do gluons travel at the speed of light

What are gluons?

These force-carrying particles are the glue that binds baryonic matter together.

Evidence for gluons

Gluon faq's answered by an expert, color charge and quantum chromodynamics, the quark-gluon plasma, additional resources, bibliography.

Gluons are suitably named because they are the 'glue' that binds quarks together to form the likes of protons and neutrons. 

They are the carriers of the strong force, one of the four fundamental forces . Force-carrying particles such as the gluon, as well as the photon for the electromagnetic force, and the W and Z bosons for the weak force, are all massless particles with a quantum spin of 1 and are referred to collectively as 'gauge bosons'.

Gluons and quarks are inextricably linked. Quarks are the elementary particles that form hadrons, which are particles made of two or three quarks. For example, protons and neutrons , which form atomic nuclei, are hadrons, and therefore exist because of quarks and gluons. Although they are connected with gluons, quarks differ in that their quantum spin is 1/2, and they have a mass, albeit a tiny one (for example, an 'up' quark has a mass of 2.01 MeV, and a 'down' quark is slightly heavier with a mass of 4.79 MeV, which are a fifth and half the mass of a proton respectively; in terms of kilograms they are about 3.56 x 10–30kg). What quarks and gluons have in common is that neither can exist as free particles, and neither can exist without the other. 

Related: 10 mind-boggling things you should know about quantum physics

Although physicists can't see individual gluons, we know they exist because of indirect evidence that can only be explained by the presence of gluons.

Gluons were first detected in 1979, in an experiment at the Positron Electron Tandem Ring Accelerator (PETRA) at the Deutsches Elektronen-Synchrotron (DESY) Laboratory in Germany. PETRA is a 1.4-mile (2.3-km) long ring, a bit like a miniature version of the Large Hadron Collider except that PETRA accelerates leptons, specifically electrons and their antimatter equivalents positrons, rather than protons and atomic nuclei.

When matter and antimatter come together it annihilates. In the case of smashing electrons into positrons, the pair annihilate and release a quark and an antiquark. The two quarks are unable to escape each other — the farther they try to move apart, the stronger the strong force between them becomes (at least up to a point, about 10^–15 m, or a femtometer), the excess stored energy allowing the quark and antiquark pair to decay, or 'hadronize', into hadron particles that form in a conic region along the directions of travel of the original quark and antiquark. This conic region of hadron particles is called a jet , and a simple electron-positron annihilation would produce two opposite jets corresponding to the quark and antiquark. 

However, if gluons are real, then the electron-positron annihilation should also produce a gluon alongside the quark-antiquark pair, and this gluon should also hadronize into a third jet. To conserve momentum, the gluon would carry away some of the momentum from one of the quarks, changing the direction of its jet so that the hadronized jets from the quarks would no longer be directly opposite one another, while the gluon-derived jet would be off to one side. This is indeed what was seen in the PETRA experiment, and in subsequent experiments too, confirming the existence of the gluon.

We asked Markus Diehl , an expert in quantum chromodynamics at the DESY Theory Group a few frequently asked questions about gluons. 

Markus Diehl is an expert in quantum chromodynamics (QCD), the theory that covers the interactions of quarks and gluons (the strong force).  

How do we know gluons exist?

A wealth of very precise measurements is correctly explained by our theory of quarks and gluons.  A rather direct — and historically the first — manifestation of gluons is the production of three distinct sprays of particles in electron-positron collisions. These events with three hadronic jets, as we call them, were first observed at DESY’s PETRA collider in 1979.  

Why are gluons important?

Gluons are responsible for binding quarks together and thus for the formation — and many properties — of protons and neutrons, the building blocks of atomic nuclei. 

Can gluons and quarks ever be separated?

For all we know, quarks and gluons cannot be observed as free particles, but they give rise to hadronic jets.  Looking closely at the distributions of the particles in a jet, one can actually determine whether it originated most likely from a gluon or from a quark.

The quantum theory that governs the physics of the strong force that is carried by gluons to bind quarks together is called quantum chromodynamics , or QCD. Named by the famous Nobel-prize-winning particle physicist Murray Gell-Mann , QCD revolves around the existence of a property of quarks and gluons called 'color charge', as described by physicists at Georgia State University . This is neither a real color nor a real electric charge (gluons are electrically neutral) — it is so-named because it is analogous to electric charge in the sense that it is the source of the strong force's interactions between quarks and gluons, just as the charge is the source of the interaction in the electromagnetic force, while the colors are just an arbitrary, albeit quirky, way to distinguish between different quarks and the interactions they have with the strong force via gluons.

Quarks can have a color charge referred to as either red, green or blue, and there are positive and negative (anti) versions of each. The quarks are able to change color in their interactions, and the gluons conserve the color charge. For example, if a green quark changes to a blue quark, the gluon must be able to carry a color charge of green-blue. Accounting for all the different color and anti-color combinations means that there must be 8 different gluons in total, as described by John Baez . Compare this to the electromagnetic force, which operates under the theory of quantum electrodynamics (QED) in which there are only two possible charges, positive or negative. QCD is far more complex than QED! 

It's not strictly true that gluons and quarks cannot be separated, but it requires very extreme conditions that have not existed in nature since the first tiny fractions of a second after the Big Bang .

A few trillionths of a second after the Big Bang, the temperature of the tiny universe was still immense at a thousand trillion degrees. During that time , before any hadrons had even formed, the infant universe was filled with a soup of free quarks and gluons known as the quark-gluon plasma (plus leptons such as neutrinos and electrons). Because the universe was so hot, the quarks and gluons were whizzing around unbound at light speed, bouncing off each other with too much energy even for the strong force to bind them.

The universe very quickly cooled as it expanded, and by the first millionth of a second, the temperature had dropped sufficiently, to 2 trillion degrees , to allow the strong force to bind quarks and gluons together to form the first hadrons.

