The Standard Model of Particle Physics

Seventeen particles, three forces, one equation that fills a coffee mug. It is the most precisely tested theory in the history of science — and almost certainly incomplete.

If you sat down a physicist from 1900 and showed them what we now call the Standard Model, the first thing they would notice is how short the list is. All of ordinary matter — you, the chair you are sitting on, every star you have ever seen — is made of just three particles: up quarks, down quarks, and electrons. Three. Hold those together with two forces (electromagnetism and the strong nuclear force) and you have built the periodic table, chemistry, biology, and most of astronomy.

But nature was not quite that economical. When physicists started smashing particles together at higher and higher energies through the twentieth century, they kept finding heavier copies of the things they already had — particles identical to the electron in every way except mass, particles like the up and down quark but heavier still. By the 1970s, the catalogue had settled into a strange, suggestive pattern.

The catalogue

The matter particles — fermions — come in three generations. Each generation is a near-perfect copy of the one below it, only heavier. Generation I (up, down, electron, electron-neutrino) builds everything around us. Generations II and III decay almost as soon as they appear; they exist in cosmic rays, inside accelerators, in the first microseconds after the Big Bang. Nobody knows why there are three. There could have been one. There could have been seventeen. There are three.

Then there are the bosons, which carry the forces. The photon mediates electromagnetism. Gluons mediate the strong force that binds quarks into protons and neutrons. The W and Z bosons mediate the weak force, the one responsible for radioactive beta decay and the nuclear reactions that power the sun. And there is the Higgs — the strangest of the lot — which we will come to in a moment.

Forces as exchange

The deep idea hidden inside the Standard Model is that forces are not invisible tugs reaching across empty space, as Newton imagined gravity. They are exchanges of particles. Two electrons repel because one of them emits a photon that the other absorbs — like two skaters throwing a ball back and forth, recoiling apart with each throw. Every interaction in physics, except gravity, is a story of one particle tossing another to its neighbour.

This picture — quantum field theory — rewrites what a particle even is. A particle is not a tiny billiard ball. It is a localized vibration of a field that fills all of space. There is an electron field, a quark field, a photon field, a Higgs field. The particles you read about are ripples in those fields. Press your finger on a calm pond; the dimple is an electron. Withdraw it; the electron is gone, but the pond is still there.

A particle is not a thing. It is a way a field can wiggle.

The Standard Model is, written compactly, a single equation called the Lagrangian. It specifies which fields exist and how they couple to one another. From that one line, everything follows: chemistry, lasers, transistors, the colour of gold, the lifetime of the muon, the rate at which the sun fuses hydrogen. The number it predicts for the magnetic moment of the electron agrees with experiment to twelve decimal places. No theory has ever been so precisely vindicated.

The Higgs and the origin of mass

For decades, one piece refused to fit. The theory's elegant symmetries required all fundamental particles to be massless — like the photon — zipping around at the speed of light. But electrons clearly have mass. Quarks have mass. The W and Z bosons have a great deal of mass. Where does it come from?

In 1964, Peter Higgs and others proposed a way out. Imagine that the universe is suffused with an invisible field — the Higgs field — that has a nonzero value everywhere, even in empty space. Different particles interact with this field with different strengths. A photon ignores it entirely and stays massless. An electron drags through it lightly. A top quark drags as if wading through molasses. That drag is what we call mass.

Confirming this required finding the particle associated with the field — the Higgs boson itself — and that took half a century, a 27-kilometre tunnel under the French-Swiss border, and ten thousand physicists. On the 4th of July 2012, CERN announced a discovery at the Large Hadron Collider that matched the predicted Higgs to within experimental error. Higgs and Englert shared the Nobel Prize the next year. The theory was complete.

What it does not explain

The Standard Model describes three of the four known forces with extraordinary precision. It does not include gravity. It says nothing about dark matter, which appears to outweigh ordinary matter five to one in the cosmos. It cannot account for why the universe is full of matter and almost empty of antimatter, even though the two were produced in equal measure at the Big Bang. It treats neutrino masses as an awkward afterthought, with no natural reason for their being so tiny. And it leaves about twenty constants — particle masses, coupling strengths — as free parameters that have to be measured rather than derived.

So the Standard Model is, in the same breath, the most successful physical theory ever written and a theory we know is wrong. Or rather, incomplete. Something larger contains it — the way Newtonian gravity is contained inside Einstein's relativity. We do not yet know what that larger thing is. String theory, supersymmetry, loop quantum gravity, grand unified theories, technicolour — each is a candidate, and none has been confirmed. The next century of physics is the search for whatever the Standard Model is the low-energy shadow of.

What is humbling is that the shadow itself is enough to build a civilization. Every chip, every magnet, every reactor, every chemical bond runs on the rules in that one Lagrangian. We do not yet have the final theory of nature. But we have, in seventeen particles and one equation, a remarkably faithful sketch.