QUANTUM FIELD THEORY

body of physical principles designed to account for subatomic phenomena. The theory also has found applications in other branches of physics. The theory arises from the attempt to combine the principles of quantum mechanics with those of relativity in an effort to describe processes such as high-energy collisions in which particles may be created or destroyed. The prototype of quantum field theories is quantum electrodynamics (QED), which describes the interaction of electrically charged particles via electromagnetic fields. Here, electric and magnetic forces are regarded as arising from the emission and absorption of exchange particles or photons. These can be represented as disturbances of electromagnetic fields, much as ripples on a lake are disturbances of the water. Under suitable conditions, photons may become entirely free of charged particles; they are then detectable as light and other forms of electromagnetic radiation. Similarly, particles such as electrons are themselves regarded as disturbances of their own quantized fields. Numerical predictions based on QED agree with experimental data to within one part in 10,000,000 in some cases. There is a widespread conviction among physicists that other forces in naturethe weak force responsible for radioactive beta-decay; the strong force, which binds together the constituents of atomic nuclei; and perhaps also gravitational forcescan be described by theories similar to QED. These theories are known collectively as gauge theories. Each of the forces is mediated by its own set of exchange particles, and differences between the forces are reflected in the properties of these particles. For example, electromagnetic and gravitational forces operate over long distances, and their exchange particles (the photon and the graviton) have no mass. The weak and strong forces operate only over distances shorter than the size of an atomic nucleus. They are mediated by massive particles, which can travel only short distances during the exchange process. It is also hoped that all the forces can be encompassed in a single gauge field theory. In such a unified theory, all the forces would have a common origin and would be related by mathematical symmetries. The simplest result would be that all the forces had identical properties. A mechanism called spontaneous symmetry breaking is used to account for the observed differences. A unified theory of electromagnetic and weak forces already has considerable experimental support; it is likely that this theory can be extended to include the strong force. There also exist theories that include the gravitational force, but these are more speculative.