It is possible to replicate the primordial quark-gluon plasma in particle accelerator experiments, such as those at CERN or at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory . 

The atomic nuclei of heavy elements such as gold or lead are smashed together at almost the speed of light , resulting in a miniature fireball that for a brief moment is hot enough to dissolve hadrons into a quark-gluon plasma. 

Almost instantaneously the fireball cools and the quarks and gluons recombine to form jets of hadrons, including mesons that consist of two quarks, and baryons that are made of three quarks. The quark-gluon plasma is extremely dense, and often the hadron jets struggle to get through and lose energy. Physicists call this 'quenching', as described by physicists at CERN , and the amount of quenching, as well as the overall distribution and energy of the jets, can provide great insights into the properties of the quark-gluon plasma. For example, physicists have learned that it behaves more like a perfect fluid that flows with zero viscosity, than a gas. By learning about properties such as these, recreating the quark-gluon plasma in particle accelerators can give scientists a window back in time to the very birth of the universe and the immediate aftermath of the Big Bang when matter first came into being.

Read the story of the discovery of gluons in 1979, as told by DESY physicists Ilka Flegel and Paul Söding in the CERN Courier . Discover the history of QCD , as told by one of its pioneers, Harald Fritzsch. Explore quarks and gluons in more detail with these resources from The Department of Energy . Explore the discovery of the gluon and journey back in time to the 70s with this article from DESY .  

Follow Keith Cooper on Twitter @21stCenturySETI. Follow us on Twitter @Spacedotcom and on Facebook .  

Particle Physics, by Brian R. Martin (2011, One-World Publications)

Origins of the Universe: The Cosmic Microwave Background and the Search for Quantum Gravity, by Keith Cooper (2020, Icon Books)

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Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of "The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence" (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.

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• rod https://forums.space.com/threads/quarks-what-are-they.58460/ Som recent discussion on quarks and gluons, relative to how the BB cosmology created this interesting stuff in nature :) Reply

Admin said: Discover how gluons bind quarks together to form protons and neutrons and explore the form weird form of matter in which they existed just after the Big Bang. What are gluons? : Read more

Sean Autrey said: But would gluons be the "line" and not the inductant spheres in this model correct. Or is it defines as an elementary particle that's resonances equal it's mass. By the spaces between the objects? Isn't it the same as gravity... And a chip off a larger piece of metal?... It behaves differently because of it's scale? Volume wise pressure over induction would be faster I guess... So it's hard to say whether soft or hard came first... Maybe it's just this rotational energy...and nothing else .. but it behaves differently based on the shape right...

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The Glue That Holds the World Together

The most we learn about subatomic paricles called gluons, the more the universe seems to be made of nothing at all.

You do not know what stuff is, you who hold it in your hands. Atoms? Yes, stuff is made of atoms. And every atom is a nucleus orbited by electrons. Every nucleus is built of protons. Every proton is - but there you reach the end of the line. Inside the proton lies the deep, unsettling truth: Stuff is made of nothing, or almost nothing, held together by glue, lots of glue. Physicists first began to suspect this in 1973. Lately it has been proved by experiment.

Frank Wilczek was a 21-year-old graduate student at Princeton University when hehelped develop this theory of the proton in 1973. He didn't really understand it himself at first; he was just following where the math led him. "We did the calculations, but we didn't have a simple intuitive understanding," Wilczek says. "The physical picture came later." That picture took a while even for physicists to absorb, because it really isn't simple. Even today, if you ask a physicist to describe a proton, you'll first get a cartoon version—the one that says a proton is made simply of three smaller particles called quarks. That description is not exactly false—it's just low resolution. It's true the way a picture of Times Square from 30,000 feet away is true. The close-up reality, the one Wilczek and his colleagues got a glimpse of long ago, is far more madding and strange.

A proton is made of three quarks, yes, but the quarks are infinitesimal—just 2 percent or so of the proton's total mass. They're rattling around at near light speed inside the proton, but they're imprisoned in flickering clouds of other particles—other quarks, which materialize briefly and then vanish and, above all, gluons, which transmit the force that binds the quarks together. Gluons are massless and evanescent, but they carry most of the proton's energy. That is why it is more accurate to say protons are made of gluons rather than quarks. Protons are little blobs of glue—but even that picture conveys something too static and substantial. All is flux and crackling energy inside a proton; it is like an unending lightning storm in a bottle, a bottle less than .1 trillionth of an inch in diameter. "It's a very rich, dynamic structure," says Wilczek. "And it's very pleasing that we have a theory that can reproduce it."

Especially if you happen to be one of the guys who invented the theory, and if, more than two decades later, that theory is actually being verified by experiment. At a particle accelerator called HERA in Hamburg, Germany, physicists have been firing electrons into protons for the past eight years, showing just how insubstantial the quarks are inside. Meanwhile, other accelerator experiments may soon reveal how the universe assembled all its protons in the first place. In February, physicists at CERN, the European Laboratory for Particle Physics outside Geneva, announced "compelling evidence" that they had succeeded in melting large numbers of protons, creating for an instant the kind of quark-gluon plasma that last existed a microsecond after the Big Bang. All the protons around now congealed from that soup. At Brookhaven National Laboratory on Long Island in New York, a new and more powerful accelerator is getting set to cook quark-gluon soup on a daily basis this summer. By next year physicists may have a much better idea of what the universe was like when it was a billion times hotter than the surface of the sun, and quarks and gluons—not yet trapped inside protons, inside nuclei, inside atoms, inside us—could play freely in the quantum fields.

