Proton

Exactly Opposite

Exactly opposite means as far as anyone can measure, and the measurements are extreme. Fill a metal sphere with a heavy gas and shake it with an alternating electric field. If the gas molecules carried even a trace of charge, the gas would start to hum. It stays silent: a proton’s charge and an electron’s cancel to better than one part in 10²¹.

That precision matters because electricity is so much stronger than gravity. Between two protons, the electric push is about 10³⁶ times their gravitational pull. If the two charges differed by only one part in 10¹⁸, every hydrogen atom would carry a sliver of charge, and any two atoms would push apart harder than gravity pulls them together. Gas could never gather into stars. A mismatch of just that size, two parts in 10¹⁸, was once proposed as the force driving the galaxies apart. Within months a laboratory test showed atoms to be neutral far more precisely than that, and today’s limit is two thousand times tighter than the proposal.

The match is not luck. The proton’s charge comes from three smaller particles inside it, called quarks, with charges of plus two thirds, plus two thirds and minus one third. They add up to exactly one. The Standard Model, the theory of all the known particles, only holds together mathematically if the quarks’ charges and the electron’s balance in exactly this way. Tilt them by any amount, and its equations contradict themselves.

Something Inside

The proton’s name is Greek for first, and for years it was pictured as a point carrying a charge. The first sign of more came from its magnetism. A bare point with the proton’s charge and mass should have one unit of it. The first measurement found about two and a half; today’s figure is 2.79. Whatever the proton is, it is not a point.

To see inside, physicists used the trick that had found the nucleus inside the atom: fire small things at it and watch how they bounce. They fired electrons moving at nearly the speed of light into protons. If a proton’s charge were a smooth cloud, every electron would pass with a gentle nudge. Instead, far more came off at steep angles than a smooth cloud allows, kicked hard by something small inside. Those small things were quarks, with no size any experiment has found, among them the two up quarks and one down quark that give the proton its charge.

How big is a proton? Its charge spreads over a radius of about 0.84 femtometers, less than a trillionth of a millimeter. Even that number took a fight. Hydrogen with a muon in place of its electron gave a proton 4 percent smaller than ordinary hydrogen had. The muon, the electron’s cousin two hundred times heavier, holds its cloud nearly two hundred times closer to the proton, so it feels the proton’s size far more, and for a decade the two answers would not agree. Most measurements since have landed on the smaller value, among them the most recent. A few results from ordinary hydrogen still sit a little higher, so the last word is not quite in.

One Percent

Weigh the three quarks on their own and they come to about one percent of the proton’s mass. The other ninety-nine percent is energy.

Pin anything into a space this small and quantum mechanics forbids it to sit still: the tighter the space, the faster it has to move. Quarks held inside a proton move at nearly the speed of light. What holds them in is a field of gluons, the carriers of the strong force, and it never settles either. It stirs the quark field around the three, and experiments that strike a proton hard find extra pairs of quarks and antiquarks there as well.

There are no little balls in there, though, and no box. The inside of a proton is one churning field. The three quarks are smeared through it as glows, and the extra pairs are ripples stirred up in it, not little balls either.

Energy has mass, so all that motion and all that field, with the quarks’ own small masses, weigh exactly what the proton weighs, on a scale and against a push. A supercomputer calculation of the strong force has split the proton’s mass into parts. Where one part ends and the next begins is partly a matter of definition, but in a widely used split, about a third is the quarks’ motion. More than half is the gluon field. The last tenth comes from the masses of quarks: not only the three, but the extra pairs too. The gluons themselves weigh nothing. Almost every kilogram you have ever lifted is energy held inside protons and neutrons.

A Third of the Spin

Like the electron, the proton carries spin, a fixed amount of real turning with no spinning ball behind it, and its spin makes it a small magnet. For decades the explanation seemed simple: two of its three quarks have their spins pointing opposite ways and cancel, and the third supplies the proton’s spin. An experiment tested this by firing muons with aligned spins into protons with aligned spins, and found that the quarks’ own spins supply only a small part. Today’s best figure is about a third. The rest comes from the spin of the gluons and from the turning of everything inside about the center, in shares that are still being measured. A new collider being built to fire electrons into protons is designed partly to finish that count.

