Gluon

A Field That Gathers

Between a positive and a negative charge, the electric field spreads out through all of space. Photons carry no charge, so the field lines ignore one another and fan out, and the pull between the charges weakens with the square of the distance: twice as far, a quarter of the pull.

Between a quark and an antiquark, the field does something no electric field ever does. It pulls itself together into a narrow tube. The field itself carries color, so its lines attract one another and crowd inward instead of spreading. A tube keeps its width as it gets longer, so its pull does not weaken as it stretches. Supercomputer simulations of the strong force show the tube directly. That one picture is the root of almost everything below.

Color Charge

Electric charge comes in one kind, with a plus and a minus. The strong force’s charge comes in three kinds, named red, green and blue. The names have nothing to do with real color; they are a reminder that the three together cancel, the way red, green and blue light add up to white. A quark carries one color and an antiquark one anticolor. A gluon carries both at once: one color and one anticolor, or a blend of such pairs.

That lets the field pass color around, and the calculations picture it one gluon at a time. A red quark gives off a gluon carrying red and antigreen, and turns green; the green quark that takes it in turns red. The red has traveled from one quark to the other inside the gluon, and at every step the total is the same: one red, one green, one blue, which adds up to no color at all. A single gluon thrown from quark to quark is a cartoon, one step in the calculation. In a real proton the three quarks sit in one color field that is never still. That field is the binding, and only the total color is ever definite.

Eight, Not Nine

Three colors and three anticolors make nine possible pairings, so you might expect nine gluons. There are eight. Six of the pairings join a color to a different anticolor, and those are gluons as they stand. The other three join a color to its own anticolor, red with antired and so on, and those three can be blended. Two of the blends still weigh the colors unevenly, so they carry color too. The third is the perfectly even blend of all three, and it carries no color at all.

A gluon like that would be trapped by nothing. It would travel any distance, and the strong force would reach across a room, acting on every proton and neutron in it. It does not exist. The symmetry the strong force is built on, a precise rule for turning the three colors into one another, has room for exactly eight messengers and leaves the even blend out. Every gluon that exists carries color, so every one is caught, and that absent ninth is why the strong force does not reach across a room. What leaks from one proton or neutron to the next travels in the fields of colorless particles that have mass, which reach only a little way, so the strong force ends at the edge of a nucleus.

Why a Quark Cannot Leave

Now pull a quark away from its partner. Once the two are about a proton’s radius apart, the field has gathered into its tube, and from then on the pull holds steady: a force of roughly fifteen tonnes’ weight that does not weaken with distance. Every step costs as much energy as the last, so the energy stored in the tube grows with every step.

There is a cheaper way out, and that is what happens. Before the quarks are even a proton’s width apart, the tube holds enough energy to make a new quark and antiquark, and it breaks, with the new pair at the broken ends. Where there was one bound pair there are two, and no quark is free. No stretch, however violent, has ever ended differently. This is confinement, and it holds everywhere in today’s cold cosmos.

A caveat the word “breaks” invites: the animation is a cartoon. The tube does not snap at a weak point, the way a rope does. The new pair is made out of the tube’s own energy, in a quantum event that can happen anywhere along it. Arrays of atoms built to mimic simpler cousins of the strong force have recently shown such breaks unfolding step by step.

Free Up Close

The same property has a second face, and at first it looks like a contradiction. Bring two quarks very close together and color loses its grip: the number that says how strongly color pulls drops the closer you look. Up close, two quarks pull much as two electric charges do, and a quark struck hard flies off almost as if it were free. This is asymptotic freedom.

The reason is again the gluons. Around any charge, the fields of empty space are stirred, and the stir acts like pairs of opposite charges that partly hide it. For electric charge they screen: from far away the charge looks smaller, and closing in, with fewer pairs in the way, you find more of it. Around a color charge the gluons do the opposite. They carry color themselves, so they spread the quark’s color out into a cloud around it, and closing in, you are inside the cloud and find less of it. Quark pairs still screen, and the two effects compete. The gluons win: their weight in the balance is 33, against 2 for each kind of quark, and nature has six kinds, so 33 beats 12. With seventeen kinds of quark the balance would tip the other way, and color would grip harder up close, not more loosely.

So the strong force runs opposite to the electric force at both ends. Far off, the electric pull fades, while the color pull does not fade at all. Up close, more electric charge comes into view, while less color does.

