# The Quantum, the Thermal and the Gravitational Reconciled: Physics and Philosophy in the Varied Landscape of the Intersections (24 - 26 June 2022)

## Idea & Motivation

Quantum, thermal, and gravitational theories constitute the three great pillars of modern fundamental physics. Each is, prima facie, conceptually and physically independent of the other two, and finds its application in regimes well separated from those of the others. Indeed, central principles of each are in manifest tension with those of the other, if not outright contradiction: general relativity admits causal and topological structures seemingly inconsistent with quantum mechanical dynamics; the superpositions of quantum mechanics conflict with general relativity's fixed causal structures; and both are in manifest tension if not outright contradiction with the most characteristic features of thermodynamics, viz., irreversibility and temporal asymmetry. It is thus of paramount importance that we investigate how our three best, most fundamental theories fit together, if indeed they do at all. In the past and in recent times, physicists and philosophers have worked together in remarkably fruitful ways in each of the three fundamental fields.

This conference will explore these philosophical and foundational issues that arise where two or more of these three frameworks intersect. Our aim is to foster such collaboration at the intersections of the three. As such, we will be guided by the irenic spirit (and person) of Jeremy Butterfield, whose work, magisterially spanning these topics, has inspired and continues to inspire, all of us.

## Confirmed Speakers

- Guido Bacciagaluppi, Utrecht University
- Harvey Brown, University of Oxford
- Fay Dowker, Imperial College London
- Flaminia Giacomini, Perimeter Institute
- Stephan Hartmann, LMU Munich
- Klaas Landsman, Radboud University
- Dennis Lehmkuhl, University of Bonn
- Michela Massimi, University of Edinburgh
- John Norton, University of Pittsburgh
- Kasia Rejzner, University of York
- Jos Uffink, University of Minnesota
- Francesca Vidotto, University of Western Ontario
- Lena Zuchowski, University of Bristol

## Registration

Public-health conditions permitting, we plan to hold the conference in-person, with no video connections, although the talks and Q&A will be recorded (pending agreement by the speakers).

Please send registration requests by 29 May 2022 using the following form: https://forms.gle/aNk8ujNmaPK1woCT6

We are happy to announce that thanks to the generous support of the DFG the registration fees have been waived. Only the conference dinner on Friday, June 24th at Biergarten Chinesischer Turm remains on dutch treat for everyone.

## Day 1 (24 June 2022) - Location: Venue Change! LMU Munich - Room E216

Session Chairs:

Friday Morning: Laura Mersini

Friday Afternoon: Bryan Roberts

Time | Event |
---|---|

09:15 - 09:45 | Registration & Coffee |

09:45 - 10:00 | Introductory Remarks |

10:00 - 11:15 | Francesca Vidotto: "ℏ, κ, G: it takes three to bring them together" |

11:15 - 11:45 | Coffee Break @ Deli Star, Amalienstraße 40 |

11:45 - 13:00 | Guido Bacciagaluppi: "Bell Inequality Violations and Relativity of Pre- and Post-selection" (joint work with Ronnie Hermens) |

13:00 - 14:30 | Lunch Break |

14:30 - 15:45 | John Norton: "Causation and Symmetry" |

15:45 - 17:00 | Klaas Landsman: "Determinism: From Physics to Philosophy" |

17:00 - 17:15 | Coffee Break @ Deli Star, Amalienstraße 40 |

17:15 - 18:30 | Michela Massimi: "Perspectival models as inferential blueprints" |

19:00 - 22:00 | Conference Dinner @ Biergarten Chinesischer Turm |

## Day 2 (25 June 2022) - Location: IBZ

Session Chair:

Saturday Morning: Daniele Oriti

Saturday Afternoon: Miklos Redei

Time | Event |
---|---|

09:15 - 09:45 | Registration & Coffee |

09:45 - 10:00 | Introductory Remarks |

10:00 - 11:15 | Stephan Hartmann: "The Open Systems View" |

11:15 - 11:45 | Coffee Break @ Deli Star, Amalienstraße 40 |

11:45 - 13:00 | Kasia Rejzner: “Measurement and Observation in Quantum Field Theory On Curved Spacetime” |

13:00 - 14:30 | Lunch Break |

14:30 - 15:45 | Harvey Brown: "What justifies the common claim that symmetries explain conservation principles?" |

15:45 - 16:15 | Coffee Break @ Deli Star, Amalienstraße 40 |

16:15 - 17:30 | Lena Zuchowski: "From Randomness to the Arrow of Time" |

## Day 3 (26 June 2022) - Location: IBZ

Session Chair:

Sunday Morning: Neil Dewar

Sunday Afternoon: Henrique Gomes

Time | Event |
---|---|

09:15 - 09:45 | Registration & Coffee |

09:45 - 10:00 | Introductory Remarks |

10:00 - 11:15 | Fay Dowker: "Black hole entropy in the causal set approach to quantum gravity" |

