Quantum gravity

Quantum gravity is the field of theoretical physics attempting to unify the theory of quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. The ultimate goal of some is a unified framework for all fundamental forces—a theory of everything.

Related Topics:
Theoretical physics - Fundamental force - Gravity - Theory of everything

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Much of the difficulty in merging these theories comes from the radically different assumptions that these theories make on how the universe works. Quantum field theory depends on particle fields embedded in the flat space-time of special relativity. General relativity models gravity as a curvature within space-time that changes as mass moves. The most obvious ways of combining the two (such as treating gravity as simply another particle field) run quickly into what is known as the renormalization problem. Gravity particles would attract each other and adding together all of the interactions results in many infinite values which cannot easily be cancelled out mathematically to yield sensible, finite results. This is in contrast with quantum electrodynamics where the interactions sometimes evaluate to infinite results, but those are few enough in number to be removable via renormalization.

Related Topics:
Quantum field theory - Special relativity - General relativity - Space-time - Renormalization - Quantum electrodynamics

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Both quantum mechanics and general relativity have been highly successful. Unfortunately, the energies and conditions at which quantum gravity effects are likely to be important are inaccessible to current laboratory experiments. The result is there are no experimental observations which would provide any hints as to how to combine the two.

Related Topics:
Quantum mechanics - General relativity

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The general approach taken in deriving a theory of quantum gravity is to assume that the underlying theory will be simple and elegant and then to look at current theories for symmetries and hints for how to combine them elegantly into an overarching theory. One problem with this approach is that it is not known if quantum gravity will be a simple and elegant theory.

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Such a theory is required in order to understand those problems involving the combination of very large mass or energy and very small dimensions of space, such as the behaviour of black holes, and the origin of the universe.

Related Topics:
Black hole - Origin of the universe

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