Unified field theory
In
Classically, however, a duality of the fields is combined into a single physical field.
The goal of a unified field theory has led to a great deal of progress for future theoretical physics, and progress continues.[citation needed]
Introduction
Forces
All four of the known fundamental forces are mediated by fields, which in the Standard Model of particle physics result from the exchange of gauge bosons. Specifically, the four fundamental interactions to be unified are:
- Strong interaction: the interaction responsible for holding quarks together to form hadrons, and holding neutrons and also protons together to form atomic nuclei. The exchange particle that mediates this force is the gluon.
- Electromagnetic interaction: the familiar interaction that acts on electrically charged particles. The photonis the exchange particle for this force.
- .
- Gravitational interaction: a long-range attractive interaction that acts on all particles. The postulated exchange particle has been named the graviton.
Modern unified field theory attempts to bring these four forces and matter together into a single framework.
History
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 and 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 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.[5] 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[6] and, two years later, that of Theodor Kaluza, who extended General Relativity to five dimensions.[7] 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. 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
After
Since then there have been several proposals for Grand Unified Theories, e.g. the
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
- Sheldon Glashow
- Unification (physics)
References
- S2CID 27691986.
- ^ "How the search for a unified theory stumped Einstein to his dying day". phys.org.
- ISBN 978-1-59777-508-3.
- ISBN 978-0-8053-6968-7.
- ^ 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 On the History of Unified Field Theories Archived 2011-08-05 at the Wayback Machine.
- ^ Erhard Scholtz (ed) (2001), Hermann Weyl's Raum - Zeit- Materie and a General Introduction to His Scientific Work, Basel, Birkhäuser.
- ^ Daniela Wuensch (2003), "The fifth dimension: Theodor Kaluza's ground-breaking idea", Annalen der Physik, vol. 12, p. 519–542.
Further reading
- Jeroen van Dongen Einstein's Unification, Cambridge University Press (July 26, 2010)
- Varadarajan, V.S. Supersymmetry for Mathematicians: An Introduction (Courant Lecture Notes), American Mathematical Society (July 2004)
External links
- On the History of Unified Field Theories, by Hubert F. M. Goenner