The full, readable lecture — the sub-atomic zoo and antimatter, the four fundamental forces, the quarks and leptons of the Standard Model, how they bind into hadrons (baryons and mesons), the conservation laws every reaction must obey, how detectors photograph invisible particles, why we build giant accelerators, and the triumph of the Standard Model with the Higgs boson. As you scroll, the panel on the right plays out each idea with an everyday object you already know — a family photo, a tug-of-war, lego bricks, a balancing ledger, a jet's vapour trail, smashed watches.
1 — The sub-atomic zoo & antimatter
The atom is not the end of the story. Inside it live a whole zoo of sub-atomic particles, and each one has a mirror-image twin called its antiparticle — same mass, but opposite charge (and opposite other quantum numbers). Think of a family photo where every member stands beside their mirror reflection.
- Electron (e⁻) — the familiar light, negative particle that orbits the nucleus. Its antiparticle is the positron (e⁺), identical mass but positive.
- Proton (p) & antiproton (p̄) — the proton is positive; the antiproton is its negative twin.
- Antimatter — matter built from antiparticles. It is real and made in the lab, but extremely rare in our Universe.
- Annihilation — when a particle meets its antiparticle they vanish, converting their entire mass into energy as two gamma-ray photons: e⁻ + e⁺ → 2γ.
Mass turns into energy (annihilation)e⁻ + e⁺ → γ + γ · total energy E = 2mc²
(each electron's rest energy is mc² ≈ 0.511 MeV, so two 0.511 MeV photons are released)
Real use: a PET scanner (Positron Emission Tomography) in a hospital relies on exactly this — a tracer emits positrons, each annihilates with a nearby electron, and the back-to-back gamma photons are detected to image the body.
2 — The four fundamental forces
Everything that happens in the Universe is governed by just four fundamental forces. Picture a tug-of-war ranked by raw strength: the strong force heaves at the front, then electromagnetism, then the weak force, and gravity — astonishingly — pulls feeblest of all, even though it shapes galaxies.
| Force | Relative strength | Range | What it does |
|---|
| Strong | 1 (strongest) | ~10⁻¹⁵ m (nucleus) | binds quarks & holds the nucleus together |
| Electromagnetic | ~10⁻² | infinite | between charges; chemistry, light, friction |
| Weak | ~10⁻⁶ | ~10⁻¹⁸ m | radioactive β-decay; changes one quark to another |
| Gravity | ~10⁻³⁹ (weakest) | infinite | between masses; planets, stars, galaxies |
- Strong & weak act only over nuclear distances — they switch off beyond the nucleus.
- Electromagnetic & gravity have infinite range but fall off as 1/r².
- Each force is carried by an exchange particle (gluon, photon, W/Z bosons, and the as-yet-unconfirmed graviton).
Why is gravity so weak? A tiny fridge magnet lifts a paperclip against the gravity of the entire Earth — that single picture shows electromagnetism crushing gravity by an enormous factor.
3 — Quarks & leptons
The Standard Model says all matter is built from two families of truly fundamental (point-like) particles — quarks and leptons. They are the irreducible lego bricks of matter: you cannot split them into anything smaller.
- Quarks — feel the strong force and carry fractional charge. The two lightest are the up quark (+⅔ e) and the down quark (−⅓ e). Quarks never appear alone — they are always bound together.
- Leptons — do not feel the strong force. The everyday ones are the electron (−1 e) and the electron-neutrino (0 charge), a ghostly, almost massless particle pouring out of the Sun.
| Particle | Type | Charge | Found in |
|---|
| Up quark (u) | quark | +⅔ e | protons, neutrons |
| Down quark (d) | quark | −⅓ e | protons, neutrons |
| Electron (e⁻) | lepton | −1 e | atoms (the cloud) |
| Neutrino (νₑ) | lepton | 0 | β-decay, the Sun |
Exam point: the proton and neutron are NOT fundamental — they are made of quarks. The electron is fundamental — as far as we know it has no internal parts.
4 — Hadrons: baryons & mesons
Quarks are never found alone — the strong force locks them together into composite particles called hadrons. Just as a few standard lego bricks click into many shapes, the up and down quarks combine into the everyday matter of the nucleus.
- Baryons — made of three quarks (qqq). The proton is uud and the neutron is udd. Baryons are heavy hadrons; the proton and neutron are the ones inside every nucleus.
- Mesons — made of a quark + an antiquark (q q̄). They are unstable and short-lived; the pion (π) is the classic example, exchanged inside the nucleus.
