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Quantum gravity (QG) is the field of theoretical physics attempting to unify quantum mechanics with...

Quantum gravity (QG) is the field of theoretical physics attempting to unify quantum mechanics with general relativity in a self-consistent manner, or more precisely, to formulate a self-consistent theory which reduces to ordinary quantum mechanics in the limit of weak gravity (potentials much less than c) and which reduces to "classical" general relativity in the limit of large actions (action much larger than reduced Planck's constant). The theory must be able to predict the outcome of situations where both quantum effects and strong-field gravity are important (at the Planck scale, unless extra dimensional theories are correct). Motivation for quantizing gravity comes from the remarkable success of the quantum theories of the other three fundamental interactions. Although some quantum gravity theories such as string theory and other so-called theories of everything also attempt to unify gravity with the other fundamental forces, others such as loop quantum gravity make no such attempt at unification, they simply quantize the gravitational field while keeping it separate from other force fields. Observed physical phenomena in the early 21st century can be described well by