Zero-size objects in Riemann-Cartan spacetime

Abstract

We use the conservation law of the stress-energy and spin tensors to study the motion of massive zero-size objects in Riemann-Cartan geometry. The resultant world line equations turn out to exhibit a novel spin-curvature coupling. In particular, the spin of the Dirac particle does not couple to the background curvature. This is a consequence of its truly zero size which consistently rules out the orbital degrees of freedom. As a test of consistency, the wave packet solution of the free Dirac equation is considered. It is shown that the wave packet spin and orbital angular momentum disappear simultaneously in the zero-size limit.