will have the opposite distribution of black, white, and shades of gray, from the object that comes out of the time machine. For developed black and white films (i.e., negatives) have the opposite shades of gray from the objects they are pictures of. But since the object that comes out of the time machine is the developed film itself it we surely have a paradox.

However, it does not take much thought to realize that there is no paradox here. What will happen is that a uniformly gray picture will emerge, which produces a developed film that has exactly the same uniform shade of gray. No matter what the sensitivity of the film is, as long as the dependence of the brightness of the developed film depends in a continuous manner on the brightness of the object being photographed, there will be a shade of gray that, when photographed, will produce exactly the same shade of gray on the developed film. This is the essence of Wheeler and Feynman’s idea. Let us first be a bit more precise and then a bit more general.

For simplicity let us suppose that the film is always a uniform shade of gray (i.e., at any time the shade of gray does not vary by location on the film). The possible shades of gray of the film can then be represented by the (real) numbers from 0, representing pure black, to 1, representing pure white.

Let us now distinguish various stages in the chronological order of the life of the film. In stage $S_1$ the film is young; it has just been placed in the camera and is ready to be exposed. It is then exposed to the object that comes out of the time machine. (That object in fact is a later stage of the film itself). By the time we come to stage $S_2$ of the life of the film, it has been developed and is about to enter the time machine. Stage $S_3$ occurs just after it exits the time machine and just before it is photographed. Stage $S_4$ occurs after it has been photographed and before it starts fading away. Let us assume that the film starts out in stage $S_1$ in some uniform shade of gray, and that the only significant change in the shade of gray of the film occurs between stages $S_1$ and $S_2$. During that period it acquires a shade of gray that depends on the shade of gray of the object that was photographed. In other words, the shade of gray that the film acquires at stage $S_2$ depends on the shade of gray it has at stage $S_3$. The influence of the shade of gray of the film at stage $S_3$, on the shade of gray of the film at stage $S_2$, can be represented as a mapping, or function, from the real numbers between 0 and 1 (inclusive), to the real numbers between 0 and 1 (inclusive). Let us suppose that the process of photography is such that if one imagines varying the shade of gray of an object in a smooth, continuous manner then the shade of gray of the developed picture of that object will also vary in a smooth, continuous manner. This implies that the function in question will be a continuous function. Now any continuous function from the real numbers between 0 and 1 (inclusive) to the real numbers between 0 and 1 (inclusive) must map at least one number to itself. One can quickly convince oneself of this by graphing such functions. For one will quickly see that any continuous function $f$ from $[0,1]$ to $[0,1]$ must intersect the line $x=y$ somewhere, and thus there must be at least one point $x$ such that $f(x)=x$. Such points are called fixed points of the function. Now let us think about what such a fixed point represents. It represents a shade of gray such that, when photographed, it will produce a developed film with exactly that same shade of gray. The existence of such a fixed point implies a solution to the apparent paradox.

Let us now be more general and allow color photography. One can represent each possible color of an object (of uniform color) by the proportions of blue, green and red that make up that color. (This is why television screens can produce all possible colors.) Thus one can represent all possible colors of an object by three points on three orthogonal lines $x, y$ and $z$, that is to say, by a point in a three-dimensional cube. This cube is also known as the “Cartesian product” of the three line segments. Now, one can also show that any continuous map from such a cube to itself must have at least one fixed point. So color photography can not be used to create time travel paradoxes either!

Even more generally, consider some system $P$ which, as in the above example, has the following life. It starts in some state $S_1$, it interacts with an object that comes out of a time machine (which happens to be its older self), it travels back in time, it interacts with some object (which happens to be its younger self), and finally it grows old and dies. Let us assume that the set of possible states of $P$ can be represented by a Cartesian product of $n$ closed intervals of the reals, i.e., let us assume that the topology of the state-space of $P$ is isomorphic to a finite Cartesian product of closed intervals of the reals. Let us further assume that the development of $P$ in time, and the dependence of that development on the state of objects that it interacts with, is continuous. Then, by a well-known fixed point theorem in topology (see, e.g., Hocking & Young 1961: 273), no matter what the nature of the interaction is, and no matter what the initial state of the object is, there will be at least one state $S_3$ of the older system (as it emerges from the time travel machine) that will influence the initial state $S_1$ of the younger system (when it encounters the older system) so that, as the younger system becomes older, it develops exactly into state $S_3$. Thus without imposing any constraints on the initial state $S_1$ of the system $P$, we have shown that there will always be perfectly ordinary, non-paradoxical, solutions, in which everything that happens, happens according to the usual laws of development. Of course, there is looped causation, hence presumably also looped explanation, but what do you expect if there is looped time?

Unfortunately, for the fan of time travel, a little reflection suggests that there are systems for which the needed fixed point theorem does not hold. Imagine, for instance, that we have a dial that can only rotate in a plane. We are going to put the dial in the time machine. Indeed we have decided that if we see the later stage of the dial come out of the time machine set at angle $x$, then we will set the dial to $x+90$, and throw it into the time machine. Now it seems we have a paradox, since the mapping that consists of a rotation of all points in a circular state-space by 90 degrees does not have a fixed point. And why wouldn’t some state-spaces have the topology of a circle?

However, we have so far not used another continuity assumption which is also a reasonable assumption. So far we have only made the following demand: the state the dial is in at stage $S_2$ must be a continuous function of the state of the dial at stage $S_3$. But, the state of the dial at stage $S_2$ is arrived at by taking the state of the dial at stage $S_1$, and rotating it over some angle. It is not merely the case that the effect of the interaction, namely the state of the dial at stage $S_2$, should be a continuous function of the cause, namely the state of the dial at stage $S_3$. It is additionally the case that path taken to get there, the way the dial is rotated between stages $S_1$ and $S_2$ must be a continuous function of the state at stage $S_3$. And, rather surprisingly, it turns out that this can not be done. Let us illustrate what the problem is before going to a more general demonstration that there must be a fixed point solution in the dial case.

Forget time travel for the moment. Suppose that you and I each have a watch with a single dial neither of which is running. My watch is set at 12. You are going to announce what your watch is set at. My task is going to be to adjust my watch to yours no matter what announcement you make. And my actions should have a continuous (single valued) dependence on the time that you announce. Surprisingly, this is not possible! For instance, suppose that if you announce “12”, then I achieve that setting on my watch by doing nothing. Now imagine slowly and continuously increasing the announced times, starting at 12. By continuity, I must achieve each of those settings by rotating my dial to the right. If at some point I switch and achieve the announced goal by a rotation of my dial to the left, I will have introduced a discontinuity in my actions, a discontinuity in the actions that I take as a function of the announced angle. So I will be forced, by continuity, to achieve every announcement by rotating the dial to the right. But, this rotation to the right will have to be abruptly discontinued as the announcements grow larger and I eventually approach 12 again, since I achieved 12 by not rotating the dial at all. So, there will be a discontinuity at 12 at the latest. In general, continuity of my actions as a function of announced times can not be maintained throughout if I am to be able to replicate all possible settings. Another way to see the problem is that one can similarly reason that, as one starts with 12, and imagines continuously making the announced times earlier, one will be forced, by continuity, to achieve the announced times by rotating the dial to the left. But the conclusions drawn from the assumption of continuous increases and the assumption of continuous decreases are inconsistent. So we have an inconsistency following from the assumption of continuity and the assumption that I always manage to set my watch to your watch. So, a dial developing according to a continuous dynamics from a given initial state, can not be set up so as to react to a second dial, with which it interacts, in such a way that it is guaranteed to always end up set at the same angle as the second dial. Similarly, it can not be set up so that it is guaranteed to always end up set at 90 degrees to the setting of the second dial. All of this has nothing to do with time travel. However, the impossibility of such set ups is what prevents us from enacting the rotation by 90 degrees that would create paradox in the time travel setting.

Let us now give the positive result that with such dials there will always be fixed point solutions, as long as the dynamics is continuous. Let us call the state of the dial before it interacts with its older self the initial state of the dial. And let us call the state of the dial after it emerges from the time machine the final state of the dial. There is also an intermediate state of the dial, after it interacts with its older self and before it is put into the time machine. We can represent the initial or intermediate states of the dial, before it goes into the time machine, as an angle $x$ in the horizontal plane and the final state of the dial, after it comes out of the time machine, as an angle $y$ in the vertical plane. All possible $\langle x,y\rangle$ pairs can thus be visualized as a torus with each $x$ value picking out a vertical circular cross-section and each $y$ picking out a point on that cross-section. See figure 1 .

Figure 1 [An extended description of figure 1 is in the supplement.]

Suppose that the dial starts at angle $i$ which picks out vertical circle $I$ on the torus. The initial angle $i$ that the dial is at before it encounters its older self, and the set of all possible final angles that the dial can have when it emerges from the time machine is represented by the circle $I$ on the torus (see figure 1 ). Given any possible angle of the emerging dial, the dial initially at angle $i$ will develop to some other angle. One can picture this development by rotating each point on $I$ in the horizontal direction by the relevant amount. Since the rotation has to depend continuously on the angle of the emerging dial, circle $I$ during this development will deform into some loop $L$ on the torus. Loop $L$ thus represents all possible intermediate angles $x$ that the dial is at when it is thrown into the time machine, given that it started at angle $i$ and then encountered a dial (its older self) which was at angle $y$ when it emerged from the time machine. We therefore have consistency if $x=y$ for some $x$ and $y$ on loop $L$. Now, let loop $C$ be the loop which consists of all the points on the torus for which $x=y$. Ring $I$ intersects $C$ at point $\langle i,i\rangle$. Obviously any continuous deformation of $I$ must still intersect $C$ somewhere. So $L$ must intersect $C$ somewhere, say at $\langle j,j\rangle$. But that means that no matter how the development of the dial starting at $I$ depends on the angle of the emerging dial, there will be some angle for the emerging dial such that the dial will develop exactly into that angle (by the time it enters the time machine) under the influence of that emerging dial. This is so no matter what angle one starts with, and no matter how the development depends on the angle of the emerging dial. Thus even for a circular state-space there are no constraints needed other than continuity.

Unfortunately there are state-spaces that escape even this argument. Consider for instance a pointer that can be set to all values between 0 and 1, where 0 and 1 are not possible values. That is, suppose that we have a state-space that is isomorphic to an open set of real numbers. Now suppose that we have a machine that sets the pointer to half the value that the pointer is set at when it emerges from the time machine.

Figure 2 [An extended description of figure 2 is in the supplement.]

Suppose the pointer starts at value $I$. As before we can represent the combination of this initial position and all possible final positions by the line $I$. Under the influence of the pointer coming out of the time machine the pointer value will develop to a value that equals half the value of the final value that it encountered. We can represent this development as the continuous deformation of line $I$ into line $L$, which is indicated by the arrows in figure 2 . This development is fully continuous. Points $\langle x,y\rangle$ on line $I$ represent the initial position $x=I$ of the (young) pointer, and the position $y$ of the older pointer as it emerges from the time machine. Points $\langle x,y\rangle$ on line $L$ represent the position $x$ that the younger pointer should develop into, given that it encountered the older pointer emerging from the time machine set at position $y$. Since the pointer is designed to develop to half the value of the pointer that it encounters, the line $L$ corresponds to $x=1/2 y$. We have consistency if there is some point such that it develops into that point, if it encounters that point. Thus, we have consistency if there is some point $\langle x,y\rangle$ on line $L$ such that $x=y$. However, there is no such point: lines $L$ and $C$ do not intersect. Thus there is no consistent solution, despite the fact that the dynamics is fully continuous.

Of course if 0 were a possible value, $L$ and $C$ would intersect at 0. This is surprising and strange: adding one point to the set of possible values of a quantity here makes the difference between paradox and peace. One might be tempted to just add the extra point to the state-space in order to avoid problems. After all, one might say, surely no measurements could ever tell us whether the set of possible values includes that exact point or not. Unfortunately there can be good theoretical reasons for supposing that some quantity has a state-space that is open: the set of all possible speeds of massive objects in special relativity surely is an open set, since it includes all speeds up to, but not including, the speed of light. Quantities that have possible values that are not bounded also lead to counter examples to the presented fixed point argument. And it is not obvious to us why one should exclude such possibilities. So the argument that no constraints are needed is not fully general.

An interesting question of course is: exactly for which state-spaces must there be such fixed points? The arguments above depend on a well-known fixed point theorem (due to Schauder) that guarantees the existence of a fixed point for compact, convex state spaces. We do not know what subsequent extensions of this result imply regarding fixed points for a wider variety of systems, or whether there are other general results along these lines. (See Kutach 2003 for more on this issue.)

A further interesting question is whether this line of argument is sufficient to resolve Consistency (see also Dowe 2007). When they apply, these results establish the existence of a solution, such as the shade of uniform gray in the first example. But physicists routinely demand more than merely the existence of a solution, namely that solutions to the equations are stable—such that “small” changes of the initial state lead to “small” changes of the resulting trajectory. (Clarifying the two senses of “small” in this statement requires further work, specifying the relevant topology.) Stability in this sense underwrites the possibility of applying equations to real systems given our inability to fix initial states with indefinite precision. (See Fletcher 2020 for further discussion.) The fixed point theorems guarantee that for an initial state $S_1$ there is a solution, but this solution may not be “close” to the solution for a nearby initial state, $S'$. We are not aware of any proofs that the solutions guaranteed to exist by the fixed point theorems are also stable in this sense.

Time travel has recently been discussed quite extensively in the context of general relativity. General relativity places few constraints on the global structure of space and time. This flexibility leads to a possibility first described in print by Hermann Weyl:

Every world-point is the origin of the double-cone of the active future and the passive past [i.e., the two lobes of the light cone]. Whereas in the special theory of relativity these two portions are separated by an intervening region, it is certainly possible in the present case [i.e., general relativity] for the cone of the active future to overlap with that of the passive past; so that, in principle, it is possible to experience events now that will in part be an effect of my future resolves and actions. Moreover, it is not impossible for a world-line (in particular, that of my body), although it has a timelike direction at every point, to return to the neighborhood of a point which it has already once passed through. (Weyl 1918/1920 [1952: 274])

A time-like curve is simply a space-time trajectory such that the speed of light is never equaled or exceeded along this trajectory. Time-like curves represent possible trajectories of ordinary objects. In general relativity a curve that is everywhere timelike locally can nonetheless loop back on itself, forming a CTC. Weyl makes the point vividly in terms of the light cones: along such a curve, the future lobe of the light cone (the “active future”) intersects the past lobe of the light cone (the “passive past”). Traveling along such a curve one would never exceed the speed of light, and yet after a certain amount of (proper) time one would return to a point in space-time that one previously visited. Or, by staying close to such a CTC, one could come arbitrarily close to a point in space-time that one previously visited. General relativity, in a straightforward sense, allows time travel: there appear to be many space-times compatible with the fundamental equations of general relativity in which there are CTC’s. Space-time, for instance, could have a Minkowski metric everywhere, and yet have CTC’s everywhere by having the temporal dimension (topologically) rolled up as a circle. Or, one can have wormhole connections between different parts of space-time which allow one to enter “mouth $A$” of such a wormhole connection, travel through the wormhole, exit the wormhole at “mouth $B$” and re-enter “mouth $A$” again. CTCs can even arise when the spacetime is topologically $\mathbb{R}^4$, due to the “tilting” of light cones produced by rotating matter (as in Gödel 1949’s spacetime).

General relativity thus appears to provide ample opportunity for time travel. Note that just because there are CTC’s in a space-time, this does not mean that one can get from any point in the space-time to any other point by following some future directed timelike curve—there may be insurmountable practical obstacles. In Gödel’s spacetime, it is the case that there are CTCs passing through every point in the spacetime. Yet these CTCs are not geodesics, so traversing them requires acceleration. Calculations of the minimal fuel required to travel along the appropriate curve should discourage any would-be time travelers (Malament 1984, 1985; Manchak 2011). But more generally CTCs may be confined to smaller regions; some parts of space-time can have CTC’s while other parts do not. Let us call the part of a space-time that has CTC’s the “time travel region” of that space-time, while calling the rest of that space-time the “normal region”. More precisely, the “time travel region” consists of all the space-time points $p$ such that there exists a (non-zero length) timelike curve that starts at $p$ and returns to $p$. Now let us turn to examining space-times with CTC’s a bit more closely for potential problems.

In order to get a feeling for the sorts of implications that closed timelike curves can have, it may be useful to consider two simple models. In space-times with closed timelike curves the traditional initial value problem cannot be framed in the usual way. For it presupposes the existence of Cauchy surfaces, and if there are CTCs then no Cauchy surface exists. (A Cauchy surface is a spacelike surface such that every inextendable timelike curve crosses it exactly once. One normally specifies initial conditions by giving the conditions on such a surface.) Nonetheless, if the topological complexities of the manifold are appropriately localized, we can come quite close. Let us call an edgeless spacelike surface $S$ a quasi-Cauchy surface if it divides the rest of the manifold into two parts such that

every point in the manifold can be connected by a timelike curve to $S$, and
any timelike curve which connects a point in one region to a point in the other region intersects $S$ exactly once.

It is obvious that a quasi-Cauchy surface must entirely inhabit the normal region of the space-time; if any point $p$ of $S$ is in the time travel region, then any timelike curve which intersects $p$ can be extended to a timelike curve which intersects $S$ near $p$ again. In extreme cases of time travel, a model may have no normal region at all (e.g., Minkowski space-time rolled up like a cylinder in a time-like direction), in which case our usual notions of temporal precedence will not apply. But temporal anomalies like wormholes (and time machines) can be sufficiently localized to permit the existence of quasi-Cauchy surfaces.

Given a timelike orientation, a quasi-Cauchy surface unproblematically divides the manifold into its past (i.e., all points that can be reached by past-directed timelike curves from $S)$ and its future (ditto mutatis mutandis ). If the whole past of $S$ is in the normal region of the manifold, then $S$ is a partial Cauchy surface : every inextendable timelike curve which exists to the past of $S$ intersects $S$ exactly once, but (if there is time travel in the future) not every inextendable timelike curve which exists to the future of $S$ intersects $S$. Now we can ask a particularly clear question: consider a manifold which contains a time travel region, but also has a partial Cauchy surface $S$, such that all of the temporal funny business is to the future of $S$. If all you could see were $S$ and its past, you would not know that the space-time had any time travel at all. The question is: are there any constraints on the sort of data which can be put on $S$ and continued to a global solution of the dynamics which are different from the constraints (if any) on the data which can be put on a Cauchy surface in a simply connected manifold and continued to a global solution? If there is time travel to our future, might we we able to tell this now, because of some implied oddity in the arrangement of present things?

It is not at all surprising that there might be constraints on the data which can be put on a locally space-like surface which passes through the time travel region: after all, we never think we can freely specify what happens on a space-like surface and on another such surface to its future, but in this case the surface at issue lies to its own future. But if there were particular constraints for data on a partial Cauchy surface then we would apparently need to have to rule out some sorts of otherwise acceptable states on $S$ if there is to be time travel to the future of $S$. We then might be able to establish that there will be no time travel in the future by simple inspection of the present state of the universe. As we will see, there is reason to suspect that such constraints on the partial Cauchy surface are non-generic. But we are getting ahead of ourselves: first let’s consider the effect of time travel on a very simple dynamics.

The simplest possible example is the Newtonian theory of perfectly elastic collisions among equally massive particles in one spatial dimension. The space-time is two-dimensional, so we can represent it initially as the Euclidean plane, and the dynamics is completely specified by two conditions. When particles are traveling freely, their world lines are straight lines in the space-time, and when two particles collide, they exchange momenta, so the collision looks like an “$X$” in space-time, with each particle changing its momentum at the impact. [ 2 ] The dynamics is purely local, in that one can check that a set of world-lines constitutes a model of the dynamics by checking that the dynamics is obeyed in every arbitrarily small region. It is also trivial to generate solutions from arbitrary initial data if there are no CTCs: given the initial positions and momenta of a set of particles, one simply draws a straight line from each particle in the appropriate direction and continues it indefinitely. Once all the lines are drawn, the worldline of each particle can be traced from collision to collision. The boundary value problem for this dynamics is obviously well-posed: any set of data at an instant yields a unique global solution, constructed by the method sketched above.

What happens if we change the topology of the space-time by hand to produce CTCs? The simplest way to do this is depicted in figure 3 : we cut and paste the space-time so it is no longer simply connected by identifying the line $L-$ with the line $L+$. Particles “going in” to $L+$ from below “emerge” from $L-$ , and particles “going in” to $L-$ from below “emerge” from $L+$.

Figure 3: Inserting CTCs by Cut and Paste. [An extended description of figure 3 is in the supplement.]

How is the boundary-value problem changed by this alteration in the space-time? Before the cut and paste, we can put arbitrary data on the simultaneity slice $S$ and continue it to a unique solution. After the change in topology, $S$ is no longer a Cauchy surface, since a CTC will never intersect it, but it is a partial Cauchy surface. So we can ask two questions. First, can arbitrary data on $S$ always be continued to a global solution? Second, is that solution unique? If the answer to the first question is $no$, then we have a backward-temporal constraint: the existence of the region with CTCs places constraints on what can happen on $S$ even though that region lies completely to the future of $S$. If the answer to the second question is $no$, then we have an odd sort of indeterminism, analogous to the unwritten book: the complete physical state on $S$ does not determine the physical state in the future, even though the local dynamics is perfectly deterministic and even though there is no other past edge to the space-time region in $S$’s future (i.e., there is nowhere else for boundary values to come from which could influence the state of the region).

In this case the answer to the first question is yes and to the second is no : there are no constraints on the data which can be put on $S$, but those data are always consistent with an infinitude of different global solutions. The easy way to see that there always is a solution is to construct the minimal solution in the following way. Start drawing straight lines from $S$ as required by the initial data. If a line hits $L-$ from the bottom, just continue it coming out of the top of $L+$ in the appropriate place, and if a line hits $L+$ from the bottom, continue it emerging from $L-$ at the appropriate place. Figure 4 represents the minimal solution for a single particle which enters the time-travel region from the left:

Figure 4: The Minimal Solution. [An extended description of figure 4 is in the supplement.]

The particle “travels back in time” three times. It is obvious that this minimal solution is a global solution, since the particle always travels inertially.

