Strong Nuclear Field
Strongest Force in Nature
The Strongest Force, Felt Only Up Close
Gravity shapes galaxies. Electromagnetism holds atoms together. Neither comes close to the strong nuclear force. Inside a nucleus, it holds two neighboring protons together dozens of times harder than their electric charges push them apart. Between the quarks inside a proton it is stronger still. There it pulls some 10³⁹ times harder than gravity pulls two protons the same distance apart. Both comparisons need a distance attached, and that is not a technicality. The electric and weak forces’ charges shift a little in strength with how closely you look. This force’s charge shifts far more: seen across a proton it is strong, and over a thousandth of that distance several times weaker, for a reason that comes below. Two quarks that close still pull on each other far harder than across a proton, as electric charges pull harder the nearer they are. Yet the force is felt only within a femtometer or two (a femtometer is 10⁻¹⁵ meters), about the width of a proton. Everything we can hold is colorless, in a sense that comes below, and the little of the force that leaks out between colorless particles fades within that distance. This is why you never feel it in daily life, even though it is holding every nucleus in your body together right now. The masses of a proton’s three quarks add up to about one percent of it. Most of the rest is energy: the strong field’s, and that of the quarks’ own motion, since a quark held in so small a space can never sit still. So that energy makes up most of your weight.
Color Charge
Think of mixing light. Red, green, blue. Shine all three together and you get white. Electromagnetism has one type of charge: positive and negative. The strong force is richer. It has three types of charge, whimsically named after colors: red, green, and blue. Each quark carries a color charge. Anti-quarks carry anti-colors. Gluons, the ripples of the strong field itself, carry color and anti-color together. Three colors and three anti-colors could pair nine ways, but there are only eight gluons: six pair a color with a different anti-color, and two are blends. The ninth would be a blend carrying no color at all, free to reach out of a proton as far as light does, and it does not exist.
Every particle that can be caught on its own is color neutral. Inside a proton, the three quarks must always carry red, green, and blue between them, combining to white. Yet no quark there has a fixed color of its own: the three are in a blend in which each could be any color, as long as all three differ. Pictures of gluons handing colors from quark to quark, like the one below, are a way of keeping the books, not a film of what happens. In a meson, a quark and an anti-quark carry a color and its anti-color. No color-charged particle has ever been seen alone. This rule is called color confinement, and it is one of the deepest rules in physics. It has never been broken, and computer simulations of the theory show it plainly, but it has not yet been proved from the theory’s equations. A closely related puzzle is one of the seven Millennium Prize Problems, with a million dollars offered for the answer. Gluons themselves have no mass, yet every ripple the strong field can make on its own seems to have some: the simulations find it, but a proof that it must is still missing.
Confinement
Picture a cord that pulls back with the same force however far you stretch it. Now try pulling a quark away from the antiquark it is bound to. In electromagnetism, field lines spread out with distance and the force weakens. In the strong force, the field does not spread. The gluon field lines squeeze into a narrow, intense tube of energy called a flux tube. Pull and the tube stretches, pulling back as hard at every length, with a force of about fifteen tonnes’ weight. So energy builds up inside it, and before the quarks are even a proton’s width apart the tube snaps. But it does not release a quark. The stored energy goes into a brand new quark and antiquark at the broken ends. You started with one pair and ended with two, still confined. Color charge is never left on its own. Below, each quark is drawn as a ball and each antiquark as a ring.
One honest caveat, since the word "snap" invites it: do not picture this as a little mechanical event you could film frame by frame. Past a little more than a femtometer, the lowest-energy state of the pair turns smoothly from one stretched tube into two separate particles, each a quark bound to an antiquark. And which particles come out of a real collision is a matter of chance. The snapping tube is how we draw it.
Asymptotic Freedom
Stranger still, at very short distances the strong force loses its special grip. Close together, quarks pull on each other much as electric charges do, with a strength that keeps falling the closer they are. Probe quarks inside a proton at high energy with particle accelerators, and over that brief, hard hit they behave almost as free particles. This property is called asymptotic freedom. Accelerators saw quarks acting free before the theory could say why.