Adding the quark chargesproton = u + u + d = (+⅔) + (+⅔) + (−⅓) = +1 e ✓
neutron = u + d + d = (+⅔) + (−⅓) + (−⅓) = 0 ✓
Check it yourself: the proton's charge of +1 and the neutron's charge of 0 fall straight out of the fractional quark charges — strong evidence that quarks are real.
5 — Conservation laws
Not every reaction you can imagine actually happens. Nature keeps a strict ledger: certain quantities must add up to the same total before and after any particle reaction — exactly like balancing a chemical equation. If the books don't balance, the reaction is forbidden.
- Charge (Q) — total electric charge is always conserved.
- Baryon number (B) — each baryon counts +1, each antibaryon −1, everything else 0. The total B never changes (this is why the proton is so stable).
- Lepton number (L) — each lepton counts +1, each antilepton −1, others 0. The total L is conserved.
Beta-minus decay — every ledger balancesn → p + e⁻ + ν̄ₑ
charge: 0 → (+1) + (−1) + 0 = 0 ✓
baryon B: 1 → 1 + 0 + 0 = 1 ✓
lepton L: 0 → 0 + (+1) + (−1) = 0 ✓
Why the antineutrino? The electron adds lepton number +1, so an antineutrino (L = −1) must appear to keep the lepton ledger at zero. That balancing requirement is exactly how the neutrino was first predicted.
6 — Particle detection
Particles are far too small to see, but we can photograph their tracks. A high-flying jet writes a white contrail across the sky long after the plane has passed — a cloud chamber and a bubble chamber do the same for charged particles.
- Cloud chamber — a charged particle ionises a supersaturated vapour, and tiny droplets condense along its path, drawing a visible white track.
- Bubble chamber — the same idea in superheated liquid, where the path is marked by a string of bubbles.
- Curved tracks — apply a magnetic field and the track curls. The radius of curvature is proportional to the particle's momentum (r = p / qB), and the direction of the curl tells you the sign of the charge.
Curvature of a charged track in a fieldr = p / (qB) ⟹ faster (more momentum) = gentler curve (bigger r)
opposite charges curl in opposite directions
The positron's discovery (1932): Carl Anderson saw a cloud-chamber track that curved the "wrong" way — a positive particle as light as an electron. That single curling vapour trail confirmed antimatter.
7 — Accelerators
How do you find out what is inside something you can never open? You smash two of them together and watch the pieces fly out — like crashing two pocket-watches to reveal the cogs and springs hidden inside. That is exactly what a particle accelerator does.
- Accelerator — uses powerful electric fields to push charged particles to nearly the speed of light, and magnetic fields to steer them around a ring.
- The LHC — the Large Hadron Collider at CERN is a 27 km ring that smashes protons head-on at enormous energy.
- E = mc² in action — the huge kinetic energy of the collision is converted into the mass of brand-new particles, including ones too heavy to exist anywhere else on Earth.
Collision energy creates masscollision K.E. → new particles · E = mc²
the higher the energy, the heavier the particles we can make and study
Why so big? Higher energy needs a longer ring (or stronger magnets) to bend the fast particles, which is why the LHC is 27 kilometres around — it was the energy needed to make the Higgs boson.
8 — The Standard Model & the Higgs · recap
Put it all together and you have the Standard Model — the most successful theory in physics. It lists every fundamental particle (the quarks and leptons), the force-carrier particles (gluon, photon, W and Z), and the Higgs boson, the particle of the field that gives the others their mass. The Higgs was the last piece, predicted in 1964 and finally discovered at the LHC in 2012.
annihilation energy
How much photon energy is released when an electron meets a positron? (mc² = 0.511 MeV each)
E = 2 × mc² = 2 × 0.511 = 1.022 MeV (shared by two 0.511 MeV gamma photons)
reading the quark content
Confirm the charge of a neutron made of u d d.
Q = (+⅔) + (−⅓) + (−⅓) = 0 — a neutral particle, as observed.
- Every particle has an antiparticle; matter + antimatter annihilate to energy (2γ).
- Four forces: strong > electromagnetic > weak > gravity, with very different ranges.
- Fundamental bricks: quarks (u, d) and leptons (electron, neutrino).
- Hadrons: baryons = 3 quarks (p = uud, n = udd); mesons = quark + antiquark.
- Reactions conserve charge, baryon number and lepton number.
- Detectors show charged tracks; a field curls them, r ∝ momentum.
- Accelerators (LHC) convert collision energy into new mass (E = mc²).
- The Standard Model + Higgs boson is our complete map of matter and force.