Quantum chromodynamics , or QCD, the theory that Wilczek and his colleagues invented, is a type of quantum field theory, and quantum field theory for beginners goes something like this. First, E=mc2, as Einstein discovered. That is, energy can be converted into mass and vice versa. Second, empty space is not empty: What we call a vacuum is actually seething with all kinds of energy fields, and the energy is constantly manifesting itself as "virtual" particles that pop into existence and then disappear again in something less than a trillionth of a nanosecond. "It wouldn't be very useful to us to see this structure of the vacuum," says Wilczek, who is now a professor at the Institute for Advanced Study at Princeton, where he lives in Einstein's old house. "It wouldn't help us avoid saber-toothed tigers or raise our children, so evolution has taught us to ignore it. But it's there."

Third (and last in our field-theory primer), when two subatomic particles exert a force on each other, they are interacting via one of those energy fields, which they themselves help to create. Specifically, they are exchanging one of those virtual particles—the quantum of the quantum field. An electron, for instance, has an electric charge of -1, and that charge generates an electric field. If the electron is moving, as it always is—spinning on its axis and orbiting the atomic nucleus—it generates a magnetic field as well; all magnetic fields are ultimately created by moving electric charges. When two charged particles interact—when, say, an electron is scattered off a proton in HERA—they exchange a virtual photon, the quantum of the electromagnetic field. The theory that describes such interactions, invented by Richard Feynman, among others, is called quantum electrodynamics, or QED.

In QED each individual electron is surrounded by a cloud of short-lived virtual particles—photons, but also other electrons paired with positrons, their positively charged antimatter twins. Those swarming particle pairs form a screen that partially cancels the electron field—at least as seen from outside the screen. From inside the screen, on the other hand, the field seems stronger than you might expect, like a bare lightbulb once a lamp shade has been removed. "OK, so that's screening, and that's pretty easy to understand," says Wilczek. "What happens in QCD, inside the proton, is just the opposite: It's antiscreening. It was a surprise even to us, so it can't be too simple. But we've come to understand it in more elementary terms over the years."

The early 1970s, when Wilczek did his pioneering work, was a heady time for particle physicists. Their air is somewhat different from what the rest of us breathe, and QCD was in it. A few years earlier, researchers at the Stanford Linear Accelerator Center had for the first time accelerated electrons to energies high enough to penetrate protons. The electrons didn't seem to be plunging into mush; they seemed to bounce off pointlike nuggets within the protons. Quarks, which had been postulated in 1964 as purely mathematical entities, began to seem as if they might really exist inside protons—and the question of how the whole thing was held together became urgent and exciting. While Wilczek and his adviser, David Gross, were discovering the answer at Princeton, another graduate student named David Politzer was discovering it independently at Harvard. Gross was the old man of the trio at 31. "We were lucky to have been young then," he wrote later, "when we could stroll along the newly opened beaches and pick up the many beautiful shells that experiment had revealed."

One shell was stranger than the rest. The Stanford experiments had seemed to show that the force between quarks—known as the strong force—actually got weaker as the quarks got closer together. That was very strange. The forces we encounter on a daily basis, electromagnetism and gravity, act in just the opposite way: They are stronger at short distances from their source and weaker at long ones. That is the intuitive way; that is how things should be. Otherwise, for instance, magnets would fly off your refrigerator to cling to the distant alluring stove—except that from a distance the fridge would start to look good to them again. And yet, weak as the strong force was at very short distances, it was strong enough at longer ones to frustrate physicists who had tried hard and failed to pull a quark out of a proton. No one had managed to observe one in isolation.

Wilczek and Gross went looking for a quantum field theory that could explain such a force. Actually Gross expected them to fail; he wanted to prove that field theory was a dead end. But one kind of mathematical formalism, called non-Abelian gauge theory, had not yet been tried. Wilczek spent half a year filling a notebook with calculations of how particles would interact through a non-Abelian quantum field. At the end he had a force that got stronger at close quarters, as a good force should, as Gross expected—but unlike what had been measured at Stanford. Wilczek checked the lengthy calculations over and over again. Then Gross spotted a single plus sign in the field equation that should have been a minus. That sign change changed everything: QCD was born.

Inside a proton, according to QCD, the quarks are "asymptotically free," as the Stanford results suggested, meaning they move almost as if there were no force between them at all. But the freedom is an illusion: A quark can never escape its partners. As the distance between them increases, so does the force, and so a wayward quark is inevitably reined in, like a bungee jumper. That's why no one ever has or ever will see a lone quark, which, when you think about it, is deeply weird. "The proton has parts, but it can't be taken apart," says Alvaro de Rújula, a theoretical physicist at CERN. "You can hold an electron in your hand. You can't hold a quark or a gluon in your hand."

The weirdness comes from the gluons. Quantum chromodynamics, the force that holds protons together, is modeled closely on quantum electrodynamics, the force that holds atoms together—but the gluons change screening to antiscreening, intuitive to bizarre. In quantum chromodynamics, quarks carry a new kind of charge, called color—which has nothing to do with ordinary color—and those charges generate a color field (hence the name chromodynamics). The quantum of the field, and the transmitter of the strong force, is the gluon. Like the photon that transmits the electromagnetic force, a gluon is massless. But unlike the photon, a gluon is charged. It generates its own color field, exerts its own strong force, and ineracts with other gluons. It leads a rich life.

The color field, like the electromagnetic one, can be thought of as having two components—call them color electric and color magnetic. A fast-moving color charge—gluons move at light speed—generates a strong color-magnetic field. Gluons are thus like little dipole magnets. The gluons that surround a quark align themselves parallel to its color field, as magnets would, and so instead of weakening it, as virtual particles do an electron's field, they strengthen it. They antiscreen the quark, amplifying its field. Here the lamp analogy no longer works—the quark is a dim bulb that somehow becomes brighter outside the shade.

That's what holds a proton together, and that's what gives it a bizarre internal structure. If one quark manages to get inside another's gluon cloud, it feels only a feeble attraction. But the farther away it goes, the more it feels the added pull of gluons—gluons emitted by the quarks, gluons emitted by other gluons, gluons that materialize into virtual quark-antiquark pairs, which exchange more gluons. "The quarks trigger the whole thing, but once it starts, it's a very powerful process, because the gluons interact," says Wilczek. "It's a sort of runaway process."