How Gluons Were Seen

A quark cannot be caught alone, and neither can a gluon. Yet physicists saw gluons by colliding electrons with positrons, and they did it with both faces of the strong force at once. When an electron and a positron meet and vanish, their energy can reappear as a quark and an antiquark flying apart. Close together and moving fast, the two are nearly free, so each flies straight. As they separate, the tube between them breaks again and again, and each arrives at the detector dressed as a narrow spray of ordinary particles, pointing the way it went. Most collisions showed two sprays, back to back.

Sometimes one of the quarks throws off a gluon on its way out, and the gluon is dressed into a spray of its own. Those collisions showed three sprays, lying in one plane like the arms of a star, and they came at the rate the theory predicted. The angles between the sprays showed that the gluon carries the same amount of spin as a photon. Nobody has caught a gluon on its own. What was seen is the trail one leaves, and for a particle that can never be alone, that is what seeing it means.

Mass From Motion

The same gluons are behind most of your weight. The three quarks inside a proton add up to about 9 MeV of its 938; the other 99% is energy: the quarks’ motion, held in a space less than two trillionths of a millimeter across, and the gluon field that holds them there, never still. In one widely used way of dividing it, a supercomputer calculation puts more than half of the proton’s mass in the gluon field and about a third in the quarks’ motion. The small remainder comes from the masses of quarks, the pairs of quarks and antiquarks stirred up in the quark field included. Gluons weigh nothing themselves, yet the energy of their field is more than half the mass of visible matter. That energy resists being pushed and is pulled by gravity exactly as mass is, because it is mass. Almost every kilogram you have ever lifted is the energy of quarks and gluons, most of it the gluon field’s.

i Mostly not three quarks, but the field between them ×

Glue on Its Own

If gluons attract one another, gluons alone should be able to stick together, with no quarks at all, as long as their colors cancel out. The theory says they can, and supercomputer calculations of glue alone put the lightest of these glueballs at about 1.7 GeV, nearly twice the mass of a proton. None is confirmed yet, because a glueball mixes with ordinary particles of similar mass and looks like them. The strongest candidate is for a heavier kind: the X(2370), a particle recently shown to spin and reflect the way a glueball of that kind and mass should. Its decays also point to a particle that treats the light kinds of quark alike, as a glueball should. The experiment that measured them reads it as made mostly of glue, though a share of ordinary quarks in it is hard to rule out. A confirmed one would be matter built entirely out of a force, something electromagnetism can never make, because photons do not stick to photons.

i Every strand carries color; the knot as a whole carries none ×

When the Glue Melts

Heat the tubes enough and they dissolve. For about the first twenty millionths of a second after the Big Bang, universe was hotter than about 1.8 trillion degrees, too hot for confinement: quarks and gluons were not locked inside protons, neutrons or any other particle, but mingled in one soup. This state, quark-gluon plasma, is the hottest matter ever made in a laboratory. It came with a surprise: it behaves not like a gas but like a nearly perfect liquid. Measured against its entropy, roughly how many ways its parts can be arranged, its internal friction is lower than that of any other fluid ever measured.

Physicists recreate it in big colliders, among them the Large Hadron Collider near Geneva, by smashing gold or lead nuclei together at nearly light speed. For some 10⁻²³ seconds a tiny droplet forms, then cools and freezes back into ordinary particles. Studying that droplet tells us about conditions that existed when universe was younger than a heartbeat.

No detector sees the drop itself, only the spray of particles it leaves, and the spray is how we know it flowed. Nuclei that meet a little off center overlap in a region shaped like an almond. A liquid with almost no friction bursts out fastest across the almond’s narrow side, where its pressure falls off most steeply, so the particles fly out lopsided, more of them to those two sides. A gas of free quarks and gluons would have spread out evenly. Physicists call the lopsided spray elliptic flow, and it matches what a nearly perfect liquid predicts.

The Messenger Is the Message

Every other known force carrier escapes this one way or another: photons barely touch each other, and the W bosons, which do interact, are too heavy to reach much beyond a seven-hundredth of a proton’s width. Only the gluon carries its own charge with nothing to hold it back. From that one property came the tube where a spreading field was expected, eight gluons instead of nine, and quarks that can never leave yet move almost freely when struck hard. From it came the sprays that showed gluons exist, almost all the mass of the matter you are made of, and a liquid that once filled all of space. It is also why quantum chromodynamics, the theory of the strong force, remains one of the hardest theories in physics to compute. The messenger is also the message.