Theory of everything
From Wikipedia, the free encyclopedia.
A theory of everything (TOE) is a theory of theoretical physics and mathematics that fully explains and links together all known physical phenomena (i.e. "everything"). Initially the term was used with an ironical connotation, to refer to various overgeneralized theories. For example, an uncle of Ijon Tichy— a famous hero of Stanislaw Lem's science fiction stories of early 1970s — was known to work on "General Theory of Everything" (Polish: "Ogólna Teoria Wszystkiego"). Over time, the term stuck in popularizations of quantum physics to describe a theory that would unify the theories of the four fundamental interactions of nature, possibly due to the influence of The Theory of Everything, a book with material written by Stephen Hawking but disowned by him.
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Mainstream physics
Albert Einstein was the first serious scientist who spent most of his life trying to find a TOE; he believed that the only task was to unify general relativity and electromagnetism.
Current mainstream physics concepts require that a TOE unify the four fundamental interactions of nature: gravity, the strong nuclear force, the weak nuclear force, and the electromagnetic force; it should also explain the spectrum of elementary particles. There has been progress toward a TOE in unifying electromagnetism and the weak nuclear force in an electroweak unified field theory and in unifying all of the forces except for gravity (which in the present theory of gravity general relativity is not a force) in grand unified theory. One missing piece in a theory of everything involves combining quantum mechanics and general relativity into a theory of quantum gravity.
The only serious candidate for a theory of everything at the moment is superstring theory / M-theory; current research on loop quantum gravity may eventually play a fundamental role in a TOE, but that is not its primary aim. These theories attempt to deal with the renormalization problem by setting up some lower bound on the length scales possible. Also, early 21st century theories of everything tend to suppose that the universe actually has more dimensions than the easily observed three of space and one of time. The motivation behind this approach began with the Kaluza-Klein theory in which it was noted that adding one dimension to general relativity would produce the electromagnetic Maxwell's equations. This has led to efforts to work with theories with large number of dimensions in the hopes that this would produce equations which are similar to known laws of physics. The notion of extra dimensions also helps to resolve the hierarchy problem which is the question of why gravity is so much weaker than any other force. The common answer involves gravity leaking into the extra dimensions in ways that the other forces do not.
In the late 1990s, it was noted that one problem with several of the candidates for theories of everything was that they did not constrain the characteristics of the predicted universe. For example, many theories of quantum gravity can create universes with arbitrary numbers of dimensions or with arbitrary cosmological constants. One bit of speculation is that there may indeed be a huge number of universes, but that only a small number of them are habitable, and hence the fundamental constants of the universe are ultimately the result of the anthropic principle rather than a consequence of the theory of everything.
There is also a philosophical debate within the physics community as to whether or not a "theory of everything" should be seen as the fundamental law of the universe. One view is the hard reductionist view that the TOE is the fundamental law of the universe and that all other theories of the universe are a consequence of the TOE. Another view is that there are laws which Steven Weinberg calls free floating laws which govern the behavior of complex systems, and while these laws are related to the theory of everything, they cannot be seen as less fundamental than the TOE. Some argue that this explanation would violate Occam's Razor if a completely valid TOE were formulated.
Other possibilities which may frustrate the explanatory capacity of a TOE may include sensitivity to the boundary conditions of the universe, or the existence of mathematical chaos in its solutions, making its predictions precise, but useless.