Phase time and Ehrenfest’s theorem in relativistic quantum mechanics and quantum gravity

We revisit the concept of phase time, which has been previously proposed as a solution to the problem of time in quantum gravity. Concretely, we show how the geometry of configuration space together with the phase of the wave function of the Universe can, under certain conditions, lead to the definition of a positive-definite squared norm from which probabilities can be defined. The norm is conserved under time evolution, and we obtain a version of Ehrenfest's theorem that is analogous to the one in ordinary quantum mechanics. We address how the present approach avoids some difficulties that were encountered in previous attempts at defining an Ehrenfest theorem relative to phase time in the context of relativistic quantum mechanics and canonical quantum gravity, and how it is connected to a notion of gauge fixing the inner product. We conclude with a brief outlook.