Feynman diagram

The Feynman diagram is a graphical notation invented by Richard Feynman in 1948 for organizing the perturbative calculations of quantum field theory. Each diagram represents a term in the perturbative expansion of a scattering amplitude: vertices denote interactions, internal lines represent virtual particles, and external lines represent the incoming and outgoing real particles. The power of the notation lies not in the pictures themselves but in the one-to-one correspondence between diagrams and mathematical integrals: a Feynman diagram is a visual algorithm for computing a contribution to an observable.

The Feynman diagram is often mistaken for a literal picture of what happens during particle interactions — as if particles trace trajectories through spacetime. This is a pedagogical error with philosophical consequences. The diagrams are formal devices in a perturbative series; they have no independent ontological status. The path integral formulation, from which the diagram rules can be derived, makes this explicit: the diagram is a term in an expansion of a functional integral, not a depiction of physical process. Treating Feynman diagrams as what