But the same initial state on $S$ is also consistent with other global solutions. The new requirement imposed by the topology is just that the data going into $L+$ from the bottom match the data coming out of $L-$ from the top, and the data going into $L-$ from the bottom match the data coming out of $L+$ from the top. So we can add any number of vertical lines connecting $L-$ and $L+$ to a solution and still have a solution. For example, adding a few such lines to the minimal solution yields:

Figure 5: A Non-Minimal Solution. [An extended description of figure 5 is in the supplement.]

The particle now collides with itself twice: first before it reaches $L+$ for the first time, and again shortly before it exits the CTC region. From the particle’s point of view, it is traveling to the right at a constant speed until it hits an older version of itself and comes to rest. It remains at rest until it is hit from the right by a younger version of itself, and then continues moving off, and the same process repeats later. It is clear that this is a global model of the dynamics, and that any number of distinct models could be generating by varying the number and placement of vertical lines.

Knowing the data on $S$, then, gives us only incomplete information about how things will go for the particle. We know that the particle will enter the CTC region, and will reach $L+$, we know that it will be the only particle in the universe, we know exactly where and with what speed it will exit the CTC region. But we cannot determine how many collisions the particle will undergo (if any), nor how long (in proper time) it will stay in the CTC region. If the particle were a clock, we could not predict what time it would indicate when exiting the region. Furthermore, the dynamics gives us no handle on what to think of the various possibilities: there are no probabilities assigned to the various distinct possible outcomes.

Changing the topology has changed the mathematics of the situation in two ways, which tend to pull in opposite directions. On the one hand, $S$ is no longer a Cauchy surface, so it is perhaps not surprising that data on $S$ do not suffice to fix a unique global solution. But on the other hand, there is an added constraint: data “coming out” of $L-$ must exactly match data “going in” to $L+$, even though what comes out of $L-$ helps to determine what goes into $L+$. This added consistency constraint tends to cut down on solutions, although in this case the additional constraint is more than outweighed by the freedom to consider various sorts of data on ${L+}/{L-}$.

The fact that the extra freedom outweighs the extra constraint also points up one unexpected way that the supposed paradoxes of time travel may be overcome. Let’s try to set up a paradoxical situation using the little closed time loop above. If we send a single particle into the loop from the left and do nothing else, we know exactly where it will exit the right side of the time travel region. Now suppose we station someone at the other side of the region with the following charge: if the particle should come out on the right side, the person is to do something to prevent the particle from going in on the left in the first place. In fact, this is quite easy to do: if we send a particle in from the right, it seems that it can exit on the left and deflect the incoming left-hand particle.

Carrying on our reflection in this way, we further realize that if the particle comes out on the right, we might as well send it back in order to deflect itself from entering in the first place. So all we really need to do is the following: set up a perfectly reflecting particle mirror on the right-hand side of the time travel region, and launch the particle from the left so that— if nothing interferes with it —it will just barely hit $L+$. Our paradox is now apparently complete. If, on the one hand, nothing interferes with the particle it will enter the time-travel region on the left, exit on the right, be reflected from the mirror, re-enter from the right, and come out on the left to prevent itself from ever entering. So if it enters, it gets deflected and never enters. On the other hand, if it never enters then nothing goes in on the left, so nothing comes out on the right, so nothing is reflected back, and there is nothing to deflect it from entering. So if it doesn’t enter, then there is nothing to deflect it and it enters. If it enters, then it is deflected and doesn’t enter; if it doesn’t enter then there is nothing to deflect it and it enters: paradox complete.

But at least one solution to the supposed paradox is easy to construct: just follow the recipe for constructing the minimal solution, continuing the initial trajectory of the particle (reflecting it the mirror in the obvious way) and then read of the number and trajectories of the particles from the resulting diagram. We get the result of figure 6 :

Figure 6: Resolving the “Paradox”. [An extended description of figure 6 is in the supplement.]

As we can see, the particle approaching from the left never reaches $L+$: it is deflected first by a particle which emerges from $L-$. But it is not deflected by itself , as the paradox suggests, it is deflected by another particle. Indeed, there are now four particles in the diagram: the original particle and three particles which are confined to closed time-like curves. It is not the leftmost particle which is reflected by the mirror, nor even the particle which deflects the leftmost particle; it is another particle altogether.

The paradox gets it traction from an incorrect presupposition. If there is only one particle in the world at $S$ then there is only one particle which could participate in an interaction in the time travel region: the single particle would have to interact with its earlier (or later) self. But there is no telling what might come out of $L-$: the only requirement is that whatever comes out must match what goes in at $L+$. So if you go to the trouble of constructing a working time machine, you should be prepared for a different kind of disappointment when you attempt to go back and kill yourself: you may be prevented from entering the machine in the first place by some completely unpredictable entity which emerges from it. And once again a peculiar sort of indeterminism appears: if there are many self-consistent things which could prevent you from entering, there is no telling which is even likely to materialize. This is just like the case of the unwritten book: the book is never written, so nothing determines what fills its pages.

So when the freedom to put data on $L-$ outweighs the constraint that the same data go into $L+$, instead of paradox we get an embarrassment of riches: many solution consistent with the data on $S$, or many possible books. To see a case where the constraint “outweighs” the freedom, we need to construct a very particular, and frankly artificial, dynamics and topology. Consider the space of all linear dynamics for a scalar field on a lattice. (The lattice can be though of as a simple discrete space-time.) We will depict the space-time lattice as a directed graph. There is to be a scalar field defined at every node of the graph, whose value at a given node depends linearly on the values of the field at nodes which have arrows which lead to it. Each edge of the graph can be assigned a weighting factor which determines how much the field at the input node contributes to the field at the output node. If we name the nodes by the letters a , b , c , etc., and the edges by their endpoints in the obvious way, then we can label the weighting factors by the edges they are associated with in an equally obvious way.

Suppose that the graph of the space-time lattice is acyclic , as in figure 7 . (A graph is Acyclic if one can not travel in the direction of the arrows and go in a loop.)

Figure 7: An Acyclic Lattice. [An extended description of figure 7 is in the supplement.]

It is easy to regard a set of nodes as the analog of a Cauchy surface, e.g., the set $\{a, b, c\}$, and it is obvious if arbitrary data are put on those nodes the data will generate a unique solution in the future. [ 3 ] If the value of the field at node $a$ is 3 and at node $b$ is 7, then its value at node $d$ will be $3W_{ad}$ and its value at node $e$ will be $3W_{ae} + 7W_{be}$. By varying the weighting factors we can adjust the dynamics, but in an acyclic graph the future evolution of the field will always be unique.

Let us now again artificially alter the topology of the lattice to admit CTCs, so that the graph now is cyclic. One of the simplest such graphs is depicted in figure 8 : there are now paths which lead from $z$ back to itself, e.g., $z$ to $y$ to $z$.

Figure 8: Time Travel on a Lattice. [An extended description of figure 8 is in the supplement.]

Can we now put arbitrary data on $v$ and $w$, and continue that data to a global solution? Will the solution be unique?

In the generic case, there will be a solution and the solution will be unique. The equations for the value of the field at $x, y$, and $z$ are:

Solving these equations for $z$ yields

which gives a unique value for $z$ in the generic case. But looking at the space of all possible dynamics for this lattice (i.e., the space of all possible weighting factors), we find a singularity in the case where $1-W_{zx}W_{xz} - W_{zy}W_{yz} = 0$. If we choose weighting factors in just this way, then arbitrary data at $v$ and $w$ cannot be continued to a global solution. Indeed, if the scalar field is everywhere non-negative, then this particular choice of dynamics puts ironclad constraints on the value of the field at $v$ and $w$: the field there must be zero (assuming $W_{vx}$ and $W_{wy}$ to be non-zero), and similarly all nodes in their past must have field value zero. If the field can take negative values, then the values at $v$ and $w$ must be so chosen that $vW_{vx}W_{xz} = -wW_{wy}W_{yz}$. In either case, the field values at $v$ and $w$ are severely constrained by the existence of the CTC region even though these nodes lie completely to the past of that region. It is this sort of constraint which we find to be unlike anything which appears in standard physics.

Our toy models suggest three things. The first is that it may be impossible to prove in complete generality that arbitrary data on a partial Cauchy surface can always be continued to a global solution: our artificial case provides an example where it cannot. The second is that such odd constraints are not likely to be generic: we had to delicately fine-tune the dynamics to get a problem. The third is that the opposite problem, namely data on a partial Cauchy surface being consistent with many different global solutions, is likely to be generic: we did not have to do any fine-tuning to get this result.

This third point leads to a peculiar sort of indeterminism, illustrated by the case of the unwritten book: the entire state on $S$ does not determine what will happen in the future even though the local dynamics is deterministic and there are no other “edges” to space-time from which data could influence the result. What happens in the time travel region is constrained but not determined by what happens on $S$, and the dynamics does not even supply any probabilities for the various possibilities. The example of the photographic negative discussed in section 2, then, seems likely to be unusual, for in that case there is a unique fixed point for the dynamics, and the set-up plus the dynamical laws determine the outcome. In the generic case one would rather expect multiple fixed points, with no room for anything to influence, even probabilistically, which would be realized. (See the supplement on

Remarks and Limitations on the Toy Models .

It is ironic that time travel should lead generically not to contradictions or to constraints (in the normal region) but to underdetermination of what happens in the time travel region by what happens everywhere else (an underdetermination tied neither to a probabilistic dynamics nor to a free edge to space-time). The traditional objection to time travel is that it leads to contradictions: there is no consistent way to complete an arbitrarily constructed story about how the time traveler intends to act. Instead, though, it appears that the more significant problem is underdetermination: the story can be consistently completed in many different ways.

Echeverria, Klinkhammer, and Thorne (1991) considered the case of 3-dimensional single hard spherical ball that can go through a single time travel wormhole so as to collide with its younger self.

Figure 9 [An extended description of figure 9 is in the supplement.]

The threat of paradox in this case arises in the following form. Consider the initial trajectory of a ball as it approaches the time travel region. For some initial trajectories, the ball does not undergo a collision before reaching mouth 1, but upon exiting mouth 2 it will collide with its earlier self. This leads to a contradiction if the collision is strong enough to knock the ball off its trajectory and deflect it from entering mouth 1. Of course, the Wheeler-Feynman strategy is to look for a “glancing blow” solution: a collision which will produce exactly the (small) deviation in trajectory of the earlier ball that produces exactly that collision. Are there always such solutions? [ 4 ]

Echeverria, Klinkhammer & Thorne found a large class of initial trajectories that have consistent “glancing blow” continuations, and found none that do not (but their search was not completely general). They did not produce a rigorous proof that every initial trajectory has a consistent continuation, but suggested that it is very plausible that every initial trajectory has a consistent continuation. That is to say, they have made it very plausible that, in the billiard ball wormhole case, the time travel structure of such a wormhole space-time does not result in constraints on states on spacelike surfaces in the non-time travel region.

In fact, as one might expect from our discussion in the previous section, they found the opposite problem from that of inconsistency: they found underdetermination. For a large class of initial trajectories there are multiple different consistent “glancing blow” continuations of that trajectory (many of which involve multiple wormhole traversals). For example, if one initially has a ball that is traveling on a trajectory aimed straight between the two mouths, then one obvious solution is that the ball passes between the two mouths and never time travels. But another solution is that the younger ball gets knocked into mouth 1 exactly so as to come out of mouth 2 and produce that collision. Echeverria et al. do not note the possibility (which we pointed out in the previous section) of the existence of additional balls in the time travel region. We conjecture (but have no proof) that for every initial trajectory of $A$ there are some, and generically many, multiple-ball continuations.

Friedman, Morris, et al. (1990) examined the case of source-free non-self-interacting scalar fields traveling through such a time travel wormhole and found that no constraints on initial conditions in the non-time travel region are imposed by the existence of such time travel wormholes. In general there appear to be no known counter examples to the claim that in “somewhat realistic” time-travel space-times with a partial Cauchy surface there are no constraints imposed on the state on such a partial Cauchy surface by the existence of CTC’s. (See, e.g., Friedman & Morris 1991; Thorne 1994; Earman 1995; Earman, Smeenk, & Wüthrich 2009; and Dowe 2007.)

How about the issue of constraints in the time travel region $T$? Prima facie , constraints in such a region would not appear to be surprising. But one might still expect that there should be no constraints on states on a spacelike surface, provided one keeps the surface “small enough”. In the physics literature the following question has been asked: for any point $p$ in $T$, and any space-like surface $S$ that includes $p$ is there a neighborhood $E$ of $p$ in $S$ such that any solution on $E$ can be extended to a solution on the whole space-time? With respect to this question, there are some simple models in which one has this kind of extendability of local solutions to global ones, and some simple models in which one does not have such extendability, with no clear general pattern. The technical mathematical problems are amplified by the more conceptual problem of what it might mean to say that one could create a situation which forces the creation of closed timelike curves. (See, e.g., Yurtsever 1990; Friedman, Morris, et al. 1990; Novikov 1992; Earman 1995; and Earman, Smeenk, & Wüthrich 2009). What are we to think of all of this?

The toy models above all treat billiard balls, fields, and other objects propagating through a background spacetime with CTCs. Even if we can show that a consistent solution exists, there is a further question: what kind of matter and dynamics could generate CTCs to begin with? There are various solutions of Einstein’s equations with CTCs, but how do these exotic spacetimes relate to the models actually used in describing the world? In other words, what positive reasons might we have to take CTCs seriously as a feature of the actual universe, rather than an exotic possibility of primarily mathematical interest?

We should distinguish two different kinds of “possibility” that we might have in mind in posing such questions (following Stein 1970). First, we can consider a solution as a candidate cosmological model, describing the (large-scale gravitational degrees of freedom of the) entire universe. The case for ruling out spacetimes with CTCs as potential cosmological models strikes us as, surprisingly, fairly weak. Physicists used to simply rule out solutions with CTCs as unreasonable by fiat, due to the threat of paradoxes, which we have dismantled above. But it is also challenging to make an observational case. Observations tell us very little about global features, such as the existence of CTCs, because signals can only reach an observer from a limited region of spacetime, called the past light cone. Our past light cone—and indeed the collection of all the past light cones for possible observers in a given spacetime—can be embedded in spacetimes with quite different global features (Malament 1977, Manchak 2009). This undercuts the possibility of using observations to constrain global topology, including (among other things) ruling out the existence of CTCs.

Yet the case in favor of taking cosmological models with CTCs seriously is also not particularly strong. Some solutions used to describe black holes, which are clearly relevant in a variety of astrophysical contexts, include CTCs. But the question of whether the CTCs themselves play an essential representational role is subtle: the CTCs arise in the maximal extensions of these solutions, and can plausibly be regarded as extraneous to successful applications. Furthermore, many of the known solutions with CTCs have symmetries, raising the possibility that CTCs are not a stable or robust feature. Slight departures from symmetry may lead to a solution without CTCs, suggesting that the CTCs may be an artifact of an idealized model.

The second sense of possibility regards whether “reasonable” initial conditions can be shown to lead to, or not to lead to, the formation of CTCs. As with the toy models above, suppose that we have a partial Cauchy surface $S$, such that all the temporal funny business lies to the future. Rather than simply assuming that there is a region with CTCs to the future, we can ask instead whether it is possible to create CTCs by manipulating matter in the initial, well-behaved region—that is, whether it is possible to build a time machine. Several physicists have pursued “chronology protection theorems” aiming to show that the dynamics of general relativity (or some other aspects of physics) rules this out, and to clarify why this is the case. The proof of such a theorem would justify neglecting solutions with CTCs as a source of insight into the nature of time in the actual world. But as of yet there are several partial results that do not fully settle the question. One further intriguing possibility is that even if general relativity by itself does protect chronology, it may not be possible to formulate a sensible theory describing matter and fields in solutions with CTCs. (See SEP entry on Time Machines; Smeenk and Wüthrich 2011 for more.)

There is a different question regarding the limitations of these toy models. The toy models and related examples show that there are consistent solutions for simple systems in the presence of CTCs. As usual we have made the analysis tractable by building toy models, selecting only a few dynamical degrees of freedom and tracking their evolution. But there is a large gap between the systems we have described and the time travel stories they evoke, with Kurt traveling along a CTC with murderous intentions. In particular, many features of the manifest image of time are tied to the thermodynamical properties of macroscopic systems. Rovelli (unpublished) considers a extremely simple system to illustrate the problem: can a clock move along a CTC? A clock consists of something in periodic motion, such as a pendulum bob, and something that counts the oscillations, such as an escapement mechanism. The escapement mechanism cannot work without friction; this requires dissipation and increasing entropy. For a clock that counts oscillations as it moves along a time-like trajectory, the entropy must be a monotonically increasing function. But that is obviously incompatible with the clock returning to precisely the same state at some future time as it completes a loop. The point generalizes, obviously, to imply that anything like a human, with memory and agency, cannot move along a CTC.

Since it is not obvious that one can rid oneself of all constraints in realistic models, let us examine the argument that time travel is implausible, and we should think it unlikely to exist in our world, in so far as it implies such constraints. The argument goes something like the following. In order to satisfy such constraints one needs some pre-established divine harmony between the global (time travel) structure of space-time and the distribution of particles and fields on space-like surfaces in it. But it is not plausible that the actual world, or any world even remotely like ours, is constructed with divine harmony as part of the plan. In fact, one might argue, we have empirical evidence that conditions in any spatial region can vary quite arbitrarily. So we have evidence that such constraints, whatever they are, do not in fact exist in our world. So we have evidence that there are no closed time-like lines in our world or one remotely like it. We will now examine this argument in more detail by presenting four possible responses, with counterresponses, to this argument.

Response 1. There is nothing implausible or new about such constraints. For instance, if the universe is spatially closed, there has to be enough matter to produce the needed curvature, and this puts constraints on the matter distribution on a space-like hypersurface. Thus global space-time structure can quite unproblematically constrain matter distributions on space-like hypersurfaces in it. Moreover we have no realistic idea what these constraints look like, so we hardly can be said to have evidence that they do not obtain.

Counterresponse 1. Of course there are constraining relations between the global structure of space-time and the matter in it. The Einstein equations relate curvature of the manifold to the matter distribution in it. But what is so strange and implausible about the constraints imposed by the existence of closed time-like curves is that these constraints in essence have nothing to do with the Einstein equations. When investigating such constraints one typically treats the particles and/or field in question as test particles and/or fields in a given space-time, i.e., they are assumed not to affect the metric of space-time in any way. In typical space-times without closed time-like curves this means that one has, in essence, complete freedom of matter distribution on a space-like hypersurface. (See response 2 for some more discussion of this issue). The constraints imposed by the possibility of time travel have a quite different origin and are implausible. In the ordinary case there is a causal interaction between matter and space-time that results in relations between global structure of space-time and the matter distribution in it. In the time travel case there is no such causal story to be told: there simply has to be some pre-established harmony between the global space-time structure and the matter distribution on some space-like surfaces. This is implausible.

Response 2. Constraints upon matter distributions are nothing new. For instance, Maxwell’s equations constrain electric fields $\boldsymbol{E}$ on an initial surface to be related to the (simultaneous) charge density distribution $\varrho$ by the equation $\varrho = \text{div}(\boldsymbol{E})$. (If we assume that the $E$ field is generated solely by the charge distribution, this conditions amounts to requiring that the $E$ field at any point in space simply be the one generated by the charge distribution according to Coulomb’s inverse square law of electrostatics.) This is not implausible divine harmony. Such constraints can hold as a matter of physical law. Moreover, if we had inferred from the apparent free variation of conditions on spatial regions that there could be no such constraints we would have mistakenly inferred that $\varrho = \text{div}(\boldsymbol{E})$ could not be a law of nature.

Counterresponse 2. The constraints imposed by the existence of closed time-like lines are of quite a different character from the constraint imposed by $\varrho = \text{div}(\boldsymbol{E})$. The constraints imposed by $\varrho = \text{div}(\boldsymbol{E})$ on the state on a space-like hypersurface are:

local constraints (i.e., to check whether the constraint holds in a region you just need to see whether it holds at each point in the region),
quite independent of the global space-time structure,
quite independent of how the space-like surface in question is embedded in a given space-time, and
very simply and generally stateable.

On the other hand, the consistency constraints imposed by the existence of closed time-like curves (i) are not local, (ii) are dependent on the global structure of space-time, (iii) depend on the location of the space-like surface in question in a given space-time, and (iv) appear not to be simply stateable other than as the demand that the state on that space-like surface embedded in such and such a way in a given space-time, do not lead to inconsistency. On some views of laws (e.g., David Lewis’ view) this plausibly implies that such constraints, even if they hold, could not possibly be laws. But even if one does not accept such a view of laws, one could claim that the bizarre features of such constraints imply that it is implausible that such constraints hold in our world or in any world remotely like ours.

Response 3. It would be strange if there are constraints in the non-time travel region. It is not strange if there are constraints in the time travel region. They should be explained in terms of the strange, self-interactive, character of time travel regions. In this region there are time-like trajectories from points to themselves. Thus the state at such a point, in such a region, will, in a sense, interact with itself. It is a well-known fact that systems that interact with themselves will develop into an equilibrium state, if there is such an equilibrium state, or else will develop towards some singularity. Normally, of course, self-interaction isn’t true instantaneous self-interaction, but consists of a feed-back mechanism that takes time. But in time travel regions something like true instantaneous self-interaction occurs. This explains why constraints on states occur in such time travel regions: the states “ ab initio ” have to be “equilibrium states”. Indeed in a way this also provides some picture of why indeterminism occurs in time travel regions: at the onset of self-interaction states can fork into different equi-possible equilibrium states.

Counterresponse 3. This is explanation by woolly analogy. It all goes to show that time travel leads to such bizarre consequences that it is unlikely that it occurs in a world remotely like ours.

Response 4. All of the previous discussion completely misses the point. So far we have been taking the space-time structure as given, and asked the question whether a given time travel space-time structure imposes constraints on states on (parts of) space-like surfaces. However, space-time and matter interact. Suppose that one is in a space-time with closed time-like lines, such that certain counterfactual distributions of matter on some neighborhood of a point $p$ are ruled out if one holds that space-time structure fixed. One might then ask

Why does the actual state near $p$ in fact satisfy these constraints? By what divine luck or plan is this local state compatible with the global space-time structure? What if conditions near $p$ had been slightly different?

