Abstracts

Time reversing quantum systems

Cristian López (University of Lausanne)

In the last few years, there has been some debate around the concept of time reversal in (unitary) standard quantum mechanics (SQM) (Albert 2000, Callender 2000, Earman 2002, Roberts 2017, Allori 2019, Lopez 2019, 2021, Callender 2022.). According to the orthodox view, time reversal in SQM is represented by an anti-unitary, non-linear time reversal operator that keeps Schrödinger’s equation invariant. Some have argued that the orthodoxy is wrong: time reversal should be represented differently, and under a more appropriate representation, SQM turns out non-time-reversal invariant (Albert 2000, Callender 2000, Lopez 2019). Others have defended the orthodoxy on different grounds (Roberts 2017, Allori 2019, Lopez 2021). In this presentation I first provide an overview of the discussion, showing pros and cons of both sides. Then, I argue that the orthodoxy can only be sustained under two assumptions: temporal relationalism and a normative, rule-prescribing view of symmetries (in particular, time-reversal symmetry). If any of these assumptions is challenged, then the orthodoxy loses much of its persuasive force. This debate, I conclude, is crucial for the discussion on the arrow of time in physics.

Super-substantivalism, explanatory dependence, and general relativity

Manuel Herrera (University of Buenos Aires)

Super-substantivalism is a set of theses that seek to account for the status of space-time and material objects in our ontology (Schaffer, 2009; Skow, 2005; Morganti, 2011; Dumsday, 2016; Giberman, 2021). One of the main super-substantivalist theses holds that space-time is more fundamental than (or ontologically prior to) matter. Given that super-substantivalism confers a privileged status to space-time as a substance ontologically prior to matter, it is therefore important to determine what exactly is meant by “fundamental” or “more fundamental than”. And the general theory of relativity (hereafter, GR) is a favorable scenario to address this discussion since this is precisely a physical theory that aims to explain the relationship between space-time and matter. 

One of the papers that, directly or indirectly, address this problem within the framework of GR is Lehmkuhl (2016). Based on a notion of ontological priority in existential terms, the author proposes two arguments coming from GR that could play in favor of the ontological priority of space-time over matter: the existence of vacuum solutions, which would indicate that space-time can exist in the absence of matter; and the impossibility of the metric field disappearing at some point or region of space-time, which would imply that matter cannot exist without a previous definition of the metric field. The author concludes that whether these arguments are sufficient to establish an ontological priority of space-time over matter will depend on the notion of ontological priority/fundamentality that we consider. Recently, Dürr and Calosi (2021) have deepened Lehmkuhl’s proposal, stating that the best candidate to establish the dependence relationship between space-time and matter is explanatory dependence (Correia, 2005; Schnieder, 2006). Broadly speaking, explanatory dependence is an approach that holds that x ontologically depends on iif and only if some feature F of y necessarily explains the existence of x. Under this view of ontological dependence, it would be necessary to determine if, indeed, within the framework of GR there is some characteristic F of space-time that explains or grounds the existence of matter. The authors point out, in response to the previous concern, that the geometric approach to GR can be seen as a perspective that supports the view of explanatory dependence for the case of space-time and matter.

According to the previous paragraph, this presentation aims to evaluate one of the super-substantivalist central thesis, i.e., space-time is more fundamental than matter, in the light of metaphysical concepts of fundamentality and ontological dependence in the context of general relativity. In particular, I defend two theses. In the first place, a negative thesis, which holds that when we evaluate the explanatory dependence (in the frame of general relativity) to give an account of the relationship between space-time and matter it turns out inadequate. In the second place, a positive thesis, in which I offer a couple of arguments for defending that the fundamentality relationship between space-time and matter must be given in ontological terms and not in epistemological terms (or explanatory terms).

Pure shape dynamics, self-subsisting structures, and the nature of time

Antonio Vassallo (WUT)

Pure Shape Dynamics (PSD) is a new framework for constructing relational theories of motion. PSD differs from standard Shape Dynamics in that it supplies a description of the evolution of a physical system solely in terms of the geometric properties of an unparametrized curve defined over the relational configuration space of the theory (called shape space) without invoking externally-fixed parametrizations. A remarkable feature of classical models of PSD is the spontaneous generation of a gravitational arrow of time whereby the system's dynamical evolution is driven towards an increase in complexity, defined as a measure of the clustering of the system's shape as well as how pronounced the clusters are. A high degree of complexity is a prerequisite for storing dynamically preserved localized information, which can be interpreted as records of the past. Hence, the arrow of time can be identified as the direction of this record generation, meaning that the shape of the system carries its ``time-stamp'' through its complexity.