In 1974, De Rújula, Politzer, Wilczek, and a few other physicists proposed this gluonization of the proton and suggested how it might one day be measured. Two decades later, scientists at HERA started doing just that. HERA is a ring-shaped accelerator, nearly four miles around, in which electrons doing 47,000 laps a second are smashed into protons going the other way. The greater the energy of the collision, the deeper an electron can bore into a proton before being deflected. By measuring how the electron is deflected in millions of collisions, physicists can collect information on the internal components that are doing the deflecting. It's like taking a picture of the inside of a proton, Wilczek says, pixel by pixel—and the results fit the proposal he and his colleagues made decades ago.

"It's only at the crudest level that a proton is made of three quarks," Wilczek says. "When you look close and get inside these clouds and start seeing the basic structure, you see that it's mostly glue."

Which makes all the more interesting the question of how the universe ever managed to design such a thing.

To visit a particle-physics laboratory is to be struck by a contrast in scale—between the smallness of the objects being studied and the hugeness of the resources—people, machinery, computing power, electric power, space, money—that are being devoted to it. (If that infrastructure seems a high price to pay for information on the origin of the universe, consider that the Web—designed to help scientists communicate enormous data files—is a CERN brainchild.) CERN consumes almost as much electricity as the city of Geneva, its neighbor. CERN's biggest accelerator, the Large Electron-Positron collider, occupies a circular tunnel 17 miles around. In the coming years it will be dismantled and replaced by an even more powerful accelerator, the Large Hadron Collider, which will re-create the energy that existed a trillionth of a second after the Big Bang. (Hadrons are nuclear particles, like protons, that are made of quarks and gluons.) To make a quark-gluon plasma, you don't need to go that far back in time—the first microsecond will do—so you need a less powerful accelerator.

Still, the Super Proton Synchrotron at CERN is more than four miles around. It's housed in a tunnel that lies 300 feet underground, and in that tunnel, inside an aluminum tube just a few inches wide, lead nuclei are accelerated by magnets to 99.9 percent of the speed of light. They are then guided up to the surface and into an immense, factorylike hall, 300 yards long. The aluminum tube forks into different detectors, where the lead nuclei—each one consisting of 208 protons and neutrons, the electrically neutral twins of protons—smash into a piece of lead foil that is just a few hundred micrometers thick.

What happens next, in theory, is simple: The collision creates a fireball intense enough to melt the protons and neutrons. The quarks and gluons circulate freely, as they do deep inside a proton, but now over a region that is many protons wide, forming a quark-gluon plasma. "If you run the movie of the Big Bang backward, it gets denser and denser, hotter and hotter," says Reinhard Stock of the University of Frankfurt, who helped design one of the CERN detectors, "and we know that all bound structures break up when their energy density exceeds their binding energy." Quantum chromodynamics demands that a quark-gluon plasma exists at a certain energy density, "but you have to prove that it exists," says Stock. "And that's why we have been here for the last 15 years."

The problem is that the laboratory fireball expands rapidly and cools rapidly, just as the primordial one must have done. The plasma survives for only 10–22 of a second before the quarks and gluons condense again into protons and other hadrons. What physicists actually detect is a spray of thousands of such particles coming out the back of the lead foil. In Stock's detector, the particles then fly through a room-sized box of argon gas, knocking electrons off argon atoms. Counters record the electrons, and computers reconstruct the particle tracks, which reveal their identity. Once physicists have analyzed the data deluge—each collision yields 10 megabytes of data, and Stock and his colleagues have recorded millions of collisions—they may learn something about the plasma-producing fireball.

Over the past few years, Stock's detectors and others at CERN have confirmed that the fireballs are hot enough and dense enough to produce quark-gluon plasma. They have found an excess of particles containing "strange" quarks and a dearth of ones containing "charm"—both of which are side effects predicted by quantum chromodynamics. (Strange quarks and charm quarks are exotic kinds not found in ordinary particles.) It all amounts to a powerful case that CERN has been creating quark-gluon plasma—but it does not amount to proof. "The problem is they haven't been able to observe it directly," says Tom Ludlam, a physicist at Brookhaven National Laboratory.

Brookhaven's Relativistic Heavy Ion Collider, which is scheduled to begin doing physics this summer, will collide nuclei of gold at 10 times the energy possible at CERN and so achieve temperatures that will allow a quark-gluon plasma to survive a tiny moment longer. As the quarks cool down from a trillion degrees, they should emit heat radiation, in the form of gamma-ray photons. That is the most direct possible signal from the quark-gluon plasma, and Brookhaven's collider should be able to detect it. You can think of it as the flash of light the universe emitted when all its protons were born.

Even the scientists at Brookhaven won't be able to finally answer the question of where protons come from. A central mystery will remain. Inside a proton, as gluons come and go, as quarks and antiquarks come and go in their numberless swarms, one number remains constant: There are always three more quarks than there are antiquarks. Those are the quarks that, "at the very crudest level," as Wilczek puts it, make up the proton. Why are they matter and not antimatter? Why is the universe made of matter and not antimatter? The answer goes beyond quantum chromodynamics. A slight imbalance between quarks and antiquarks, if Wilczek and other theorists are right, was already present in the primordial quark-gluon plasma. Understanding its origin will require accelerators that reach even higher energies, such as CERN's Large Hadron Collider. It will require new kinds of theories—some of which, unfortunately, might demand that we start thinking of particles as tiny loops of string and the universe as having many more dimensions than the four we know and love.

