**Collection Number**C21004

**Collection Date**

**Collection Type**Conference/School

## Relational dynamics: interacting clocks and systems and quantum time dilation

Mehdi Ahmadi
Santa Clara University

Time is absolute in quantum mechanics, whereas it is dynamical in general relativity. This is considered as one of the main obstacles towards unifying quantum theory and gravity. Relational quantum dynamics offers a possible solution by treating clocks as internal quantum systems, which promotes time to a dynamical quantity. This talk begins with a quick overview of time in relational quantum dynamics. We then explain that the inclusion of an interaction term coupling the clock and system causes the system dynamics to be governed by a time-nonlocal Schrödinger equation.

## Relative subsystems and quantum reference frame transformations.

Esteban Castro Ruiz
Université Libre de Bruxelles (ULB)

Transformations between reference frames play a crucial role in our understanding of physical processes. In practice, reference frames are realised by physical systems, which are standardly treated as classical. However, assuming that every physical system is ultimately quantum, it is interesting to ask how a theory of transformations wrt quantum reference frames would look like, and what implications it would have for our description of spacetime.

## Quantum measurements of time

Lorenzo Maccone
Universita degli Studi di Pavia Italy

"We propose a time-of-arrival operator in quantum mechanics by conditioning on a quantum clock. This allows us to bypass some of the problems of previous proposals, and to obtain a Hermitian time of arrival operator whose probability distribution arises from the Born rule and which has a clear physical interpretation. The same procedure can be employed to measure the ""time at which some event happens"" for arbitrary events (and not just specifically for the arrival time of a particle).
This talk is based on the paper: L. Maccone, K. Sacha, Quantum measurements of time, Phys. Rev. Lett.

## (Quantum) frame covariance: from foundations via gauge theories to gravity

Philipp Hoehn
Okinawa Institute of Science and Technology (OIST)

I will sketch how the perspective-neutral approach to (quantum) frame covariance brings together some recent developments on dynamical reference frames in quantum foundations, gauge theories and gravity. The survey will touch on spatial frames, quantum clocks and the problem of time, edge modes, and the relativity of subsystems.

## Discussion Session 2

## The emergence of quantum mechanics and space, from a fundamental, active time

Lee Smolin
Perimeter Institute for Theoretical Physics

"We propose a realist completion of quantum mechanics, in the sense of a complete description of individual events. The proposed fundamental theory assumes that time, events, causal structure, momentum and energy are fundamental. But space and the wave function are emergent.
The beables of the theory are the views of the events, which are a subset of their causal pasts.

## "TIME IN QUANTUM GRAVITY - From the fundamental level to the classical limit"

Claus Kiefer
Universität zu Köln

Time cannot be both absolute (as in quantum mechanics) and dynamical (as in general relativity). I present general arguments for the absence of time at the most fundamental level of quantum gravity. I discuss possible concepts that could replace it and present the recovery of standard time as an approximate concept. My discussion is restricted to quantum geometrodynamics, but I argue for the validity of my conclusions beyond that scheme.

## Space and Time in a Lorentzian path integral

Bianca Dittrich
Perimeter Institute for Theoretical Physics

I will present a quantum gravity approach based on a Lorentzian path integral for quantum geometries. The properties of quantum space time can be measured using geometric operators. This allows also to discuss fluctuations of causal structure as well as violations of (micro-) causality. I will explain how the Lorentzian path integral comes with various options regarding which quantum space times to sum over: e.g. whether to include causality violations or not, or whether to allow also for space times with Euclidean signatures in Lorentzian path integrals.

## A New Perspective on Time Reversal Motivated by Quantum Gravity

Abhay Ashtekar
Pennsylvania State University

Time Reversal T is usually discussed in the traditional framework of quantum mechanics in which T is represented by an anti-unitary operator. But quantum gravity may well need generalization of standard quantum mechanics which may not preserve even its linear structure, let alone the unitarity of dynamics and anti-unitarity of T. Then the currently used arguments to conclude that T violation is a fundamental aspect of Nature will break down.