A new spin test for the equivalence principle

The existing impressive tests for the strong equivalence principle are reviewed and their classical nature is emphasized. The possibility is raised here that intrinsic quantum spins may behave differently from orbital angular momentum in gravitational fields. The techniques developed to measure the electric dipole moment of the neutron are shown to offer hopes of testing this hypothesis. Einstein's theory predicts a null result for this experiment. This would constitute the first quantum test for the strong equivalence principle. Deviation from a null result would invalidate Einstein's theory of gravitation, as well as indicate the failure of the discrete symmetries (P, T) in gravitation. (author)