Four forces, three in the model
Everything that happens is governed by four fundamental forces. Gravity is by far the weakest but shapes the cosmos because it is always attractive and infinite in range. Electromagnetism binds electrons to nuclei and atoms into matter. The strong force glues quarks into hadrons and hadrons into nuclei. The weak force changes one kind of quark or lepton into another and drives radioactive beta decay.
Force as exchange, and why range varies
In quantum field theory a force is not action at a distance but the exchange of a gauge boson. Two electrons repel by tossing a photon between them, the way two skaters recoil by throwing a ball back and forth. The properties of the carrier fix the character of the force.
The carrier's mass sets the force's range. A rough uncertainty-principle estimate gives the range as the Compton wavelength of the carrier: heavy carrier, short reach.
The range of a force carried by a boson of mass m. A massless carrier gives infinite range.
This explains a great deal at a glance. The photon is massless, so electromagnetism reaches across the universe with its familiar inverse-square falloff. The W and Z weigh about 80 and 91 GeV — enormous — so the weak force reaches barely 10^{-18} m, only inside a nucleon. That is why it is 'weak': not intrinsically feeble, but crushed into a tiny range.
Feynman diagrams: bookkeeping for interactions
A Feynman diagram is a cartoon of an interaction that doubles as a precise calculational recipe. Time runs one way, space the other; solid lines are fermions, wavy lines are force carriers, and every vertex — where lines meet — is a fundamental interaction contributing a factor of the force's coupling strength.
The internal line is a virtual particle — the exchanged photon here. It is 'off-shell', not obeying E^2=(pc)^2+(mc^2)^2, and can never be caught in a detector. To find the probability of a process you sum the amplitudes of all diagrams that connect the same start and end, then square: the observable cross-section is |\text{amplitude}|^2. More vertices mean more factors of the small coupling, so the simplest diagrams usually dominate — that is why the expansion is useful.
QED, QCD and the weak force up close
The electromagnetic theory is quantum electrodynamics (QED), the most accurately verified theory known. Its coupling is the dimensionless fine-structure constant \alpha — small, which is exactly why the Feynman series converges so beautifully.
The fine-structure constant sets the strength of every electromagnetic vertex.
The strong theory is quantum chromodynamics (QCD), built on colour charge. Its twist is that gluons themselves carry colour, so they interact with each other — unlike neutral photons. This self-interaction produces two hallmark behaviours: asymptotic freedom (the force weakens at very short distances, so quarks rattle almost freely inside a proton) and confinement (it strengthens with separation, so pulling a quark out costs ever more energy until a new quark-antiquark pair pops into being instead).
The weak force, carried by the massive W and Z, is the only one that can change a particle's flavour — turning an up quark into a down, or an electron into a neutrino. That is exactly what happens inside a nucleus in beta decay, where a down quark in a neutron emits a W and becomes an up quark, converting the neutron into a proton.
Beta-minus decay: at the quark level, d \to u + W^- and the W^- becomes an electron and antineutrino.