# Foundations of Observational, Classical and Semi-Classical Gravitational Physics and The Problem of Agency and Laws of Nature (28-31 March 2023)

## Idea & Motivation

The conference of "Foundations of Gravitational Physics and The Problem of Agency and Laws of Nature" will address various topics in theoretical physics and foundations of physics, focusing on gravitational physics and on agency and physical laws. Invited physicists and philosophers have been chosen to represent different approaches to such topics (semi-classical gravity, holography, quantum gravity theories, foundations of physics) and to discuss different kinds of issues (locality, the nature of black hole entropy, approaches to the Information-Loss Paradox, the emergence of classicality in approaches to QG, quantum reference frames, epistemology and methodology of theory and of observation, ontology, inter alia), in an attempt to foster a dialogue among the different fields. Contributed talks from theoretical physicists, observational astronomers and philosophers will be chosen in part so as to complement the perspectives of the invited speakers. Such a gathering will spur new, innovative approaches to the above mentioned problems, as well as connecting and invigorating work on existing approaches. It will also provide young researchers with a comprehensive introduction to the state of the art of the various central fields of research, and established researchers with a comprehensive overview.

## Confirmed Speakers

- Goncalo Araujo-Regado
- Ali Barzegar
- Jibril Ben Achour
- Časlav Brukner
- Eugenia Colafranceschi
- Erik Curiel
- Shep Doeleman
- John Dougherty
- Jamee Elder
- Felix Finster
- Christophe Goeller
- Henrique Gomes
- Anne-Catherine de la Hamette
- Stefan Hollands
- Vincent Lam
- Luca Marchetti
- Niels Martens
- Siddharth Muthukrishnan
- Ramesh Narayan
- Daniele Pranzetti
- Renato Renner
- Ana Alonso Serrano
- Ronak Soni
- Marika Taylor
- Jos Uffink
- Aron Wall

We are committed to fostering diversity and equality in our programs. Submissions from underrepresented groups are particularly welcome. The conference will be organized and run under the MCMP's code of conduct.

## Registration

The conference will be held in person.

Please send registration requests **by 15 March 2023** using the following form: https://forms.gle/gqaxG15547yoHfu1A

There are no fees for the conference as well for the conference dinners.

## Conference Venue

Geschwister-Scholl-Platz 1, room E 216, 80539 München.

## Organizers

For information about practical matters and registration, please contact one of the organisers.

## Acknowledgement

The conference is supported by the Munich Center For Mathematical Philosophy, the LMU/Cambridge Strategic Partnership project on semi-classical gravity and black hole physics as well by the FQXi grant: “The Epistemic Nature of Physical Laws, From Intelligent Agents to Quantum Gravity and Cosmology”.

## Program

The conference is spanned over four days. Each day will be about a different topic. The speakers of the first day will discuss agency, law of nature as well semiclassical gravity. The second day is dedicated to the topic of holography and quantum gravity approaches. The third day is devoted to black hole physics. Finally, on the last day of the conference, the speakers present their work on quantum reference frames and the information loss paradox.

### Day 1 (28 March 2023)

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

09:00 - 12:15 | Free Morning |

12:15 - 13:30 | Registration |

13:30 - 14:15 | John Dougherty: "The substantial role of Weyl symmetry in deriving general relativity from string theory" |

14:15 - 15:00 | Daniele Pranzetti: "Local holography and celestial symmetries" |

15:00 - 15:30 | Coffee Break |

15:30 - 16:15 | Luca Marchetti: "Emergent cosmological physics from Quantum Gravity" |

16:15 - 17:00 | Vincent Lam: "Laws of nature and spacetime" |

17:00 - 18:00 | Discussion |

19:00 | Conference Dinner |

## Day 2 (29 March 2023)

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

09:00 - 09:30 | Registration |

09:30 - 10:15 | Aron Wall: "Cauchy Slice Holography" |

10:15 - 11:00 | Siddharth Muthukrishnan: "Unpacking Black Hole Complementarity" |

11:00 - 11:30 | Coffee Break |

11:30 - 12:15 | Gonçalo Araujo Regardo: "Holographic Formulation of Quantum Gravity in a Closed Universe" |

12:15 - 13:30 | Lunch Break |

13:30 - 14:15 | Ana Alonso Serrano: "Thermodynamics of Spacetime and Quantum Gravity Phenomenology" |

14:15 - 15:00 | Jos Uffink: "Black Holes and Thermodynamics: a mere analogy or an identification?" |

15:00 - 15:30 | Coffee Break |

15:30 - 16:15 | Felix Finster: "An introduction to causal fermion systems and the causal action principle" |

16:15 - 17:00 | Henrique Gomes: "Waves of marble and waves of wood: what's the difference?" |

17:00 - 18:00 | Discussion |

### Day 3 (30 March 2023)

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

09:00 - 09:30 | Registration |

09:30 - 10:15 | Renato Renner: "The information loss paradox and reference frames" |