And one might take it that the lack of normal answers to these questions indicates that it is very implausible that our world, or any remotely like it, is such a time travel universe. However the proper response to these question is the following. There are no constraints in any significant sense. If they hold they hold as a matter of accidental fact, not of law. There is no more explanation of them possible than there is of any contingent fact. Had conditions in a neighborhood of $p$ been otherwise, the global structure of space-time would have been different. So what? The only question relevant to the issue of constraints is whether an arbitrary state on an arbitrary spatial surface $S$ can always be embedded into a space-time such that that state on $S$ consistently extends to a solution on the entire space-time.

But we know the answer to that question. A well-known theorem in general relativity says the following: any initial data set on a three dimensional manifold $S$ with positive definite metric has a unique embedding into a maximal space-time in which $S$ is a Cauchy surface (see, e.g., Geroch & Horowitz 1979: 284 for more detail), i.e., there is a unique largest space-time which has $S$ as a Cauchy surface and contains a consistent evolution of the initial value data on $S$. Now since $S$ is a Cauchy surface this space-time does not have closed time like curves. But it may have extensions (in which $S$ is not a Cauchy surface) which include closed timelike curves, indeed it may be that any maximal extension of it would include closed timelike curves. (This appears to be the case for extensions of states on certain surfaces of Taub-NUT space-times. See Earman, Smeenk, & Wüthrich 2009). But these extensions, of course, will be consistent. So properly speaking, there are no constraints on states on space-like surfaces. Nonetheless the space-time in which these are embedded may or may not include closed time-like curves.

Counterresponse 4. This, in essence, is the stonewalling answer which we indicated in section 1. However, whether or not you call the constraints imposed by a given space-time on distributions of matter on certain space-like surfaces “genuine constraints”, whether or not they can be considered lawlike, and whether or not they need to be explained, the existence of such constraints can still be used to argue that time travel worlds are so bizarre that it is implausible that our world or any world remotely like ours is a time travel world.

Suppose that one is in a time travel world. Suppose that given the global space-time structure of this world, there are constraints imposed upon, say, the state of motion of a ball on some space-like surface when it is treated as a test particle, i.e., when it is assumed that the ball does not affect the metric properties of the space-time it is in. (There is lots of other matter that, via the Einstein equation, corresponds exactly to the curvature that there is everywhere in this time travel worlds.) Now a real ball of course does have some effect on the metric of the space-time it is in. But let us consider a ball that is so small that its effect on the metric is negligible. Presumably it will still be the case that certain states of this ball on that space-like surface are not compatible with the global time travel structure of this universe.

This means that the actual distribution of matter on such a space-like surface can be extended into a space-time with closed time-like lines, but that certain counterfactual distributions of matter on this space-like surface can not be extended into the same space-time. But note that the changes made in the matter distribution (when going from the actual to the counterfactual distribution) do not in any non-negligible way affect the metric properties of the space-time. (Recall that the changes only effect test particles.) Thus the reason why the global time travel properties of the counterfactual space-time have to be significantly different from the actual space-time is not that there are problems with metric singularities or alterations in the metric that force significant global changes when we go to the counterfactual matter distribution. The reason that the counterfactual space-time has to be different is that in the counterfactual world the ball’s initial state of motion starting on the space-like surface, could not “meet up” in a consistent way with its earlier self (could not be consistently extended) if we were to let the global structure of the counterfactual space-time be the same as that of the actual space-time. Now, it is not bizarre or implausible that there is a counterfactual dependence of manifold structure, even of its topology, on matter distributions on spacelike surfaces. For instance, certain matter distributions may lead to singularities, others may not. We may indeed in some sense have causal power over the topology of the space-time we live in. But this power normally comes via the Einstein equations. But it is bizarre to think that there could be a counterfactual dependence of global space-time structure on the arrangement of certain tiny bits of matter on some space-like surface, where changes in that arrangement by assumption do not affect the metric anywhere in space-time in any significant way . It is implausible that we live in such a world, or that a world even remotely like ours is like that.

Let us illustrate this argument in a different way by assuming that wormhole time travel imposes constraints upon the states of people prior to such time travel, where the people have so little mass/energy that they have negligible effect, via the Einstein equation, on the local metric properties of space-time. Do you think it more plausible that we live in a world where wormhole time travel occurs but it only occurs when people’s states are such that these local states happen to combine with time travel in such a way that nobody ever succeeds in killing their younger self, or do you think it more plausible that we are not in a wormhole time travel world? [ 5 ]

An alternative approach to time travel (initiated by Deutsch 1991) abstracts away from the idealized toy models described above. [ 6 ] This computational approach considers instead the evolution of bits (simple physical systems with two discrete states) through a network of interactions, which can be represented by a circuit diagram with gates corresponding to the interactions. Motivated by the possibility of CTCs, Deutsch proposed adding a new kind of channel that connects the output of a given gate back to its input —in essence, a backwards-time step. More concretely, given a gate that takes $n$ bits as input, we can imagine taking some number $i \lt n$ of these bits through a channel that loops back and then do double-duty as inputs. Consistency requires that the state of these $i$ bits is the same for output and input. (We will consider an illustration of this kind of system in the next section.) Working through examples of circuit diagrams with a CTC channel leads to similar treatments of Consistency and Underdetermination as the discussion above (see, e.g., Wallace 2012: § 10.6). But the approach offers two new insights (both originally due to Deutsch): the Easy Knowledge paradox, and a particularly clear extension to time travel in quantum mechanics.

A computer equipped with a CTC channel can exploit the need to find consistent evolution to solve remarkably hard problems. (This is quite different than the first idea that comes to mind to enhance computational power: namely to just devote more time to a computation, and then send the result back on the CTC to an earlier state.) The gate in a circuit incorporating a CTC implements a function from the input bits to the output bits, under the constraint that the output and input match the i bits going through the CTC channel. This requires, in effect, finding the fixed point of the relevant function. Given the generality of the model, there are few limits on the functions that could be implemented on the CTC circuit. Nature has to solve a hard computational problem just to ensure consistent evolution. This can then be extended to other complex computational problems—leading, more precisely, to solutions of NP -complete problems in polynomial time (see Aaronson 2013: Chapter 20 for an overview and further references). The limits imposed by computational complexity are an essential part of our epistemic situation, and computers with CTCs would radically change this.

We now turn to the application of the computational approach to the quantum physics of time travel (see Deutsch 1991; Deutsch & Lockwood 1994). By contrast with the earlier discussions of constraints in classical systems, they claim to show that time travel never imposes any constraints on the pre-time travel state of quantum systems. The essence of this account is as follows. [ 7 ]

A quantum system starts in state $S_1$, interacts with its older self, after the interaction is in state $S_2$, time travels while developing into state $S_3$, then interacts with its younger self, and ends in state $S_4$ (see figure 10 ).

Figure 10 [An extended description of figure 10 is in the supplement.]

Deutsch assumes that the set of possible states of this system are the mixed states, i.e., are represented by the density matrices over the Hilbert space of that system. Deutsch then shows that for any initial state $S_1$, any unitary interaction between the older and younger self, and any unitary development during time travel, there is a consistent solution, i.e., there is at least one pair of states $S_2$ and $S_3$ such that when $S_1$ interacts with $S_3$ it will change to state $S_2$ and $S_2$ will then develop into $S_3$. The states $S_2, S_3$ and $S_4$ will typically be not be pure states, i.e., will be non-trivial mixed states, even if $S_1$ is pure. In order to understand how this leads to interpretational problems let us give an example. Consider a system that has a two dimensional Hilbert space with as a basis the states $\vc{+}$ and $\vc{-}$. Let us suppose that when state $\vc{+}$ of the young system encounters state $\vc{+}$ of the older system, they interact and the young system develops into state $\vc{-}$ and the old system remains in state $\vc{+}$. In obvious notation:

Similarly, suppose that:

Let us furthermore assume that there is no development of the state of the system during time travel, i.e., that $\vc{+}_2$ develops into $\vc{+}_3$, and that $\vc{-}_2$ develops into $\vc{-}_3$.

Now, if the only possible states of the system were $\vc{+}$ and $\vc{-}$ (i.e., if there were no superpositions or mixtures of these states), then there is a constraint on initial states: initial state $\vc{+}_1$ is impossible. For if $\vc{+}_1$ interacts with $\vc{+}_3$ then it will develop into $\vc{-}_2$, which, during time travel, will develop into $\vc{-}_3$, which inconsistent with the assumed state $\vc{+}_3$. Similarly if $\vc{+}_1$ interacts with $\vc{-}_3$ it will develop into $\vc{+}_2$, which will then develop into $\vc{+}_3$ which is also inconsistent. Thus the system can not start in state $\vc{+}_1$.

But, says Deutsch, in quantum mechanics such a system can also be in any mixture of the states $\vc{+}$ and $\vc{-}$. Suppose that the older system, prior to the interaction, is in a state $S_3$ which is an equal mixture of 50% $\vc{+}_3$ and 50% $\vc{-}_3$. Then the younger system during the interaction will develop into a mixture of 50% $\vc{+}_2$ and 50% $\vc{-}_2$, which will then develop into a mixture of 50% $\vc{+}_3$ and 50% $\vc{-}_3$, which is consistent! More generally Deutsch uses a fixed point theorem to show that no matter what the unitary development during interaction is, and no matter what the unitary development during time travel is, for any state $S_1$ there is always a state $S_3$ (which typically is not a pure state) which causes $S_1$ to develop into a state $S_2$ which develops into that state $S_3$. Thus quantum mechanics comes to the rescue: it shows in all generality that no constraints on initial states are needed!

One might wonder why Deutsch appeals to mixed states: will superpositions of states $\vc{+}$ and $\vc{-}$ not suffice? Unfortunately such an idea does not work. Suppose again that the initial state is $\vc{+}_1$. One might suggest that that if state $S_3$ is

one will obtain a consistent development. For one might think that when initial state $\vc{+}_1$ encounters the superposition

it will develop into superposition

and that this in turn will develop into

as desired. However this is not correct. For initial state $\vc{+}_1$ when it encounters

will develop into the entangled state

In so far as one can speak of the state of the young system after this interaction, it is in the mixture of 50% $\vc{+}_2$ and 50% $\vc{-}_2$, not in the superposition

So Deutsch does need his recourse to mixed states.

This clarification of why Deutsch needs his mixtures does however indicate a serious worry about the simplifications that are part of Deutsch’s account. After the interaction the old and young system will (typically) be in an entangled state. Although for purposes of a measurement on one of the two systems one can say that this system is in a mixed state, one can not represent the full state of the two systems by specifying the mixed state of each separate part, as there are correlations between observables of the two systems that are not represented by these two mixed states, but are represented in the joint entangled state. But if there really is an entangled state of the old and young systems directly after the interaction, how is one to represent the subsequent development of this entangled state? Will the state of the younger system remain entangled with the state of the older system as the younger system time travels and the older system moves on into the future? On what space-like surfaces are we to imagine this total entangled state to be? At this point it becomes clear that there is no obvious and simple way to extend elementary non-relativistic quantum mechanics to space-times with closed time-like curves: we apparently need to characterize not just the entanglement between two systems, but entanglement relative to specific spacetime descriptions.

How does Deutsch avoid these complications? Deutsch assumes a mixed state $S_3$ of the older system prior to the interaction with the younger system. He lets it interact with an arbitrary pure state $S_1$ younger system. After this interaction there is an entangled state $S'$ of the two systems. Deutsch computes the mixed state $S_2$ of the younger system which is implied by this entangled state $S'$. His demand for consistency then is just that this mixed state $S_2$ develops into the mixed state $S_3$. Now it is not at all clear that this is a legitimate way to simplify the problem of time travel in quantum mechanics. But even if we grant him this simplification there is a problem: how are we to understand these mixtures?

If we take an ignorance interpretation of mixtures we run into trouble. For suppose that we assume that in each individual case each older system is either in state $\vc{+}_3$ or in state $\vc{-}_3$ prior to the interaction. Then we regain our paradox. Deutsch instead recommends the following, many worlds, picture of mixtures. Suppose we start with state $\vc{+}_1$ in all worlds. In some of the many worlds the older system will be in the $\vc{+}_3$ state, let us call them A -worlds, and in some worlds, B -worlds, it will be in the $\vc{-}_3$ state. Thus in A -worlds after interaction we will have state $\vc{-}_2$ , and in B -worlds we will have state $\vc{+}_2$. During time travel the $\vc{-}_2$ state will remain the same, i.e., turn into state $\vc{-}_3$, but the systems in question will travel from A -worlds to B -worlds. Similarly the $\vc{+}$ $_2$ states will travel from the B -worlds to the A -worlds, thus preserving consistency.

Now whatever one thinks of the merits of many worlds interpretations, and of this understanding of it applied to mixtures, in the end one does not obtain genuine time travel in Deutsch’s account. The systems in question travel from one time in one world to another time in another world, but no system travels to an earlier time in the same world. (This is so at least in the normal sense of the word “world”, the sense that one means when, for instance, one says “there was, and will be, only one Elvis Presley in this world.”) Thus, even if it were a reasonable view, it is not quite as interesting as it may have initially seemed. (See Wallace 2012 for a more sympathetic treatment, that explores several further implications of accepting time travel in conjunction with the many worlds interpretation.)

We close by acknowledging that Deutsch’s starting point—the claim that this computational model captures the essential features of quantum systems in a spacetime with CTCs—has been the subject of some debate. Several physicists have pursued a quite different treatment of evolution of quantum systems through CTC’s, based on considering the “post-selected” state (see Lloyd et al. 2011). Their motivations for implementing the consistency condition in terms of the post-selected state reflects a different stance towards quantum foundations. A different line of argument aims to determine whether Deutsch’s treatment holds as an appropriate limiting case of a more rigorous treatment, such as quantum field theory in curved spacetimes. For example, Verch (2020) establishes several results challenging the assumption that Deutsch’s treatment is tied to the presence of CTC’s, or that it is compatible with the entanglement structure of quantum fields.

What remains of the grandfather paradox in general relativistic time travel worlds is the fact that in some cases the states on edgeless spacelike surfaces are “overconstrained”, so that one has less than the usual freedom in specifying conditions on such a surface, given the time-travel structure, and in some cases such states are “underconstrained”, so that states on edgeless space-like surfaces do not determine what happens elsewhere in the way that they usually do, given the time travel structure. There can also be mixtures of those two types of cases. The extent to which states are overconstrained and/or underconstrained in realistic models is as yet unclear, though it would be very surprising if neither obtained. The extant literature has primarily focused on the problem of overconstraint, since that, often, either is regarded as a metaphysical obstacle to the possibility time travel, or as an epistemological obstacle to the plausibility of time travel in our world. While it is true that our world would be quite different from the way we normally think it is if states were overconstrained, underconstraint seems at least as bizarre as overconstraint. Nonetheless, neither directly rules out the possibility of time travel.

If time travel entailed contradictions then the issue would be settled. And indeed, most of the stories employing time travel in popular culture are logically incoherent: one cannot “change” the past to be different from what it was, since the past (like the present and the future) only occurs once. But if the only requirement demanded is logical coherence, then it seems all too easy. A clever author can devise a coherent time-travel scenario in which everything happens just once and in a consistent way. This is just too cheap: logical coherence is a very weak condition, and many things we take to be metaphysically impossible are logically coherent. For example, it involves no logical contradiction to suppose that water is not molecular, but if both chemistry and Kripke are right it is a metaphysical impossibility. We have been interested not in logical possibility but in physical possibility. But even so, our conditions have been relatively weak: we have asked only whether time-travel is consistent with the universal validity of certain fundamental physical laws and with the notion that the physical state on a surface prior to the time travel region be unconstrained. It is perfectly possible that the physical laws obey this condition, but still that time travel is not metaphysically possible because of the nature of time itself. Consider an analogy. Aristotle believed that water is homoiomerous and infinitely divisible: any bit of water could be subdivided, in principle, into smaller bits of water. Aristotle’s view contains no logical contradiction. It was certainly consistent with Aristotle’s conception of water that it be homoiomerous, so this was, for him, a conceptual possibility. But if chemistry is right, Aristotle was wrong both about what water is like and what is possible for it. It can’t be infinitely divided, even though no logical or conceptual analysis would reveal that.

Similarly, even if all of our consistency conditions can be met, it does not follow that time travel is physically possible, only that some specific physical considerations cannot rule it out. The only serious proof of the possibility of time travel would be a demonstration of its actuality. For if we agree that there is no actual time travel in our universe, the supposition that there might have been involves postulating a substantial difference from actuality, a difference unlike in kind from anything we could know if firsthand. It is unclear to us exactly what the content of possible would be if one were to either maintain or deny the possibility of time travel in these circumstances, unless one merely meant that the possibility is not ruled out by some delineated set of constraints. As the example of Aristotle’s theory of water shows, conceptual and logical “possibility” do not entail possibility in a full-blooded sense. What exactly such a full-blooded sense would be in case of time travel, and whether one could have reason to believe it to obtain, remain to us obscure.

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How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

Adlam, Emily, unpublished, “ Is There Causation in Fundamental Physics? New Insights from Process Matrices and Quantum Causal Modelling ”, 2022, arXiv: 2208.02721. doi:10.48550/ARXIV.2208.02721
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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

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From millennium-skipping Victorians to phone booth-hopping time traveler teenagers, the term time travel often summons our most fantastic visions of what it means to move through the fourth dimension. But of course you don't need a time machine or a fancy wormhole to jaunt through the years.

As you've probably noticed, we're all constantly engaged in the act of time travel. At its most basic level, time is the rate of change in the universe -- and like it or not, we are constantly undergoing change. We age, the planets move around the sun, and things fall apart.

We measure the passage of time in seconds, minutes, hours and years, but this doesn't mean time flows at a constant rate. In fact Einstein's theory of relativity determines that time is not universal. Just as the water in a river rushes or slows depending on the size of the channel, time flows at different rates in different places. In other words, time is relative.

But what causes this fluctuation along our one-way trek from the cradle to the grave? It all comes down to the relationship between time and space. Human beings frolic about in the three spatial dimensions of length, width and depth. Time joins the party as that most crucial fourth dimension . Time can't exist without space, and space can't exist without time. The two exist as one: the space time continuum . Any event that occurs in the universe has to involve both space and time.

In this article, we'll look at the real-life, everyday methods of time travel in our universe, as well as some of the more far-fetched methods of dancing through the fourth dimension.

Did you know your GPS devices rely on time-travel calculations to help you navigate around town? It's true! GPS satellite clocks are about 3 8 seconds longer per day than a clock closer to earth due to the gravitational frequency shift. They make up for this discrepancy by using time travel calculations or they could be way off from your current location and time.

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

A Student Just Proved Paradox-Free Time Travel Is Possible

Now we can all go back to 2019.

This follows recent research observing that the present is not changed by a time-traveling qubit.
It's still not very nice to step on butterflies, though.

In a peer-reviewed paper, an honors undergraduate student says he has mathematically proven the physical feasibility of a specific kind of time travel. The paper appears in Classical and Quantum Gravity .

University of Queensland student Germain Tobar, who the university’s press release calls “prodigious,” worked with UQ physics professor Fabio Costa on this paper . In “ Reversible dynamics with closed time-like curves and freedom of choice ,” Tobar and Costa say they’ve found a middle ground in mathematics that solves a major logical paradox in one model of time travel. Let’s dig in.

The math itself is complex, but it boils down to something fairly simple. Time travel discussion focuses on closed time-like curves (CTCs), something Albert Einstein first posited. And Tobar and Costa say that as long as just two pieces of an entire scenario within a CTC are still in “causal order” when you leave, the rest is subject to local free will.

“Our results show that CTCs are not only compatible with determinism and with the local 'free choice' of operations, but also with a rich and diverse range of scenarios and dynamical processes,” their paper concludes.

In a university statement, Costa illustrates the science with an analogy:

“Say you travelled in time, in an attempt to stop COVID-19's patient zero from being exposed to the virus. However if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place. This is a paradox, an inconsistency that often leads people to think that time travel cannot occur in our universe. [L]ogically it's hard to accept because that would affect our freedom to make any arbitrary action. It would mean you can time travel, but you cannot do anything that would cause a paradox to occur."

Some outcomes of this are grouped as the “ butterfly effect ,” which refers to unintended large consequences of small actions. But the real truth, in terms of the mathematical outcomes, is more like another classic parable: the monkey’s paw. Be careful what you wish for, and be careful what you time travel for. Tobar explains in the statement:

“In the coronavirus patient zero example, you might try and stop patient zero from becoming infected, but in doing so you would catch the virus and become patient zero, or someone else would. No matter what you did, the salient events would just recalibrate around you. Try as you might to create a paradox, the events will always adjust themselves, to avoid any inconsistency.”

While that sounds frustrating for the person trying to prevent a pandemic or kill Hitler, for mathematicians, it helps to smooth a fundamental speed bump in the way we think about time. It also fits with quantum findings from Los Alamos , for example, and the way random walk mathematics behave in one and two dimensions.

At the very least, this research suggests that anyone eventually designing a way to meaningfully travel in time could do so and experiment without an underlying fear of ruining the world—at least not right away.

preview for Pop News: Airports, Lava Floors and Movie Stunts

Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all.

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Time travel is possible, but it’s a one-way ticket

Chenoa van den Boogaard , Physics and Astronomy editor

The ability to travel through time, whether it is to fix a mistake in the past or gain insight into the future, has long been embraced by science fiction and debated by theoretical physicists. While the debate continues over whether travelling into the past is possible, physicists have determined that travelling to the future most certainly is. And you don’t need a wormhole or a DeLorean to do it.

Real-life time travel occurs through time dilation, a property of Einstein’s special relativity . Einstein was the first to realize that time is not constant, as previously believed, but instead slows down as you move faster through space.

As part of his theory, Einstein re-envisioned space itself. He coined the phrase “spacetime,” fusing the three dimensions of space and one dimension of time into a single term. Instead of treating space as a flat and rigid place that holds all the objects in the universe, Einstein thought of it as curved and malleable, able to form gravitational dips around masses that pull other objects in, just as a bowling ball placed in the centre of a trampoline would cause any smaller object placed on the trampoline to slide towards the centre.

