Unified field theory

In physics, unified field theory is an attempt to unify all the fundamental forces and the interactions between elementary particles into a single theoretical framework. The term was coined by Einstein who attempted to reconcile the general theory of relativity with electromagnetism in a single field theory. His quest proved elusive and a unified field theory, sometimes grandiosely referred to as the Theory of Everything (TOE, for short), has remained the holy grail for physicists, the long-sought theory which would explain the nature and behavior of all matter.

History

Historically, the first unified field theory was developed by James Clerk Maxwell. In 1831, Michael Faraday made the observation that time-varying magnetic fields could induce electric currents. Until then, electricity and magnetism had been thought as unrelated phenomena. In 1864, Maxwell published his famous paper on a dynamical theory of the electromagnetic field. This was the first example of a theory that was able to encompass previous theories (namely electricity and magnetism) to provide a unifying theory of electromagnetism. However, today we know that the classical electrodynamics developed by Maxwell eventually breaks down near the quantum limit (for large momentum and energy transfer). A complete quantum description of the electromagnetic force was achieved in the 1940s, a theory known as quantum electrodynamics (QED). This theory represents the interactions of charged particles mediated by force carriers named photons. The theory is based on a space-time symmetry of the field called gauge (really phase) symmetry. The theory was so successful that the principle of continuous gauge symmetry was soon adopted for all forces.

Related Topics:
James Clerk Maxwell - Michael Faraday - Quantum electrodynamics - Gauge symmetry

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In 1967, two Americans Sheldon Glashow and Steven Weinberg and a Pakistani Abdus Salam proposed independently a theory unifying electromagnetism and the weak nuclear forces. They found that in seeking a quantum gauge field theory of the weak forces they were forced to introduce an additional force. They demonstrated that the gauge field from the weak interaction was structurally identical to the electromagnetic field. Quantum electrodynamics is then a consequence of a spontaneous symmetry breaking in a theory in which initially the weak and electromagnetic interactions are unified. This unified theory was governed by the exchange of four particles: the photon for electromagnetic interactions, and a neutral Z particle and two charged W particles for weak interaction. As a result of the spontaneous symmetry breaking the weak force becomes short range and the Z and W bosons acquire masses of the order of 90 GeV/c^2 . Their theory was given experimental support by the discovery, in 1983, of the Z and W bosons at CERN by Carlo Rubbia's team. For their insights, Glashow, Weinberg and Salam were awarded the Nobel Prize in Physics in 1979. Carlo Rubbia and Simon van der Meer received the Prize in 1984.

Related Topics:
Sheldon Glashow - Steven Weinberg - Abdus Salam - Weak nuclear force - Spontaneous symmetry breaking - Z particle - W particle - CERN - Carlo Rubbia - Nobel Prize in Physics - Simon van der Meer

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The next logical step towards the unification of the fundamental forces of nature

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was to include the strong interaction with the electroweak forces in a theory called the Grand Unified Theory (GUT). A quantum theory of the strong force had been developed in the 1970s under the name of Quantum Chromodynamics.

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The strong interaction acts between quarks via the exchange of particles called gluons. There are eight types of gluons, each carrying a color charge and an anti-color charge. Based on this theory, Sheldon Glashow and Howard Georgi proposed the first grand unified theory in 1974, which applied to energies above 1000 GeV. Since then there have been several proposals for GUTs, although none is currently universally accepted. A major problem for expermimental tests of such theories is the energy scale involved, which is well beyond the reach of current accelerators. However, there are some falsifiable predictions that have been made for low energy processes that do not involve accelerators. One of these predictions is that the proton is unstable and can decay. It is at present unknown if the proton can decay although experiments have determined a lower bound of 10^{35} years for its lifetime. It is therefore uncertain, at the present time, whether any GUT can provide an accurate description of matter.

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Gravity has yet to be included in a theory of everything. Theoretical physicists have been so far incapable of formulating a consistent theory that combines general relativity and quantum mechanics. The two theories have proved to be incompatible and the quantization of gravity remains an outstanding problem in the field of physics. In recent years the quest for a unified field theory has largely focused on string theory. Much hope has been put on one of its offshoots known as M-theory (M. Kaku, B. Greene). Others theories that attempt to explain the quantization of gravity are twistor theory (R. Penrose and W. Rindler), Noncommutative geometry (A. Connes, J. Madore) and loop quantum gravity (L. Smolin, R. Gambini and J. Pullin).

Related Topics:
Gravity - General relativity - String theory - M-theory - M. Kaku - B. Greene - Twistor theory - R. Penrose - Noncommutative geometry - A. Connes - J. Madore - Loop quantum gravity - L. Smolin - R. Gambini - J. Pullin

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See also dynamic theory of gravity, generalized theory of gravitation.

Related Topics:
Dynamic theory of gravity - Generalized theory of gravitation

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