PART IV · Third Room · Quanta and Beyond

The Standard Model — the
17 fundamental particles that build the universe

If you break a stone apart, and break it again and again, what remains? The answer from 20th-century physics is astonishingly simple — all the matter and forces in the entire universe can be described by just 17 kinds of fundamental particles. This map, often called the "periodic table" of matter, is the Standard Model.

§1The bricks that build matter — fermions

Our bodies, the stars, and the galaxies are all ultimately made of two kinds of fundamental particle — quarks and leptons. There are six quarks (up, down, charm, strange, top, bottom) and six leptons (the electron, muon, and tau, plus three neutrinos). These 12 particles are called fermions, and they are the raw material of "matter."

Quarks never travel alone; they are always bound together. A proton is up-up-down (uud), a neutron is up-down-down (udd) — composite particles in which three quarks are tightly bound by the strong force. The electrons orbiting the nucleus are leptons. So ordinary matter is essentially a combination of just three particles: up quarks, down quarks, and electrons. The heavier particles appear only briefly, in the high energies of the early universe or in particle colliders.

In plain words If chemistry's periodic table organized every substance in the world using some hundred elements, the Standard Model goes much deeper still, organizing even the "parts that make the elements" into just 17 particles. It's like a complete list of every type of Lego brick.

§2The messengers that carry force — bosons

How do particles push and pull on one another? In the Standard Model, force is transmitted by exchanging particles called bosons. Electromagnetism is carried by the photon, the strong force (which binds the nucleus) by the gluon, and the weak force (which drives radioactive decay) by the W and Z bosons. Add the Higgs boson, discovered in 2012, and the particles of the Standard Model number 17 in all.

The Higgs is special. The degree to which a particle is resisted as it collides with the Higgs field that fills all of space is precisely its mass. Without the Higgs field, electrons and quarks would be massless and scatter away at the speed of light, and no atoms, no stars, and no us could exist.

§3An imperfect map — the missing pieces

The Standard Model is the most precisely tested theory humankind has ever built. Experiments have found, one by one, the particles it predicted, and some of its values match to ten decimal places. It even predicted and confirmed the existence of antimatter (antiquarks, positrons, and so on) that mirrors matter. Yet this map has clear blank spaces.

The Standard Model deals with only three forces — electromagnetism, the strong force, and the weak force. The fourth force, gravity, does not fit into it. Nor can it explain the dark matter and dark energy that make up most of the universe, or why neutrinos have mass at all (however tiny). These blanks are exactly where the search for a quantum theory of gravity, such as string theory, begins.

The biggest blank — gravity

The Standard Model brilliantly unified three forces, but it cannot include gravity. When you try to describe gravity in the language of quantum mechanics, the calculations blow up to infinity. The attempt to hold all three forces plus gravity within a single framework — that is the starting point of quantum gravity theories, including string theory, which we will meet in the next chapter.

Key points

  • Standard Model = 12 matter particles (6 quarks + 6 leptons) + 4 force-carrying bosons + the Higgs = 17 in total
  • Fermions (spin ½) make up matter; bosons (spin 1, and the Higgs spin 0) carry force and mass
  • Proton = uud, neutron = udd — composite particles of quarks bound by gluons (the strong force)
  • Interaction with the Higgs field gives fundamental particles their mass (discovered 2012)
  • It explains three forces (electromagnetic, strong, weak) but cannot include gravity
EXP.14 — Particle Collider1 GeV
10 GeV
Observe — Smash two protons (uud) head-on, and by E = mc² the collision energy is converted into heavy particles that fly off in all directions. The higher you push the energy slider, the heavier the particles that are "unlocked" in turn — from the electron (0.5 MeV) up to the Higgs (125 GeV) and the top quark (173 GeV). Check the chips at the bottom to see which particles you can make right now.
EXP.14b — Particle Zoo (the 17 Standard Model particles)
This is the periodic table of matter. The three left columns are the 1st, 2nd, and 3rd generation quarks (purple) and leptons (cyan); the fourth column is the force-carrying gauge bosons (gold); and on the right is the Higgs (red), which gives mass. Click a particle to see its charge, mass, and role below.
PARTICLE ZOOHiggs boson
Click a particle cell to display its details
Higgs boson (H) — charge 0 · mass 125 GeV · role: gives mass
DEEP DIVE — Equations & History
Mass–energy equivalence · fermions and bosonsE = mc²
$E = mc^2 \;\;\Rightarrow\;\; m = \frac{E}{c^2}$
In the collider the kinetic energy E of two protons is converted, at the instant of collision, into the mass m of new particles — which is why the more energy you have, the heavier the particles you can create (this connects to special relativity in Part II). The particles produced are either spin-½ fermions (quarks and leptons = matter), or integer-spin bosons (the photon, gluon, W, and Z = force; the Higgs = mass). Two fermions cannot occupy the same state (the Pauli exclusion principle), which gives matter its volume, while bosons can overlap and so carry force.
The three forces and their carrier particlesGauge bosons
$\text{Electromagnetism} \to \gamma \qquad \text{Strong force} \to g \qquad \text{Weak force} \to W^{\pm},\,Z^0$
The Standard Model unifies electromagnetism, the strong force, and the weak force by having each mediated by its own boson. The photon (γ) and gluon (g) are massless, but the weak force's W and Z bosons are very heavy, at 80.4 and 91.2 GeV respectively — and this mass, too, comes from the Higgs mechanism. The particle that would carry the fourth force, gravity (the hypothetical "graviton"), is still absent from this table.
HISTORY — A Standard Model timeline
1968
Glashow, Weinberg, and Salam propose the electroweak theory, uniting electromagnetism and the weak force (1979 Nobel Prize)
1995
Fermilab discovers the last quark, the top quark (173 GeV)
2012
CERN's LHC discovers the Higgs boson (125 GeV) — completing the last piece of the Standard Model (2013 Nobel Prize)
← PREV
Quantum Entanglement