Causality and flow of time in realist quantum models

          @misc{ pirsa_PIRSA:25060085,
            doi = {10.48660/25060085},
            url = {https://pirsa.org/25060085},
            author = {Cohen, Eliahu},
            keywords = {},
            language = {en},
            title = {Causality and flow of time in realist quantum models},
            publisher = {Perimeter Institute},
            year = {2025},
            month = {jun},
            note = {PIRSA:25060085 see, \url{https://pirsa.org}}
          }
          

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].