John Archibald Wheeler
He pioneered work in the quantization of general relativity and, in particular, developed canonical quantum gravity, manifestly covariant methods, and heat kernel algorithms.  DeWitt formulated the Wheeler–DeWitt equation for the wave function of the universe with John Archibald Wheeler and advanced the formulation of Hugh Everett's many-worlds interpretation of quantum mechanics. With his student Larry Smarr, he originated the field of numerical relativity.