The Standard Model

If you kept cutting anything in half, over and over, you would eventually reach pieces that cannot be cut any smaller. Those pieces are the fundamental particles, and it turns out there are not that many of them.

Matter is made from two kinds of building block. Quarks stick together to make the protons and neutrons in the centre of every atom. Leptons include the electron, which whizzes around the outside of atoms and carries electricity. Almost everything you see is built from just up quarks, down quarks and electrons. Nature also keeps two heavier copies of each of these, which we only make in labs or find in cosmic rays, and they quickly decay.

Then there are the forces, and here is the surprising part. A force, in this picture, is particles being thrown back and forth. Light itself, the photon, is the messenger of the electric and magnetic force. Other messenger particles carry the two forces that live inside the atomic nucleus.

One more particle, the Higgs, is special. It comes from an invisible field spread through all of space, and that field is what gives many of the other particles their mass. It was the last piece to be found, in 2012. The whole scheme is called the Standard Model, and it works astonishingly well, though it still leaves gravity out.

The particles split into two great families by a property called spin. The matter particles are fermions: six quarks and six leptons. The force carriers are bosons. That distinction is not just a label. Fermions cannot pile into the same state, which is why matter takes up space; bosons happily share a state, which is why forces can build up.

The twelve matter particles come in three generations, three near-identical sets that differ only in mass. The first generation, the up and down quarks and the electron with its neutrino, makes up all stable matter. The second and third generations, the charm and strange, top and bottom quarks, the muon and tau and their neutrinos, are heavier and unstable, decaying quickly into first-generation particles. Nobody knows why nature made three copies.

Quarks carry a kind of charge called colour, and it comes with a strict rule: colour charges must cancel out, so quarks are never found alone. They are locked together in twos and threes inside particles like protons and neutrons. Try to pull two quarks apart and the force between them does not weaken; it stays strong until there is enough energy to make new quarks, which snap onto the ends. This is called confinement.

Three of the four forces appear here as carrier particles. The photon carries electromagnetism. Gluons carry the strong force that binds quarks. The W and Z bosons carry the weak force, responsible for certain radioactive decays. The Higgs boson is the ripple of the Higgs field, and the drag that field exerts on particles as they move is where much of their mass comes from. The glaring absence is gravity, which the Standard Model does not describe at all.

A quantum field theory. The Standard Model is not really a list of particles but a theory of fields. Every particle is a ripple, a quantised excitation, of an underlying field that fills spacetime: an electron is a knot in the electron field, a photon a ripple in the electromagnetic field. Its structure is fixed by a gauge symmetry, written \(SU(3) \times SU(2) \times U(1)\). The \(SU(3)\) part governs the strong force and colour, and \(SU(2) \times U(1)\) governs the unified electroweak force.

Electroweak symmetry breaking. At high energies electromagnetism and the weak force are two faces of one electroweak interaction, and their carriers are massless. The Higgs field acquires a non-zero value everywhere, which breaks that symmetry: the W and Z bosons become heavy while the photon stays massless, and the fermions gain mass in proportion to how strongly they couple to the field. The 2012 discovery of the Higgs boson at the Large Hadron Collider confirmed this last mechanism directly.

How precise it is. Quantities like the electron's magnetic moment are predicted and measured to more than ten decimal places, and they agree. Very little else in science is tested this hard. Within its domain the Standard Model is arguably the most successful physical theory ever written.

What it cannot do. It omits gravity entirely, and marrying it to general relativity is the great unsolved problem. It gives neutrinos no mass, yet neutrinos do oscillate and so must have some. It offers no candidate for dark matter or dark energy, and does not explain why matter so overwhelmingly outnumbers antimatter. So the Standard Model is spectacularly right and clearly not the whole story, which is exactly why physicists keep pushing at its edges.