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Theory of everything
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Theory of everything

In physics a theory of everything (TOE) is a theory that unifies the four interactions of nature: gravity, the strong nuclear force, the weak nuclear force, and the electromagnetic force. There has been progress toward a TOE in unifying electromagentism 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.

One of the most popular candidates for a theory of everything at the moment is string 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 1990's, 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.

Speculative ideas

Attempts to create theories of everything are common among people outside the professional physics community. Unfortunately some of these theories suffer from the inability to make quantifiable and/or falsifible predictions. For example, a theory of everything would provide some insight into the relative strength of forces, and predictions of particle lifetimes and cross sections. It would need to be shown to explain all known universal phenomena.

Unlike professional physicists, who are generally aware that their proposed theory is incomplete, untested, and likely to be wrong and who are aware of the huge difficulties and challenges involved in creating a TOE, amateurs who create TOE's tend to be unaware of what work has already been done, the mechanisms for testing scientific theories and the fact that most proposed theories (logically, all but one) are wrong. See List of alternative, speculative and disputed theories.

See also: The Theory of Everything, a book with material written by Stephen Hawking but disowned by him.

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