The study of the properties of electromagnetic radiation and the way in which it interacts with charged matter in terms of quantum mechanics. The collision of a moving electron with a proton, in this theory, can be visualized by a space–time diagram (Feynman diagram) in which photons are exchanged.
*Perturbation-theory calculations using Feynman diagrams enable an agreement between theory and experiment to a greater accuracy than one part in 109 to be obtained. Because of this, QED is the most accurate theory known in physical science. Although many of the effects calculated in QED are very small (about 4 × 10−6 eV), such as energy level splitting in the spectra of atoms, they are of great significance for demonstrating the physical reality of fluctuations and polarization in the vacuum state. QED is a gauge theory for which the gauge group is Abelian (see group theory).
An electron–proton collision.