The Standard Model and the Fundamental Forces

Chemistry has its periodic table — a single chart that lays out every element and hints at how they combine. Particle physics has one too, and it is far smaller and far stranger. Everything you have ever seen, touched, eaten or breathed — every star, every atom, every beam of light — is built from just a handful of truly fundamental particles, held together by just four forces. That chart is the Standard Model, and it is the most precisely tested theory in the history of science.

It sorts every known fundamental particle into two great families. The fermions are the particles of matter — the "stuff". The bosons are the particles that carry the forces — the "glue". Understand how those two families fit together and you understand what the Universe is made of, at the deepest level we have yet reached.

The matter particles: quarks and leptons

The fermions — the matter particles — come in two kinds. Quarks feel the strong force and are never found alone; they clump together to build protons and neutrons. Leptons do not feel the strong force; the e^- electron is the famous one, and each charged lepton has a ghostly, almost massless partner called a neutrino.

Remarkably, nature repeats itself. The matter particles come in three generations — three near-identical copies, each heavier than the last. Each generation has two quarks and two leptons:

Here is the punchline that surprises everyone: ordinary matter is made from the first generation alone. Up and down quarks build every proton and neutron — a proton is uud (charge +\tfrac{2}{3}+\tfrac{2}{3}-\tfrac{1}{3}=+1) and a neutron is udd (charge +\tfrac{2}{3}-\tfrac{1}{3}-\tfrac{1}{3}=0) — and the electron completes the atom. The heavier generations are made fleetingly in cosmic rays and accelerators and decay away in a flash. Two whole extra copies of matter exist that you are simply never made of.

The four fundamental forces

Every interaction in the Universe — a magnet snapping to a fridge, the Sun burning, an atom decaying, an apple falling — is one of just four fundamental forces. They differ enormously in strength and in range:

Roughly, if we set the strong force to a strength of 1, then electromagnetism is about 10^{-2}, the weak force about 10^{-6}, and gravity about 10^{-39} — a spread of thirty-seven orders of magnitude.

Force carriers: forces are an exchange

So how does one particle "push" or "pull" another across empty space? In the Standard Model the answer is beautifully concrete: the particles throw a boson back and forth. Each force has its own carrier particle — a gauge boson — and the force is the continual exchange of these carriers:

A neat pattern falls out of this: the range of a force is set by the mass of its carrier. A massless carrier (photon, gluon) can in principle reach any distance; a massive carrier (W, Z) can only flit across a whisker before it must be "repaid". The strong force's short range is a subtler story (gluons pull on each other), but the rule of thumb holds: heavier carrier, shorter reach.

Three traps that catch out almost everyone meeting the Standard Model:

The whole model on one chart

Below is the Standard Model laid out as its own periodic table: three columns of matter (the three generations of quarks and leptons) and a column of force-carrying bosons, with the Higgs sitting apart at the bottom. Use the switch to light up one generation and watch how each is just a heavier echo of the last — or highlight the force carriers to pick out the gluon, photon and W/Z bosons. Notice the Higgs stays outside that highlight: it is not a force carrier.

The Higgs boson: where mass comes from

One tile on the chart is different from all the others. The Higgs boson (H) is not a matter particle and not a force carrier. It is a ripple in the Higgs field — an invisible field thought to fill all of space. As particles move through this field, they are "dragged" by it, and that drag is what we experience as mass. Particles that couple strongly to the field (like the top quark) are heavy; those that ignore it (like the photon) are massless and fly at the speed of light.

Without the Higgs field, the electron would be massless, atoms could not form, and chemistry — and you — would not exist. It was the last piece of the Standard Model to be found, predicted in 1964 but not confirmed until 2012.

In July 2012, two vast detectors at CERN's Large Hadron Collider — a 27 km ring under the French–Swiss border — announced they had found the Higgs boson. You cannot see a Higgs directly: it exists for less than 10^{-22}\,\text{s} before decaying. Instead, physicists smashed protons together billions of times and looked for the faint statistical bump in the debris where Higgs bosons had briefly appeared and fallen apart — for example into two photons.

The press nicknamed it the "God particle" (a name physicists rather dislike). Its discovery earned Peter Higgs and François Englert the 2013 Nobel Prize — nearly fifty years after they wrote down the idea. And it was not the first time: the Standard Model has a habit of predicting particles decades before anyone finds them. The top quark (predicted in the 1970s) was not seen until 1995; the tau neutrino not until 2000. The theory writes the guest list long before the guests arrive.

What the Standard Model does not explain

For all its stunning success, the Standard Model is known to be incomplete. It is the best map we have — but there is clearly land off the edges:

So the Standard Model is not the final word. It is a magnificent, ferociously well-tested theory that also, honestly, points to everything it still cannot say. That gap is where the next century of physics lives.