As physics evolves, the image of the proton that quantum chromodynamics has given us may come to seem reassuringly concrete and solid—although solid is just what a proton is not. Flying into one—if you can imagine doing that, riding the strong force in a kind of subnuclear glider—would be like falling through Earth's atmosphere. The upper atmosphere of the proton is a thin cirrus of virtual quark-antiquark pairs; they form a shield for what lies below. As you fall past them, the atmosphere gets denser and denser, the clouds thicker and thicker. Your plane is struck with increasing frequency and force by flashes of color lightning—the gluons. And then, perhaps four fifths of the way through your descent, you emerge from the cloud cover. The ride is calmer now. The lightning bolts have not disappeared; they have fused to a continuous sheet, and somehow you feel at once featherlight and immune from all forces. You're near the center of the proton now, utterly trapped as you fall toward the asymptote of utter freedom, and you are finding . . . not much.

"The closer you look, the more you find the proton is dissolving into lots of particles, each of which is carrying very, very little energy," says Wilczek. "And the elements of reality that triggered the whole thing, the quarks, are these tiny little things in the middle of the cloud. In fact, if you follow the evolution to infinitely short distances, the triggering charge goes to zero. If you really study the equations, it gets almost mystical."

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Gluons, along with quarks, are the building blocks of protons and neutrons, which are, in turn, the building blocks of atomic nuclei. Just like photons transmit electromagnetic force between two charged particles, gluons are the exchange particles between quarks. So, they are responsible for binding protons and neutrons together in atomic nuclei.

They are fundamental particles and thus cannot be broken down into smaller ones. They are named gluons because these particles function as the ‘glue’ that holds the nucleus and its constituent baryons together.

A Feynman diagram often represents gluon interactions. The emitted gluons are shown as helices that generate a color change for the quarks. They are assumed to be bi-colored, carrying a unit of color and a unit of anti-color. Gluons convert one color of the quark to the other and vice versa, as shown in the diagram below:

Common Properties of Gluons

• They have insignificant mass.
• They travel at the speed of light.
• Having a spin of 1, it is a type of vector boson.
• Like quarks, the gluons carry a strong charge known as color.
• They can interact with each other within their range of about a Fermi and produce virtual quark-antiquark pairs.
• Apart from having a positive or negative electric charge, they possess a property called color charge: positive and negative redness, greenness, and blueness. They are not actual colors but names given to them for their detection. For each color charge, there is an anti-color variant.

How are Gluons Involved in Forces within the Atom

Particles of matter transfer energy by exchanging force-carrying particles, known as bosons, with one another. Gluon being a type of boson, its primary role is to actively carry the strong forces that bind protons and neutrons in an atom .

The force connecting positive and negative color charges is a strong nuclear force . Much more potent than the three other fundamental forces: gravity, electromagnetism, and the weak nuclear forces. Because of such high nuclear forces, it is tough to separate quarks and gluons inside a composite particle, making the particle extremely stable.

The only way of separating is by creating a state of matter known as quark-gluon plasma. In such plasma, the density and temperature are very high, which allows protons and neutrons to melt.

• Gluons – Open.edu
• Gluons – Hyperphysics.phy-astr.gsu.edu
• Gluon – Sciencedirect.com
• Quarks and Gluons – Energy.gov
• Gluon – Chemeurope.com

Article was last reviewed on Friday, October 7, 2022

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gluon, an elementary particle that mediates, or carries, the strong, or nuclear, force. In quantum chromodynamics (QCD), the quantum field theory of strong interactions , the interaction of quarks (to form protons , neutrons , and other elementary particles ) is described in terms of gluons—so called because they “glue” the quarks together. Gluons are massless, travel at the speed of light, and possess a property called color. Analogous to electric charge in charged particles, color is of three varieties, arbitrarily designated as red, blue, and yellow, and—analogous to positive and negative charges—three anticolor varieties. Quarks change their color as they emit and absorb gluons, and the exchange of gluons maintains proper quark color balance.

Unlike other forces, the force between quarks increases as the distance between the quarks increases. Up to distances about the diameter of a proton, quarks behave as if they were free of one another, a condition called asymptotic freedom. As the quarks move farther apart, the gluons that move between them utilize the energy that they draw from the quark's motion to create more gluons—the larger the number of gluons exchanged among quarks, the stronger the binding force. The gluons thus appear to lock the quarks inside the elementary particles, a condition called confinement. Gluons can also bind with one another to form composite particles called glueballs.

According to QCD, only colorless objects may exist in isolation. Therefore, individual gluons and individual quarks cannot exist in nature, and only indirect evidence of their existence can be detected. In 1979, compelling evidence was found when quarks were shown to emit gluons during studies of particle collisions at the German national high-energy physics laboratory in Hamburg.

See J. Diasdebens and S. Costa Ramos, ed., The Physics of Quark-Gluon Plasma (1988); F. J. Yndurain, The Theory of Quark and Gluon Interactions (1993).

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2024, Columbia University Press. All rights reserved.

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What is the speed of light? Here’s the history, discovery of the cosmic speed limit

On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it’s one of the most important constants that appears in nature and defines the relationship of causality itself.

As far as we can measure, it is a constant. It is the same speed for every observer in the entire universe. This constancy was first established in the late 1800’s with the experiments of Albert Michelson and Edward Morley at Case Western Reserve University . They attempted to measure changes in the speed of light as the Earth orbited around the Sun. They found no such variation, and no experiment ever since then has either.

Observations of the cosmic microwave background, the light released when the universe was 380,000 years old, show that the speed of light hasn’t measurably changed in over 13.8 billion years.

In fact, we now define the speed of light to be a constant, with a precise speed of 299,792,458 meters per second. While it remains a remote possibility in deeply theoretical physics that light may not be a constant, for all known purposes it is a constant, so it’s better to just define it and move on with life.

How was the speed of light first measured?

In 1676 the Danish astronomer Ole Christensen Romer made the first quantitative measurement of how fast light travels. He carefully observed the orbit of Io, the innermost moon of Jupiter. As the Earth circles the Sun in its own orbit, sometimes it approaches Jupiter and sometimes it recedes away from it. When the Earth is approaching Jupiter, the path that light has to travel from Io is shorter than when the Earth is receding away from Jupiter. By carefully measuring the changes to Io’s orbital period, Romer calculated a speed of light of around 220,000 kilometers per second.