A computer-generated representation of Einstein’s curved spacetime. The Earth creates a gravitational dip in the fabric of spacetime which is deepest at its core. Courtesy and Â© of NASA

The closer an object gets to the centre of the dip, the faster it accelerates. The centre of the Earth’s gravitational dip is located at the Earth’s core, where gravitational acceleration is strongest. According to Einstein’s theory, because time moves more slowly as you move faster through space, the closer an object is to the centre of the Earth, the slower time moves for that object.

This effect can be seen in GPS satellites, which orbit 20,200 kilometres above the Earth’s surface. These satellites have highly precise clocks onboard that gain an average of 38 microseconds per day due to time dilation. While this time gain seems insignificant, GPS satellites rely on their onboard clocks to maintain precise global positioning. Running 38 microseconds fast would result in a positioning error of nearly 10 kilometres, an error that would increase daily if the time difference were not constantly corrected.

A more dramatic example of time dilation can be seen in the movie Interstellar when Matthew McConaughey and his crew land on a planet with an extreme gravitational field caused by a nearby black hole. Because of the black hole’s intense gravitational influence, time slows dramatically for the crew on the planet, making one hour on the surface equal to seven years on Earth. This is why, when the crew returns to Earth, Matthew McConaughey’s daughter is an old woman while he appears to be the same age as when he left.

So why hasn’t humanity succeeded in making such drastic leaps forward in time? The answer to this question comes down to velocity. In order for humanity to send a traveller years into the future, we would either have to take advantage of the intense gravitational acceleration caused by black holes or send the traveller rocketing into space at close to the speed of light (about 1 billion km/h). With our current technology , jumping a few microseconds into the future is all humans can manage.

But if technology one day allows us to send a human into the future by travelling close to the speed of light, would there be any way for the traveller to use time dilation to return to the past and report her findings? “Interstellar travel reaching close to the speed of light might be possible,” says Dr. Jaymie Matthews , professor of astrophysics at the University of British Columbia, “[but] this voyage is one way into the future, not back to the past.”

If we can’t use time dilation to return to the past, does this mean that the past is forever inaccessible? Perhaps not. Einstein proposed that time travel into the past could be achieved through an Einstein-Rosen bridge, a type of wormhole. Wormholes are theoretical areas of spacetime that are warped in a way that connects two distant points in space.

A visualization of a wormhole: The fabric of spacetime curves back upon itself, forming a bridge between two distant locations. Image by Panzi , CC-BY 3.0

Einstein’s equations suggested that this bridge in space could hypothetically connect two points in time instead if it were stable enough. “At the moment, even an Einstein-Rosen bridge cannot [be used to] go back in the past because it doesn’t live long enough – it is not stable,” Matthews explains.

“Even if it was stable, it [requires] other physics, which we don’t have. Hypothetical particles and states of matter that have “exotic” physical properties that would violate known laws of physics, such as a particle having a negative mass. That is why “wormholes” are only science fiction.”

While it would be fascinating to travel back in time to see the dinosaurs or to meet Albert Einstein and show him the reality of time travel, perhaps it is best if the past remains untouched. Travelling to the past invites the possibility of making an alteration that could destroy the future. For example, in Back to the Future , Marty McFly travels to the past and inadvertently prevents his parents from meeting each other, nearly preventing his own existence. But if he had undone his own existence, how could he have travelled back in time in the first place?

Marty’s adventures are a variation of the grandfather paradox: what happens if you go back in time and kill your grandfather before your father is conceived? If you are successful, how is it possible that you’re alive to kill your grandfather in the first place?

A recent study at the University of Queensland may have the answer to this baffling paradox. In this study, the researchers prove mathematically that paradox-free time travel is possible, showing that the universe will self-correct to avoid inconsistencies. If this is true, then even if we could travel back in time, we would never be able to alter events to create a different future.

While these new findings are enlightening, there appears to be more evidence that, although time dilation can allow us to glimpse the future, we will never be able to visit the past. As the late Stephen Hawking said in his book Black Holes and Baby Universes , “The best evidence we have that time travel [into the past] is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Banner image by Alex Lehner, CC BY 2.0

240 thoughts on “ Time travel is possible, but it’s a one-way ticket ”

How do I go about time travel? what do I need how do I get those required things?

Very large ring magnets and some mathematics and will to see it in reality.

How about a sphere magnet ship…

hoe about 3d time and hemi synch or portals augmented reality,power of suggestion..drugs pcp binural tones frequency amplitude .virtual computing ie.

I’m a time traveling tourist, Stephen Hawking was wrong.

Time is simply a measurement of space under the amount given its mass and the amount of light and dark in which governs its mass in a 4dimensional reality step outside of the force in which permenates its flow one would reside there would be no past present or future there be a fixed permance of a constant here and now and so ok then what is to come.

Very well explained article !!

But I think if physics says time travel can be possible then it’s definitely possible. Considering not to go back to your childhood and fix things but rather can go to the past but as invisible person to them. So that,

No actions by you would impact your future.

Regards, Kirankumar DR

Tell me more

Yes.. I wish I can do this too 🙂

We will understand it better, by and by…

I have a theory for warp speed, but nasa would have to put it to the test…check my Facebook

I am reading for this drive , i am ready , without think my life safe or not

@Ravi chandila English translation please?

Please someone help me I just want to send a message to myself in my past.,to get the love of my life, he never revealed to me his feelings now my life is ruined by the decision of my elders Please help me, it’s question of my life and death. Nazneen

Is time travel machine is their, if the time travel machine is true can it move to the past . To bring back my lost life

That’s the problem you know.. it is not there that’s why we aren’t able to travel time..and yes it it will be built then you will be able to do so…..

damn my life is also lost and broken but still no one can give a time machine for free

DO NOT change the future. That’s why people like you couldn’t go. One wrong person to ruin it for the rest of us

On the point of time reversal, it is evidently impossible. The Uncertainty Principle prohibits spacetime reversal. The Universe is unable to remember its past (as a consequence of the Uncertainty Principle), therefore the Universe cannot reorganise itself.

Can I have to go on my past with another time travel it is a possible when just tell me about one thing that can I have to go in my past one year

we dont need magnets.we need a strong gravitational force to warp spacetime allowing us to travel through with speed of sound or speed of light or faster.we need to learn how to control such force carefully or it could be lethal.gravity slows down time.but it can theoratically work both ways.if we can reverse the gravity’s natural reaction we could speed up a spacecraft faster than light(its all relative(and theoratical))

I WAS ACTUALLY JUST THINKING THE SAME THINFG BEFORE READING YOUR PIECE. VERY WELL EXPLAINED, AND IT DOES MAKE ALOT OF SENSE. WELL DONE.

oh and I forgot to add it can be the key to look into the universe and also travelling time(theoratical).speed and gravity are the key to the universe(theory not proved)

All you really need is a crystal diode with 16 sides, a large pain of glass, and a frequency transmitter near a bathtub full of ice cold water….if you reach the right frequency you can travel through time forward and reverse…

Magnetized metal(VCR Reading Head), to read time out of the Magnetosphere all around earth. The Magnetosphere kills 2 birds with one stone- it protects earth and it records human time:

Mystery solved and I will explain, I was in a coma 3 months and I experienced things, I traveled time forward and backward, it is not a one way ticket. Movies and songs are recorded on magnetic tape in a VCR tape Cartridge or Cassette tape,Â Â Magnetic tape recording works by converting electrical signals into magnetic energy, which imprints a record of the signal onto a moving tape covered in magnetic particles.Â Â 3D life on earth(a movie), and the Magnetosphere all around earth coming from the core of earth(MAGNETIC ACTIVITY) without Atom Made Tape, is like a movie on magnetic Atom made tape in a VCR tape cartridge. Revolution and Rotation is the motor(VCR).

This is why people have those freaky Deji’vuÂ feelings like they have lived this before, BECAUSE YOU HAVE, and how people can be psychic, and how there is Prophecy in the Bible. When a person dies, their Spirit- MIND(Thoughts, Feelings, Urges(Physical and mental personality)) breaks out of human body- a stopped heart is what releases the spirit from the human body. Then the Soul(Life) with the memory of your existence in it breaks out of spirit and goes back to your birthday with a erased memory, meanwhile your spirit goes back in time to when you were a teenager starting the mental puberty, maturity from that adult spirit you died with in last life.In that old movie Star Wars or maybe it was the Empire Strikes Back, there is a scene where Princess LayaÂ plays like a 3D movie, that is EXACTLY how its of life on earth.

Mr Snow, I believe you as I have seen it too. As humans we have deep knowledge of things we cannot rationally explain but you have done a great job here.

I thought that Analogy would be a better and easier way to explain, or in a picture of the earth from far out in space with the atmosphere around it looks like a DVD disk and the earth being the center sticker but is in 3D.

Actually you are on to several things here. I have also had the infusion of knowledge that also had to do with comparing life to recorded movies and music. I know you were using it to explain your theory, but I do think there is something there, I always have. When you watch a movie you are seeing the past. Why canâ€™t you somehow use a recording as a base to go back into? I agree with everything you said here, and itâ€™s worth looking into.

Jeffrey, very interesting idea!! Could be something to that. As far as your coma experiences, I think there are things we just do not understand and are nearly impossible to explain. Perhaps time IS like a video tape, or a DVD? Magnetism is one of the forces of nature. I too have had some odd experiences that suggest that we are able to perceive things beyond our five known senses.

I think if you have had a near death experience, such as being in a coma, then you have experienced the powerful hallucinations provided by the chemical substance DMT which your body creates naturally in times of extreme trauma, but also found in most plants and used recreationally by some who are brave enough and into that kind of thing. Your theory is interesting, but completely unproven and as far as I know untested. If things were so simple, I’m sure many scientists would have already thought of such an idea and tested it.

How do I travel through time

Be alive and live life to the fullest is the best way to travel through time ! OR Befriend grey aliens../ They may hold the key to the sum of all knowledge in the universe..

Sounds good will it work

Really log vaps mil sakte hau h kya

Can you plz explain I didn’t get it

You dont first all you are not experienced in the field of the space time continum and you could you upset the already fragile and multitude of alternate realitys that have looping due irresponsible ones who somehow gotten the technology causing another altered time frame there are a disarray multiple reality which are looping in earths 4dimensonal time frame time traveling is not for a vacation or just to get a joy ride its a serious and complex reality not be joked about it is a real thing and certain individual have are upset the balance of earths original time zone note now the gaurdians of this region of milky way the galatic order of the light keepers Angelic gaurdians of the (names with held)are working over time ooh nice pun (over TIME) ha wow to restore Earth back to a original time continum

Who said I want a joy ride, my life is devastated even my kids are suffering, I want to commit suicide but can’t leave my kids back, Being captive for most of my life, if my life is changed nothing will be disturbed, only thing happens is 3 life’s will be saved. And more so over I don’t want to travel I just want to send a message to myself in my past plz on the date of 30th May 1996. My life is ruined plz help me, it was my dad,brother, sister who pushed me into the dungeon and my husband and his family took over the charge of torturing me. Nazneen

I want to go back in time and tell my 5 year old self to burn the creepy dolls that my mom bought cause there is demons in it at the same time I will kidnap and torture my dad right now go back in time and show the younger version of my dad show him what will happen to his future self if he donâ€™t get rid of those possessed objects and keeps letting my mom buy those antiques Iâ€™m 18 now Iâ€™m single no girlfriend no friend alone nothing very depressed too and I try to remember the positive things that happened in my life which there arenâ€™t many tho but the demons keep squeezing my memory brain and my mom keeps on making so much loud noise including her damn mouth I have attempted to burn the demonic dolls but I only burned them for a minute or two with gas cause I was worried I might accidentally set my whole neighborhood on fire but then my mom threw it all in the recycle instead of the trash so the demons just keep bothering me its driving me nuts he he.

Access to a Quantum Computer Network on the web would be a good start. A series of ChatBots and webhook sites strategically placed in not only space, but in time. A series of algorithms and I think information can be transferred backwards to ones self…

How do we know that there are no horde of tourists among ourselves?

How do we know we’re all not tourists?

We’re all time travelers. We all travel into the future daily. 1 second at a time. Lol…

Agreed! I had the same thought!

Excellent question

If is possible, I would like to go back to: January the 1st 1975 & relive the 70â€™s as I prefer that decade to the awful one I am facing now, Back then We had more police our streets & left our front doors open, Those days were far much more better .

https://3netra.co.in/61-2/

Please do comment on my blog post regarding time travel

how about you ask the flash to help you

I need the time travel so I’m fails so many times i love time travel i have to go fast and future so i have no idea im travel is a my dream so my dream solution plz say me i have time travel so please help me someone please…..

I think you are over reacting

When we look at the stars now it is what they looked like years ago so what if we go to the stars and look down?

You cant go to the stars. It will just take billions and billions of years to go even to the next nearest star than our Sun- proxima centuri. Sorry to say, but do you think that you will be alive all those years??

You can do that without going to the stars… our planet reflects light as well thus making it visible from other parts of the universe…. has the word â€œreflectionâ€ crossed your mind ? 😉

Contact me on my hangout I will help you [email protected]

bro just time travel its not that hard

Please help me to time travel, can I see myself when I go back in time like Harmaini sees herself in Harry potter?? Or can I send messages to myself I know the particular date when to send. It’s not the mistake I had done in my past but it was done by my father and brother who are safe, happy enjoying their lives,my life is totally ruined Please help me. Nazneen

I want to go back in time to save my wife .it was a bad mastake she died .that could be changed i need to go back and save her. Please help me.yours gordon sutcliffe

Would love to hear more how itâ€™s possible, as I am really so desperate to go back in time. I lost my wife 6mons back because of COVID and I will do the impossible things to make it happen.

DMT Experience

what is that?

Dmt experience. Time travel, out of body and sometimes superhuman capabilities.

Jump into a black hole

We have to lose something(the past) to gain something(the future) in time travel.Time cannot be played with.Am I correct.

you need to have d e t e r m i n a t i o n

Time machine is possible

speeder than light LOL

speeder than light cuz if the light break it limits it will move backward in time

Don’t Just don’t disturb the past

I want to go back in time and see my dad. I miss him.

mee too raina I lost my father the day before you posted the comment 18th may, crap it hurts me so much. I would rather die to bring those moments back….

Everything is connected . Time isn’t real .

It is universe we travel to and not a time line in one universe

Ask trump….Mandela effect…. dmt 5th dimension

u need an X-WING starfighter and a lightsaber to fight the knights at past and a R2-B2 to track

The fact that no one has time travelled to the past is the proof that time travelling will NEVER exist.

Others have. Portals open most of the time. Example: Miami Fl. Magnetic Material gets bombarded by the sun. Which fractures and formed portals within that area. Ley lines can lead to the portals of travel within miami for just to start. One can laugh or wonder if. In my experience jumping for the better the word of it (Movie Jumper) can be done. You can either Teleport or Time Travel. Our sun open these portals everyday. The best time when Sun spots start to emerge. All that electrons traveling at light speed is enough to rupture our magnetic fields on Earth. You will return of course. Like water on a lake or an ocean time will corrects itself. Your inner clock is your ticket back home. With a little math,fourth dimensional thinking,a magnetic meter, the right location,history research and luck. You may get to expirence it. First clue….cold spots…it may not be a ghost.

Plz can you help me please help me you can save my life

I wish I could help you, I can sense your sufferings.

You need a bag of hyperlink modules to start, then nuclear beepbeep gatangas, when you have that come back here and I will tell you what you need next.

You need high voltage beepbeep gatangas and a large broonasic magnet of about 450 Gauss, come back here when you have these and I will tell you the rest.

you need an old fashioned police box

If you rotate the center of the earth in the opposite direction, then the whole earth can be moved back in time, on the other hand, if you move the center of the earth and change its position by separating it from the part of the earth, then you will be able to time correctly. Let’s reach the other side.

How I could time travel any time travel machines inverted

give audition in the flash series..

I think that to go back in time you’d to travel faster than the speed of light since time stops at the speed of light but if you wanted to go back to say mlk’s assassination you would need to go at least 10 times the speed of light

You don’t want to, the moment you wrote that message is a historical point in time.

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It’s Close I can feel it

Yes it becomes a history but my life also in the past changes and the present also with it. The way I’m suffering from the pain and want to end my life I’m 100% sure at least sure no one around me is or was as hopeless and horrible as my hubby I’m devastated I really want to send a message to my past it may not start but it will definitely change. I was forced, not given any option, my father and brother gave me wrong information and had no concerns for me. It was just survival for me. I repent for not killing myself when I had time, but now if I have a chance why not. Now when I’m out of my marriage I come to know a guy then had feelings for me, was madly in love and wanted to ask for my hand, now I want to inform my self and change everything plz help me.

I too would like to go back in time. I just wish he lived a happy eternal life. I would just like to repeat to come back in 2020.

I heard from a guy in Idaho that time travel is possible. You’ll need to go online and purchase a pogo stick looking device and make sure not to forget the crystals.

I think u need a black-hole-proof spaceship, go to the centre, escape the black hole and viola! You are now in the past. If you can’t escape, then you’d travel to a time where that black hole didn’t exist.

Believe me you time travel! If not physically then you do mentally,like you through dreams.

Though they sale it online, it would not take the chance. It is as simple as beating the speed of light and having some system to send you to the time you want. Time however is not real, and were just traving universes. It will all be in the open in 2028 according to other travelers.

All you need base on how to travel to time is very simple but had to find firstly find a way to get to space through a space rocket secondly find a very perfect consifigration for traveling to tiTme then find a very fast rocket that could create a form of force reaction in space in order yo enable fast speed in space for the break through of non gravity in space and make sure that while doing all you activities is not far away from planet and not also to close to planet earth and make sure that you are with wristwatchs whose time is set disame then you can to the future

Man you can get all you need for too build a time machine in your local store man, man I sure wished Iâ€™d kept mine but it frightened the heck off me man, sometimes when I fart I find a grape in my pants

time travel is a fake, baseless and delusional idea. If you believe in that crap then tell us if we are living in the future or in the past. To travel backward the entire system has to return all along with nature and events, it won’t be for you alone except time travel only happens in the mind.

you would need to get about 1,000,000 pounds of silicon and then somehow conduct enough energy to make 500 cars run without an engine and then go to a nuqular power plant and somehow make a portal. but the whole world could go out of orbit if you do that so I wouldent sugest it.

Time machine is good and bad because,with the time machine you will know about your future which is not good.

Is time travel actually a real thing because if it is then I need it because I am trying to go back in time to fix all of my mistakes

So what if time travel is the reason that we now believe there are other realities in our own world.this could be that a Time traveler we could only go back and couldn’t come back, and on doing so if you do something to change the past in stead make a new reality.making other things are deferent and ours realty stays the same . sometimes reality gets mixed up make the mandela effect that we see today

Time in the future it is faster then now. The past is slower so you can travel . It is up to you. One way is to meditate. You can travel and see any body you want right now. You can fly faster then light. That is one way. You go to the future. To go to the past you sleep for a long time. Some time you go to the future or the past. Your heart well stop and your body gets cold. Sometimes you can control it sometimes you can’t.

but how do we know that is really true ? i mean i want to figure this out, i want to time travel, but how is it that simple ? so many people have been trying to figure this out for many years and its that simple ?

Yeah what if you get stuck in there what do you do than

You cant go there in the first place. Dont worry. With current technology, we will only end up messing some few microseconds. Highly doubtful, if we can end up getting the news of travelling hundreds of years in our lifetime.

wait what would happen if someone saw you while you where in past/future i’m curious

Time is an illusion based on perceived reality and is only relative to our limitations. Time isnâ€™t what it seems and all things canâ€™t be figured out

Im on a school computer looking this up and i found this article and scrolling trough it and ive not heard one statement here as good as yours bro

This is blowing my mind people, then I see the school boy on the post. Great stuff, whoever reads this is already capable of travelling through time. Think about all people who have posted on this thread, now think about who will read mine. Now think of those â‚¬opposite trolls $ who never ever bother posting on you tube thread etc. But ONE comment from one of the time travellers who wrote on this thread. So that opposite troll is me,I donâ€™t normally post.however because of previous comments Iâ€™m posting here. And I love the DMT shit I loved that and lived that one out in real life,,,,another day.

So my point is ifOne or two threads have made me write this….then what will my post make others write , think…..then I could travel back and not write this…. then what. Love the conception of time how can u travel something that doesnâ€™t YOU perceive to be time, like a train can only run on its train tracks, a car can only drive on a road etc Itâ€™s posibble I know it is. Sometimes when u have fun times moves swift but locked in jail it goes snail pace. U c me. I write letters to myself from past from future. Remember everything that happens in present becomes part the past. But the future is what you hold in your hands. Question is, now you know….what the f are u gonna do about it?.. 01/04 ==== 21

Hahahah only realised school boy is named BIG dick pissing myself laughing I gotta go pee. Respect certified

so not halal mode

True so were not traveling in time. It is just different universe (on what we call) different time, day, tears, etc.

You would be scared for life

you will desepear

Maybe it has happened before and we just don’t know that they’re from the future. If people in the future time traveled, the would know that it’s dangerous to mess with the past and would pretend to be part of the past.

I believe time travel is already possible, however we cannot fix past mistakes without altering future predicaments. Say we stop JFK’s assasination, that would completely change the future from that point forward to one none of us can know/guess or conclude the effects? Other time travel purposes go to the future I think that from now our world will die off before 2096 basdd on overpopulation, global warming & polution as such creating islands of plastic waste in our oceans. The best thing my opinion go back to the garden of Eden, kill that Serpent Satan before he tricks Eve into the forbidden fruit. Then let God raise, enlighten & teach us how to be humanly sustainable on his planet & I guarantee technology & smart phones? Ain’t no part of it!!

Time travel possible but one n only theory of Stephen hawking

How it is possible to jump in time …??

Many ways. The most used is creating a black hole which can be done in a few ways. 1) traveling forwards or backwords faster than the speed of light 2) been known during heavy lightning strikes. Each way is a fast movement that opens the black hole. It has been done by the Government since the 1980s though they claimed they never beet the speed of light until 2002. However, Time is a illusion and their for we are actually traveling different universe that are differnt than ours even if the difference is by 1 thing. Each universe may have (what we call) different time, days and years. And each time we change that time line we created a new one. It is belief as CERN has said they destroy 5 universe, that they can travel to them. Since 2012 it has seem we been shifting and is now belief they have possibly came together. The event is known as The Mandela Effect.

