Quantum theory of gravity

Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. One ultimate goal hoped to emerge as a result of this is a unified framework for all fundamental forces—a "theory of everything", or "Grand Unified Theory" (GUT).
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A theory of gravity which explains the force in terms of quantum particles known as gravitons. As yet there is no satisfactory quantum theory of gravity. One is needed so that it can be incorporated into the grand unified theory, to produce a unified field theory which will explain all the forces of nature at once.