During the 1910s and 1920s, the power of quantum theory became obvious to most physicists. Its success was demonstrated most notably in the explanation of atomic phenomena, as in the Bohr model of the atom and its subsequent refinements. The natural next step was to apply quantum theory to the field of electromagnetic phenomena. The development of quantum electrodynamics (QED) was begun in the late 1920s by Paul Dirac and extended by a number of theorists, including Werner Heisenberg, Wolfgang Pauli, and Enrico Fermi.

QED theory attempts to use quantum principles to explain the properties of an electromagnetic field and its interaction with electrically charged particles. For example, it tries to describe what happens when an electron travels through an electromagnetic field.

The simplest electromagnetic phenomena yield rather easily to quantum analysis. For example, in describing an electromagnetic field, Dirac pictured the field as consisting of...