No one has the right theory in my thinking. Only a few things are wrong. It is universes with (what we call) different time, days and years we are traveling to and not time itself as it is a illusion. Their is no stop to how much we can do, or where we can go. No limit as such say.

There is no God. No magical serpent or Garden of Eden ever existed. Basing a scientific theory on archaic stories does no one any good.

You choose a hopeless eternity. I choose hope through the promise of salvation through Christ for those who believe. You see, I have child in heaven. Thankfully, have a hopeful reality that I can embrace. There is a God. Our known universe is only 14 or so billion years old… is it mathematically possible that random molecules out of the Big Bang mixed in just the right way from to form a complex cellular organism… with DNA… and result in humans and such diversity of life forms? Itâ€™s naive to accept this as a result of chance. Think about it. How is that remotely possible without a creator?

Hahaha. You make it seem as tho the big bang happened, and we just popped into existence? Naw it’s called evolution baby, we started out as microscopic organisms, seriously, when did you drop out of school? But that’s like saying a some guy writes a book to explain away natural phenomenons that they were to stupid (un-evolved) to grasp and the concept good and bad and the eternal damnation, And thus, the Bible, and boom, everyone now was made by God, hahaha. When you can prove he/she exists, and that the Bible was a autobiography, and not just some twisted piece of Fiction, that has no real basis in reality, and cannot be proved to be more that a work of Fiction. Rather than being used as the16th Century control tact, ‘be good or you’ll go to hell’. But I guess that’s what they mean when they say ignorance is bliss, (maybe if I was as ignorant as y’all believers I’d believe to). But I can’t see how a ‘GOD’ would ever ask one of its creations to kill another.. Genocide, Crusades, all the ethnic cleansing.. All In the name of God Almighty! Hahahahahhaaa. Aliens are more believable than this shit, and theirs no proof they exist either. Hahahahaha. Fug’n Bible thumpers. ‘Step out side your faith and see the world for what it really is, a complex organism, mad of gravity and dust, quite a unique specimen! And we, yes Bible bangers, this includes you, are destroying it like the bubonic plague.’. ‘The end is coming and it’s our fault’

Have you taken the time to read The Old Testament and the prophecies therein that came to be ?.

How do you explain that ?.

My last post should read GS not G

You have not had an encounter yet with God. Don’t be so certain on yuour theory of evolution. He came and shook my reality to it’s core. Made thing possibly that no one could ever explain.

What are you talking about? Ur so wrong and funny in every way.

BlissfullyInformed just told me his comment was all an April fools prank. He believes in Jesus and was just fooling.

Time travel is very much possible just as you decided to come existence in this century meaning one can decide to be in another time zone . life is all about numbers, you just have to work on numbers

I’m pretty sure ppl don’t decide to come into existence. If that were true I wouldn’t be replying to your comment.

Un like your other reply, I understand what you mean. Each timeline (or universe as some see it) can easily be traveled to at will. No different than traveling threw your time you want to visit.

Science has proven a few things from the Bible is true. God does exist. Christians are confused with time and what it says. For a example. God created the world, as science even belives it was God who created the big bang, yet the bang has happen itself creating the moon, planets and stars. Christians also fail to understand chapter 1 and 2 of gen. spoke of two different creations which can be why we see dinosaurs before humans as chapter 1 spoke of animals first and humans 2nd. Their also was different time than, as without the moon a full day is 6 hours. It would take 4 days back than to equal are 1 day. Time is lost and Christians are just confuse on that time. That does not proof their is no God. As they have already found the robes of Jesus and remains of Noah’s ark, it proves much did happen. The bible only has less than 50% of what was written.

Changing the past is impossible, because if we went back into the past, that means we were already there during the time you experienced it.

We all know how to get into time travel but how do we get out……..

You don’t need time travel – all you need is life. And what is life? Life is the evolution of the impossible into the inevitable over an infinite amount of time.

if it is shown that if something, such as a solution to a particular class of equations, were possible, then two mutually contradictory things would be true, such as a number being both even and odd. The contradiction implies that the original premise is impossible.

This is called proof by impossibility. Thus if some traveled back in time far enough to kill his grandfather, we have the contradiction and therefore it is impossible.

You could argue that he would be able to time travel, but not kill his grandfather. However almost anything a person does going back in time would cause the same contradiction, thererfore it is the traveling back in time that is impossible.

Actually, it probably is possible to travel back in time, however to do so, you would also have to travel so far in space that you cannot see anything that happened before your current time due to the speed of light, because this to could affect the future.

The reason I am here is that, i really want to go back the day when our matriculation exam was just finished. Everything around me is peaceful and happy. Currently, I am living in dire situation. People are dying outside on the streets. Smokes everywhere. Everything is in doom. Ah, yeah. I really miss my past. If you are reading this, you can judge me in anyways. I just want to live peacefully and happily.

You must live in Portland

I entirely know what you say and how you feel, Robin. I am totally convinced that future is no promise to offer a better place to live. World is becoming unnecessarily more complex and more horrible and more insecure. Therefore, travelling back in time to a point where things were still far away from such ordeals is what I aspire. But I think if it is possible to travel back in time without the possibility of carrying our lived experiences with us, it will be useless as we will be repeating the same mistakes over and over again. Now, this begs the questions “in what type of physique could we imagine ourselves back there if such time travel becomes possible? That is, becoming younger again in a physical regression (as I said this would be a torture without having learned from all these later years)? Or appearing at our desired times in our present physique and age? I believe the most ideal one would be if we appeared at our desired point in time at the same age that we were at that point of time with a good feeling of our later lived experiences.

Mam all u need to do is just run faster as much as u can or visit the black hole because in both condition time just slow it down ….

Time travel is simple. If you do happen to travel to the past you create a new time line not affecting the time line you left. In essence you going to the past is now your future. Even if you were able to return you may never know if you remained in your time-line or created a new one. So even if you changed something in your travels it would happen in the future not the past.

Sorry time traveling is not possible, there is no way you can go into the past or the future â€â™‚ï¸. You can only be in the time you are already in.

Incorrect. General relativity allows time travel into the future. You need a space ship that can travel extremely fast though, approaching the speed of light, or you need to get close to a supermassive black hole.

It is travel into the past that there is no known practical way to do, and is probably impossible.

So what happens when we Die? Where do we go? I want to go back in time so I can meet my childhood friends…

Simple question from a simple mind:

At what point, when a person says they are from the future, do we stop throwing them in the funny farm and actually start listening??

When they show actual proof. Not just some random prediction of the future.

I don’t believe that “glimpses into the future” could be possible. If it were so, we could glimpse blueprints of the future that we could bring back to the present and build before they were invented. My personal.beleif is in any time frame there is only one active time which is the present. The past no longer exists and the future hasn’t occurred yet, so there is no such thing as ‘time travel’ except for the frame we are in now.

First off time is not real we make time if you travel anywhere all you are doing is beating the Earth speed try this for a mathematical equation the Earth travels a thousand miles per hour you’re not beating human time that is your own equation the Earth travels a thousand miles per hour a space shuttle travel 17,000 mph you can beat time that you made so time is not real you are only beating the Earth speed if you go in a space shuttle and go around the earth 17,000 miles per hour the Earth only travels a thousand miles per hour plus it has all types of gravitational pull from the Moon Earth’s access on the til t you figure out the mathematical equation I cannot time travel is real if you can beat the Earth speed and we can it has nothing to do with its 12:00 it’s 1:00 that’s not real time is made up as a mathematical equation you can beat the Earth speed you can go back into the Earth’s time in a space shuttle but you’re not beating anything except the Earth’s speed think about that one time is not real at all all it is is a mathematical equation think about that one real long

What I’m trying to say is this a space shuttle travel 17,000 mph the Earth travels a thousand you beat it 16 times faster that’s all you did you’re not beating any time you’re not beating 1:00 you’re not beating 3:00 all you’re doing is beating the Earth’s time you can go in reverse around the Earth 17,000 mph okay you can go forward with the Earth’s centrifugal force 17,000 miles per hour you’re not beating anything you’re beating a mathematically equation that we we created astronauts been traveling time for instance for years and haven’t told us because of the space shuttle that does travel 17,000 mph it beats the Earth speed 16 times a boggles my mind you have the Earth access the moon gravitational pull but you can get in a space shuttle and travel 17,000 miles per hour and beat the Earth’s speed 17 times think about it

If any scientist or anybody can actually answer this question how do you set up this equation with the Earth spinning a thousand miles per hour you have the moon pulling gravity the Earth’s access on until I want to know tell me then wondering for a while this equation popped into my head about 2 years ago I’m not a math whiz or anything I just thought about it weird how the mind works I’m not into space or any space stuff at all I’m Samanthas boy friend John antos wrote this

I liked your post and the knowledge you given. I also written a post on Time Travel.

how would any of that stuff be true because e’*34+Em would stop all the forss of vissecs and how would we do it if you now what i mean??? also thanks for the scuff for my project

I would love it if I had a real life time machine here with me now which could take me to anytime I want, the past, present or future. If I had a time machine here with me now, I would go to the past in September 2004 when I was born and give myself to another family that is actually rich and not this horrible family that I have now.

that not nice

Close but not quite right scientists of the idiotic variety, yes, you don’t want people to travel back in time to mess with their own pasts, of course, but you say it’s impossible, but it’s not, and I’m always ignored with my crazed crackpot theories, so what’s the harm in telling the truth as I see it, while it could be possible to travel to the past, here in lies the problem with rewriting the future, while some believe it’s possible to travel back in time, but it’s very expensive and definitely a one-way trip to the future or to the past. Basically Doc Brown got the mechanism for time travel almost right but the energy out put needs to be quadrupled instead, allowing for the ‘physical item, being or vehicle’ to transport through time without killing the time traveler in question. Wormholes are unpredictable, until warp speed for spaceships are a thing, it is not possible for the space ships to achieve time travel, unless they want to enter a black hole, which I would not recommend. as you need warp speed to survive the emptiness of the black hole, without being ripped to shreds. Say for example, Back to the future 1, the timeline doesn’t erase it continues on without the ‘said time traveler’ in existence basically the Marty from Wimpy George’s timeline did time travel to the past and messed with his parent’s meeting so to speak, but never return to the same timeline therefore Marty A went known as a Missing Child in timeline A, while it continues on without him, however Marty A became Marty B/C, in the Successful George Timeline. So that is what I’m talking about. the timeline changes only for the time traveler themselves the ones who are left behind don’t experience a thing of timeline rewritten-ism, as it would never happen in the first place. The other thing is if you want to mess with your own childhood, to make a better life for the past self, the key thing to remember it’s not really you. It’s an alternative version of you, that you interfered with. creating a parallel timeline to it’s original, yet slightly different. Yes it would be awkward to raise yourself. but as long as you are staying in the past, nothing should happen until the age you traveled back in time, unless of course you touched your past self and suddenly de-aged and merged with your past self, is an option 1, option 2 the future self explodes spreading guts all over the place and therefore the past self, of you became a murderer of your future self, I am more inclined to believe option 1 as option 2 seems a little too out there. Basically you would have two memories one of the former timeline and one of the current different timeline. Still traveling through time is truly a one way trip and if you want to travel through time, you would need some time travel mechanism, the way you scientist talk is basically a dream version, or an OBE version (OUT-OF-BODY-EXPERIENCE) which is basically a vivid/lucid dream which is not true time travel, the true time travel is based on the BTTF Trilogy not the idiotic versions you preach about. I believe I’ve said enough.

Mystery solved and I will explain, I was in a coma 3 months and I experienced things, I traveled time forward and backward, it is not a one way ticket. Movies and songs are recorded on magnetic tape in a VCR tape Cartridge or Cassette tape, Magnetic tape recording works by converting electrical signals into magnetic energy, which imprints a record of the signal onto a moving tape covered in magnetic particles. 3D life on earth(a movie), and the Magnetosphere all around earth coming from the core of earth(MAGNETIC ACTIVITY) without Atom Made Tape, is like a movie on magnetic Atom made tape in a VCR tape cartridge. Revolution and Rotation is the motor(VCR).

This is why people have those freaky Dejiâ€™vu feelings like they have lived this before, BECAUSE YOU HAVE, and how people can be psychic, and how there is Prophecy in the Bible. When a person dies, their Spirit- MIND(Thoughts, Feelings, Urges(Physical and mental personality)) breaks out of human body- a stopped heart is what releases the spirit from the human body. Then the Soul(Life) with the memory of your existence in it breaks out of spirit and goes back to your birthday with a erased memory, meanwhile your spirit goes back in time to when you were a teenager starting the mental puberty, maturity from that adult spirit you died with in last life.In that old movie Star Wars or maybe it was the Empire Strikes Back, there is a scene where Princess Laya plays like a 3D movie, that is EXACTLY how its of life on earth.

If only wish I could undo everything what I’ve done wrong in the past, I’d be more happier

And that my friend is absolutely what you do not or would not know. Everyone focuses on what they don’t or haven’t had rather than what positives they do have around them. To change the ingredients of a past life only changes the flavour you have in this life, it does not make you happier.

No, travel to the future is not possible. Like, future is unpredictable and always have been so give up on that field

Already has been, and has been proven.

Time travel is not so possible for every one , but there are already time travelers on earth #@*

Who are these time travelers?

Depends if it is the Governments (they done it since the 80s), or if it was a Accidental travel, or a simple us creating our own machine. Either way, one can easily find storys, and other evidence with a good research. I have a website that shows the effects of change cause by time travel.

They are out their (done by the government since the 80s) but the future is open with time travel (told its open since 2028) so they travel back much.

Time travel 101-

Create a closed loop circuit around a full metal structure, hermetically seal it and bring O2, Use two tesla coils to create north and south poles. (Artificial Magneto sphere.) Make sure to pain the outside in lead to prevent any cosmic rays from penetrating the materials on the inside. (Radiation = bad). Connect a ball made of w/e with wires that alternate the current from the coils to w/e panel on the outside of the structure to make it move via inductive magnetic / electric Lorentzo (Lorentzo = ExMfield = Velocity. = Antigravity) Create Antigravity by using forces from the inside reactor. (Pressurized Mercury, and Tesla Turbine.) Then Move 10-100x faster than light depending on the charged field, Friction will be added to the electric field instead of the craft allowing the G-forces not to crush you inside. The field will take the pressures of outer space, The temperature of space will allow for super conductivity of the structure.

Eventually you will arrive in the future, if you stay in one place. but account for the movement of earth in your travel log. To see outside you will need a monitor / camera system, as any leaks through a viewing area will cause death by radiation from the cosmic rays from the field you have created.

The O2 can be used as a backup generator, through air pressure and the tesla turbine.

There are many different ways to make wormholes, but the curvature of space is really hard to calculate to send a machine far out to the end and create a link with the machine that wants to travel there. And leaving one behind to get back.

If you can imagine it, it can be done. You just need the knowledge of not dying to complete it.

U.S.S. Tourist, You’re a time traveler or just insanely smart.

You don’t need to go the speed of light. Human Time is recorded in the magnetospere as a movie is record, ed on magnet VCR Tape or a song on a record. A VCR or record does not have to go light speed to retrieve the recorded info. All of life is recorded in 3D by our Magnetosphere. My Analogy is imagine a VCR tape cartridge being the earth, imagine life on earth being the movie but in 3D with out adom made tape, imagine Rotation and Revolution of Earth being the VCR putting all in to motion- playing. That is how its done, the magnetosphere kills two birds with one stone, it protects earth and records time, human time is in a magnetic bubble that is why the Bible refers our time is different from gods time and this is how God the maker(PLANET OF UNITED SUPREME BEINGS) can flip through our time to know everything. By the way long before life on earth, he built the original 7 wonders of world(Pyramids) to Pump the Seven gasses into the atmosphere of this planet found in the goldilocks zone, so Life can live on it, and that life of all types is his technological cyborgs that grow and multiply on earth also he seeded it with plant, trees, sea creature and things that fly,. Anyway that above is how time is recorded.

Until recently, I thought my neighbor was a crackpot until he actually invented a time machine. He utilized an ordinary closet, and showed me the sophisticated (to me) instrumentation he had installed. I was very skeptical at first, until he offered a small demonstration and entered the time coordinates and energized his invention. To my amazement, when I opened the door, the clock on the wall was 30 minutes later than when we stepped into the machine. OMG!!! Destroy this thing before it destroys us!!!.

So happy to have my husband back after 6 months of separation. get any kind of relationship/marriage help you want from….Robinsonbuckler11 @gmail com………………………

I find it odd that people say time travel isn’t possible yet… If time travel is possible, it has always existed. Meaning, there is not past present it future, only our perception of time. What we know as past present and future have always been occurring simultaneously, so travel was invited the moment the universe wss formed. Dinosaurs are roaming the earth right now, and forever. A version of me is typing this and has always been typing this, within this perceived moment of “time” and time travel has always happened, whether or not we exist in that reality at the right “time” to observe time travel is the only question.

I find it odd that people say time travel isn’t possible yet… If time travel is possible, it has always existed. Meaning, there is no past present or future, only our perception of time. What we know as past present and future have always been occurring simultaneously, so travel was invited the moment the universe was formed. Dinosaurs are roaming the earth right now, and forever. A version of me is typing this has always been typing this, within this perceived moment of “time” and time travel has always happened, whether or not we exist in that reality at the right “time” to observe time travel is the only question.

Their had to be one point however, when it was created and started, and for that, there was nothing but the current time. Once it was created, than we had a pass, present and future to which we can go back to millions of years to see Adam and Eve with the dinosaurs or go millions of years in the future. However, given the events that changes, each time a new time line has been created. We also have destroyed the planet and repopulated many times in the last million years. Each event changed, or something we do different (without traveling) enters a new universe where some things may be different or the same. Today are universe are shifting a lot.

To be fair, even if it is a one way trip into the past, that doesn’t stop machines going back. We could send a machine back and order it to do anything we want and then tell it to meet us at a certain time in the future. We send it back, then go straight to the meeting point we agreed and then we’ll be able to prove if it worked or not.

I’m a girl who has read a book about seeing future through a box. So is it actually possible?

Time travel has been done on purpose by the Government since the late 1980s. From research, the mostly use kids, or future Presidents. Their are some cases where people have been struck by lightning or came across some tragically event that cause them to leave their timeline either forward or behind in time. The Mandela Effect is the current cause of how things go wrong when time travel is not done right. Click on my name to see the website.

Even as traveling to a location as a future or pass date is possible as what people here mean. However, as you said, it is numbers. Time is a illusion and we do not travel threw time, just universe that are different than ours. What we call time dates and months is what changes each universe. We are all from different universes today as they came together. The mandela effect is a fine example.

thx to eleon wont we soon be able to digitize our conscious being, then accelerate that data pass the speed of light some how then download it into some android or something…..i dunno…..just a thought

I want to go to my elementary school again. Someone help me out, I know its Idiotic but stil.. I am not good at science. As far I understood, 1) we can trace through time if we travel fast than speed of light.. I think memory os the only thing that is faster than light, Yeah I can go to Paris within 1 sec in my memory but yeah its illustion, i want in real 2) Through Blackhole – I think its Bermuda triangle

if you travel back in time you will still be your age now. That is how it worked with others. No one gets younger otherwise traveling to far back would kill you. No school would let you return to school as a adult so not possible.

Plz help me I just want to send a message to myself in my past and save my self from a beast plz help Nazneen

Would love to experience many moments in life again for the first time again!

I think that time traveling should be left alone, for the sake of humanity. There are some things we’re not ready for yet.

Well stephen hawking may be wrong. I mean, the study proved that the universe self corrects itself to prevent inaccuracies. So maybe tourists from past do visit us but we don’t remember them as the universe alters our memory. If you guys have read about Butterfly Effect, a simple mistake today may grow through years to become a giant disaster in future so if you think of it, oncoming tourists from future may cause giant inaccuracies. Imagine this, You have travelled to past. You brought two cakes for yourself, so you pay the shopkeeper 20$. The shopkeeper invests the 20$ in stocks, strikes gold there and becomes a rich businessman.His daughter goes to Cambridge and marries someone else than the person she was supposed to marry according to time. Can you imagine the magnitude of inaccuracy after 100 years? Therefore, whatever the tourists from future do, is corrected by the universe and we don’t remember it. Creepy, but food for thought.It also adds a special meaning to the word ‘Fate’.

How much wacky terbacky (i.e. weed) you be smokin’ JOE JOE?

Hmmmm…. As brilliant of a mind as Stephen Hawkins was, how is he so sure that he would even recognize hordes of tourists from the future? Almost everyone is aware of the warning of the Butterfly Effect. So I’m sure any future visitors Intelligent enough for Past-Time travel would be amply attuned to this.

Most future people coming to the pass (our time) seems careless and not intelligent. Most are taking FBI lie detector test and telling us what is happening in the future. That is a bad idea, because if you tell us (example) who is the next President, and the Government does not like the person they than can change that event to let someone else in (as seen in 2020) One should never acknowledge who he or she is or why they are their. Most traveling is to get knowing of the pass or to pick up certain things. Since are pass is changing, events are changing and are timelines are messed up, someone made a mistake. The Mandela Effect is a fine example.

Wow that’s great plz help me go to my past plz,I can’t do it by my own at least help me send a msg to myself in my past Nazneen

I think it is possible, but time traveling is really just changing universe created by different time lines. Our whole solar system is in a whole different place now and Earth is much smaller in this universe from the one I grew up end. Someone has already changed the timeline.

Roads? Where we’re going, you don’t need roads!

Youre wrong about your measurement of speed for traveling, in order for time to slow down, with inside an object compared to outside. Scientists proved that time with inside an object at an excelorated speed actually appeared to have slown down during the duration of time for the test. The speed was far less then the terminal speed of a rocket for NASA at 256,000 kms p/h.

In to the volicity of space. Generating a vacuum of space, could be no different the the actual transport of matter over frequency where in fact matter can be carried by sound. It is believed that an alien civilization harnessed this energy in the form of bolisks that where believed to carry the same properities and in consideration of harmonic resinance, the simularities could be used in order to carry large weight. In accordance with a documentry on theoretical science.

