GoodBye and Closing Remarks

Alexander Smith Dartmouth College

Flaminia Giacomini Perimeter Institute for Theoretical Physics
Interaction and Evolution in Classical and Quantum Physics, and Indefinite Causal Structure
Erik Curiel
LudwigMaximiliansUniversitiät München (LMU)
In classical mechanics, the representations of dynamical evolutions of a system and those of interactions the system can have with its environment are different vector fields on the space of states: evolutions and interactions are conceptually, physically and mathematically different in classical physics, and those differences arise from the generic structure of the very dynamics of classical systems ("Newton's Second Law"). Correlatively, there is a clean separation of the system's degrees of freedom from those of its environment, in a sense one can make precise.
Hierarchy of Theories with Indefinite Causal Structures: A Second Look at the Causaloid Framework
Nitica Sakharwade
Perimeter Institute for Theoretical Physics
"The Causaloid framework [1] is useful to study Theories with Indefinite Causality; since Quantum Gravity is expected to marry the radical aspects of General Relativity (dynamic causality) and Quantum Theory (probabilisticness). To operationally study physical theories one finds the minimum set of quantities required to perform any calculation through physical compression.
Time Symmetry in Decoherence and Stable Facts
Anirban Ganguly
AixMarseille University
It has been previously discussed how events (interactions) in quantum mechanics are timesymmetric and an arrow of time is only due to the arrow of inference in the paper “Quantum information and the arrow of time”, arXiv:2010.05734 by Andrea Di Biagio, Pietro Dona, and Carlo Rovelli. In the relational interpretation of Quantum Mechanics, these interactions are relative facts. Stable facts result from relative facts through the process of decoherence as shown in the paper "Di Biagio, A., Rovelli, C., Foundations of Physics 51, 30 (2021)".
Arrows of time and locally mediated toymodels of entanglement
"Making progress in quantum gravity requires resolving possible tensions between quantum mechanics and relativity. One such tension is revealed by Bell's Theorem, but this relies on relativistic Local Causality, not merely the timereversal symmetric aspects of relativity. Specifically, it depends on an arrowoftime condition, taken for granted by Bell, which we call No FutureInput Dependence. One may replace this condition by the weaker Signal Causality arrowoftime requirement  only the latter is necessary, both for empirical viability and in order to avoid paradoxical causal loops.
Representing time and time's arrow
Bryan Roberts
London School of Economics and Political Science
What does it mean to say that a curve in state space describes change with respect to time, as opposed to space or any other parameter? What does it mean to say it's time is asymmetric? Inspired by the WignerBargmann analysis of the Poincaré group, I discuss a general framework for understanding the meaning of time evolution and temporal symmetry in terms of the representation of a semigroup that includes "time translations", amongst the automorphisms of a state space.
An experiment to detect the Discreteness of time
Marios Christodoulou
The University of Hong Kong (HKU)
To this date no empirical evidence contradicts general relativity. In particular, there is no experimental proof a quantum theory of gravity is needed. Surprisingly, it appears likely that the first such evidence would come from experiments that involve non relativistic matter and extremely weak gravitational fields. The conceptual key for this is the Planck mass, a mesoscopic mass scale, and how it relates with what remains of general relativity in the Newtonian limit: time dilation.
What does the Path Integral imply for Quantizing Time?
"Even though pathintegral formulations of quantum theory are thought to be equivalent to statebased approaches, pathintegrals are rarely used to motivate answers to foundational questions. This talk will summarize a number of implications concerning time and timesymmetry which result from the pathintegral viewpoint. Such a perspective sheds serious doubt on dynamical collapse theories, and also pushes against efforts to extend configuration space to include multiple time dimensions.
Times of arrival and gauge invariance
Siddhant Das
LudwigMaximiliansUniversitiät München (LMU)
"We revisit the arguments underlying two wellknown arrivaltime distributions in quantum mechanics, viz.,
the AharonovBohm and Kijowski (ABK) distribution, applicable for freely moving particles, and the quantum
flux (QF) distribution. An inconsistency in the original axiomatic derivation of Kijowski’s result is pointed out,
along with an inescapable consequence of the “negative arrival times” inherent to this proposal (and generalizations thereof).
Inequivalent clocks in quantum cosmology
Steffen Gielen
University of Sheffield
Quantum cosmology faces the problem of time: the Universe has no background time, only interacting dynamical degrees of freedom within it. The relational view is to use one degree of freedom (which can be matter or geometry) as a clock for the others. In this talk we discuss a cosmological model of a flat FLRW universe filled with a massless scalar field and a perfect fluid.