Unified Field Theory

Unified Field Theory, in physics, 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 (see Inflationary Theory).

unified field theory

in particle physics, an attempt to describe all fundamental forces and the relationships between elementary particles in terms of a single theoretical framework. In physics, forces can be described by fields that mediate interactions between separate objects. In the mid-19th century James Clerk Maxwell formulated the first field theory in his theory of electromagnetism. Then, in the early part of the 20th century, Albert Einstein developed general relativity, a field theory of gravitation. Later, Einstein and others attempted to construct a unified field theory in which electromagnetism and gravity would emerge as different aspects of a single fundamental field. They failed, and to this day gravity remains beyond attempts at a unified field theory.

At subatomic distances, fields are described by quantum field theories, which apply the ideas of quantum mechanics to the fundamental field. In the 1940s quantum electrodynamics (QED), the quantum field theory of electromagnetism, became fully developed. In QED, charged particles interact as they emit and absorb photons (minute packets of electromagnetic radiation), in effect exchanging the photons in a game of subatomic "catch." This theory works so well that it has become the prototype for theories of the other forces.

During the 1960s and '70s particle physicists discovered that matter is composed of two types of basic building block--the fundamental particles known as quarks and leptons. The quarks are always bound together within larger observable particles, such as protons and neutrons. They are bound by the short-range strong force, which overwhelms electromagnetism at subnuclear distances. The leptons, which include the electron, do not "feel" the strong force. However, quarks and leptons both experience a second nuclear force, the weak force. This force, which is responsible for certain types of radioactivity classed together as beta decay, is feeble in comparison with electromagnetism.

At the same time that the picture of quarks and leptons began to crystallize, major advances led to the possibility of developing a unified theory. Theorists began to invoke the concept of local gauge invariance, which postulates symmetries of the basic field equations at each point in space and time (see gauge theory). Both electromagnetism and general relativity already involved such symmetries, but the important step was the discovery that a gauge-invariant quantum field theory of the weak force had to include an additional interaction--namely, the electromagnetic interaction. Sheldon Glashow, Abdus Salam, and Steven Weinberg independently proposed a unified "electroweak" theory of these forces based on the exchange of four particles: the photon for electromagnetic interactions, and two charged W particles and a neutral Z particle for weak interactions.

During the 1970s a similar quantum field theory for the strong force, called quantum chromodynamics (QCD), was developed. In QCD, quarks interact through the exchange of particles called gluons. The aim of researchers now is to discover whether the strong force can be unified with the electroweak force in a grand unified theory (GUT). There is evidence that the strengths of the different forces vary with energy in such a way that they converge at high energies. However, the energies involved are extremely high, more than a million million times as great as the energy scale of electroweak unification, which has already been verified by many experiments.

Grand unified theories describe the interactions of quarks and leptons within the same theoretical structure. This gives rise to the possibility that quarks can decay to leptons and specifically that the proton can decay. Early attempts at a GUT predicted that the proton's lifetime must be in the region of 10³² years. This prediction has been tested in experiments that monitor large amounts of matter containing on the order of 10³² protons, but there is no evidence that protons decay. If they do in fact decay, they must do so with a lifetime greater than that predicted by the simplest GUTs. There is also evidence to suggest that the strengths of the forces do not converge exactly unless new effects come into play at higher energies. One such effect could be a new symmetry called "supersymmetry."

A successful GUT will still not include gravity. The problem here is that theorists do not yet know how to formulate a workable quantum field theory of gravity based on the exchange of a hypothesized graviton. See also quantum field theory.