Why? Because gluons carry color charge. Unlike photons, which carry no electric charge, gluons interact with each other. They pull on each other. This self-interaction creates an anti-screening effect. In electromagnetism, the field round an electron hides part of its charge, as if pairs of charges in the space round it turned their plus ends inward, so it looks weaker from far away. The quark field’s pairs hide part of a color charge too, but the gluon field, colored itself, spreads it outward more, so it looks stronger the farther away you probe. Its strength falls at short range and grows toward the size of a proton. That fits confinement, though it does not by itself prove it. A grip that never lets go, however far you pull, and a strength that is weaker the closer you look.
Nuclear Binding
Think of a sealed box full of magnets. The box itself is not magnetic. But press another box right against it, and you feel a faint tug. The force holding protons and neutrons together inside a nucleus is not the strong force between quarks directly. It is what leaks out of them. Each proton and neutron is color neutral as a whole, so no color reaches out of it. But within about a proton’s width of another, the two can reach each other through a colorless field, whose ripples are a quark bound to an antiquark, and some of the force gets across.
Why does the leak reach so short a way? Two nucleons pull on each other through the field of a particle, and the heavier the particle, the shorter the field’s reach. From the size of nuclei, its mass was worked out before anyone had seen it: a few hundred times the electron’s. Then it was found, about 270 times as heavy as the electron. It is the pion, a quark bound to an antiquark, the lightest particle whose field can reach out of a nucleon. Its mass sets the reach at about one and a half femtometers. Each nucleon is wrapped in a thin cloud of pion field that fades within that distance, and where two clouds overlap, the nucleons pull on each other. A charged pion field carries charge across, so a proton and a neutron held this way keep trading places: the proton becomes a neutron and the neutron a proton. A neutral pion changes neither. When a free proton and neutron are first caught like this, the energy freed by their binding leaves as a flash of gamma light. Below, the pions are drawn one at a time, as if thrown, and the proton glows warm and the neutron cool.
Up close the force turns around. Pushed nearer than about half a femtometer, two nucleons repel, so a nucleus cannot collapse. This is not the force between quarks growing stronger up close. Pressed into one another, their six quarks would have to share one small space. The rules for identical particles, together with a magnet-like part of the color force, make that cost energy, so they push back. Each nucleon holds on only to its nearest neighbors, so every nucleus, light or heavy, has about the same density, some 230 million tonnes to the cubic centimeter. Against this short grip, the protons’ electric push reaches across the whole nucleus. Heavy nuclei need extra neutrons, which add grip and no push. But each extra neutron has to sit in a higher energy level than the last, until turning one into a proton would lower the nucleus’s energy, so the trick runs out. Beyond lead no nucleus is stable for good.
Nucleons are held most tightly in nuclei around iron and nickel. Light nuclei release energy by fusing toward them, heavy ones by splitting toward them. That is what powers nuclear reactors and nuclear weapons, and every star you see in the night sky: the Sun fuses hydrogen into helium, with the weak force turning protons into neutrons along the way, as every sun that ever shone has done.
Quark-Gluon Plasma
Heat matter enough and its atoms come apart into nuclei and electrons, and its nuclei into protons and neutrons. Heat it vastly more and the protons and neutrons come apart too. Under ordinary conditions, quarks and gluons are permanently confined inside them. But around 1.8 trillion degrees (more than a hundred thousand times the temperature of the Sun’s core), the strong force loosens its grip. It does so smoothly, over a range of temperatures, not at one sharp point the way ice melts. Quarks and gluons are then no longer held in separate particles but spread through all of the hot matter, a state called quark-gluon plasma. This state filled all of space for about the first twenty millionths of a second after the Big Bang, until universe cooled enough for quarks to bind into protons and neutrons.
Physicists have recreated it by smashing heavy nuclei together at nearly the speed of light: gold in one collider, and lead in the Large Hadron Collider. For a brief instant, a drop of quark-gluon plasma appears. Surprisingly, it behaves not like a gas but like a nearly perfect liquid. Measured against its entropy, a measure of how many ways its parts can share their energy, its internal friction is the lowest of any known substance, close to a floor that some theories suggest.
The clue is its shape. Nuclei that meet a little off center overlap in a region shaped like an almond, and the drop bursts out fastest across the almond’s narrow side, where its pressure falls off most steeply. A gas of free quarks would spread evenly. The particles it leaves fly out lopsided in just the way a liquid with almost no friction predicts. It is the kind of matter that filled universe in its first millionths of a second.