11:15 - 11:45 | Coffee Break |

11:45 - 13:00 | Jos Uffink: "Black Holes and Thermodynamics: a mere analogy or an identification?" |

13:00 - 14:30 | Lunch Break |

14:30 - 15:45 | Flaminia Giacomini: "Quantum reference frames: towards a quantum description of space and time" |

15:45 - 16:15 | Coffee Break |

16:15 - 17:30 | Dennis Lehmkuhl: "The Petrov-Pirani-Penrose classification of spacetimes and its role in the renaissance of general relativity" |

17:30 - 18:45 | Jeremy Butterfield: “What Remains to Be Done at the Intersections of Physics and Philosophy” |

## Abstracts

### Guido Bacciagaluppi (Utrecht University): Bell Inequality Violations and Relativity of Pre- and Post-selection" (joint work with Ronnie Hermens)

The Bell inequalities can be violated by postselecting on the results of a measurement of the Bell states. If information about the original state preparation is available, we point out how the violation can be reproduced classically by postselecting on the basis of this information. We thus propose a variant of existing experiments that rules out such alternative explanations by having the preparation and the postselection at spacelike separation. Unlike the timelike case in which one can sharply distinguish Bell inequality violations based on pre-or postselection of a Bell state, in our scenario the distinction between these physical effects becomes foliation dependent. We call this “relativity of pre-and postselection” and argue that it has consequences for the interpretation of collapse. top

### Jeremy Butterfield (University of Cambridge): What Remains to Be Done at the Intersections of Physics and Philosophy

TBAtop

### Harvey Brown (University of Oxford): What justifies the common claim that symmetries explain conservation principles?

It is widely claimed by physicists that symmetries have explanatory priority when it comes to the link between them and conservation principles. In the literature on Noether's first theorem doubts have been raised about this claim in the light of the lesser-known converse theorem that Noether also proved in 1918. But her converse theorem does not in fact underpin these doubts, and nor did she claim it does. However, an important generalisation of Noether's work due independently to Martínez Alonso in 1979 and Olver in 1986 includes a converse result which does seem to strengthen the doubts. I will argue that what justifies the physicists' claim is therefore not modern Noetherian logic, nor an alleged metaphysical fundamentality concerning symmetries, but rather pragmatic considerations related to the way symmetries are exploited in the standard model in particle physics.top

### Fay Dowker (Imperial College London): Black hole entropy in the causal set approach to quantum gravity

Quantum, thermal and gravitational physics are all implicated in the values of the temperature and entropy of a black hole. The value of the entropy -- the number of Planck-sized plaquettes tiling the horizon -- speaks of discreteness and is one of the strongest motivations for the causal set approach to the problem of quantum gravity. I will review work towards a statistical mechanical account of black hole entropy -- and causal horizon entropy more generally -- within causal set theory.top

### Flaminia Giacomini (Perimeter Institute): Quantum reference frames: towards a quantum description of space and time

In physics, observations are typically made with respect to a frame of reference. Although reference frames are usually not considered as degrees of freedom, in most practical situations use physical system as reference frames. Can a quantum system be considered as a reference frame and, if so, which description would it give of the world? In the talk, I will introduce a general method to associate a reference frame to a quantum system, which generalises the usual reference frame transformation to a "superposition of coordinate transformations”. Such quantum reference frames transformations imply that the notion of entanglement and superposition are not given a priori, but depend on the choice of the quantum reference frame even in a non-relativistic setting. Quantum reference frames could be a useful tool at the intersection of gravity and quantum theory. For instance, they allow one to generalise the Einstein’s equivalence principle to superpositions of gravitational fields, and to describe the behaviour of quantum clocks ticking in a superposition of times relative to one another.top

### Stephan Hartmann (MCMP/LMU Munich): The Open Systems View

In classical mechanics, the state of a physical system tells us everything we need to know about the possible interactions that it can enter into. The quantum analogue of the classical state description is the state vector, but in this talk I will argue that it is not the state vector but its probabilistic generalisation, the density operator, that most fundamentally characterises a system in standard quantum theory. Density operators represent open systems in standard quantum theory, but there is a general framework for describing open systems—the general quantum theory of open systems—that I will argue should be thought of as more fundamental than standard quantum theory. In particular I will argue that no matter how one interprets quantum mechanics, just so long as one takes it to be complete, there are good reasons to think of it as grounded in the general quantum theory of open systems. The paper is based on joint work with Michael Cuffaro (MCMP) and can be downloaded from the arXiv: https://arxiv.org/abs/2112.11095. top

### Klaas Landsman (Radboud University): Determinism: From Physics to Philosophy

Starting from the idea that Determinism is a family resemblance, I will review its status in quantum theory and general relativity, with special attention to Jeremy’s contributions e.g. to the hole argument and to philosophical thought in physics in general. The conclusion will be that the topic is too difficult at least for the speaker.top

### Dennis Lehmkuhl (University of Bonn): The Petrov-Pirani-Penrose classification of spacetimes and its role in the renaissance of general relativity

