The Standard Model
Everything you have ever seen or touched — your own body, the chair you are sitting on, the air, the oceans, the distant stars — is built from a surprisingly short list of building blocks. Physicists have sorted that list into a single chart called the Standard Model of particle physics. Think of it as the Universe's parts catalogue. The pieces it lists are elementary particles: the smallest known bits of nature, things that — as far as anyone has ever been able to tell — are not made of anything smaller and cannot be split apart.
The catalogue sorts everything into two kinds of thing. There are the bricks — the bits of solid matter that things are made of — and the mortar — packets of pure energy that pass between the bricks and hold them together. We will take the bricks first, then the mortar, and finish with one special particle that gives everything its weight.
The bricks: particles of matter. The matter particles are called fermions — that is simply the family name for the bits that make up stuff. They come in two groups. The first group is the quarks. Quarks are never found alone; they always clump together in threes. Two quarks of one type and one of another lock together to make a proton or a neutron — the heavy particles packed into the tiny core, or nucleus, at the centre of every atom. The second group is the leptons. The most familiar lepton is the electron, the tiny negatively charged particle that swarms around the outside of every atom and whose flow through wires we call electricity. The electron has a near-weightless, barely-there cousin called the neutrino: trillions of them stream through your body every second without bumping into anything at all.
Oddly, nature made three almost identical copies of this set of bricks, which physicists call three generations. Only the lightest generation — the two lightest quarks plus the electron and its neutrino — makes up the ordinary, stable world around you. The two heavier generations are the very same particles built heavier; they appear only in violent collisions, such as when fast-moving particles from space slam into the upper atmosphere, and they fall apart again almost instantly.
The mortar: particles of force. The mortar particles are called bosons. They are not lumps of matter; each is a packet of energy passed between the bricks, and that exchange is what we feel as a force. These are the four forces and their carriers from the previous lesson — §1.1, The Four Fundamental Forces, explains what each force does and how a carrier works. Here we simply add the carriers to the catalogue as particles in their own right: the photon (γ) for electricity and magnetism, the gluon (g) for the strong force that glues quarks together, and the heavy W and Z for the weak force.
The Higgs: the giver of weight. At the heart of the chart sits one more particle, the Higgs. It belongs to an invisible something — a field — that fills all of empty space. Particles gain their mass (their heft, their resistance to being pushed around) by dragging through this field, the way you feel heavier and slower wading through water. Without it, the bricks would all rush around at the speed of light and could never slow down to settle into atoms, planets, or people. The Higgs was the last piece of the catalogue to be found, finally spotted in 2012 inside a giant machine that smashes particles together.
Antimatter: the mirror twins. Finally, every particle in the catalogue has a mirror twin called an antiparticle — identical in mass but carrying the opposite electric charge. The electron's twin, for example, is the positively charged positron. Matter and antimatter cannot live side by side: the instant a particle meets its antiparticle, the two destroy each other completely and vanish in a flash of light. That looks like a curiosity here, but it turns out to be the key to the whole second half of the story.
The chart below lays all of this out at once. The matter bricks sit on the left — the quarks along the top, the leptons just below — arranged in three columns for the three generations, lightest on the left and heaviest on the right. The force-carrying mortar (photon, gluon, W and Z) stands in its own column to the right, and the Higgs sits apart on its own. Click any tile to read what that particle is and what it does.
The Origin of the Particles
Where did all of these particles come from? Rewind to the first tiny fractions of a second after the Big Bang — the hot, dense beginning from which the Universe has been expanding ever since — and you find no matter at all. There were no bricks and no atoms, only energy, in the form of intensely hot radiation. Every bit of matter that exists today condensed out of that raw energy.
This is possible because mass and energy are really two forms of the same thing. Einstein captured the link in his famous equation E = mc² — energy equals mass multiplied by the speed of light times itself. In plain terms it says that energy can be turned into mass, and mass can be turned back into energy. The early Universe was doing exactly that, constantly.
In the furnace of those first moments, packets of energy crashed together and condensed into matter through a process called pair production: a collision that creates a particle and its antiparticle together, always as a matched pair. The opposite happened just as often — a particle would meet its antiparticle and the two would annihilate, turning straight back into energy. Creation and destruction balanced each other, and the cosmos stayed a churning mix of energy and short-lived matter.
Then the expansion tipped that balance. As the Universe grew it also cooled, in the same way a gas cools as it spreads out and thins. Once the temperature dropped low enough, collisions no longer carried enough energy to create a particular kind of particle. From that moment on, that particle was no longer being replaced as fast as it was destroyed, so the ones that happened to survive became permanent. Physicists call this freezing out. Heavier particles take more energy to make, so they froze out earliest, while the Universe was still ferociously hot.
The order of appearance followed that rule. The force carriers settled out first, as the single unified force of the newborn Universe split into the four we know today — the story told in §1.1. The matter bricks came next, heaviest first. For about the first millionth of a second the Universe was a quark–gluon plasma — a soup so hot that the quarks could not yet stick together; only as it cooled did they lock into protons and neutrons, the first step toward building atoms that §1.3, Synthesis of Elements, picks up. Around one second in, the ghostly neutrinos stopped interacting with everything else and began drifting freely through space, where they still travel today. By about one minute, the lightest charged brick — the electron — froze out, and the main era of matter-making came to an end.
That leaves one last puzzle, the most important of all. If matter and antimatter were always made in equal amounts and always destroy each other on contact, then as the Universe cooled they should have wiped each other out completely, leaving nothing behind but light. We should not exist. The escape was a tiny imbalance: for reasons physicists are still working out, the early Universe made a very slight surplus of matter — roughly a billion-and-one matter particles for every billion antimatter particles. When the great wave of annihilation swept through, every antiparticle found a matter partner and both vanished — but that single leftover matter particle in every billion had no partner, and survived. Everything solid in the Universe today — every galaxy, every star, every living thing — is built from that one-in-a-billion residue of survivors. The annihilation also flooded the cosmos with light — the relic glow that still bathes the whole sky today, which we will meet as the Cosmic Microwave Background in §1.4.
Press play on the animation below — or step through it with the arrow keys — to watch the whole story unfold. While the Universe is hot, light and matter trade places freely: light turns into matter–antimatter pairs (drawn as a filled dot and a matching ring), and those pairs annihilate straight back into light, in balance. As space cools, creation switches off and every pair annihilates for good. Because matter outnumbered antimatter by about one particle in a billion, that tiny surplus is left with nothing to cancel against — and the lone survivor it shows you stands for all the matter there is.
A short catalogue of particles, three repeating generations, and one Higgs that grants them weight — all of it conjured out of pure energy, and saved from total annihilation by a single extra particle in every billion.