Gravitational couplings of intrinsic spin

The gravitational couplings of intrinsic spin are briefly reviewed. A consequence of the Dirac equation in the exterior gravitational field of a rotating mass is considered in detail, namely, the difference in the energy of a spin-½ particle polarized vertically up and down near the surface of a rotating body is ℏΩsin θ. Here θ is the latitude and Ω = 2GJ/(c²R³), where J and R are, respectively, the angular momentum and radius of the body. It seems that this relativistic quantum gravitational effect could be measurable in the foreseeable future.