Experimental tests for some quantum effects in gravitation

Abstract

The existing impressive tests for general relativity are shown not to yield very useful information on the possible quantum gravitational interactions. The possibility is raised here that intrinsic spins may behave differently from orbital angular momenta in external gravitational fields. The dominant spin interactions are most generally characterized by three parameters α 1, α 2, α 3. All the metric theories of gravitation predict α 1 = α 2 = 0. Indirect limits posed on these parameters by existing data are not very meaningful ( α i ≲ 10 10). Feasible experiments based on the neutron electric dipole moment measurement techniques are discussed and shown to offer the possibility of measuring α 1 ∼ 1. Other possible experimental set ups are also briefly reviewed. The existence of these effects is shown to imply the breakdown of the equivalence principle. In particular, α 1 ≠ 0, α 2 ≠ 0 also implies the breakdown of discrete symmetries in gravitation ( C, P, T). Theoretical frameworks that accommodate such effects are analyzed. A reinterpretation of Einstein's generalized gravitational theory as well as a recent theoretical proposal of Hayashi are shown to be sufficiently general for this purpose. Other important implications of these quantum effects are discussed in detail.