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In physics, a unified field theory is a type of field theory that allows all of the fundamental forces between elementary particles to be written in terms of a single field. There is no accepted unified field theory yet, and this remains an open line of research. The term was coined by Albert Einstein who attempted to unify the general theory of relativity with electromagnetism. A Theory of Everything is closely related to unified field theory, but differs by not requiring the basis of nature to be fields, and also attempts to explain all physical constants of nature.

This article describes unified field theory as it is currently understood in connection with quantum theory. Earlier attempts based on classical physics are described in the article on classical unified field theories.

There may be no a priori reason why the correct description of nature has to be a unified field theory; however, this goal has led to a great deal of progress in modern theoretical physics and continues to motivate research. Unified field theory is only one possible approach to unification of physics.

Introduction

According to our current understanding of physics, forces between objects (e.g. gravitation) are not transmitted directly between the two objects, but instead go through intermediary entities called fields. All four of the known fundamental forces are mediated by fields, which in the Standard Model of particle physics result from exchange of bosons (integral-spin particles). Specifically the four interactions to be unified are (from strongest to weakest):

• Strong nuclear interaction: the interaction responsible for holding quarks together to form neutrons and protons, and holding neutrons and protons together to form nuclei. The exchange particle that mediates this force is the gluon.
• Electromagnetic interaction: the familiar interaction that acts on electrically charged particles. The photon is the exchange particle for this force.
• Weak nuclear interaction: a repulsive short-range interaction responsible for radioactivity, that acts on electrons, neutrinos and quarks. It is governed by the W boson.
• Gravitational interaction: a long-range attractive interaction that acts on all particles with mass. The postulated exchange particle has been named the graviton.

Modern unified field theory attempts to bring these four force-mediating fields together into a single framework. Quantum theory seems to limit any deterministic theory's descriptive power (in simple terms, no theory can predict events more accurately than allowed by the Planck constant).

History

The first successful (classical) unified field theory was developed by James Clerk Maxwell. In 1820 Hans Christian Oersted discovered that electric currents exerted forces on magnets, while in 1831, Michael Faraday made the observation that time-varying magnetic fields could induce electric currents. Until then, electricity and magnetism had been thought of 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 separate field theories (namely electricity and magnetism) to provide a unifying theory of electromagnetism. Later, in his theory of special relativity Albert Einstein was able to explain the unity of electricity and magnetism as a consequence of the unification of space and time into an entity we now call spacetime.

In 1921 Theodor Kaluza extended General Relativity to five dimensions and in 1926 Oscar Klein proposed that the fourth spatial dimension be curled up (or compactified) into a small, unobserved circle. This was dubbed Kaluza-Klein theory. It was quickly noticed that this extra spatial direction gave rise to an additional force similar to electricity and magnetism. This was pursued as the basis for some of Albert Einstein's later unsuccessful attempts at a unified field theory. Einstein and others pursued various non-quantum approaches to unifying these forces; however as quantum theory became generally accepted as fundamental, most physicists came to view all such theories as doomed to failure.

Modern progress

In 1963 American physicist Sheldon Glashow proposed that the weak nuclear force and electricity and magnetism could arise from a partially unified electroweak theory. In 1967, Pakistani Abdus Salam and American Steven Weinberg independently revised Glashow's theory by having the masses for the W particle and Z particle arise through spontaneous symmetry breaking with the Higgs mechanism. This unified theory was governed by the exchange of four particles: the photon for electromagnetic interactions, 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 80.4 and 91.2 ${\displaystyle GeV/c^{2}}$, respectively. Their theory was first given experimental support by the discovery of weak neutral currents in 1973. In 1983, the Z and W bosons were first produced at CERN by Carlo Rubbia's team. For their insights, Salam, Glashow and Weinberg were awarded the Nobel Prize in Physics in 1979. Carlo Rubbia and Simon van der Meer received the Prize in 1984.

After Gerardus 't Hooft showed the Glashow-Weinberg-Salam electroweak interactions was mathematically consistent, the electroweak theory became a template for further attempts at unifying forces. In 1974, Sheldon Glashow and Howard Georgi proposed unifying the strong and electroweak interactions into a Grand Unified Theory, which would have observable effects for energies much above 100 GeV.

Since then there have been several proposals for Grand Unified Theories, although none is currently universally accepted. A major problem for experimental tests of such theories is the energy scale involved, which is well beyond the reach of current accelerators. Grand Unified Theories make predictions for the relative strengths of the strong, weak, and electromagnetic forces, and in 1991 LEP determined that supersymmetric theories have the correct ratio of couplings for a Georgi-Glashow Grand Unified Theory. Many Grand Unified Theories predict that the proton can decay, and if this were to be seen, details of the decay products could give hints at more aspects of the Grand Unified Theory. It is at present unknown if the proton can decay, although experiments have determined a lower bound of ${\displaystyle 10^{35}}$ years for its lifetime.

A new Grand Unified Theory, called An Exceptionally Simple Theory of Everything, has been proposed by Antony Garrett Lisi, and has garnered some support by experts in the field. Lisi's theory is based on the group known as E₈.

The current state of unified field theories

Gravity has yet to be successfully included in a theory of everything. Simply trying to combine the graviton with the strong and electroweak interactions runs into fundamental difficulties (the resulting theory is not renormalizable). Theoretical physicists have not yet formulated a widely accepted, consistent theory that combines general relativity and quantum mechanics. The incompatibility of the two theories remains an outstanding problem in the field of physics. Some theoretical physicists currently believe that a quantum theory of general relativity may require frameworks other than field theory itself, such as string theory or loop quantum gravity. One promising string theory is the heterotic string which can tie together gravity and the three other forces into a tight connection. Other candidate string theories do not have this feature of unifying the forces and gravity in a compelling manner. Loop quantum gravity does not appear to link the electroweak and strong forces to gravity, and if so, it would fail as a unified field theory. Ultimately, nature may not be best understood in terms of a unified field theory; this conceptualization may not be correct, although it has led to advances in physics.

References

1. Surfer dude stuns physicists with theory of everything, Telegraph, November 14, 2007.

See also

• Classical unified field theories
• Electromagnetic force
• Weak nuclear force
• Grand Unification
• MSSM
• General Relativity
• Theory of Everything

• Articles lacking sources from April 2007
• Particle physics
• Theories of gravitation
• Unsolved problems in physics