Is the equivalence principle useful for understanding general relativity?

Abstract

The equivalence principle (EP) is at the heart of general relativity (GR), tested in many aspects. In discussion, it is often referred to explaining qualitatively the influence of gravity on physical phenomena, but is this qualitative approach supported by the theory? We compare clock rates, frequency shifts, light deflection, and time delay in simple static spacetimes to the analogous phenomena seen by accelerated observers in Minkowski space. In contrast to previous studies, we do not assume that the gravitational field is weak and see, as we proceed, how the field is constrained by the EP. Special care is taken that results are only observer-, but not coordinate-dependent. By this, we clarify some of the issues raised in the literature and show which gravitational effects can and which cannot be simulated by acceleration. The paper may also serve as a starting point for discussions on the implications of the EP.