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By the time the twentieth century was well underway, physicists had realised that the huge diversity of forces encountered in everyday life — friction, tension, drag, the push of a spring, the pull of gravity, the shocks of chemical reactions, the binding of a nucleus — could all be reduced to just four fundamental forces: the gravitational, electromagnetic, strong nuclear and weak nuclear interactions. Every observed physical process is mediated by one (or a combination) of these four. The Standard Model of particle physics accounts for three of them (all except gravity), and describes them in a unified framework in which every force is mediated by the exchange of a characteristic "gauge boson".
This lesson surveys the four fundamental forces, their relative strengths, their ranges, and which particles they act on. It also gathers together the conservation laws that govern every particle interaction — rules you have already seen individually in earlier lessons, but which deserve their own summary treatment. It covers parts of section 6.4.4 of the OCR A-Level Physics A specification (H556).
Gravity is the force between masses. You are familiar with its effects from GCSE onwards: falling apples, planetary orbits, the bending of light by the Sun. It is described classically by Newton's law of universal gravitation (F = GMm/r^2) and more completely by Einstein's general theory of relativity (1915).
Key features:
F \propto 1/r^2, never reaches zero).10^{-36} times the strength of the electromagnetic force between two protons.Because gravity is so weak, it is utterly negligible at the scale of individual particles. Two protons in a nucleus attract each other gravitationally with a force so small that it is swamped by the other three forces by many orders of magnitude. Gravity dominates at astronomical scales only because masses at that scale are enormous and because (unlike electric charges) there is no cancellation.
Electromagnetism is the force between electric charges, unified with magnetism by Maxwell in the 1860s and described quantum-mechanically by quantum electrodynamics (QED) — the most precisely tested theory in all of science.
Key features:
F \propto 1/r^2).γ), massless and chargeless.Because charges come in opposite signs that cancel out at macroscopic scales (matter is almost exactly electrically neutral), electromagnetism does not dominate bulk behaviour the way gravity does. But at the atomic scale, where individual charges cannot be shielded, it is the principal architect of the material world.
The strong nuclear force — sometimes just "the strong force" — is the interaction that holds quarks together inside hadrons, and that glues nucleons into nuclei. It was postulated in the 1930s to explain why nuclei do not fly apart despite the electrostatic repulsion between protons, and its modern description (quantum chromodynamics, QCD) dates from the 1970s.
Key features:
\sim 10^{-15} m, i.e. about the size of a nucleon). Beyond a few femtometres, it switches off sharply.g).The short range is why the strong force does not extend beyond the nucleus: once you get more than a few fm away, its influence vanishes. This is why atoms can exist at all — outside the nucleus, the strong force plays no role, leaving electromagnetism in charge.
The residual strong force between nucleons is the nuclear force you met in Lesson 4. The original Yukawa theory (1935) described it as a pion exchange between nucleons; the modern QCD picture sees pion exchange as just one of many ways that the underlying quark-gluon interaction "leaks" out of individual nucleons. Either way, the range and strength are consistent with what we measure.
The weak nuclear force is responsible for radioactive beta decay and related processes, including neutrino interactions and the decay of heavier quarks and leptons into lighter ones. It is the only force capable of changing the flavour of a quark (e.g. d \to u in beta-minus decay).
Key features:
\sim 10^{-18} m, even shorter than the strong force). The range reflects the mass of the force carriers.M_W \approx 80 GeV/c², M_Z \approx 91 GeV/c²).The weak force is the only one that can change a quark from one flavour to another, or a lepton from one flavour to another. Without it, beta decay, muon decay, and neutrino interactions would all be impossible. Stars, pulsars and supernovae are all partly powered by weak processes; in particular, the proton-proton chain in the Sun involves weak interactions that turn protons into neutrons.
The weak and electromagnetic forces are unified at high energies into a single electroweak force (Glashow, Weinberg and Salam, Nobel 1979). They appear separate at low energies because the W and Z are heavy while the photon is massless.
| Force | Relative strength | Range | Acts on | Carrier(s) |
|---|---|---|---|---|
| Strong nuclear | 1 | \sim 10^{-15} m | quarks, hadrons | gluons |
| Electromagnetic | 10^{-2} | infinite | charged particles | photon (γ) |
| Weak nuclear | 10^{-6} | \sim 10^{-18} m | all fermions | W^\pm, Z^0 |
| Gravitational | 10^{-38} | infinite | everything with mass | graviton (?) |
Note how the "strengths" span 38 orders of magnitude. Relative strengths depend on the distance scale and the process in question; this table gives rough values at nuclear distances.
Every interaction and decay obeys a set of strict conservation laws. In an OCR exam you will be asked to check candidate reactions against these laws to see if they are allowed.
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