Classic theory The first successful
classical unified field theory was developed by
James Clerk Maxwell. In 1820,
Hans Christian Ørsted 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 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. 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 field in a classical field theory and, two years later, that of
Theodor Kaluza, who extended General Relativity to
five dimensions. Continuing in this latter direction, Oscar Klein proposed in 1926 that the fourth spatial dimension be
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 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, in an attempt to unify general relativity with quantum mechanics. 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 , 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 numerous proposals for Grand Unified Theories involving larger and larger unifying groups. While these theories are self-consistent, no proposal produced a solution to outstanding issues in cosmology, like the
baryon asymmetry problem or the missing mass now attributed to
dark matter. Experimental tests of such theories requires an energy scale well beyond the reach of current
accelerators so no empirical evidence outside of cosmology can guide theory. 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 (but not
Pati–Salam) 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 1035 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
renormalizable. The incompatibility of the two theories remains an outstanding problem in the field of physics. == See also ==