study of the quantum mechanical interaction of elementary particles and fields. Elementary particles (electrons, protons, neutrons, quarks, etc.) are the most basic physical constituents of the universe, and fields are regions throughout which a force may be exerted. Electromagnetic radiation is the energy radiated in the form of a wave as a result of the motion of electric charges. Quantum theory is the modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles. Therefore, QED is the theory that describes h

QED is often referred to as 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. Now, the basis of QED has been applied to the theories of strong, weak, and gravitational forces.

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 each of them can be represented graphically by means of the diagrams developed by Feynman.

QED deals with the creation of elementary particles from electromagnetic energy, and with the reverse processes in which a pa

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