Feynman diagrams illustrate the interaction between subatomic particles. Photons or force carrying particles are emitted and reabsorbed, matter particles are deflected and this is all shown on a Feynman diagram.. On a Feynman diagram there are separate axes for time and the three space dimensions – shown as a single axis. The simplest Feynman diagram, illustrating the interaction between two electrons, is shown below.
An electron enters from the bottom left and emits a photon which travels up the page. The photon is absorbed by another electron. The photon is the particle which carries the electromagnetic force between charged particles. Notice that in the diagram above, when the photon passes one one electron to the other, the distance between them increases: the electrons repel each other. Notice also that as the particles pass in the diagram from left to right, time is increasing, though on a quantum scale time can fluctuate, and move forwards and backwards, and this can also be shown in Feynman diagrams.