Feynman diagram

A Feynman diagram is a simple picture, invented by Richard Feynman, that represents an interaction between particles in quantum field theory. Lines stand for particles travelling through spacetime, and the points where lines meet, called vertices, mark where particles interact — for instance an electron emitting or absorbing a photon. At a glance the diagram tells the story of who came in, what was exchanged, and who went out.

But a Feynman diagram is much more than a cartoon. Each diagram corresponds to a precise mathematical expression, assembled by following a fixed set of rules, that contributes to the probability amplitude for a process. To compute how likely a given outcome is, physicists sum the contributions of all the diagrams that could produce it, from the simplest up through ever more elaborate ones with more vertices, each typically smaller than the last.

This bookkeeping turned forbidding calculations into something almost visual and made quantum electrodynamics the most accurately tested theory in science, with predictions matching experiment to a dozen decimal places. It is worth being honest, though, that the internal lines of a diagram are not literal journeys of real particles. They are mathematical terms — often labelled virtual particles — in an approximation, and the diagram is a tool for organising the calculation, not a photograph of what physically happens.