is a field theory
in physics combining supersymmetry
and general relativity
. A supergravity theory is generally the zero-length limit of a superstring theory
(i.e., the limit in which the string is approximated as having zero length, and treated as a dimensionless point-particle), with the exception of "maximal" 11-dimensional supergravity, which is a limit of M-theory
(most likely the limit in which the membranes are treated as having zero volume).