However the properties, present the fact that a working property controdicts your counter intuative theory of gravitational deceloration of matter to colide within itself to absorb all things into non existance as to the transfer of matter into energy, rather then your idiolisms of transfer between dimentional space to another destination that is not linked or the transfer between time that isnt, either.

However to reproduce the fabric of time within space in a practical measurement as I have mentioned, would put an end to all the lunacy of an unmeasureable field, which people fail to identify. Like running into a glass window. Only to not know what forcefield is present.

Time travel into the past can be achieved simply going faster than the speed of light.

The closer you get to the speed of light the slower time goes

If you reach the speed of light time stops

If you go faster than the speed of light it starts to reverse

Why does no one seem to know this?

Christopher Reeves did this in Superman 3 brah.

Any time travel, pass and future, is by going faster than the speed of light. It is said by reversing that that you can go back in time. However, I assume since the Government has done this since the 80s they have better ways (maybe tying in a date) and not having to go to a unknown date.

I want to send a message to myself in the past on a particular date plz can you help me, this means a lot lot lot to me,plz help me Nazneen

Why don’t we drop the declaratory statements that it “is or isn’t possible!” Until someone actually does so. Just say “maybe”.

People have and their are records both to the pass and future. The Government has done it since the 80s as part of the “star wars project” and are much better at it today. This explains the black holes in the sky of 2019, and the CERN destroying 5 parallel universes in 2013. We also see changes because of time travel events changing time. The Mandela Effect is a find example.

I want to send a msg to myself and my family in the past ,is it possible plz help me my life will be saved one who helps me saves me and my kids from a pack of beasts,

The worst idea ever. We all want to do this and where does it stop. A lottery win does not sound bad if you knew the actual location, time and place. After a while though, would you not want to write that hit song, become the author of the Harry Potter books, stop 9/11? The idea of giving your pass self (a time time travel was not proven) information of the future could change things in a major way. This would cause one small thing to change creating many others to change. This has already happen in simple ways of the The Berenstein Bears changing to The Berenstain Bears. This is a small event but this event “The Mandela Effect” now has over 3,000 changes.

What if you decided to give your pass self information about a lottery ticket that would be a winner, bought late at night and he was hit by a car on the way to get it. Changes the whole future. However, If detailed right, done right, with no large changes, it may not effect much, but to know your being given info from yourself in a future time (when that was not known much or provrn back than) You would either assume it is a joke or you gone crazy.

I don’t want to win a lottery, my decision about my career and studying was right but my family and their cruelty has put me into this worst condition I just want to go back complete my studies and live a life like a human not like a animal or slave,help me plz Nazneen

Can someone take me to 2013? i can pay later to all of you in bitcoins so its a win win and you dont need to do anything, just wait

LOL but still complicating on my side

You travel in your dreams where time and space colloids ..That’s y sometimes the dream which you dreamt might be a 10 mins reel time but you felt dreaming whole time like 6 to 8hrs .. Probably even traveling to parallel universe

I agree. Dreams as we know it is not a simple sleep. The part of the brain we do not use while awake, we use at night. This is the phenomenon part of the brain that can do thing we feel a human can not do. We of course use less than 30% of our brain. By the use of 100% of the brain we would use both sides and be able to do common things such as read thoughts, move things without touching them etc. The idea of using this side of the brain, would be the theory we can leave our bodies and visit different universe, see what could of happen shall we done something different, and even see future events. This may be why we notice different memories to some things as we could of held some from another reality.

It would be very weird, however, if we were trapped in that universe, or another body and fail to return to ours. Is that how people die in their sleep?

i just fell like going to late 70’s, where i can see majority of family.. i am willing to trade life for it…..

Time travel to the pass is just as common as the future. However, as both has been done it is NOT travel threw time. Time is a illusion we created. We are actually traveling threw different universe with (what we call) different time, dates, years, etc. The Mandela Effect is a find example how traveling threw different reality’s change the time lines.

As a add on to the above, Time travel is not a theory, has been proven, and has been done by the Government since the 1980s. Their is many residue in our history to even show some time travel storys to be real.

Where can one get a reverse watch, is it really possible to go back in past with its help, is it sooo easy ,plz help me ??????? Nazneen

US20060073976A1- search this patent number,this describes the process for time travelling,I really don’t think magnetic energy will work,maybe heat focused on a specific point could expand the fabric of space and make a hole in it.even then I will the hole take you to another time.it would be one thing to time travel but selecting a point in time would be impossible.you could only travel to the time you device was built?

Is there a watch which back travels in time or reverse time watch? Is it true? How to get one? But with that how can I send a message to myself in my past, plz help Nazneen

I don’t believe such a watch exist and their are plenty of smart minds with huge funds trying to travel.right now there are only theories.

Thank you very much for your response. I just want to send a message to myself in my past. Nothing much will be changed but 3 literally dying devastating lives will be saved. We are suffering for the mistakes and egoistic arrogance of others so if possible plz help me

Traveling back in time isn’t just a when problem, it’s a *where* problem. Where was the place you’re standing right now a thousand years ago, or a thousandth of a second ago? There is no useful answer to those questions, so there’s nowhere to travel back in time to.

Traveling forward in time? You’re doing it now.

when you step through a door is time lost when you come back through? lets say you return days Later how much time did you loose. what exactly is Time,.? is dialation a safe way to return ,. a Blackhole will assist you in in travel, the question is will you arrive safe,.

Traveling back in time is impossible. 2 reasons why that are never taken into account.

A) The stuff you are made of ( subatomic material) is being used by something else. It I not like you are a facsimile of the already existing material. What you are made of is exactly the same existing material. The problem is exact stuff can not exist in 2 different places in the same point in time. You will either : Decompile or fall out of phase with the universe. Both bad outcomes for the time traveler.

B) Lets look at it from logical commonsense. You have a bar of gold . You intend to send the bar back 1 second in time. Now you have 2 bars of gold . You send those 2 bars back one second . You have 4 bars …… do that 50 times . You have over 900 trillion bars of gold. All made of the exact subatomic particles. The more the bars back the more the existing mass of the universe increase. What are the consequences of changing the mass of the universe . Hence the paradox . Information can not be destroyed., It also can not be created.

At least this is the way my brain perceives going back in time.

Time is a function of change. None of the 4 forces The strong force , The weak force , Electromagnetism and Gravity can not work without time.

I will figure out time travel one day but only for the past.

I wish I could travel back to 18th of June to save my mom.

Is time travel really a one way ticket? Theoretically, if you can go one way, you should be able to go back.

Time is not one way. It’s consequences are however irreparable given certain circumstances and is not something that should be taken lightly or thought of in a manner of disregard. I’ve only very recently decided to take to your social platforms regarding space and time.

You can try finding me on Instagram. I’m not familiar with these platforms to better direct you there. My Instagram name is johnrvh

On Twitter it seems to be @_JohnRvH

If I go forward I will have to pay extra bills and taxes. I donâ€™t think I can afford it.

You’re the first person I’ve come across in this timeline that has a sense of humor. Thankfully, going forward is not possible if that future hasn’t been created yet.

timetraval is no joke if its created the whole universe could go out of orbit.

Cauchy problem converging to non minimal terraces as t â†’ +âˆž

Stephen Hawking may he rest in peace a genius but not all knowing. As far as he knows we haven’t been flocked by tourists, in the same maybe these UFO sightings are actually time travelers from the future coming to the past to view how we really lived why things really happened the way they did, etc. To limit the imagination of possible and impossible is wrong then you create fantasy. And we have learned from history that there is truth in fantasy. I.e. the different mythos of the different ancient cultures from around the world including those of the Norse. Improbable and probable should be more appropriate. It’s possible because it can be imagined improbable die to the right math or this or that not existing or matching up. I also believe that if time travel to the past were possible that the changing of something in the past would create a new timeline running current with your timeline at which will inevitably collide and will cause the collapse of the universe at which point a new universe will be born.

so i think the speed of light is only relative to deciding a point of destination -initially- as specific gravity of destination needs to be ascertained to calculate the frequency needed to run an alcubierre-white engine to bend space correctly to cross space ‘quickly’, the point of reference may well be jupiter in our solar system for the fact of the moons that orbit it, i surmise that by using a ‘dead end ‘ equation that usually puts notable mathematicians into the outer regions by trying to solve it may actually be the key as calculations end in a loop of 4-2-1 ie 3N+1; this process of calculation creates a sine wave over time/distance relative to specific gravity of chosen destination – as time is determined by gravity therefore if the speed of light to a destination can be used to ascertain the specific gravity of a ‘body’ to visit ie a star or sun due to receivable resonant frequencies emitted by the body, then the constrictions of the speed of light do not exist other than to give a constant, by using the 3N+1 method of calculation ,once the speed of light and returning resonant frequencies of a destination are determined the calculation can be extrapolated to match the distance giving the end point -in doing this the sine wave required can be ascertained and be condensed to create a wormhole and allow the alcubierre-white engine to ‘bend or distort space enough so that the bubble you are in matches the required specific gravity of the destination – the frequency of the body nearest to the destination point should be used and resonated inside the bubble to create synchronicity of frequency and cause attraction i also believe that travelling through space require the ability to see things from different perspectives and it requires the ability to navigate through a series of what may be described as “Aims Windows” where your point of view needs to change inherently with a given position at a given point in the galaxy

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Time travel is theoretically possible, calculations show. But that doesn't mean you could change the past.

Time travel is possible based on the laws of physics, according to researchers.
But time-travelers wouldn't be able to alter the past in a measurable way, they say.
And the future would essentially stay the same, according to the reseachers.

Imagine you could hop into a time machine, press a button, and journey back to 2019, before the novel coronavirus made the leap from animals to humans.

What if you could find and isolate patient zero? Theoretically, the COVID-19 pandemic wouldn't happen, right?

Not quite, because then future-you wouldn't have decided to time travel in the first place.

For decades, physicists have been studying and debating versions of this paradox: If we could travel back in time and change the past, what would happen to the future?

A 2020 study offered a potential answer: Nothing.

"Events readjust around anything that could cause a paradox, so the paradox does not happen," Germain Tobar, the study's author previously told IFLScience .

Tobar's work, published in the peer-reviewed journal Classical and Quantum Gravity in September 2020, suggests that according to the rules of theoretical physics, anything you tried to change in the past would be corrected by subsequent events.

The grandfather paradox

Physicists have considered time travel to be theoretically possible since Albert Einstein came up with his theory of relativity. Einstein's calculations suggest it's possible for an object in our universe to travel through space and time in a circular direction, eventually ending up at a point on its journey where it's been before – a path called a closed time-like curve.

Still, physicists continue to struggle with scenarios like the coronavirus example above, in which time-travelers alter events that already happened. The most famous example is known as the grandfather paradox: Say a time-traveler goes back to the past and kills a younger version of his or her grandfather. The grandfather then wouldn't have any children, erasing the time-traveler's parents and, of course, the time-traveler, too. But then who would kill Grandpa?

A take on this paradox appears in the movie "Back to the Future," when Marty McFly almost stops his parents from meeting in the past – potentially causing himself to disappear.

To address the paradox, Tobar and his supervisor, Dr. Fabio Costa, used the "billiard-ball model," which imagines cause and effect as a series of colliding billiard balls, and a circular pool table as a closed time-like curve.

Imagine a bunch of billiard balls laid out across that circular table. If you push one ball from position X, it bangs around the table, hitting others in a particular pattern.

The researchers calculated that even if you mess with the ball's pattern at some point in its journey, future interactions with other balls can correct its path, leading it to come back to the same position and speed that it would have had you not interfered.

"Regardless of the choice, the ball will fall into the same place," Dr Yasunori Nomura, a theoretical physicist at UC Berkeley, previously told Insider.

Tobar's model, in other words, says you could travel back in time, but you couldn't change how events unfolded significantly enough to alter the future, Nomura said. Applied to the grandfather paradox, then, this would mean that something would always get in the way of your attempt to kill your grandfather. Or at least by the time he did die, your grandmother would already be pregnant with your mother.

Back to the coronavirus example. Let's say you were to travel back to 2019 and intervene in patient zero's life. According to Tobar's line of thinking, the pandemic would still happen somehow.

"You might try and stop patient zero from becoming infected, but in doing so you would catch the virus and become patient zero, or someone else would," Tobar said, according to Australia's University of Queensland , where Tobar graduated from.

Nomura said that although the model is too simple to represent the full range of cause and effect in our universe, it's a good starting point for future physicists.

Watch: There are 2 types of time travel and physicists agree that one of them is possible

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Time travel could be possible, but only with parallel timelines

Assistant Professor, Physics, Brock University

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Have you ever made a mistake that you wish you could undo? Correcting past mistakes is one of the reasons we find the concept of time travel so fascinating. As often portrayed in science fiction, with a time machine, nothing is permanent anymore — you can always go back and change it. But is time travel really possible in our universe , or is it just science fiction?

Read more: Curious Kids: is time travel possible for humans?

Our modern understanding of time and causality comes from general relativity . Theoretical physicist Albert Einstein’s theory combines space and time into a single entity — “spacetime” — and provides a remarkably intricate explanation of how they both work, at a level unmatched by any other established theory. This theory has existed for more than 100 years, and has been experimentally verified to extremely high precision, so physicists are fairly certain it provides an accurate description of the causal structure of our universe.

For decades, physicists have been trying to use general relativity to figure out if time travel is possible . It turns out that you can write down equations that describe time travel and are fully compatible and consistent with relativity. But physics is not mathematics, and equations are meaningless if they do not correspond to anything in reality.

Arguments against time travel

There are two main issues which make us think these equations may be unrealistic. The first issue is a practical one: building a time machine seems to require exotic matter , which is matter with negative energy. All the matter we see in our daily lives has positive energy — matter with negative energy is not something you can just find lying around. From quantum mechanics, we know that such matter can theoretically be created, but in too small quantities and for too short times .

However, there is no proof that it is impossible to create exotic matter in sufficient quantities. Furthermore, other equations may be discovered that allow time travel without requiring exotic matter. Therefore, this issue may just be a limitation of our current technology or understanding of quantum mechanics.

an illustration of a person standing in a barren landscape underneath a clock

The other main issue is less practical, but more significant: it is the observation that time travel seems to contradict logic, in the form of time travel paradoxes . There are several types of such paradoxes, but the most problematic are consistency paradoxes .

A popular trope in science fiction, consistency paradoxes happen whenever there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.

For example, consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I get to the past. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

But if I cannot use the time machine, then I cannot go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it. In other words, the time machine is destroyed if and only if it is not destroyed. Since it cannot be both destroyed and not destroyed simultaneously, this scenario is inconsistent and paradoxical.

Eliminating the paradoxes

There’s a common misconception in science fiction that paradoxes can be “created.” Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves for this exact reason. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

But in physics, a paradox is not an event that can actually happen — it is a purely theoretical concept that points towards an inconsistency in the theory itself. In other words, consistency paradoxes don’t merely imply time travel is a dangerous endeavour, they imply it simply cannot be possible.

This was one of the motivations for theoretical physicist Stephen Hawking to formulate his chronology protection conjecture , which states that time travel should be impossible. However, this conjecture so far remains unproven. Furthermore, the universe would be a much more interesting place if instead of eliminating time travel due to paradoxes, we could just eliminate the paradoxes themselves.

One attempt at resolving time travel paradoxes is theoretical physicist Igor Dmitriyevich Novikov’s self-consistency conjecture , which essentially states that you can travel to the past, but you cannot change it.

According to Novikov, if I tried to destroy my time machine five minutes in the past, I would find that it is impossible to do so. The laws of physics would somehow conspire to preserve consistency.

Introducing multiple histories

But what’s the point of going back in time if you cannot change the past? My recent work, together with my students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that Novikov’s conjecture cannot resolve. This takes us back to square one, since if even just one paradox cannot be eliminated, time travel remains logically impossible.

So, is this the final nail in the coffin of time travel? Not quite. We showed that allowing for multiple histories (or in more familiar terms, parallel timelines) can resolve the paradoxes that Novikov’s conjecture cannot. In fact, it can resolve any paradox you throw at it.

The idea is very simple. When I exit the time machine, I exit into a different timeline. In that timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline I came from. Since I cannot destroy the time machine in the original timeline, which is the one I actually used to travel back in time, there is no paradox.

After working on time travel paradoxes for the last three years , I have become increasingly convinced that time travel could be possible, but only if our universe can allow multiple histories to coexist. So, can it?

Quantum mechanics certainly seems to imply so, at least if you subscribe to Everett’s “many-worlds” interpretation , where one history can “split” into multiple histories, one for each possible measurement outcome – for example, whether Schrödinger’s cat is alive or dead, or whether or not I arrived in the past.

But these are just speculations. My students and I are currently working on finding a concrete theory of time travel with multiple histories that is fully compatible with general relativity. Of course, even if we manage to find such a theory, this would not be sufficient to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes.

Time travel and parallel timelines almost always go hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science as well. General relativity and quantum mechanics tell us that time travel might be possible, but if it is, then multiple histories must also be possible.

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Updated 1994. Original by Jon Thaler.

Time Travel: Fact or Fiction?

We define time travel to mean departure from a certain place and time followed (from the traveller's point of view) by arrival at the same place at an earlier time (from the sedentary observer's point of view). Time travel paradoxes arise from the fact that departure occurs after arrival according to one observer, and before arrival according to another. In the terminology of special relativity, time travel implies that the timelike ordering of events is not invariant. This violates our intuitive notions of causality. But intuition is not an infallible guide, so we must be careful. Is time travel really impossible, or is it merely another phenomenon where "impossible" means "nature is weirder than we think?" The answer is more interesting than you might think.

The Science Fiction Paradigm

The "B-grade movie" image of the intrepid chrononaut climbing into his time machine and watching the clock outside spin backwards while those outside watch him revert to callow youth is, according to current theory, impossible. In current theory, the arrow of time flows in only one direction at any particular place. If this were not true, then one could not impose a 4-dimensional co-ordinate system on space-time, and many nasty consequences would result. Nevertheless, there is a scenario that is not ruled out by present knowledge. This usually requires an unusual spacetime topology (due to wormholes or strings in general relativity) which has not yet seen, but which may be possible. In this scenario the universe is well behaved in every local region; only by exploring the global properties does one discover time travel.

Conservation Laws

It is sometimes argued that time travel violates conservation laws. For example, sending mass back in time increases the amount of energy that exists at that time. Doesn't this violate conservation of energy? This argument uses the concept of a global conservation law, whereas relativistically invariant formulations of the equations of physics imply local conservation. A local conservation law tells us that the amount of stuff inside a small volume changes only when stuff flows in or out through the surface. A global conservation law drops the assumption that the stuff must flow through the surface: it integrates over all space and assumes that there is no flow in or out at infinity. If this integral cannot be performed, then global conservation does not follow. So, sending mass back in time might be all right, but it implies that something strange is happening. (Why shouldn't we be able to do the integral?)

General Relativity

The possibility of time travel in GR has been known at least since 1949 (by Kurt Godel, discussed in [1], page 168). The GR spacetime found by Godel has what are now called "closed timelike curves" (CTCs). A CTC is a worldline that a particle or a person can follow that ends at the same spacetime point (the same position and time) as it started. A solution to GR which contains CTCs cannot have a spacelike embedding—space must have "holes" (as in donut holes, not holes punched in a sheet of paper). A would-be time traveller must go around or through the holes in a clever way.

The Godel solution is a curiosity, but not useful for constructing a time machine. Two recent proposals, one by Morris et al. [2] and one by Gott [3], have the possibility of actually leading to practical devices (although if you believe this, I have a bridge to sell you). As with Godel, in these schemes nothing is strange locally; time travel results from the unusual topology of spacetime. The first scheme uses a wormhole (which might be the inner part of a black hole: see Figure 1 of [2]) which is held open and manipulated by electromagnetic forces. The second scheme uses the conical geometry generated by an infinitely long string with mass. If two strings pass by each other, a clever person can go into the past by travelling a figure-eight path around the strings. In this scenario, if the string has non-zero diameter and finite mass density, there is a CTC without any unusual topology.

Grandfather Paradoxes

With the demonstration that general relativity contains CTCs, people began studying the problem of self-consistency. Basically, the problem is that of the "grandfather paradox": what happens if our time traveller kills his grandfather before he himself was born? In more readily analyzable terms, one can ask what the implications are of the quantum mechanical interference of the particle with its future self. Boulware [5] shows that there is a problem: unitarity is violated. This is related to the question of when one can do the global conservation integral discussed above. It is an example of the "Cauchy problem" [1, chapter 7].

5 Bizarre Paradoxes Of Time Travel Explained

December 20, 2014 James Miller Astronomy Lists , Time Travel 58

There is nothing in Einstein’s theories of relativity to rule out time travel , although the very notion of traveling to the past violates one of the most fundamental premises of physics, that of causality. With the laws of cause and effect out the window, there naturally arises a number of inconsistencies associated with time travel, and listed here are some of those paradoxes which have given both scientists and time travel movie buffs alike more than a few sleepless nights over the years.

Types of Temporal Paradoxes

The time travel paradoxes that follow fall into two broad categories:

1) Closed Causal Loops , such as the Predestination Paradox and the Bootstrap Paradox, which involve a self-existing time loop in which cause and effect run in a repeating circle, but is also internally consistent with the timeline’s history.

2) Consistency Paradoxes , such as the Grandfather Paradox and other similar variants such as The Hitler paradox, and Polchinski’s Paradox, which generate a number of timeline inconsistencies related to the possibility of altering the past.

1: Predestination Paradox

A Predestination Paradox occurs when the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. The result is a ‘temporal causality loop’ in which Event 1 in the past influences Event 2 in the future (time travel to the past) which then causes Event 1 to occur.

This circular loop of events ensures that history is not altered by the time traveler, and that any attempts to stop something from happening in the past will simply lead to the cause itself, instead of stopping it. Predestination paradoxes suggest that things are always destined to turn out the same way and that whatever has happened must happen.

Sound complicated? Imagine that your lover dies in a hit-and-run car accident, and you travel back in time to save her from her fate, only to find that on your way to the accident you are the one who accidentally runs her over. Your attempt to change the past has therefore resulted in a predestination paradox. One way of dealing with this type of paradox is to assume that the version of events you have experienced are already built into a self-consistent version of reality, and that by trying to alter the past you will only end up fulfilling your role in creating an event in history, not altering it.

