Editing Feynman diagram (section)

== Particle-path interpretation ==
A Feynman diagram is a representation of quantum field theory processes in terms of [[Elementary particle|particle]] interactions. The particles are represented by the diagram lines. The lines can be squiggly or straight, with an arrow or without, depending on the type of particle. A point where lines connect to other lines is a vertex, and this is where the particles meet and interact. The interactions are: emit/absorb particles, deflect particles, or change particle type.

The three different types of lines are: internal lines, connecting vertices, incoming lines, extending from "the past" to a vertex, representing an initial state, and outgoing lines, extending from a vertex to "the future", representing the end state (the latter two are also known as external lines). Traditionally, the bottom of the diagram is the past and the top the future; alternatively, the past is to the left and the future to the right. When calculating [[correlation functions]] instead of [[scattering amplitude]]s, past and future are not relevant and all lines are internal. The particles then begin and end on small x's, which represent the positions of the operators whose correlation is calculated.

Feynman diagrams are a pictorial representation of a contribution to the total amplitude for a process that can happen in different ways. When a group of incoming particles scatter off each other, the process can be thought of as one where the particles travel over all possible paths, including paths that go backward in time.

Feynman diagrams are [[Graph (discrete mathematics)|graphs]] that represent the interaction of particles rather than the physical position of the particle during a scattering process. They are not the same as [[spacetime diagram]]s and [[bubble chamber]] images even though they all describe particle scattering. Unlike a bubble chamber picture, only the sum of all relevant Feynman diagrams represent any given particle interaction; particles do not choose a particular diagram each time they interact. The law of summation is in accord with the [[quantum superposition|principle of superposition]]—every diagram contributes to the total process's amplitude.