Search Talks

I will examine a foundational assumption in quantum probability assignments, that experimental data arise from an identically and independently distributed (i.i.d.) ensemble. However, this assumption becomes problematic in the regime of quantum gravity. I will outline a proposal to resolve this issue by leveraging the tool of quantum reference frames in the measurement context.

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].

Lee’s most philosophical work is a probing challenge to the "timeless" view prevailing in much of physics. I’’ll say why I think he is right that the idea of the universe as a fixed totality of events is a mirage, and one that arises– as he argued – from extending techniques appropriate for open subsystems to the universe as a whole. Time (for any system in the universe) is irresolubly open-ended, ongoing, and incomplete.

I show how to make sense of physical probability even for a single atom at a single time in an otherwise empty universe. The ideas are in https://arxiv.org/abs/2404.12954 and http://arxiv.org/abs/2505.06983. As applied to the EPRB setup, probability defined in this way is perfectly local.

A statistical mechanics of geometry based on the quasi-local energy of constantly accelerating observers outside black holes is proposed. By assuming particle statistics for the geometric quanta, the geometry condenses in the large area limit, providing a quantum gravity cutoff for the entropy of the thermal atmosphere of geometric excitations. These excitations model the fluctuating ``shape of the black hole" and form an effectively two dimensional quantum thermal geometric atmosphere. The entropy from these ``Debye" excitations is proportional to the area. Prospects of for the observation of the fluctuations are briefly discussed.