10:15 - 11:00 | Marika Taylor: "Recovering information from black holes: microstates and islands" |

11:00 - 11:30 | Coffee Break |

11:30 - 12:15 | Erik Curiel: "The Hawking Effect, Its Desiderata and Its Discontents" |

12:15 - 13:30 | Lunch Break |

13:30 - 14:15 | Sheperd Doeleman: " Event Horizon Cinema: Black Hole Imaging and Movie Making" |

14:15 - 15:00 | Niels Martens: "Taking the long view: historical, philosophical & cultural aspects of the ngEHT" |

15:00 - 15:30 | Coffee Break |

15:30 - 16:15 | Jamee Elder: "Theory-laden observation in black hole astrophysics" |

16:15 - 17:00 | Ramesh Narayan: "Is Sagittarius A* Really a Black Hole?" |

17:00 - 18:00 | Discussion |

19:00 | Conference Dinner |

### Day 4 (31 March 2023)

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

09:00 - 09:30 | Registration |

09:30 - 10:15 | Časlav Brukner: TBA |

10:15 - 11:00 | Anne-Catherine de la Hamette: "Quantum Reference Frames for Superpositions of Spacetimes" |

11:00 - 11:30 | Coffee Break |

11:30 - 12:15 | Christophe Goeller: TBA |

12:15 - 13:30 | Lunch Break |

13:30 - 14:15 | Ali Barzegar: "Towards A Minimalist Account of Agency" |

14:15 - 15:00 | Stefan Hollands: TBA |

15:45 - 15:30 | Coffee Break |

15:30 - 16:15 | Eugenia Colafranceschi: "Understanding Ryu-Takayangi as entropy without invoking holography" |

16:15 - 17:00 | Ronak Soni: "The Crossed Product Construction from Holographic Quantum Error-Correcting Codes" |

17:00 - 18:00 | Discussion |

19:00 | Farewell |

## Abstracts

### Goncalo Araujo-Regado (University of Cambridge): Holographic Formulation of Quantum Gravity in a Closed Universe

In this talk I will introduce a holographic formalism which provides a dual description of quantum cosmology on closed spatial slices. In particular, I will motivate a re-interpretation of the “wavefunction of the Universe” as the partition function of a special type of QFT living on the spatial slice. This recovers usual proposals of dS/CFT, but goes beyond them. A Wheeler-DeWitt state will turn out to be equivalent to an RG flow trajectory in the space of QFTs, defined in terms of a so-called “T^2”-deformation. I will discuss how “superpositions” of such QFT partition functions relate to the contour problem in quantum cosmology and I will explicitly calculate the flow for the case of the minisuperspace toy model of 2+1-dimensional pure gravity, recovering known no-boundary solutions. I will end by discussing potential philosophical implications from this holographic description.top

### Ali Barzegar (LMU Munich): Towards A Minimalist Account of Agency

In this talk, we adopt a top-down approach to agency aimed at developing a minimalist, scalable and naturalized account of it. After providing a general definition, we explore some possible extensions and refinements, domain of applicability, as well as a comparison with other recent accounts of agency. With respect to what we classify as strong (such as Tononi’s) and weak (such as Rovelli’s) characterizations, our notion of agency situates itself in a middle position – our intent being precisely that of spelling out the advantages of this median account within a variety of contexts, such as the interpretation of quantum mechanics, the debate on the nature of physical laws and probabilities.

Based on work with Emilia Margoni and Daniele Oriti.

### Časlav Brukner (Austrian Academy of Science): TBA

### JErik Curiel (MCMP/LMU Munich): The Hawking Effect, Its Desiderata and Its Discontents

I give a heuristic overview of the emission of radiation by black holes when quantum effects are taken into account---the "Hawking effect". I shall not work through any particular derivation of the effect in detail, as the rough, intuitive ones tend to be badly misleading, and the precise, rigorous ones are too technically demanding given the

constraints of this talk. I shall rather sketch the basic ingredients any derivation requires, the choices one must make in constructing a derivation, including what exactly it is one hopes to show, and discuss physical and conceptual problems those ingredients and conclusions raise and face. I focus on apparent inconsistencies among several of the most popular approaches.

