Supergravity

Supergravity is a theory of gravity that includes supersymmetry in its equations. The theory of supergravity arose from the problem of incorporating the uncertainty principle with general relativity. When the equations of these two theories were combined a number of infinities appeared in the solutions. These infinities were troublesome to physicists. It was apparent to them that something was missing.

All particles exhibit a property known as spin. There is no definite definition of spin. Although physicists do not have a deep understanding of what spin is they do have a mathematical description of how it behaves. This allows them to compare spin's behavior to the behavior of other things that they feel that they understand better. Fermions have half-integer spin and their ground state energies (lowest energy level) are negative. Bosons have integer spin and their ground state energies are positive. The infinities resulting in combining the uncertainty principle with general relativity are the result of having an unequal number of fermions to bosons. When the supersymmetric particles suggested by supersymmetry are added to the equations the number of fermions is exactly equal to the number of bosons. The negative ground state energies of the fermions cancel out the positive ground state energies of the bosons eliminating the infinities. By eliminating these infinities the equations make more sense to physicists.

The equations of supergravity, as well as the equations of string theory, work best if you consider the universe to contain 11 dimensions. That is 10 dimensions of space and one dimension of time. When any other number of dimensions is considered the equations become unstable and produce anomalies. Some physicists believe that the reason that gravity is so weak compared to the other forces is because it is spread out among the other dimensions. The reason that we do not see the other dimensions is that they are too small to be seen.