– Cinema Treatment

In The Time Machine (2002) movie, for instance, Dr. Alexander Hartdegen witnesses his fiancee being killed by a mugger, leading him to build a time machine to travel back in time to save her from her fate. His subsequent attempts to save her fail, though, leading him to conclude that “I could come back a thousand times… and see her die a thousand ways.” After then traveling centuries into the future to see if a solution has been found to the temporal problem, Hartdegen is told by the Über-Morlock:

“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you .”

Movies : Examples of predestination paradoxes in the movies include 12 Monkeys (1995), TimeCrimes (2007), The Time Traveler’s Wife (2009), and Predestination (2014).
Books : An example of a predestination paradox in a book is Phoebe Fortune and the Pre-destination Paradox by M.S. Crook.

2: Bootstrap Paradox

A Bootstrap Paradox is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being or existence.

– Information : George Lucas traveling back in time and giving himself the scripts for the Star War movies which he then goes on to direct and gain great fame for would create a bootstrap paradox involving information, as the scripts have no true point of creation or origin.

– Person : A bootstrap paradox involving a person could be, say, a 20-year-old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20-year-old time traveler, who travels back 21 years through time, meets a woman, and so on. American science fiction writer Robert Heinlein wrote a strange short story involving a sexual paradox in his 1959 classic “All You Zombies.”

These ontological paradoxes imply that the future, present, and past are not defined, thus giving scientists an obvious problem on how to then pinpoint the “origin” of anything, a word customarily referring to the past, but now rendered meaningless. Further questions arise as to how the object/data was created, and by whom. Nevertheless, Einstein’s field equations allow for the possibility of closed time loops, with Kip Thorne the first theoretical physicist to recognize traversable wormholes and backward time travel as being theoretically possible under certain conditions.

Movies : Examples of bootstrap paradoxes in the movies include Somewhere in Time (1980), Bill and Ted’s Excellent Adventure (1989), the Terminator movies, and Time Lapse (2014). The Netflix series Dark (2017-19) also features a book called ‘A Journey Through Time’ which presents another classic example of a bootstrap paradox.
Books : Examples of bootstrap paradoxes in books include Michael Moorcock’s ‘Behold The Man’, Tim Powers’ The Anubis Gates, and Heinlein’s “By His Bootstraps”

3: Grandfather Paradox

The Grandfather Paradox concerns ‘self-inconsistent solutions’ to a timeline’s history caused by traveling back in time. For example, if you traveled to the past and killed your grandfather, you would never have been born and would not have been able to travel to the past – a paradox.

Let’s say you did decide to kill your grandfather because he created a dynasty that ruined the world. You figure if you knock him off before he meets your grandmother then the whole family line (including you) will vanish and the world will be a better place. According to theoretical physicists, the situation could play out as follows:

– Timeline protection hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger but the gun fails to fire. Click! Click! Click! The bullets in the chamber have dents in the firing caps. You point the gun elsewhere and pull the trigger. Bang! Point it at your grandfather.. Click! Click! Click! So you try another method to kill him, but that only leads to scars that in later life he attributed to the world’s worst mugger. You can do many things as long as they’re not fatal until you are chased off by a policeman.

– Multiple universes hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger and Boom! The deed is done. You return to the “present,” but you never existed here. Everything about you has been erased, including your family, friends, home, possessions, bank account, and history. You’ve entered a timeline where you never existed. Scientists entertain the possibility that you have now created an alternate timeline or entered a parallel universe.

Movies : Example of the Grandfather Paradox in movies include Back to the Future (1985), Back to the Future Part II (1989), and Back to the Future Part III (1990).
Books : Example of the Grandfather Paradox in books include Dr. Quantum in the Grandfather Paradox by Fred Alan Wolf , The Grandfather Paradox by Steven Burgauer, and Future Times Three (1944) by René Barjavel, the very first treatment of a grandfather paradox in a novel.

4: Let’s Kill Hitler Paradox

Similar to the Grandfather Paradox which paradoxically prevents your own birth, the Killing Hitler paradox erases your own reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect,” killing Hitler would have far-reaching consequences for everyone in the world, even if only for the fact you studied him in school.

The paradox itself arises from the idea that if you were successful, then there would be no reason to time travel in the first place. If you killed Hitler then none of his actions would trickle down through history and cause you to want to make the attempt.

Movies/Shows : By far the best treatment for this notion occurred in a Twilight Zone episode called Cradle of Darkness which sums up the difficulties involved in trying to change history, with another being an episode of Dr Who called ‘Let’s Kill Hitler’.
Books : Examples of the Let’s Kill Hitler Paradox in books include How to Kill Hitler: A Guide For Time Travelers by Andrew Stanek, and the graphic novel I Killed Adolf Hitler by Jason.

5: Polchinski’s Paradox

American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which a billiard ball enters a wormhole, and emerges out the other end in the past just in time to collide with its younger version and stop it from going into the wormhole in the first place.

Polchinski’s paradox is taken seriously by physicists, as there is nothing in Einstein’s General Relativity to rule out the possibility of time travel, closed time-like curves (CTCs), or tunnels through space-time. Furthermore, it has the advantage of being based upon the laws of motion, without having to refer to the indeterministic concept of free will, and so presents a better research method for scientists to think about the paradox. When Joseph Polchinski proposed the paradox, he had Novikov’s Self-Consistency Principle in mind, which basically states that while time travel is possible, time paradoxes are forbidden.

However, a number of solutions have been formulated to avoid the inconsistencies Polchinski suggested, which essentially involves the billiard ball delivering a blow that changes its younger version’s course, but not enough to stop it from entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’ which states that a probability distortion would occur in order to prevent a paradox from happening. This also helps explain why if you tried to time travel and murder your grandfather, something will always happen to make that impossible, thus preserving a consistent version of history.

Books: Paradoxes of Time Travel by Ryan Wasserman is a wide-ranging exploration of time and time travel, including Polchinski’s Paradox.

Are Self-Fulfilling Prophecies Paradoxes?

A self-fulfilling prophecy is only a causality loop when the prophecy is truly known to happen and events in the future cause effects in the past, otherwise the phenomenon is not so much a paradox as a case of cause and effect. Say, for instance, an authority figure states that something is inevitable, proper, and true, convincing everyone through persuasive style. People, completely convinced through rhetoric, begin to behave as if the prediction were already true, and consequently bring it about through their actions. This might be seen best by an example where someone convincingly states:

“High-speed Magnetic Levitation Trains will dominate as the best form of transportation from the 21st Century onward.”

Jet travel, relying on diminishing fuel supplies, will be reserved for ocean crossing, and local flights will be a thing of the past. People now start planning on building networks of high-speed trains that run on electricity. Infrastructure gears up to supply the needed parts and the prediction becomes true not because it was truly inevitable (though it is a smart idea), but because people behaved as if it were true.

It even works on a smaller scale – the scale of individuals. The basic methodology for all those “self-help” books out in the world is that if you modify your thinking that you are successful (money, career, dating, etc.), then with the strengthening of that belief you start to behave like a successful person. People begin to notice and start to treat you like a successful person; it is a reinforcement/feedback loop and you actually become successful by behaving as if you were.

Are Time Paradoxes Inevitable?

The Butterfly Effect is a reference to Chaos Theory where seemingly trivial changes can have huge cascade reactions over long periods of time. Consequently, the Timeline corruption hypothesis states that time paradoxes are an unavoidable consequence of time travel, and even insignificant changes may be enough to alter history completely.

In one story, a paleontologist, with the help of a time travel device, travels back to the Jurassic Period to get photographs of Stegosaurus, Brachiosaurus, Ceratosaurus, and Allosaurus amongst other dinosaurs. He knows he can’t take samples so he just takes magnificent pictures from the fixed platform that is positioned precisely to not change anything about the environment. His assistant is about to pick a long blade of grass, but he stops him and explains how nothing must change because of their presence. They finish what they are doing and return to the present, but everything is gone. They reappear in a wild world with no humans and no signs that they ever existed. They fall to the floor of their platform, the only man-made thing in the whole world, and lament “Why? We didn’t change anything!” And there on the heel of the scientist’s shoe is a crushed butterfly.

The Butterfly Effect is also a movie, starring Ashton Kutcher as Evan Treborn and Amy Smart as Kayleigh Miller, where a troubled man has had blackouts during his youth, later explained by him traveling back into his own past and taking charge of his younger body briefly. The movie explores the issue of changing the timeline and how unintended consequences can propagate.

Scientists eager to avoid the paradoxes presented by time travel have come up with a number of ingenious ways in which to present a more consistent version of reality, some of which have been touched upon here, including:

The Solution: time travel is impossible because of the very paradox it creates.
Self-healing hypothesis: successfully altering events in the past will set off another set of events which will cause the present to remain the same.
The Multiverse or “many-worlds” hypothesis: an alternate parallel universe or timeline is created each time an event is altered in the past.
Erased timeline hypothesis : a person traveling to the past would exist in the new timeline, but have their own timeline erased.

Screen Rant

Marvel changed the rules of time travel again, and killed all hope of gambit's return.

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X-Men '97 Episode 8 Ending Explained

Marvel finally did an overlooked x-men character justice after 15 years of live-action disappointment, x-men '97's spider-man cameo completes a 26-year-old unresolved story.

Warning: This article contains SPOILERS for X-Men '97 episode 8!

Marvel frames Gambit's death in X-Men '97 as permanent due to new time travel rules.
X-Men '97 presents absolute points in time, preventing time travel to change events like Genosha.
The series hints at MCU ties through similar iconography of the Sacred Timeline concept.

Marvel has just changed its rules of time travel again, and this time, X-Men '97 makes Gambit's possible return from the dead hopeless. Marvel Studios still has to present its live-action version of the X-Men, with the studio only using multiverse variants played by actors who were part of Fox's X-Men franchise since Disney acquired that studio. Alongside that, Marvel Studios has used nostalgia in another key way, bringing back the world of the excellent X-Men: The Animated Series on Disney+ with an impressive roster of characters in X-Men '97 .

Since its debut, X-Men '97 has quickly become a sensation, being held highly as possibly the best adaptation of the mutants outside of the comics. It is easy to see why, as the animated series does a great job of making all of its characters matter instead of pretty much just Wolverine, as was the case in Fox's movies. One of the series' standout characters was Gambit , who sadly died in the Genosha massacre in episode 5. Now, Marvel Studios' new time travel rules make Gambit's return from the dead this season or in X-Men '97 season 2 likely impossible.

Bastion's global threat has brought the world to the brink of all-out war, with a Magneto cliffhanger that could change everything for the X-Men.

X-Men '97 Changed How Absolute Points In Time Work

Cable has brought new time travel rules.

X-Men '97 just used the concept of absolute points in time. Marvel Studios' other animated series, What If...? , explained the concept as absolute points being events that simply cannot be changed ; otherwise, that could lead to the destruction of that entire timeline. That effect was practically shown in What If...? season 1's Doctor Strange episode , where the variant of Benedict Cumberbatch's MCU character attempted to save the life of his Christine Palmer over and over. Eventually, Doctor Strange's efforts lead to the destruction of his universe, with him as the sole survivor.

X-Men '97 names the concept but introduces a different way for absolute points to work. In the animated series, Cable explains that he has tried to stop the Genosha event several times, only he is pulled back from that moment every time he tries to travel there, as was shown in X-Men '97 episode 5 when he was sent back to the future after meeting his mother, Madelyne Pryor. On Twitter, X-Men '97 showrunner Beau DeMayo commented on how Cable's explanation of absolute points means no time travel can undo Genosha .

X-Men '97 Means Gambit's Death Is Permanent

The genosha event is an absolute point.

Based on Marvel's new explanation for absolute points, Gambit's death is likely to stick. The most common theory for how the character could come back hinged on Cable 's presence in Genosha. However, the reveal that Nathan Summers has tried to undo the tragic event over and over but is always pulled from that moment in time means Gambit, Madelyne Pryor, and countless other mutants are permanently dead. As Gambit was finally shining after being done dirty in live-action, the news will surely upset fans of the character.

Marvel Studios' X-Men '97 continues to deliver, and the animated series has shown more respect for a mutant than the Fox X-Men movies ever did.

However, it is perhaps good that X-Men '97 will not walk back on Gambit's death. The moment was one of the most shocking and emotional in any Marvel series, and undoing it would take away from Gambit's sacrifice, with the character dying to stop the three-headed Sentinel and save as many mutants as he could. Adding to that, X-Men '97 has already walked back on enough major deaths , revealing that Charles Xavier has been alive all this time and that Magneto never died in Genosha. Due to that, Gambit's death should stick, showing the series does have high stakes.

Does X-Men '97's "Absolute Points" Rule Mean It's In The MCU Multiverse?

Loki established the sacred timeline concept.

X-Men '97 included a scene where a display in front of Cable showed a very similar representation of the MCU's Sacred Timeline from before Loki created Yggdrasil at the end of Loki season 2. The moment seemed to hint that X-Men '97 is part of the MCU's multiverse; however, showrunner Beau DeMayo chalked that to borrowed iconography that would make the concept of absolute points easier for fans to understand without delving too much into it.

DeMayo claims that X-Men '97 's use of absolute points was so that there was no need to introduce a new word/concept when they could use one that fans were already familiar with. However, the X-Men '97 showrunner did concede that there is some crossover between the series and the MCU. DeMayo revealed that he views the series " as its own neighboring tree " to the MCU's but claimed that " a few leaves from one tree can mingle with another at some point ." While Gambit's death seems to be permanent, it is interesting that X-Men '97 is exploring an MCU concept.

X-Men '97

X-Men '97 is the direct continuation of the popular 1990s animated series X-Men: The Animated Series. Taking up where the third season left off, Marvel's revival brings back famous mutants such as Wolverine, Storm, Rogue, Gambit, Cyclops, Beast, Magneto, and Nightcrawler, who fight villains like Mr. Sinister, the Sentinels, and the Hellfire Club.

X-Men '97 (2024)
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Taurus Zodiac Sign. Abstract night sky background

Gather ’round and root down, my babies: the new moon in Taurus is upon us.

Rising in the dark sky at 11:22 p.m. ET on Tuesday, May 7 at 18º01’ of Taurus, this is the first new moon since the quaking, quickening, total solar eclipse on April 8. Because eclipses are not a time to embark or set intentions of any kind, this spectacle is the first true new moon of the spring season and the astrological new year.

Blessedly, we’re all being born brand-new — and with a gilded set of bull horns.

Set of moon phases for new, half, and full

Ruling the second house of wealth and worth , Taurus is a sign of creature comforts, capital gains and tangible goods. Because of the relationship between Taurus, value and valuables, this new moon asks us how we invest our energies and indulge ourselves

New moons are synonymous with new beginnings, fresh starts, fertile fields and introspection. Because darkness expands the imagination and our concept of what is possible, plausible and desirable, the new moon is a celestial slate-cleaning, a summons to dream and an invitation to initiate.

The new moon marks the start of a new lunar cycle when the sun and the moon are cheek to cheek in the same zodiac sign. This month, that sign is Taurus, the fixed, stabilize to survive, masturbate to dominate, first earth sign of the cosmic wheel.

Never has the need for stability been more pronounced.

After the upheaval of eclipse season, the onset of Pluto retrograde, a conjunction between Uranus and Jupiter, and a scorched earth Mercury retrograde in Aries, we’re staggering — dazed — out of the flames, licked clean and rendered ready for a heel of bread, a home to call our own, and something light and linen to wear.

Illustration of the Taurus constellation including a drawing of a bull

This month’s theme is restructuring and integrating: how to create a foundation that can withstand and even welcome in — with open doors and a charcuterie board — the wild winds of change.

As an earth sign, Taurus belongs to the body, and under these skies, our primary lesson is to deepen our relationship with the language of the self, responding to need and staying nourished in every sense of the word.

As once said teacher/ Scorpio sage Soeuraya : “The body is a catalyst to get to know the landscape underneath. Where is my mind, what are my thoughts doing, are they dictating, or is that me?”

Super new moon

This new moon is especially potent as it is not only the first non-eclipse new moon of spring, but it is also a super new moon, meaning the moon’s orbit is closer to we humble Earthlings — thus, we will feel the pull and power of the moody mother more than ever.

Taurus is synonymous with both steadfast toil and absolute hedonism, a dynamic that suggests an understanding of the true nature of life, of energy, investment and exertion; the value of doing it right, and doing it thoroughly the first time, and the joy of taking your time when it comes to feeling good.

The moon is exalted in Taurus, meaning it can express itself freely and fully. The moon governs instinct, comfort and emotionality, and in the fertile fields of Taurus, it gets all that it needs — and then some.

This is a moon that expects satisfaction but ain’t afraid to work hard for the payoff.

“The thing about life is that you must survive. Life is going to be difficult, and dreadful things will happen. What you do is move along, get on with it, and be tough. Not in the sense of being mean to others, but being tough with yourself and making a deadly effort not to be defeated.” Katherine Hepburn, double Taurus, certified bad b–ch

Examples of celebrities with the sun and moon in Taurus include Iggy Pop and Katharine Hepburn, a pair of bulls that seem both in command and in service of their animal bodies; elegantly defiant, arrestingly physical, utterly and inescapably themselves, and ever bent on being free and getting theirs.

Here, here!

Get fed, get yours, get full, go bull

NEW YORK, NEW YORK - APRIL 12: Iggy Pop performs during the Post Pop Depression tour at United Palace Theater on April 12, 2016 in New York City. (Photo by Adela Loconte/WireImage)

As Hepburn herself extolled , “The thing about life is that you must survive. Life is going to be difficult, and dreadful things will happen. What you do is move along, get on with it, and be tough. Not in the sense of being mean to others, but being tough with yourself and making a deadly effort not to be defeated.”

The moon will be kicking it in the pleasure pastures of Taurus alongside the sun, Venus (planet of attraction), Jupiter (planet of expansion) and Uranus (planet of upheaval).

The dense concentration of those radical forces in this earth sign suggests that who we are, what we desire, how we meet those desires and what we think we deserve may radically shift in the coming weeks.

As the glutton of good, Taurus wants for itself and others to always — and in every sense — f eel full; sated with food, secure with finances, satisfied by a lover and slaked by beauty.

This moon and the season it heralds ask us to consider what makes and keeps us full: to name it, claim it and hold out our proverbial cups for more of it.

Astrologer Reda Wigle researches and irreverently reports back on planetary configurations and their effect on each zodiac sign. Her horoscopes integrate history, poetry, pop culture and personal experience.

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'The Ministry of Time' by Kaliane Bradley is our 'GMA' Book Club pick for May

Kaliane Bradley's novel is a blend of travel, romance and spy thriller.

"The Ministry of Time" by Kaline Bradley is our "GMA" Book Club pick for May.

A British-Cambodian writer and editor based in London, Bradley was the winner of the 2022 Harper’s Bazaar Short Story Prize and the 2022 V.S. Pritchett Short Story Prize.

A blend of multiple genres from romance to thriller, her debut fiction novel chronicles the journey of a civil servant who is offered the salary of her dreams working for a new government ministry gathering “expats” across history to find out if time travel is attainable.

MORE: 'Just for the Summer' by Abby Jimenez is our 'GMA' Book Club pick for April

Her responsibility is to serve as a bridge by living with, assisting and monitoring an expat known as “1847” or Commander Graham Gore.

As far as history is concerned, Commander Gore died on Sir John Franklin’s doomed 1845 expedition to the Arctic, so he’s a little disoriented to be living with an unmarried woman who is surrounded by outlandish concepts such as “washing machines,” “Spotify” and “the collapse of the British Empire.”

But with an appetite for discovery, a seven-a-day cigarette habit, and the support of a charming and chaotic cast of fellow expats, he soon adjusts. The dynamic later evolves from an uncomfortable situation into something more genuine and deeper.

PHOTO: 'The Ministry of Time' by Kaliane Bradley is our Book Club pick for May.

By the time the true shape of the Ministry’s project comes to light, Gore and the bridge are forced to confront the choices that brought them together. Can true love prevail? What does it mean to defy history, when history is living in your house?

Read an excerpt below and get a copy of the book here .

The Ministry of Time: A Novel

This month, we are also teaming up with Little Free Library to give out free copies in Times Square and at 150 locations across the U.S. and Canada. Since 2009, more than 300 million books have been shared in Little Free Libraries across the world. Click here to find a copy of "The Ministry of Time" at a Little Free Library location near you.

Read along with us and join the conversation all month on our Instagram account, @GMABookClub, and with #GMABookClub.

***************************

The interviewer said my name, which made my thoughts clip. I don’t say my name, not even in my head. She’d said it correctly, which people generally don’t.

“I’m Adela,” she said. She had an eye patch and blond hair the same color and texture as hay. “I’m the Vice Secretary.”

“Of . . . ?”

“Have a seat.”

This was my sixth round of interviews. The job I was interviewing for was an internal posting. It had been marked security clearance required because it was gauche to use the top secret stamps on paperwork with salary bands.

I’d never been cleared to this security level, hence why no one would tell me what the job was. As it paid almost triple my current salary, I was happy to taste ignorance.

I’d had to produce squeaky-clean grades in first aid, Safeguarding Vulnerable People, and the Home Office’s Life in the UK test to get this far.

I knew that I would be working closely with refugees of high-interest status and particular needs, but I didn’t know from whence they were fleeing. I’d assumed politically important defectors from Russia or China.

Adela, Vice Secretary of God knows what, tucked a blond strand behind her ear with an audible crunch.

“Your mother was a refugee, wasn’t she?” she said, which is a demented way to begin a job interview.

“Yes, ma’am.” “Cambodia,” she said. “Yes, ma’am.”

I’d been asked this question a couple of times over the course of the interview process. Usually people asked it with an upward lilt, expecting me to correct them, because no one’s from Cambodia.

You don’t look Cambodian, one early clown had said to me, then glowed like a pilot light because the interview was being recorded for staff monitoring and training purposes. He’d get a warning for that one.

People say this to me a lot, and what they mean is: you look like one of the late-entering forms of white — Spanish maybe — and also like you’re not dragging a genocide around, which is good because that sort of thing makes people uncomfortable.

There was no genocide-adjacent follow-up: Any family still there [understanding moue]? Do you ever visit [sympathetic smile]? Beautiful country [darkening with tears]; when I visited [visible on lower lid] they were so friendly...

