Quantum frames of reference and the relational flow of time

In this short review paper, relative evolution in time and related issues are analyzed within classical and quantum mechanics. We first discuss the basics of quantum frames of reference in both space and time. We then focus on the latter, and more specifically on the “timeless” approach to quantum mechanics due to Page and Wootters. We address time–energy uncertainty relations and the emergence of non-unitarity within this framework. We emphasize relational aspects of quantum time as well as unique features of non-inertial clock frames.