Supergravity is a type of quantum theory of elementary particles and their interactions that is based on the particle symmetry known as supersymmetry and that naturally includes gravity along with the other fundamental forces (the electromagnetic force, the weak nuclear force, and the strong nuclear force).

The electromagnetic and the weak forces are now understood to be different facets of a single underlying force that is described by the electroweak theory. Further unification of all four fundamental forces in a single quantum theory is a major goal of theoretical physics. Gravity, however, has proved difficult to treat with any quantum theory that describes the other forces in terms of messenger particles that are exchanged between interacting particles of matter. General relativity, which relates the gravitational force to the curvature of space-time, provides a respectable theory of gravity on a larger scale. To be consistent with general relativity, gravity at the quantum level must be carried by a particle, called the graviton, with an intrinsic angular momentum (spin) of 2 units, unlike the other fundamental forces, whose carriers (e.g., the photon and the gluon) have a spin of 1.

A particle with the properties of the graviton appears naturally in certain theories based on supersymmetry--a symmetry that relates fermions (particles with half-integral values of spin) and bosons (particles with integral values of spin). In these theories supersymmetry is treated as a "local" symmetry; in other words, its transformations vary over space-time, unlike a "global" symmetry, which transforms uniformly over space-time. Treating supersymmetry in this way relates it to general relativity, and so gravity is automatically included. Moreover, these supergravity theories seem to be free from various infinite quantities that usually arise in quantum theories of gravity. This is due to the effects of the additional particles that supersymmetry predicts (every particle must have a supersymmetric partner with the other type of spin). In the simplest form of supergravity, the only particles that exist are the graviton with spin 2 and its fermionic partner, the gravitino, with spin 3/2. (Neither has yet been observed.) More complicated variants also include particles with spin 1, spin 1/2, and spin 0, all of which are needed to account for the known particles. These variants, however, also predict many more particles than are known at present, and it is difficult to know how to relate the particles in the theory to those that do exist.

Excerpt from the Encyclopedia Britannica without permission.