Adela just nodded. I wondered if she’d go for the rare fourth option and pronounce the country dirty.

“She would never refer to herself as a refugee, or even a former refugee,” I added. “It’s been quite weird to hear people say that.”

“The people you will be working with are also unlikely to use the term. We prefer ‘expat.’ In answer to your question, I’m the Vice Secretary of Expatriation.”

“And they are expats from . . . ?” “History.”

Adela shrugged. “We have time-travel,” she said, like someone describing the coffee machine. “Welcome to the Ministry.”

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I was thrilled to get the job. I’d plateaued where I was, in the Lan- guages department of the Ministry of Defence.

I worked as a translator - consultant specializing in Southeast Asia, specifically Cambodia. I’d learned the languages I translated from at university.

Despite my mother speaking Khmer to us at home, I hadn’t retained it through my forma- tive years. I came to my heritage as a foreigner.

I liked my Languages job well enough, but I’d wanted to become a field agent, and after failing the field exams twice I was at a bit of a loss for career trajectory. It wasn’t what my parents had had in mind for me.

When I was a very small child, my mother made her ambitions known. She wanted me to be prime minister. As prime minister, I would “do something” about British foreign policy and I would also take my parents to fancy governmental dinners.

I would have a chauffeur. (My mother never learned to drive; the chauffeur was important.) Regrettably she also drilled the karmic repercussions of gossip and lying into me — the fourth Buddhist precept is unambiguous on this—and thus at the age of eight my political career was over before it began.

My younger sister was a far more skilled dissembler. I was dutiful with language, and she was evasive, pugnacious with it.

This is why I became a translator and she became a writer — or at least she tried to become a writer and became a copy editor.

I was paid considerably more than her, and my parents understood what my job was, so I would say that karma worked in my favor. My sister would say something along the lines of: Go f--k yourself. But I know she means it in a friendly way, probably.

Even on the very day we were to meet the expats, we were still arguing about the word “expat.”

“If they’re refugees,” said Simellia, one of the other bridges, “then we should call them refugees. They’re not moving to a summer cottage in Provence.”

“They will not necessarily think of themselves as refugees,” said Vice Secretary Adela.

“Has anyone asked them what they think?”

“They see themselves as kidnap victims, mostly. Nineteen-sixteen thinks he’s behind enemy lines. Sixteen-sixty-five thinks she’s dead.”

“And they’re being released to us today?”

“The Wellness team think their adjustment will be negatively im- pacted if they’re held on the wards any longer,” said Adela, dry as a filing system.

We — or rather, Simellia and Adela — were having this argument in one of the Ministry’s interminable rooms: pebble-colored with lights embedded in the ceiling, modular in a way that suggested opening a door would lead to another identical space, and then another, and then another. Rooms like this are designed to encourage bureaucracy.

This was supposed to be the final direct briefing of the five bridges: Simellia, Ralph, Ivan, Ed, and me.

We’d all gone through a six-round interview process that put the metaphorical drill to our back teeth and bored. Have you now, or ever, been convicted of or otherwise implicated in any activity that might undermine your security status? Then nine months of preparation.

The endless working groups and background checks. The construction of shell jobs in our old departments (Defence, Diplomatic, Home Office). Now we were here, in a room where the electricity was audible in the light bulbs, about to make history.

“Don’t you think,” said Simellia, “that throwing them into the world when they think they’re in the afterlife or on the western front might impede their adjustment? I ask both as a psychologist and a person with a normal level of empathy.”

Adela shrugged.

“It might. But this country has never accepted expatriates from history before. They might die of genetic mutations within the year.”

“Should we expect that?” I asked, alarmed.

“We don’t know what to expect. That’s why you have this job.”

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Saguaro National Park offers travelers an iconic slice of the Southwest

Get your camera ready for Saguaro National Park .

The park is home to the tallest cactus species in the country, the saguaro, a symbol of the Southwest. They’re particularly concentrated in the west district of the park, just outside Tuscon, Arizona.

“We like to call that one the Instagram side because you can literally go anywhere in the park, and there will be at least two or three dozen saguaros in the background of your photo,” said Cam Juárez, Community Engagement and Outreach coordinator and public information officer at the park.

Here’s what else travelers will find at Saguaro, the latest national park in USA TODAY’s yearlong series .

Why is Saguaro National Park worth seeing?

“Saguaros are super special because they only exist in this part of the world the way they do,” said Juárez. “You can visit certain parts of Mexico, you can visit certain parts of central Arizona, but you’re never going to see them the way you see them here.”

They’re not the only scenic beauties in the biodiverse park.

“Our peaks are in the neighborhood of around 9,000 feet,” Juárez said. Visitors can find aspen, pine and fir trees in its sky islands. “As you climb every few 100 feet down in elevation, you're experiencing different biomes all the way down to grasslands, to desert shrub, to basically just what you would imagine a desert to look like.”

Which side is better for Saguaro National Park?

The park is divided into east and west districts, which bookend Tucson. Tucson International is the closest airport.

“My favorite is the west district, just because I love seeing the saguaro. You're going to have 2 million, versus maybe a quarter million saguaros on the east side,” Juárez said. “But if you're a backcountry hiker and you want to hike long distances, the east district, Rincon Mountain District, is definitely better.”

He noted the Arizona Trail , from the U.S.-Mexico border up to the Grand Canyon, goes right through the east district.

Can I drive in Saguaro National Park?

Yes. There is a $25 vehicle entry fee for the park, which is cashless.

Visitors can purchase passes online at Recreation.gov or at a park kiosk using a debit or credit card.

How long does it take to go through Saguaro National Park?

You could visit both sides of the park in one day but may be better off with two.

“If you only have one day, definitely take the Cactus Forest loop ,” Juárez recommended. Cactus Forest Drive is a scenic 8-mile roadway in the east district. The west district has a 5-mile Bajada Loop Drive.

What is the best time to visit Saguaro National Park?

Juárez called October and mid-November magical. That’s just before the park’s busiest time of year, which he said runs from after Thanksgiving through April.

“Two times a year, we see a lot of rainstorms,” he added. “We have our traditional summer monsoons, and we have our winter rains. As a bimodal desert, you're going to see different blossoms at different times of the year.”

He said monsoons amplify the park’s already magnificent sunsets.

“The different colors of red and yellows and golds, coupled with purple skies, it's just everything that you see on postcards,” he said. “It just comes alive.”

What does the saguaro mean to Native Americans?

“The Tohono O’odham have a very special relationship with the park. Their whole culture revolves around the saguaro cactus, or as they call it, the Ha:san ,” Juárez said. Tribal members may harvest the fruit for traditional uses with a special permit through the Interior Department and National Park Service. “The fruit is harvested as part of their traditional medicine, their traditional foods. A lot of ceremonial wines come from that.”

He said other tribes tied to the land include Pascua Yaqui Tribe and Gila River Indian Community .

What else should visitors know about Saguaro?

“I would love for people to know that Saguaro National Park is making a concerted effort to be conscientious of the fact that the representation matters,” Juárez said. “ Friends of Saguaro National Park , in conjunction with our team here, developed what's called the Next Generation Ranger program, and it's not just a paid internship. It's an opportunity for (interns) to gain experience and gain what's called the Public Land Corps credit opportunity, which will help us diversify not just race and ethnicity but also ability and gender … so when you walk into Saguaro National Park, you're going to run into very knowledgeable, very experienced, diverse young people that are matching our country's identity.”

Nearly 45% of the Tucson population is of Hispanic or Latino heritage, according to the U.S. Census . Several park staffers speak Spanish, including Juárez.

“It comes in handy because we're not just talking our friends to the south. We're talking all the way down to Central America and South America that are visiting national parks,” he said, adding that sometimes they’ll welcome visitors in Spanish. “You should see the look on people's faces. They're like, ‘Wow, you speak my language.’ I encourage all of my coworkers and staff people to make it a point to try to at least learn how to say welcome and hello in different languages.”

National parks for every body: How to make the outdoors more accessible

The park is also making a concerted effort to welcome guests of all abilities. Both districts offer a variety of accommodations, including accessible trails , braille and large-text versions of the park brochure, touchable exhibits, captioned programs and more.

“I definitely am an active voice, as a person with a physical disability, that says we can't just give folks a smidgen of what able-bodied individuals can get,” Juárez said. “We should make a very concerted effort to expand on everything we've done so far.”

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Kendrick Lamar vs. Drake Beef Goes Nuclear: What to Know

The two rappers had circled one another for more than a decade, but their attacks turned relentless and very personal in a slew of tracks released over the weekend.

Drake dressed in dark clothing raps into a microphone, with a hand gesturing in the air. Kendrick Lamar, dressed in red and a dark ball cap worn backward, raps into a microphone.

By Joe Coscarelli

The long-building and increasingly testy rap beef between Kendrick Lamar and Drake exploded into full-bore acrimony and unverifiable accusations over the weekend with the rapid-fire release of multiple songs littered with attacks regarding race, appropriation, sexual and physical abuse, body image, misogyny, hypocrisy, generational trauma and more.

Most relentless was Lamar, a Pulitzer Prize winner from Compton who tends toward the isolated and considered but has now released four verbose and conceptual diss tracks — totaling more than 20 minutes of new music — targeting Drake in the last week, including three since Friday.

Each racked up millions of streams and the three that were made available commercially — “Euphoria,” “Meet the Grahams” and “Not Like Us” — are expected to land near the top of next week’s Billboard singles chart, while seeming to, at least momentarily, shift the public perception of Drake, long a maestro of the online public arena and meme ecosystem .

In between, on Friday night, Drake released his own broadside against Lamar — plus a smattering of other recent challengers — in a teasing Instagram interlude plus a three-part track and elaborate music video titled “Family Matters,” in which he referred to his rival as a fake activist and attempted to expose friction and alleged abuse in Lamar’s romantic relationship.

But that song was followed within half an hour by Lamar’s “Meet the Grahams,” an ominous extended address to the parents and young son of Drake, born Aubrey Graham, in which Lamar refers to his rival rapper as a liar and “pervert” who “should die” in order to make the world safer for women.

Lamar also seemed to assert that Drake had more than a decade ago fathered a secret daughter — echoing the big reveal of his son from Drake’s last headline rap beef — a claim Drake quickly denied on Instagram before hitting back in another song on Sunday. (Neither man has addressed the full array of rapped allegations directly.)

How did two of the most famous artists in the world decide to take the gloves off and bring real-life venom into an extended sparring match for rap supremacy? It was weeks, months and years in the making, with a sudden, breakneck escalation into hip-hop infamy. Here’s a breakdown.

Since late March, the much-anticipated head-to-head seemed inevitable. Following years of “will they or won’t they?” lyrical feints, Lamar hit directly on record first this year during a surprise appearance on the song “Like That” by the Atlanta rapper Future and the producer Metro Boomin, both formerly frequent Drake collaborators.

With audible disgust, Lamar invoked the track “First Person Shooter” from last year’s Drake album, “For All the Dogs,” in which a guest verse from J. Cole referred to himself, Drake and Lamar as “the big three” of modern MCs.

Lamar took exception to the grouping, declaring that there was no big three, “just big me.” He also called himself the Prince to Drake’s Michael Jackson — a deeper, more complex artist versus a troubled, pop-oriented hitmaker.

“Like That” spent three weeks at No. 1 on the Billboard Hot 100, as Future and Metro Boomin released two chart-topping albums — “We Don’t Trust You” and “We Still Don’t Trust You” — that were anchored by a parade of Drake’s past associates, each of whom seemed to share a simmering distaste toward the rapper, who later called the ambush a “20 v. 1” fight.

In early April, J. Cole fought back momentarily , releasing the song “7 Minute Drill,” in which he called Lamar overrated, before backtracking, apologizing and having the song removed from streaming services. But Drake soon picked up the baton, releasing a wide-ranging diss track called “Push Ups” less than a week later that addressed the field, with a special focus on Lamar’s height, shoe size and supposedly disadvantageous business dealings.

Less than a week later, Drake mocked Lamar’s lack of a response on “Taylor Made Freestyle,” a track released only on social media. It featured Drake taunting Lamar for being scared to release music at the same time as Taylor Swift and using A.I. voice filters to mimic Tupac and Snoop Dogg imploring Lamar to battle for the good of the West Coast.

“Since ‘Like That,’ your tone changed a little, you not as enthused,” Drake rapped in an abbreviated third verse, as himself. “How are you not in the booth? It feel like you kinda removed.” (“Taylor Made Freestyle” was later removed from the internet at the request of the Tupac Estate.)

But it was a seemingly tossed-off line from the earlier “Push Ups” that included the name of Lamar’s longtime romantic partner — “I be with some bodyguards like Whitney” — that Lamar would later allude to as a red line crossed, making all subject matter fair game in the songs to come. (It was this same alleged faux pas that may have triggered an intensification of Drake’s beef with Pusha T in 2018.)

How We Got Here

Even with Drake-dissing cameos from Future, Ye (formerly Kanye West), Rick Ross, the Weeknd and ASAP Rocky, the main event was always going to be between Drake, 37, and Lamar, 36, who have spent more than a decade subtly antagonizing one another in songs while maintaining an icy frenemy rapport in public.

In 2011, when Drake introduced Lamar to mainstream audiences with a dedicated showcase on his second album, “Take Care,” and an opening slot on the subsequent arena tour, the tone was one of side-eying competition. “He said that he was the same age as myself/and it didn’t help ’cause it made me even more rude and impatient,” Lamar rapped on “Buried Alive Interlude” of his earliest encounter with a more-famous Drake. (On his Instagram on Friday, Drake released a parody of the track, citing Lamar’s jealousy since then.)

The pair went on to appear together on “Poetic Justice,” a single from Lamar’s debut album, “Good Kid, M.A.A.D City,” in 2012, as well as “___ Problems” by ASAP Rocky the same year.

But their collaborations ceased as Drake became his generation’s premiere hitmaker across styles in hip-hop and beyond, while Lamar burrowed deeper into his own psyche on knotty concept albums that brought wide critical acclaim alongside less constant commercial success.

When asked, the two rappers tended to profess admiration for one another’s skill, but seemed to trade subtle digs in verses over the years, always with plausible deniability and in the spirit of competition, leading to something of a hip-hop cold war.

The Week It Went Nuclear

Lamar’s first targeted response, “Euphoria,” was more than six minutes long and released last Tuesday morning. In three sections that raised the temperature as they built, he warned Drake about proceeding and insisted, somewhat facetiously, that things were still friendly. “Know you a master manipulator and habitual liar too,” Lamar rapped. “But don’t tell no lie about me and I won’t tell truths ’bout you.”

He accused the biracial Drake, who was born and raised in Toronto, of imitating Black American heritage and insulting him subliminally. “I hate the way that you walk, the way that you talk, I hate the way that you dress,” Lamar said. “I hate the way that you sneak diss, if I catch flight, it’s gon’ be direct.” And he called Drake’s standing as a father into question: “Teachin’ him morals, integrity, discipline/listen, man, you don’t know nothin’ ’bout that.”

Days later, Lamar doubled down with an Instagram-only track called “6:16 in LA,” borrowing both Drake’s “Back to Back” diss tactic from his 2015 beef with Meek Mill and a song title structure lifted from what is known as Drake’s time-stamp series of raps. Opting for psychological warfare on a beat produced in part by Jack Antonoff, Swift’s chief collaborator, Lamar hinted that he had a mole in Drake’s operation and was aware of his opponent’s opposition research.

“Fake bully, I hate bullies, you must be a terrible person,” he rapped. “Everyone inside your team is whispering that you deserve it.”

That night, Drake’s “Family Matters” started with its own justification for getting personal — “You mentioned my seed, now deal with his dad/I gotta go bad, I gotta go bad” — before taking on Lamar’s fatherhood and standing as a man in excruciating detail. “They hired a crisis management team to clean up the fact that you beat on your queen,” Drake rapped. “The picture you painted ain’t what it seem/you’re dead.”

Yet in a chess move that seemed to anticipate Drake’s familial line of attack, Lamar’s “Meet the Grahams” was released almost immediately. “This supposed to be a good exhibition within the game,” Lamar said, noting that Drake had erred “the moment you called out my family’s name.” Instead of a rap battle, Lamar concluded after another six minutes of psychological dissection, “this a long life battle with yourself.”

He wasn’t done yet. Dispensing with subtlety, Lamar followed up again less than 24 hours later with “Not Like Us,” a bouncy club record in a Los Angeles style that delighted in more traditional rap beef territory, like juvenile insults, proudly unsubstantiated claims of sexual preferences and threats of violence.

Lamar, however, didn’t leave it at that, throwing one more shot at Drake’s authenticity as a rapper, calling him a greedy and artificial user as a collaborator — “not a colleague,” but a “colonizer.”

On Sunday evening, Drake responded yet again. On “The Heart Part 6,” a title taken from Lamar’s career-spanning series, Drake denied the accusation that he preyed on young women, indicated that he had planted the bad information about his fake daughter and seemed to sigh away the fight as “some good exercise.”

“It’s good to get out, get the pen working,” Drake said in an exhausted outro. “You would be a worthy competitor if I was really a predator.” He added, “You know, at least your fans are getting some raps out of you. I’m happy I could motivate you.”

Joe Coscarelli is a culture reporter with a focus on popular music, and the author of “Rap Capital: An Atlanta Story.” More about Joe Coscarelli

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7 Ways Retirees Can Go Globe Trotting on a Fixed Income

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A lot of people want to travel and retirement seems like the perfect time to do so. But just because retirees have more time on their hands doesn’t mean they’re always flush with cash.

The good news is that there are ways to travel the world and see all those exotic places — and have those unforgettable experiences — even if you’re on a fixed income. Here are just some of the ways retirees can go globe trotting without risking their financial stability in their later years.

Balance Tax-Efficient Withdrawals With Local Price Differences

“I’ve developed strategies for retirees aiming to travel while sticking to a budget. One particularly effective approach involves structuring retirement income streams to benefit from tax efficiencies while taking advantage of geographical arbitrage,” said David Blain, chartered financial analyst (CFA) at BlueSky Wealth Advisors .

Starting with the tax strategies, Blain said that retirees find ways to optimize their withdrawal sequences from their IRAs and 401(k)s to get them the most tax benefits. Not only can this reduce their tax liability, but it can also help stretch their retirement funds so they can travel.

As for geographical arbitrage, Blain said “Retirees can strategically relocate or travel to regions where their dollar stretches further due to lower cost of living . Countries in Southeast Asia and parts of Eastern Europe, for instance, offer rich cultural experiences at a fraction of the cost faced in the U.S. or Western Europe.”

Travel Last-Minute or During the Off-Season

Another way to make traveling more affordable is to take those trips during the off seasons. Travel when and where most other people aren’t and you’re likely to find better prices on things like airfare and accommodations. You might even find cheaper bundle deals that include events at your destination.

“Since retirees have more flexibility in their schedules, the key is to travel at unusual times or even last minute. If you live near a port, look for last-minute travel options. For example, cruise ships will often offer last-minute deals for unsold rooms. Last-minute deals are usually at a considerable discount,” said Jay Zigmont, Ph.D., certified financial planner (CFP) and founder of Childfree Wealth .

“Additionally, be sure to check any travel plans for peak and non-peak times. Pick the times that are the cheapest and avoid peak times like school vacations.”

Embrace the Nomadic Lifestyle

While taking frequent or lengthy trips might not be feasible for retirees living on a fixed income, embracing a more nomadic lifestyle could help.

Rather than going from city to city or country to country, choose a more central location and stay there for a while. This can lower expenses, and not just when it comes to airfare. Even long-term accommodations are often cheaper than short-term ones.

“I encourage retirees to explore the benefits of a nomadic lifestyle or part-time residency in multiple countries,” Blain said. “This can provide savings in terms of taxes, living expenses and healthcare costs.”

Plus, staying in one place is a great opportunity to really get to know the area and learn to live like a local.

Volunteer or Work Part-Time While Traveling

Just because you’ve retired doesn’t mean you can’t still use some of your free time to volunteer or work part-time.

“One overlooked strategy suitable for retirees on a fixed income is ‘voluntourism,'” said Jodi Blodgett, a professional travel photographer . This is essentially where you travel while volunteering with local organizations.

“[Voluntourism] not only helps cover parts of travel costs through organizations but also provides immersive experiences that are rich in culture and community interaction,” Blodgett said.

There are local volunteer programs you can get involved with that’ll often offer free meals and accommodation in return for certain skills or expertise. Or you can join an organization that handles the logistics for you, but charges a fee in exchange. Be sure to weigh the costs and the gains before going with the latter.

Take Advantage of Better Exchange Rates

“Spending extended periods in countries with favorable exchange rates can dramatically decrease overall living expenses,” Blain said.

Depending on where you’re from and your local currency, you could choose destinations where your money goes a lot further. You’ll still have the same amount you did before, but you’ll be able to experience much more with it at your destination.

Keep an eye on different countries’ exchange rates and wait until you find one that suits your needs financially. For the best rates, exchange currencies at a local credit union or bank.

Take Advantage of Senior Discounts

Many companies offer senior discounts to those who are of a certain age — usually 55 or older. For example, United Airlines offers discounts to those ages 65 and up on some flights. Many museums, restaurants and other local venues also provide cheaper rates to seniors.

Retirees can also combine the already built-in senior discounts with certain membership or rewards programs. For instance, the AARP allows members to save up to 10% on hotels and resorts or 30% on car rentals. There are always specific rules and limitations to be aware of, but this can also help those living on a fixed income.

Look Into Passive Income Streams

When it comes to passive income, the sky’s the limit.

One option for those who own their own home and want to spend a significant amount of time overseas is to rent out their property on sites like Airbnb or Vrbo.

In 2021, Airbnb hosts earned an average of $13,800 . While the company hasn’t released more recent earnings, renting out a home continues to be a profitable way for many to earn supplemental income.

Of course, every person’s story is different. Properties located in more desirable or touristy areas are likely to bring in more money. But even a few extra thousand dollars a year could offset the cost of traveling abroad.

Renting out a property on Airbnb isn’t the only way to earn supplemental income while traveling. It’s all about leveraging existing skills and experiences in a lucrative way — whether that’s to get into affiliate marketing or start an online course that generates monthly income.

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