Observations continued to improve until by the 19 th century astronomers and physicists had developed the sophistication to get very close to the modern value. In 1865, James Clerk Maxwell made a remarkable discovery. He was investigating the properties of electricity and magnetism, which for decades had remained mysterious in unconnected laboratory experiments around the world. Maxwell found that electricity and magnetism were really two sides of the same coin, both manifestations of a single electromagnetic force.

As Maxwell explored the consequences of his new theory, he found that changing magnetic fields can lead to changing electric fields, which then lead to a new round of changing magnetic fields. The fields leapfrog over each other and can even travel through empty space. When Maxwell went to calculate the speed of these electromagnetic waves, he was surprised to see the speed of light pop out – the first theoretical calculation of this important number.

What is the most precise measurement of the speed of light?

Because it is defined to be a constant, there’s no need to measure it further. The number we’ve defined is it, with no uncertainty, no error bars. It’s done. But the speed of light is just that – a speed. The number we choose to represent it depends on the units we use: kilometers versus miles, seconds versus hours, and so on. In fact, physicists commonly just set the speed of light to be 1 to make their calculations easier. So instead of trying to measure the speed light travels, physicists turn to more precisely measuring other units, like the length of the meter or the duration of the second. In other words, the defined value of the speed of light is used to establish the length of other units like the meter.

How does light slow down?

Yes, the speed of light is always a constant. But it slows down whenever it travels through a medium like air or water. How does this work? There are a few different ways to present an answer to this question, depending on whether you prefer a particle-like picture or a wave-like picture.

In a particle-like picture, light is made of tiny little bullets called photons. All those photons always travel at the speed of light, but as light passes through a medium those photons get all tangled up, bouncing around among all the molecules of the medium. This slows down the overall propagation of light, because it takes more time for the group of photons to make it through.

In a wave-like picture, light is made of electromagnetic waves. When these waves pass through a medium, they get all the charged particles in motion, which in turn generate new electromagnetic waves of their own. These interfere with the original light, forcing it to slow down as it passes through.

Either way, light always travels at the same speed, but matter can interfere with its travel, making it slow down.

Why is the speed of light important?

The speed of light is important because it’s about way more than, well, the speed of light. In the early 1900’s Einstein realized just how special this speed is. The old physics, dominated by the work of Isaac Newton, said that the universe had a fixed reference frame from which we could measure all motion. This is why Michelson and Morley went looking for changes in the speed, because it should change depending on our point of view. But their experiments showed that the speed was always constant, so what gives?

Einstein decided to take this experiment at face value. He assumed that the speed of light is a true, fundamental constant. No matter where you are, no matter how fast you’re moving, you’ll always see the same speed.

This is wild to think about. If you’re traveling at 99% the speed of light and turn on a flashlight, the beam will race ahead of you at…exactly the speed of light, no more, no less. If you’re coming from the opposite direction, you’ll still also measure the exact same speed.

This constancy forms the basis of Einstein’s special theory of relativity, which tells us that while all motion is relative – different observers won’t always agree on the length of measurements or the duration of events – some things are truly universal, like the speed of light.

Can you go faster than light speed?

Nope. Nothing can. Any particle with zero mass must travel at light speed. But anything with mass (which is most of the universe) cannot. The problem is relativity. The faster you go, the more energy you have. But we know from Einstein’s relativity that energy and mass are the same thing. So the more energy you have, the more mass you have, which makes it harder for you to go even faster. You can get as close as you want to the speed of light, but to actually crack that barrier takes an infinite amount of energy. So don’t even try.

How is the speed at which light travels related to causality?

If you think you can find a cheat to get around the limitations of light speed, then I need to tell you about its role in special relativity. You see, it’s not just about light. It just so happens that light travels at this special speed, and it was the first thing we discovered to travel at this speed. So it could have had another name. Indeed, a better name for this speed might be “the speed of time.”

Related: Is time travel possible? An astrophysicist explains

We live in a universe of causes and effects. All effects are preceded by a cause, and all causes lead to effects. The speed of light limits how quickly causes can lead to effects. Because it’s a maximum speed limit for any motion or interaction, in a given amount of time there’s a limit to what I can influence. If I want to tap you on the shoulder and you’re right next to me, I can do it right away. But if you’re on the other side of the planet, I have to travel there first. The motion of me traveling to you is limited by the speed of light, so that sets how quickly I can tap you on the shoulder – the speed light travels dictates how quickly a single cause can create an effect.

The ability to go faster than light would allow effects to happen before their causes. In essence, time travel into the past would be possible with faster-than-light travel. Since we view time as the unbroken chain of causes and effects going from the past to the future, breaking the speed of light would break causality, which would seriously undermine our sense of the forward motion of time.

Why does light travel at this speed?

No clue. It appears to us as a fundamental constant of nature. We have no theory of physics that explains its existence or why it has the value that it does. We hope that a future understanding of nature will provide this explanation, but right now all investigations are purely theoretical. For now, we just have to take it as a given.

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Getting up to speed on the proton

Scientists develop groundbreaking theory for calculating what's happening inside a proton travelling at the speed of light.

For more than 2,000 years, scientists thought the atom was the smallest particle possible. Then, they discovered that it has a nucleus made up of protons and neutrons surrounded by electrons. After that, they found that the protons and neutrons themselves have a complex inner world full of quarks and antiquarks held together by a superglue-like force created by gluons.

"Protons along with neutrons constitute over 99 percent of the visible universe, meaning everything from galaxies and stars to us," said Yong Zhao -- a physicist at the U.S. Department of Energy's (DOE) Argonne National Laboratory. "Yet, there is still much we do not know about the rich inner life of protons or neutrons."

