Elementaryparticles
Everything you see, touch and are — every stone, every plant, every thought — is built from tiny building blocks. These building blocks are called elementary particles. They are so small that a single glass of water contains more of them than there are grains of sand on Earth. This page explains which particles exist, how they relate to one another, and why they form the foundation of modern physics.
Three families — one world
Physics today recognizes exactly 17 distinct elementary particles — particles that, as far as we currently know, cannot be broken down any further. They fall into three large groups: quarks make up matter, leptons are independent matter particles, and bosons carry the forces.
The particle table
Much like the periodic table of elements in chemistry, physics has an overview of all known elementary particles — the Standard Model. It is one of the most precise and successful theories science has ever developed. The particles are arranged into three generations: each higher generation is heavier and shorter-lived than the one before it.
| Generation I | Generation II | Generation III | Bosons | |||||
|---|---|---|---|---|---|---|---|---|
| Quarks ↑ | u Up +⅔ 2.3 MeV | c Charm +⅔ 1.27 GeV | t Top +⅔ 173 GeV | γ Photon 0 0 | g Gluon 0 0 | |||
| Quarks ↓ | d Down −⅓ 4.8 MeV | s Strange −⅓ 95 MeV | b Bottom −⅓ 4.18 GeV | |||||
| Leptons | e⁻ Electron −1 0.511 MeV | μ Muon −1 105.7 MeV | τ Tau −1 1.777 GeV | W W boson ±1 80.4 GeV | H Higgs 0 125 GeV | |||
| Neutrinos | νₑ e-neutrino 0 ≈ 0 | νμ μ-neutrino 0 ≈ 0 | ντ τ-neutrino 0 ≈ 0 | Z Z boson 0 91.2 GeV | ||||
Quarks — the innermost core of matter
Quarks are the smallest known building blocks of matter — but they never show up alone. They always exist in bound groups: two or three quarks together form what are called hadrons. The best known of these are protons and neutrons, the building blocks of every atomic nucleus. A proton consists of two up quarks and one down quark, a neutron of one up quark and two down quarks.
There are six different quark types, which physicists call "flavors": up, down, charm, strange, top and bottom. These names are pure labels — quarks obviously have no actual flavor. What they do have is a special property called color charge, which carries the strong nuclear force. Every quark carries one of three "colors" — red, green or blue. Combine them so that they become "colorless" and a stable particle results.
The quarks of the second and third generations — charm/strange and top/bottom — are far heavier than those of the first. The top quark weighs about as much as an entire gold atom, even though it is a point-like elementary particle. These heavy quarks arise only in extremely high-energy collisions inside particle accelerators and decay almost instantly into lighter, first-generation particles.
Leptons — the free spirits
Leptons are matter particles that, unlike quarks, know no confinement — they can exist freely in nature. The best known lepton is the electron, which forms the shell of every atom and is responsible for chemical bonds. Without electrons there would be no molecules, no chemistry, no life. The electron has been known for over a hundred years and was one of the first elementary particles science ever discovered.
Besides the electron there are two more charged leptons: the muon (μ) and the tau (τ). They behave exactly like the electron but are much heavier: the muon is roughly 207 times heavier, the tau about 3,477 times. Why nature created these heavier copies of the electron remains one of the great puzzles of physics to this day. Both particles are unstable and decay within the briefest of times.
The three neutrinos are especially fascinating particles: no electric charge, no color charge, and a mass so tiny that physicists spent decades thinking they had none at all. Every second, billions of neutrinos fly through your body — from the sun or from distant supernovae — without leaving the slightest trace. They are the most elusive particles we know.
Bosons — messengers of the forces
Alongside the matter particles there is a second class: the force particles, the bosons. They are the mediators between matter particles — the messengers that carry forces from one particle to another. The photon is the boson of the electromagnetic force: every time light shines, a magnet pulls, or lightning strikes, photons are at work. The photon is massless and always moves at the speed of light.
The gluon mediates the strong nuclear force — the force that holds quarks together inside protons and neutrons. Its name comes from the English word "glue." At short distances this force is by far the strongest of the four fundamental forces — strong enough to overcome the repulsion between the positively charged protons in an atomic nucleus. Without gluons there would be no stable atomic nuclei.
The W and Z bosons mediate the weak nuclear force, which is responsible for radioactive decay. The Higgs boson plays a special role: it is the quantum of the Higgs field, which gives the other particles their mass. Its discovery in 2012 at CERN in Geneva was a historic triumph — after decades of searching. It is the only known elementary spin-0 particle.
Fermions and bosons — two worlds
All 17 elementary particles can be sorted by spin into two fundamentally different categories. Fermions carry half-integer spin (½) — this includes all quarks and leptons. They obey the Pauli exclusion principle: two fermions cannot occupy the same quantum state at the same time. This is precisely why atoms are stable — electrons don't all pile into the lowest energy state but instead occupy different orbitals, which is what makes the diversity of chemical elements possible in the first place.
Bosons, by contrast, have integer spin (0, 1) and are not subject to the Pauli principle. Any number of bosons can occupy the same state — this is the basis for lasers (many photons in the same state), superconductivity, and Bose-Einstein condensates, one of the most exotic states of matter there is. This distinction follows directly from combining relativity and quantum mechanics — it is not an arbitrary scheme but a deep mathematical necessity.
Antimatter — the mirror image
Every elementary particle has an antiparticle with opposite charge but identical mass. The antiparticle of the electron is the positron, that of the proton the antiproton. At the Big Bang, matter and antimatter formed in nearly equal amounts — yet today's universe consists almost entirely of ordinary matter. Why a tiny imbalance arose that prevented complete mutual annihilation is one of the deepest open questions in physics.
When matter meets antimatter, they annihilate each other completely — their entire mass is converted into pure energy (E = mc²). This process is called annihilation. Medicine already puts this effect to use: in positron emission tomography (PET), positrons are injected into the body, where they annihilate with electrons and produce gamma rays that yield a precise 3D image of metabolism.
Three generations — why?
The particles of the Standard Model are arranged into three generations. The first generation — electron, electron neutrino, up quark and down quark — forms all the stable, everyday matter. Every atom in the universe is built from these four particles. The second and third generations are heavier and unstable: they arise in particle accelerators or from cosmic rays and decay within the briefest of times into lighter, first-generation particles.
Why nature chose exactly this threefold structure, nobody knows. In principle there could just as well be four or five generations. Precise measurements at CERN have shown, however, that there are exactly three light neutrino types — which caps the number of complete generations at three. This fact is firmly built into the Standard Model but still not truly explained to this day. It is one of many signs that the Standard Model, for all its successes, is not the last word.
Are these really the smallest building blocks?
The word "elementary" means: not further divisible. By the current state of science, the 17 particles of the Standard Model are considered point-like — they have no measurable extent. Yet the history of physics shows that particles once thought fundamental have repeatedly turned out to be composite: atoms were once considered indivisible, then came electrons and atomic nuclei, then protons and neutrons, then quarks. Many physicists suspect there could be an even deeper layer still.
The Standard Model is brilliant — and yet incomplete. It doesn't explain gravity at the quantum level, gives no account of dark matter (about 27% of the universe), and doesn't explain why there are exactly three generations of particles. The search for a more comprehensive theory is the great project of 21st-century physics.