Can Quantum Thermodynamics Save Time?

Abstract

The thermal time hypothesis (TTH) is a proposed solution to the problem of time: a coarse-grained state determines a thermal dynamics according to which it is in equilibrium, and this defines the flow of time in generally covariant quantum theories. This article raises a series of objections to the TTH as developed by Alain Connes and Carlo Rovelli. Two technical challenges concern the relationship between thermal time and proper time and the possibility of implementing the TTH in classical theories. Three conceptual problems concern the flow of time in nonequilibrium states and the extent to which the TTH is background independent and gauge invariant.