At very small distances, the principles of quantum mechanics are necessary to accurately describe physical phenomena. Developing a theory that incorporates both the principles of quantum mechanics and gravity, a theory of "quantum gravity", has proven to be extremely difficult. String theory is the first real hope of providing such a theory.
a theory that successfully merges quantum mechanics and general relativity, possibly involving modifications of one or both. String theory is an example of a theory of quantum gravity.
A theory that unifies relativity with quantum mechanics under one single framework. It has not been tested.
A theory of physics (yet to be discovered) that encompasses both gravity and quantum mechanics, and therefore correctly describes matter on both the largest and the smallest scales. What is inside a black hole
Physical theories now being designed to cope with conditions before the universe was 10 -43 seconds old. At 10 -43 seconds, the force of gravity within the universe becomes comparable to the strong nuclear force. At such a magnitude, gravity may be modified by quantum mechanical effects.
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. The ultimate goal of some is a unified framework for all fundamental forces—a "theory of everything".