This talk will focus on the development of new mathematical methods during the 1960s that allowed for new ways of understanding the solution space of the Einstein equations. The focus will be on the classification scheme for vacuum solutions first developed by Aleksei Petrov in 1954 and then applied to the question of how to give an coordinate-independent definition of the presence of gravitational radiation by Felix Pirani in 1957. I will review Pirani’s definition and rationale for proposing his definition, and then discuss Penrose’s 1960 re-derivation and elaboration of the Petrov classification in the context of his spinor formulation of GR, and his criticism of Pirani’s definition of gravitational radiation. Starting from there, I will review the emerging discussion of how the different Petrov classes should be interpreted, and thus how the solution space of the Einstein equations could to be understood, indeed how it could be used as a map of spacetimes and their interpretation. I shall argue that present-day philosophy of physics is still very far from having harvested all the conceptual treasures that originated from this debate in the 1950s and 1960s.top

### Michela Massimi (University of Edinburgh): Perspectival models as inferential blueprints

In this talk I present the important exploratory function that a family of models — perspectival models — play across a number of areas in science. First I explain what I mean by 'perspectival models' then I illustrate their role as 'inferential blueprints' that deliver knowledge about what is possible. I draw on my book Perspectival Realism (OUP, 2022) to illustrate the importance of perspectival models in delivering modal knowledge with examples taken from the history of nuclear physics.top

### John D Norton (University of Pittsburgh): Causation and Symmetry

Causal metaphysics has failed repeatedly to provide a factual principle of causation. Curie's principle asserts that the symmetries of causes must be preserved in their effects. It has the appearance of such a long-sought factual principle of causation. I argue that the appearance is deceptive and that the principle can only be made true by reducing it to a banal truism.top

### Kasia Rejzner (University of York): Unraveling mathematical foundations of quantum field theory

In this talk I will give an overview of recent advances in understanding the mathematics underlying quantum field theory (QFT). I will start with discussing the axiomatic approach of algebraic quantum field theory and talk about how it evolved in the last three decades, making contact with perturbative QFT. The main points of focus will be renormalization and locality.top

### Jos Uffink (University of Minesota): Black Holes and Thermodynamics: a mere analogy or an identification?

Ever since Jacob Bekenstein argued, in 1972, that the surface area of a black hole event horizon is analogous to the thermodynamical concept of entropy, the issue of "black hole thermodynamics" has been discussed in the literature. Stephen Hawking's realization, in 1974, that according to quantum field theory a black hole should emit what is now known as Hawking radiation contributed to taking this analogy more seriously. In 1999, Bob Wald presented a full-fledged analogy between all the laws of thermodynamics and corresponding statements about black holes.

Nevertheless, the issue of whether this correspondence is to be seen as a mere analogy or as establishing that black holes are genuine thermal systems in the sense of thermodynamics seems to be open for dispute.

In this talk, I will identify the distinctions that still exist between the laws of thermodynamics and their correspondence to black holes, and argue which gaps remain to be bridged if one wishes to see this analogy as an identification of black holes as genuine thermal systems.

### Francesca Vidotto (University of Western Ontario): ℏ, κ, **G**: it takes three to bring them together

The relation between thermodynamic and the quantum theory has been fertile, deeply explored and still a source 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. Yet, it may well be the key to a deeper understanding of the other relations, bringing a common conceptual framework. A possible path to such a framework may come from emphasizing the role that partitions plays in the three contexts. I will start by reviewing a selection of results in covariant thermodynamics, such as the construction of a covariant notion of thermal equilibrium by considering tripartite systems. I will then discuss how such construction requires a relational take on thermodynamics, similarly of what happens with the quantum theory and in gravity.top

### Lena Zuchowski (University of Bristol): From Randomness to the Arrow of Time

The 'arrow of time' is often defined through the 2nd Law of Thermodynamics and the predicted entropy-increase. As entropy and the 2nd Law are both contested concepts with various contingencies (e.g., on statistical or empirical interpretations; on different groundings in order, disorder and randomness etc), the direction of time as indicated by the arrow then also becomes subject to such contingencies. The talk will present a 'contingency-roadmap' and compare different routes to derive the arrow of time. It will conclude that an empirical, local interpretation of the arrow and all relevant ancestor concepts provides the epistemically safest , if ontologically most restricted, unpacking.top

## Venue

LMU Munich, Main Building, Room A120, Geschwister-Scholl-Platz, 80539 München on June 24th.

IBZ, Amalienstraße 38, 80799 München on June 25th - 26th.

## Organizers

- Erik Curiel, MCMP/LMU Munich

- Sebastian De Haro, University of Amsterdam

- Bryan Roberts, London School of Economics

- Katie Robertson, University of Birmingham

For information about any other practical matters, please contact one of the organisers.

## Acknowledgements

The conference is organized and supported by the Munich Center for Mathematical Philosophy (LMU Munich) and funded in part by the German Research Foundation (DFG).