Causality and flow of time in realist quantum models

Abstract

In a series of papers [Phys. Rev. D 102, 124027 (2020)], [Phys. Rev. D 102, 124028 (2020)] I had the pleasure to explore with Cortês, Elitzur and Smolin realist formulations of quantum theory which obey either retrocausality or disordered causality. After briefly presenting these works, I will attempt to extend them by addressing time as a quantum observable using the Page-Wootters formalism. Within this framework, the clock is treated as a quantum subsystem, endowing the remainder of the system with a notion of time through entanglement. I will show the fruitfulness of this approach by deriving new time-energy uncertainty relations [Quantum 6, 683 (2022)] and by elucidating aspects of dynamical nonlocality [Phys. Rev. A 105, 042207 (2022)]. Furthermore, I will demonstrate that in the post-Newtonian regime, perspective-dependent non-unitarity and a distinct form of time–asymmetry naturally emerge when the clocks experience acceleration or gravitational effects [Commun. Phys. 5, 298 (2022)]. Finally, I will discuss the more recent works viewing all the above as spatiotemporal quantum reference frames [Phys. Rev. A 109, 032205 (2024)], [arXiv:2503.20090].