Zhao has co-authored a paper on an innovative method for calculating the quark and gluon structure of a proton travelling at the speed of light. The name of the team's creation is large-momentum effective theory, LaMET for short, which works jointly with a theory called lattice quantum chromodynamics (QCD).

The proton is tiny -- about 100,000 times smaller than an atom, so physicists often model it as a point with no dimensions. But these new theories can predict what's happening within the speed-of-light proton as though it were a body of three dimensions.

The concept of momentum is vital to not only LaMET but physics in general. It equals the speed of an object times its mass.

More than a half century ago, Zhao explained, a simple quark model by physicists Murray Gell-Mann and George Zweig uncovered some of the inner structure of the proton while at rest (no momentum). From that model, scientists pictured the proton as consisting of three quarks and predicted their essential properties, such as electric charge and spin.

Later experiments with protons accelerated to close to the speed of light demonstrated that the proton is even more complex than originally thought. For example, it contains uncountable particles that interact with one another -- not just three quarks bound by gluons. And the gluons can briefly turn into quark-antiquark pairs before they destroy each other and become a gluon again. Particle accelerators like that at DOE's Fermi National Accelerator Laboratory produced most of these results.

"When you accelerate the proton and collide it with a target, that's when the magic happens in terms of revealing its many mysteries," Zhao said.

About five years after the simple quark model rocked the physics community, a model proposed by Richard Feynman pictured the proton travelling at near the speed of light as a beam carrying an infinite number of quarks and gluons moving in the same direction. He called these particles "partons." His parton model has inspired physicists to define a set of quantities that describe the 3D proton structure. Researchers could then measure these quantities in experiments at particle accelerators.

Earlier calculations with the best available theory at the time (lattice QCD) produced some illuminating details about the distribution of quarks and gluons in the proton. But they had a serious shortcoming: They could not accurately distinguish between fast- and slow-moving partons.

The difficulty was that lattice QCD could only calculate the properties of the proton that do not depend on its momentum. But applying Feynman's parton model to lattice QCD requires knowing the properties of a proton with infinite momentum, which means that the proton particles must all be traveling at the speed of light. Partially filling that knowledge gap, LaMET provides a recipe for calculating the parton physics from lattice QCD for large but finite momentum.

"We have been developing and refining LaMET over the last eight years," said Zhao. "Our paper summarizes this work."

Running on supercomputers, lattice QCD calculations with LaMET are generating new and improved predictions about the structure of the speed-of-light proton. These predictions can then be put to the test in a new one-of-a-kind facility called the Electron-Ion Collider (EIC). This facility is being built at DOE's Brookhaven National Laboratory.

"Our LaMET can also predict useful information about quantities that are extraordinarily difficult to measure," said Zhao. "And with powerful enough supercomputers, in some cases, our predictions could even be more precise than possible to measure at the EIC."

With deeper understanding of the 3D quark-gluon structure of matter using theory and EIC measurements, scientists are poised to reach a far more detailed picture of the proton. We will then be entering a new age of parton physics.

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Materials provided by DOE/Argonne National Laboratory . Original written by Joseph E. Harmon. Note: Content may be edited for style and length.

Journal Reference :

• Xiangdong Ji, Yizhuang Liu, Yu-Sheng Liu, Jian-Hui Zhang, Yong Zhao. Large-momentum effective theory . Reviews of Modern Physics , 2021; 93 (3) DOI: 10.1103/RevModPhys.93.035005

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Seeing beauty in the quark-gluon plasma with energy correlators

Carlota andres, fabio dominguez, jack holguin, cyrille marquet, and ian moult, phys. rev. d 110 , l031503 – published 30 august 2024.

• No Citing Articles
• Introduction.
• Theoretical approach.
• Numerical results.
• Conclusions.
• ACKNOWLEDGMENTS

Heavy quarks created in heavy-ion collisions serve as an excellent probe of the produced quark-gluon plasma (QGP). The radiation pattern of jets formed from heavy quarks as they traverse the QGP exhibits a particularly interesting structure due to the interplay of two competing effects: the suppression of small-angle radiation, also known as the “dead-cone” effect, and the enhancement of emitted gluons by medium-induced radiation. In this Letter , we propose a new observable, based on the energy correlator approach to jet substructure, which will allow us to disentangle the two scales associated to these two phenomena and to determine under which conditions the dead cone is filled by medium-induced radiation. Combined with the forthcoming high-statistics measurements of heavy-flavor jets, this work provides a novel tool to unravel the dynamics of the QGP.

• Received 22 August 2023
• Accepted 2 August 2024

DOI: https://doi.org/10.1103/PhysRevD.110.L031503

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP 3 .

Published by the American Physical Society

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• 1 CPHT, CNRS, École polytechnique, Institut Polytechnique de Paris , 91120 Palaiseau, France
• 2 LIP , Avenida Professor Gama Pinto, 2, P-1649-003 Lisboa, Portugal
• 3 Instituto Galego de Física de Altas Enerxías (IGFAE), Universidade de Santiago de Compostela , Santiago de Compostela 15782, Spain
• 4 University of Manchester , School of Physics and Astronomy, Manchester M13 9PL, United Kingdom
• 5 Department of Physics, Yale University , New Haven, Connecticut 06511, USA

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Vol. 110, Iss. 3 — 1 August 2024

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A heavy-flavor jet propagating through the QGP forms a complicated energy pattern due to an interplay of two characteristic angular scales: the dead-cone angle θ 0 and the onset angle θ on . These scales can be extracted from the asymptotic energy flux using energy correlators.

EEC of a light-quark (blue), c -quark (orange), and b -quark (green) jet in p-p (dashed) and heavy-ion (solid) collisions. Different panels correspond to different jet energies and medium parameters. All curves are normalized by the integrated vacuum result Σ vac .

