##### Wiktionary

**quantum gravity**

n. (context physics English) A branch of theoretical physics aiming to unite quantum mechanics with general relativity.

##### Wikipedia

**Quantum gravity**

**Quantum gravity** (**QG**) is a field of theoretical physics that seeks to describe the force of gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored.

The current understanding of gravity is based on Albert Einstein's general theory of relativity, which is formulated within the framework of classical physics. On the other hand, the nongravitational forces are described within the framework of quantum mechanics, a radically different formalism for describing physical phenomena based on the wave-like nature of matter. The necessity of a quantum mechanical description of gravity follows from the fact that one cannot consistently couple a classical system to a quantum one.

Although a quantum theory of gravity is needed in order to reconcile general relativity with the principles of quantum mechanics, difficulties arise when one attempts to apply the usual prescriptions of quantum field theory to the force of gravity via graviton bosons. The problem is that the theory one gets in this way is not renormalizable and therefore cannot be used to make meaningful physical predictions. As a result, theorists have taken up more radical approaches to the problem of quantum gravity, the most popular approaches being string theory and loop quantum gravity. A recent development is the theory of causal fermion systems which gives quantum mechanics, general relativity, and quantum field theory as limiting cases.

Strictly speaking, the aim of quantum gravity is only to describe the quantum behavior of the gravitational field and should not be confused with the objective of unifying all fundamental interactions into a single mathematical framework. While any substantial improvement into the present understanding of gravity would aid further work towards unification, study of quantum gravity is a field in its own right with various branches having different approaches to unification. Although some quantum gravity theories, such as string theory, try to unify gravity with the other fundamental forces, others, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other forces. A theory of quantum gravity that is also a grand unification of all known interactions is sometimes referred to as a theory of everything (TOE).

One of the difficulties of quantum gravity is that quantum gravitational effects are only expected to become apparent near the Planck scale, a scale far smaller in distance (equivalently, far larger in energy) than what is currently accessible at high energy particle accelerators. As a result, quantum gravity is a mainly theoretical enterprise, although there are speculations about how quantum gravitational effects might be observed in existing experiments.

#### Usage examples of "quantum gravity".

The unification of spacetime was ripped apart by the forces of __quantum gravity__, and space became a seething probabilistic froth, laced by wormholes.

Those with deep longing for life after death might, it seems, devote them selves to cosmology, __quantum gravity__, elementary particle phys ics, and, especially, transfinite arithmetic.

In October 1981, I went to Moscow for a conference on __quantum gravity__.

Again, her work was much more theoretical, and very much more fine-grained, but after her work with the fusion group in Da Vinci, he had a healthy respect for her ability to help in any problem that involved __quantum gravity__ and the ultramicrostructure of matter.

The search for a unification of relativity and quantum theory - respectably known as `__quantum gravity__' and often derided as a Theory of Everything - has turned up a beautiful mathematical proposal, string theory.

October 1981, I went to Moscow for a conference on __quantum gravity__.

All they were doing was trying to build a test bed for __quantum gravity__, just as advertised.

Although recently the theory group at Da Vinci had begun buzzing about developments in superstring theory and __quantum gravity__ they were making.