General relativity with spin and torsion: Foundations and prospects

A generalization of Einstein's gravitational theory is discussed in which the spin of matter as well as its mass plays a dynamical role. The spin of matter couples to a non-Riemannian structure in space-time, Cartan's torsion tensor. The theory which emerges from taking this coupling into account, the U/sub 4/ theory of gravitation, predicts, in addition to the usual infinite-range gravitational interaction mediated by the metric field, a new, very weak, spin contact interaction of gravitational origin. We summarize here all the available theoretical evidence that argues for admitting spin and torsion into a relativistic gravitational theory. Not least among this evidence is the demonstration that the U/sub 4/ theory arises as a local gauge theory for the Poincare group in space-time. The deviations of the U/sub 4/ theory from standard general relativity are estimated, and the prospects for further theoretical development are assessed. (AIP)