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From Encyclopaedia Britannica

Quantum Electrodynamics (QED), quantum field theory of the interactions of

charged  particles with the electromagnetic field. It describes mathematically not onlyall interactions of  light with matter   but also those of charged particles with one

another. QED is a relativistic theory in that Albert Einstein’s theory of  special

relativity is built into each of its equations. Because the behaviour of atoms and

molecules is primarily electromagnetic in nature, all of  atomic physics can be

considered a test laboratory for the theory. Some of the most precise tests of QED

have been experiments dealing with the properties of subatomic particles known as

muons. The magnetic moment of this type of particle has been shown to agree with

the theory to nine significant digits. Agreement of such high accuracy makes QED

one of the most successful physical theories so far devised.

In 1928 the English physicist P.A.M. Dirac laid the foundations for QED with his

discovery of a wave equation that described the motion and spin of  electrons and

incorporated both quantum mechanics and the theory of  special relativity. The QED

theory was refined and fully developed in the late 1940s by Richard P. Feynman, Julian S. Schwinger , and Tomonaga Shin’ichirō, independently of one another. QED

rests on the idea that charged particles (e.g., electrons and positrons) interact by

emitting and absorbing  photons, the particles that transmit electromagnetic forces.

These photons are “virtual”; that is, they cannot be seen or detected in any way

 because their existence violates the conservation of energy and momentum. The photon exchange is merely the “force” of the interaction, because interacting particles

change their speed and direction of travel as they release or absorb the energy of a

 photon. Photons also can be emitted in a free state, in which case they may be

observed as light or other forms of  electromagnetic radiation. 

The interaction of two charged particles occurs in a series of processes of increasing

complexity. In the simplest, only one virtual photon is involved; in a second-order

 process, there are two; and so forth. The processes correspond to all the possible

ways in which the particles can interact by the exchange of virtual photons, and eachof them can be represented graphically by means of the so-called Feynman diagrams. 

Besides furnishing an intuitive picture of the process being considered, this type of

diagram prescribes precisely how to calculate the variable involved. Each subatomic

 process becomes computationally more difficult than the previous one, and there are

an infinite number of processes. The QED theory, however, states that the more

complex the process — that is, the greater the number of virtual photons exchanged in

the process — the smaller the probability of its occurrence. For each level of

complexity, the contribution of the process decreases by an amount given by α2 — 

where α is a dimensionless quantity called the fine-structure constant, with a

numerical value equal to (1/137). Thus, after a few levels the contribution isnegligible. In a more-fundamental way the factor α serves as a measure of the

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strength of the electromagnetic interaction. It equals e2/4πεo[planck]c, where e is the

electron charge, [planck] is Planck’s constant divided by 2π, c is the speed of light,

and εo is the permittivity of free space.

QED is often called a  perturbation theory because of the smallness of the fine-

structure constant and the resultant decreasing size of higher-order contributions.This relative simplicity and the success of QED have made it a model for other

quantum field theories. Finally, the picture of electromagnetic interactions as theexchange of virtual particles has been carried over to the theories of the other

fundamental interactions of matter, the strong force, the weak force, and the

gravitational force. See also gauge theory.