Editing Unified field theory (section)

== History ==
=== Classic theory ===
The first successful [[Classical unified field theories|classical unified field theory]] was developed by [[James Clerk Maxwell]].  In 1820, [[Hans Christian Ørsted]] discovered that [[electric current]]s exerted forces on [[magnet]]s, while in 1831, [[Michael Faraday]] made the observation that time-varying [[magnetic field]]s 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|a dynamical theory of the electromagnetic field]]. This was the first example of a theory that was able to encompass previously separate field theories (namely electricity and magnetism) to provide a unifying theory of electromagnetism. By 1905, [[Albert Einstein]] had used the constancy of the [[speed-of-light]] in Maxwell's theory to unify our notions of space and time into an entity we now call [[spacetime]]. In 1915, he expanded this theory of [[special relativity]] to a description of gravity, [[general relativity]], using a field to describe the curving geometry of four-dimensional (4D) spacetime.

In the years following the creation of the general theory, a large number of physicists and mathematicians enthusiastically participated in the attempt to unify the then-known fundamental interactions.<ref>See [[Catherine Goldstein]] & Jim Ritter (2003) "The varieties of unity: sounding unified theories 1920-1930" in A. Ashtekar, et al. (eds.), ''Revisiting the Foundations of Relativistic Physics'', Dordrecht, Kluwer, p. 93-149; Vladimir Vizgin (1994), ''Unified Field Theories in the First Third of the 20th Century'', Basel, Birkhäuser; Hubert Goenner [http://relativity.livingreviews.org/Articles/lrr-2004-2/ On the History of Unified Field Theories] {{webarchive|url=https://web.archive.org/web/20110805194546/http://relativity.livingreviews.org/Articles/lrr-2004-2/ |date=2011-08-05 }}.</ref> Given later developments in this domain, of particular interest are the theories of [[Hermann Weyl]] of 1919, who introduced the concept of an (electromagnetic) [[gauge theory|gauge field]] in a classical field theory<ref>Erhard Scholtz (ed) (2001), ''Hermann Weyl's'' Raum - Zeit- Materie ''and a General Introduction to His Scientific Work'', Basel, Birkhäuser.</ref> and, two years later, that of [[Theodor Kaluza]], who extended General Relativity to [[Five-dimensional space|five dimensions]].<ref>Daniela Wuensch (2003), "The fifth dimension: Theodor Kaluza's ground-breaking idea", ''Annalen der Physik'', vol. 12, p. 519–542.</ref> Continuing in this latter direction, Oscar Klein proposed in 1926 that the fourth spatial dimension be [[compactification (physics)|curled up]] into a small, unobserved circle. In [[Kaluza–Klein theory]], the gravitational curvature of the extra spatial direction behaves as an additional force similar to electromagnetism. These and other models of electromagnetism and gravity were pursued by Albert Einstein in his attempts at a [[classical unified field theories|classical unified field theory]]. By 1930 Einstein had already considered the Einstein-Maxwell–Dirac System [Dongen]. This system is  (heuristically) the super-classical [Varadarajan] limit of (the not mathematically well-defined) [[quantum electrodynamics]]. One can extend this system to include the weak and strong nuclear forces to get the Einstein–Yang-Mills–Dirac System. The French physicist [[Marie-Antoinette Tonnelat]] published a paper in the early 1940s on the standard commutation relations for the quantized spin-2 field. She continued this work in collaboration with [[Erwin Schrödinger]] after [[World War II]]. In the 1960s [[Mendel Sachs]] proposed a generally covariant field theory that did not require recourse to renormalization or perturbation theory. In 1965, Tonnelat published a book on the state of research on unified field theories.

=== Modern progress ===
In 1963, American physicist [[Sheldon Glashow]] proposed that the [[weak nuclear force]], 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 modelled the [[electroweak interaction]] as a force mediated by four particles: the photon for the electromagnetic aspect, a neutral Z particle, and two charged W particles for the weak aspect. As a result of the spontaneous symmetry breaking, the weak force becomes short-range and the W and Z bosons acquire masses of 80.4 and {{val|91.2|u=GeV/c<sup>2</sup>}}, 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, Glashow, Salam, 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 to be 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 the [[Georgi–Glashow model]], the first [[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, e.g. the [[Pati–Salam model]], 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 [[particle accelerator|accelerators]]. Grand Unified Theories make predictions for the relative strengths of the strong, weak, and electromagnetic forces, and in 1991 [[Large Electron-Positron Collider|LEP]] determined that [[Minimal Supersymmetric Standard Model|supersymmetric]] theories have the correct ratio of couplings for a Georgi–Glashow Grand Unified Theory.

Many Grand Unified Theories (but not Pati–Salam) predict that [[proton decay|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 10<sup>35</sup> years for its lifetime.

=== Current status ===

Theoretical physicists have not yet formulated a widely accepted, consistent theory that combines [[general relativity]] and [[quantum mechanics]] to form a [[theory of everything]]. Trying to combine the [[graviton]] with the strong and electroweak interactions leads to fundamental difficulties and the resulting theory is not [[Renormalization|renormalizable]]. The incompatibility of the two theories remains an outstanding problem in the field of physics.