Lisbon-Lausanne Workshop “On Time in the Foundations of Physics”

June 15, 2022

Jointly organized by the Centre of Philosophy of the University of Lisbon and the Section de Philosophie of the Université de Lausanne

Date: June 15, 2022: 14:00-19:30 (CEST)


Location: The workshop will be held on Zoom, people interested in attending the Seminars can register here.



Eugene Chua, University of California San Diego

Gabriele Carcassi, University of Michigan

Siddhant Das, Ludwig Maximilian University

Francesca VIdotto, University of Western Ontario

Emily Adlam, University of Western Ontario



(14:00-15:00 CEST): Eugene Chua (University of California San Diego) – The Time in Thermal Time

Abstract: Notoriously, there is a ’problem of time’ in the Hamiltonian approach to the quantization of gravity; the resultant formalism is often said to be ‘frozen’, non-dynamical, and timeless. To resolve this problem, Connes & Rovelli (1994), and Rovelli elsewhere, proposed the ‘thermal time hypothesis’: the flow of time emerges thermodynamically. While statistical states are typically defined to be in equilibrium with respect to some background time, Connes & Rovelli propose that we can instead define time in terms of these statistical states: statistical states define a time according to which they are in equilibrium. For this argument to not slip into circularity, we better have a good conceptual grasp on notions such as ‘equilibrium’ and ‘statistical state’ independent of time. Here, I’ll argue that these concepts implicitly presuppose some notion of time, hidden in the notions of probability and stationarity required for deploying these concepts.


(15:00-16:00 CEST): Gabriele Carcassi (University of Michigan) – Time as an operational definition

Abstract: We will first argue, in general, that operational definitions are necessary and sufficient to characterize physical objects. Therefore the question of the nature of time should be framed as: how do we define time operationally and how can that process be modelled? We discuss how the process of synchronization plays a fundamental role in the operation definition. We then model a clock as a series of references that define a before and an after, and we find a set of necessary and sufficient conditions for which time can be modelled as a real number. Finally, we discuss whether those conditions can be truly met in practice and what may happen when those requirements break down.


(16:00-16:15 CEST): Break 


(16:15-17:15 CEST): Siddhant Das (Ludwig Maximilian University) – Can we fix quantum arrival times before 2026? 

Abstract: I will discuss the problem of predicting the arrival (or detection) time of a quantum particle on a detector surface that remains unresolved despite the time-honoured empirical successes of quantum mechanics. Given that arrival-time or time-of-flight (TOF) measurements are the quintessence of standard experimental techniques of atomic and particle physics, this is particularly striking. Drawing upon the early history of this issue, I will discuss many (disparate) theoretical predictions for the TOF distribution of a particle suggested in the literature. These are informed by various semi-classical heuristics, principles, extensions, or (for want of a  better word) “interpretations” of quantum theory. Next, I will describe a so-called “back-wall” experiment—an arrival-time experiment developed in collaboration with my advisor (late) Prof. Detlef Dürr that can reliably distinguish the aforementioned  proposals. A key feature of our set-up is that the particle is escaping a potential barrier (the back-wall) in the direction of a distant detector, as opposed to moving completely freely. Without the back-wall, most proposals become nearly indistinguishable, which renders usual experimental tests inconclusive. I will also describe a concrete ion-trap implementation of the back-wall experiment doable with present-day technology. Time permitting, I will mention a version of this experiment featuring a spin-1/2 particle as an electron whose de Broglie-Bohm analysis predicts, thanks to quantum backflow,  intriguing spin-dependent arrival-time distributions hitherto unknown, demanding experimental inspection.


(17:15-18:15 CEST): Francesca VIdotto (University of Western Ontario) – Thermodynamics without time

Abstract: Our fundamental theories, i.e. the quantum theory and general relativity, are invariant under time reversal. Only when we treat system from the point of view of thermodynamics, i.e. averaging between many subsystem components, an arrow of time emerges. The relation between thermodynamic and the quantum theory has been fertile, deeply explored and still a source of new investigations. The relation between the quantum theory and gravity, while it has not yet brought an established theory of quantum gravity, has certainly sparkled in depth analysis and tentative new theories. On the other hand, the connection between gravity and thermodynamics is less investigated and more puzzling. I review a selection of results in covariant thermodynamics, such as the construction of a covariant notion of thermal equilibrium by considering tripartite systems. I discuss how such construction requires a relational take on thermodynamics, similarly of what happens with the quantum theory and in gravity.


(18:15-18:30 CEST): Break


(18:30-19:30 CEST): Emily Adlam (University of Western Ontario) – Two Roads to Retrocausality

Abstract: In this talk I will elucidate the relationship between retrocausality and locality. I begin by providing a brief schema of the various ways in which violations of Bell’s inequalities might lead us to consider some form of retrocausality. I then consider some possible motivations for using retrocausality to rescue locality, arguing that none of these motivations is adequate and that therefore there is no clear reason why we should prefer local retrocausal models to nonlocal retrocausal models. Next, I examine several different conceptions of retrocausality, concluding that `all-at-once’ retrocausality is more coherent than the alternative dynamical picture. I then argue that since the `all-at-once’ approach requires probabilities to be assigned to entire histories or mosaics, locality is somewhat redundant within this picture. Thus I conclude that using retrocausality as a way to rescue locality may not be the right route to retrocausality. Finally, I demonstrate that accepting the existence of nonlocality and insisting on the nonexistence of preferred reference frames leads naturally to the acceptance of a form of retrocausality, albeit one which is not mediated by physical systems travelling backwards in time. I argue that this is the more natural way to motivate retrocausal models of quantum mechanics.



Andrea Oldofredi

Cristian Lopez