In physics, it is a theory that proposes to unify the four
known interactions, or forces—the strong, electromagnetic, weak, and
gravitational forces—by a simple set of general laws. Four distinct forces are
known to control all the observed interactions in matter: gravitation,
electromagnetism, the strong force (a short-range force that holds atomic
nuclei together), and the weak force (the force responsible for slow nuclear
processes, such as beta decay). The attempts to develop a unified field theory
are grounded in the belief that all physical phenomena should ultimately be
explainable by some underlying unity.One of the first to attempt the development of such a theory was Albert Einstein, whose work in relativity had led him to the hypothesis that it should be possible to find a unifying theory for the electromagnetic and gravitational forces. Einstein tried unsuccessfully during the last 30 years of his life to develop a theory that would represent forces and material particles by fields only, in which particles would be regions of very high field intensity. The development of quantum theory, which Einstein rejected, and the discovery of many new particles, however, precluded Einstein's success in formulating a unifying theory based on relativity and classical physics alone.
An important advance in this quest was made in 1967-68 by the American physicist Steven Weinberg and the Pakistani physicist Abdus Salam. They succeeded in unifying the weak interaction and the electromagnetic interaction by using a mathematical technique known as gauge symmetry (see Elementary Particles). According to this theory the electromagnetic interaction consists of the exchange of a photon, and the weak interaction of the exchange of W and Z intermediate bosons. These bosons are believed to belong to the same family of particles as the photons. Theoretical physicists are currently attempting to combine this so-called electroweak theory with the strong nuclear force, using symmetry theories; such attempts are known as grand unification theories, or GUTs. The effort also continues to combine all four fundamental interactions, including gravitation, in what are now known as supersymmetry theories. Thus far, however, such attempts have not succeeded, although they are proving useful in current work in cosmology. (Encarta Encyclopedia)