It is a spin you may already have met in a hospital. The nucleus of nearly every hydrogen atom in the water of your body is a single proton. A magnetic resonance imaging scanner, an MRI, holds the body in a strong magnet. The protons’ tiny magnets still point every which way, with only a slight lean toward the field, as if a few in every million were lined up. A radio pulse tips that small surplus over, and as it wheels round the field, its turning magnetism drives a faint current in a coil, which is turned into a picture of soft tissue.

Pushing Out, Holding In

Quarks racing at nearly the speed of light push outward, and something has to hold them in. Physicists have worked out the pressure the quarks feel inside the proton from collisions in which an electron bounces off it and a particle of light flies out. Near the center, the push outward is about 10³⁵ pascals, roughly ten times the pressure at the heart of a neutron star. Farther out, past about 0.6 femtometers, the pressure turns around and squeezes inward: the grip of the field that holds the quarks in.

Push and squeeze have to cancel exactly. If the push won, the proton would blow apart; if the squeeze won, it would shrink. A raindrop holds together the same way: the pressure inside pushes out, its surface pulls in, and the drop keeps its size because the two balance. That measurement came with large uncertainties and was argued over, but supercomputer calculations of the strong force, counting the gluons as well as the quarks, have since found the same pattern: a push at the core and a squeeze around it, cancelling.

Why It Lasts

Inside, nothing ever settles; outside, nothing ever changes. Both are true for one reason: the proton is the lowest-energy state its quarks and gluons can be in. The storm inside is not energy waiting to run down. It is the floor. The motion that being squeezed forces on the quarks cannot be taken away, because there is no lower state to drop into, so a proton cannot slow down, cool off or wear out. That is also why every proton is the same as every other: there is one lowest state, and they are all in it.

A free neutron, by contrast, lasts about fifteen minutes. It has a lower state to fall to, because a proton, an electron and an antineutrino together weigh less than a neutron does. The proton is the lightest particle with three more quarks than antiquarks, and nothing in the Standard Model lets it turn into anything lighter.

Theories that unite the forces put quarks and electrons into a single family, which lets a quark turn into a positron, the electron’s antiparticle. So they predict that protons do decay in the end, on average after 10³⁴ years or more, nearly a trillion trillion times the present age of universe. Nobody can wait that long for one proton, but anyone can watch a great many at once. One detector, a tank of 50,000 tonnes of pure water deep in a mine in Japan, holds about 10³⁴ protons in the part it watches most closely. If protons lasted 10³⁴ years, about one of them would decay there every year. In twenty-two years of watching, not one went the way those theories favor most, into a positron and a pion. So on average a proton would take at least 2.4 times 10³⁴ years to decay that way. A successor with about eight times as much water under watch is due to start soon.

A Beam That Stops on Purpose

A fast proton fired into the body slows down as it goes, and the slower it moves, the more energy it hands to the tissue around it. So on the way in it leaves relatively little energy in each millimeter of tissue, then several times as much in its last few millimeters, and stops. Past that point, almost nothing. The spike is called the Bragg peak, after the physicist who found it. How deep it lies is set by the proton’s starting speed.

An X-ray beam behaves the other way. It gives up energy all along its path, the most near the skin, and keeps going out the far side of the body. So cancer can be treated with protons instead. Mixing beams of several speeds spreads the peak across a whole tumor, while tissue beyond it receives almost nothing. That matters most next to the spinal cord, the eyes or a child’s growing organs. More than a hundred centers around the world now treat patients this way.

A Storm at Its Floor

Put together, the proton is a storm resting on its own floor. Three quarks that weigh one percent of it race inside a space less than two trillionths of a millimeter across. The energy of the field between them is most of its mass. The field carries a share of its spin too, and the proton’s core pushes ten times harder than the heart of a neutron star against a squeeze that holds it exactly. Because all of that is already the lowest the proton can go, it never runs down and never changes, and it will outlast every star. Its charge cancels the electron’s to better than one part in 10²¹, which is what lets gravity, a force 10³⁶ times weaker, gather matter into stars and planets at all. It is the anchor of every atom, and as far as anyone has been able to measure, it is immortal.