In this talk, I will first provide an interpretation of the PSD framework in moderate ontic structuralist terms, whereby shapes are characterized as self-subsisting structures--i.e., structures individuated in purely intrinsic terms, without the need for an external embedding spacetime. I will then discuss how ``garden-variety'' temporal notions are recovered from PSD's dynamical description. In particular, I argue that time appears from such a picture by means of a dependence that takes property identity and supervenience as essential ingredients, thus qualifying as a reduction of the temporal notions to facts about self-subsisting structures. [Based on joint work with Tim Koslowski and Pedro Naranjo]

Losing time in classical and quantum cosmology

Charlotte Erika Zito (University of Geneva) and Marta Pedroni (University of Geneva)

Canonical approaches to quantum gravity are notoriously affected by the so-called “problem of time”: the Wheeler-De Witt equation that is supposed to describe the dynamics of the theory displays a static wave function without a quantity that corresponds to physical time, i.e. no time variable. This has been taken to suggest that time disappears in quantum gravity (Kuchař 1992; Wüthrich 2017; Huggett, Vistarini and Wüthrich 2013; Rickles 2004).

Time also appears to be besieged within the framework of classical cosmology, though in a different way. In particular, what seems to be missing while approaching the Big Bang singularity is the possibility of finding “clocks” (i.e. processes or scales) which can be used as a physical basis for measuring time (Rugh and Zinkernagel 2009, 2017). We will refer to this as the "scale problem for time".

In this work, we will clarify the subject matter of each problem by presenting their frameworks and contexts of applicability, with a particular focus on the notion of time implicitly presupposed. Then, we will draw a comparison between the two problems by discussing common assumptions and crucial divergences. Finally, we will investigate whether and how the "problem of time" and the "scale problem of time" can be linked to one another.

Burgess and the Bucket: The Emergence of Spacetime in Classical Theories of Gravitation

Lorenzo Cocco (University of Geneva) and Joshua Babic (University of Geneva)

The paper studies in detail a precise formal construction of spacetime from matter suggested by the logician John Burgess. We presuppose a continuous and perdurantistic matter ontology. The result is a systematic method to translate claims about the geometry of a flat relativistic, or classical, spacetime into claims about geometrical relations between matter points. The approach is extended to electric and magnetic fields by treating them as multifields defined on matter, rather than as fields in the vacuum. A few tentative suggestions are made to adapt the method to general relativity and to quantum theories. The construction supports an approach known in the literature as “enriched relationism”. It also gives a precise example of the reduction, or “emergence” of spacetime from a fundamentally nonspatiotemporal ontology.

Is spacetime an accident?

Christian Wüthrich (University of Geneva)

Spacetime is regarded by many as emerging from a more fundamental structure: what is the modal status of the emergence relation? We show that according to two of these approaches, causal set theory and group field theory, spacetime could have failed to exist. (joint work with Baptiste Le Bihan)

Baby Universes Need Parental Watch

Saakshi Dulani  (University of Geneva) and Baptiste Le Bihan (University of Geneva)

The problem of information loss in black hole evaporation has long been a challenge. Late-time Hawking radiation is highly-entangled, and a shrinking black hole fails to provide sufficient information storage capacity to support the entanglement. One potential solution is that regions of spacetime beyond black hole horizons pinch off from the universe. Such ‘baby universes’ must be capable of providing sufficient storage capacity to house the information entangled with the rest of the universe. However, we will show that this approach requires postulating the existence of an objective universal time that coordinates the original and baby universes—a 'parental watch.' We will then briefly review the literature to show that several proponents of the baby universe proposal do in fact seek to rehabilitate an objective 'now.' We thereby plan to convince you that the link between the baby universe proposal and the claim that spacetime admits of a single, privileged foliation is no accident.

Causation and quantum gravity

Sam Baron (Australian Catholic University)

Approaches to quantum gravity seek to reconcile our two most successful theories at the moment, general relativity and quantum mechanics. In some approaches to quantum gravity, such as causal set theory and causal dynamical triangulation, physicists appeal to causation. But does causation in any philosophically interesting sense have a role to play in quantum gravity? Or are physicists using ‘causation’ differently to how it is used in philosophy? I argue that in at least some cases, the use of causation in quantum gravity corresponds to causation in the usual sense found in philosophy, and so there’s some reason to suppose that causation has a role to play in fundamental physics.