Top half of each panel: ratio of the EEC of a b -jet with respect to a c -jet in p-p (blue) and heavy-ion (red) collisions. Bottom half of each panel: ratio of the EEC of b -jet ( c -jet) in p-p with respect to heavy-ion collisions in green (pink). The colored bands are meant to guide the eye towards the region where the dead cone of the heavy-ion b -jet is filled, as explained in the text.

Ratio of the EEC correlator of a c -jet with respect to light-quark jet in p-p (blue) and heavy-ion (red) collisions. Each panel corresponds to different jet energies and medium parameters. The colored bands are meant to guide the eye towards the region where the dead cone of the heavy-ion c -jet is filled by medium-induced radiation. They are defined as explained in the text for Fig.  3 replacing bottom with charm and charm with massless.

EEC of a c -quark (orange) and b -quark (green) jet in p-p (dashed) and heavy-ion (solid) collisions. The curves were obtained using the single scattering approximation for the medium-induced splitting. Different panels correspond to different jet energies and medium parameters. All curves are normalized by the integrated vacuum result Σ vac .

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How bright is the universe’s glow? Study offers best measurement yet

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Banner image: Artist's depiction of the New Horizons spacecraft billions of miles from Earth. (Credit: NASA, APL, SwRI, Serge Brunier/ESO, Marc Postman/STScI, Dan Durda)

Scientists have traveled to the edges of the solar system, virtually, at least, to capture the most accurate measurements to date of the faint glow that permeates the universe—a phenomenon known as the cosmic optical background. 

The new study, published Aug. 28 in The Astrophysical Journal , draws on observations from NASA’s New Horizons spacecraft, which whizzed past Pluto in 2015 and is now nearly 5.5 billion miles from Earth. The research seeks to answer a deceptively simple question, said co-author Michael Shull, an astrophysicist at CU Boulder.

“Is the sky really dark?” said Shull, professor emeritus in the Department of Astrophysical and Planetary Sciences .

Space may look black to human eyes, but scientists believe that it’s not completely dark. Since the dawn of the cosmos, trillions of galaxies containing countless stars have formed and died, leaving behind an imperceptibly faint light. Think of it as the night light in space. 

Shull and the team, led by Marc Postman at the Space Telescope Science Institute in Baltimore, calculated just how bright that glow is. Their findings suggest that the cosmic optical background is roughly 100 billion times fainter than the sunlight that reaches Earth’s surface—far too faint for humans to see with the naked eye.

The results could help scientists shine a light on the history of the universe since the Big Bang.

“We’re kind of like cosmic accountants, adding up every source of light we can account for in the universe,” Shull said.

Click to enlarge Map revealing the regions in space, marked by circles and triangles, where New Horizons measured the cosmic optical background. The team pointed the spacecraft's LORRI instrument above and below the plane of the Milky Way Galaxy, along the map's equator, to avoid light from the galaxy. (Credit: Postman et al., 2024, The Astrophysical Journal)

Into the dark

It’s a type of number crunching that has captured the imagination of scientists for nearly 50 years, he added.

Shull explained that, after decades of research, astrophysicists think they have a pretty good idea of how the cosmos evolved. The first galaxies formed during an epoch known as the Cosmic Dawn several hundred million years after the Big Bang. The starlight from galaxies in the distant universe reached its brightest point about 10 billion years ago and has been dimming ever since. 

Precise measurements of the cosmic optical background could help scientists confirm whether this picture of the cosmos makes sense—or if there are mysterious, as-of-yet-undiscovered objects casting light into space.

Taking those kinds of measurements, however, isn’t easy, especially not from Earth.

Earth’s neighborhood is teeming with tiny grains of dust and other debris. Sunlight glints off this mess, washing out any signals that might be coming from the cosmic optical background. 

“A metaphor I use is if you want to see the stars, you need to get out of Denver,” Shull said. “You have to go way out, right to the northeast corner of Colorado where all you have ahead of you are South Dakota and Nebraska.”

New Horizons has given scientists a once-in-a-lifetime opportunity to do something similar in space.

Cosmic accounting

The mission has uniquely Colorado origins. Alan Stern, who studied as a graduate student at CU Boulder under Shull and former Senior Research Associate Jack Brandt, leads the New Horizons mission. He’s currently based at the Southwest Research Institute in Boulder, Colorado. The spacecraft also carries the Student Dust Counter , an instrument designed and built by students at the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder.

Over the course of several weeks in summer 2023, the researchers pointed New Horizons’ Long Range Reconnaissance Imager (LORRI) at about two dozen patches of sky.

Even at the edge of the solar system, the team still had a lot of extra light to contend with. The Milky Way Galaxy, for example, sits within a halo that, like our solar system, gathers dust.

“You can’t get away from dust,” Shull said. “It’s everywhere.”

He and his colleagues estimated how much light that halo could generate, then subtracted it from what they were viewing with LORRI. After getting rid of additional sources of light, the team was left with the cosmic optical background.

In scientific terms, that background amounts to about 11 nanowatts per square meter per steradian. (A steradian is a patch of sky with a width about 130 times the diameter of the moon).

Shull said that this value lines up well with how many galaxies scientists believe should have formed since the Big Bang. Put differently, there don’t seem to be any strange objects, such as exotic kinds of particles, out there in space producing a lot of light. But the researchers can’t rule out such anomalies completely.

The team’s measurements are likely to be the best estimates of the universe’s glow for a long time. New Horizons is using its remaining fuel supplies to pursue other scientific priorities, and no other missions are currently heading toward those cold and dark corners of space. 

“If they put a camera on a future mission, and we all wait a couple of decades for it to get out there, we could see a more exact measurement,” Shull said.

Other co-authors of the new study include SWRI’s Alan Stern and Tod Lauer at the U.S. National Science Foundation National Optical Infrared Astronomy Research Laboratory. Researchers from the Johns Hopkins University Applied Physics Laboratory, University of Texas at San Antonio and University of Virginia also participated.

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