### Eugenia Colafranceschi (UC Santa Barbara): Understanding Ryu-Takayangi as entropy without invoking holography

In appropriate semiclassical limits, the so-called island formula computes the entropy of non-gravitational quantum systems entangled with a gravitational theory. This is a special case in which the quantum-corrected Ryu-Takayanagi formula has been shown to compute a von Neumann entropy using only properties of the gravitational path integral and, in particular, without relying on the existence of a holographic dual field theory. It is thus natural to claim that a similar conclusion should hold more broadly, and that any asymptotically-AdS gravitational theory will define an algebra for any boundary region such that, in appropriate limits, the entropy of any state on that algebra is computed by the quantum-corrected Ryu-Takayanagi formula. Recent works by Chandrasekaran, Pennington and Witten have used the theory of von Neumann algebras to derive results of this form in various special contexts. We argue here that the above claim holds more generally, whenever the Euclidean path integral of the gravitational theory satisfies a set of standard axioms. We thus allow finite values of all coupling constants and do not require taking any special limits. Since our axioms do not restrict ultra-violet bulk structures, they may be expected to hold equally well for successful formulations of string field theory, spin-foam models, or any other approach to constructing a UV-complete theory.top

### Sheperd Doeleman (Harvard University): Event Horizon Cinema: Black Hole Imaging and Movie Making

Black holes are cosmic objects so small and dense, that nothing, not even light can escape their gravitational pull. Until recently, no one had ever seen what a black hole actually looked like. Einstein's theories predict that a distant observer should see a ring of light encircling the black hole, which forms when radiation emitted by infalling hot gas is lensed by the extreme gravity near the event horizon. The Event Horizon Telescope (EHT), a global array of radio dishes linked together by a network of atomic clocks to form an Earth-sized virtual telescope, has now imaged and detected these rings towards two sources: the galaxy M87, and the compact radio source SgrA* in the center of the Milky Way. These observations confirm the theory of General Relativity at the boundary of a black hole, but more importantly signal the emergence of precision measurements near event horizons. Of course, the question to ask is "What's next?" This talk will cover how the first images were made, the basic results, future directions that will enable real-time black hole movies, and the new questions we may be able to answer.top

### John Dougherty (MCMP/LMU Munich): The substantial role of Weyl symmetry in deriving general relativity from string theory

String theory reduces to general relativity in appropriate regimes. Huggett and Vistarini have given an account of this reduction that includes a deflationary thesis about symmetry: although the usual derivation of general relativity from string theory appeals to a premise about the theory’s symmetry, Huggett and Vistarini argue that this premise plays no logical role. In this article I disagree with their deflationary thesis and argue that their analysis is based on a popular but flawed conception of the interaction between symmetry and quantization. I argue that a better conception recognizes a distinction between ordinary, broken, and anomalous symmetries.top

### Jamee Elder (Harvard University): Theory-laden observation in black hole astrophysics

In this talk I will discuss the ways that contemporary observations of black holes—particularly those of the LIGO-Virgo and Event Horizon Telescope Collaborations (LVC and EHTC)—are theory- or model-laden. Although theory-ladenness is not always epistemically problematic, it may become so if it leads to a vicious circularity, where the theoretical assumptions made by the experimenters—in the physical design of the experiment or in subsequent inferences from the empirical data—guarantee that the observation will confirm the theory being tested. General relativistic assumptions enter into the methods of both experiments through the use of models and simulations of black hole spacetimes. This includes numerical relativity simulations in the case of the LVC and general relativistic magnetohydrodynamic simulations in the case of the EHTC. While these simulations play "ampliative" roles in both experiments, the epistemic consequences vary—vicious circularities do not always arise. Much of this talk will be devoted to discussing examples of theory-laden observation from each, including parameter estimation and theory testing by the LVC; and mass measurements and variability monitoring by the EHTC. Of these, the LVC parameter estimation is (I argue) the most problematic case of theory-ladenness. The final part of this talk will involve diagnosing features of the epistemic situation of black hole astrophysics that lead observations to be at risk of vicious circularity and how the LVC and EHTC counteract this danger.

Based on joint work with Juliusz Doboszewski.top

### Felix Finster (University of Regensburg): An introduction to causal fermion systems and the causal action principle

The theory of causal fermion systems is an approach to describe fundamental physics. It gives quantum mechanics, general relativity and quantum field theory as limiting cases and is therefore a candidate for a unified physical theory. The dynamics of a causal fermion system is described by a nonlinear variational principle, the causal action principle.

In the talk I will give a simple introduction, with an emphasis on the basic structures of spacetime.top

### Christophe Goeller (LMU Munich): TBA

### Henrique Gomes (University of Cambridge): Waves of marble and waves of wood: what's the difference?

I'll give a conceptual exposition of aspects of gravitational radiation, especially in relation to energy. My motive for doing so is that the strong analogies with electromagnetic radiation are not widely appreciated. In particular, this is a reply to some recent papers in the philosophy of physics literature that seem to deny that gravitational waves carry energy. The main argument of the talk is based on two points: (i) that for both electromagnetism and gravity, in the presence of material sources, radiation is an effective concept, unambiguously emerging only in certain regimes or solutions of the theory; and (ii) similarly, energy conservation is only unambiguous in certain regimes or solutions of general relativity. Crucially, the domain of (i), in which radiation is meaningful, overlaps significantly with the domain of (ii), in which energy conservation is meaningful. Conceptually, the overlap of regimes is no coincidence: the long-standing question about the existence of gravitational waves was settled precisely by finding a consistent manner to articulate their energy and momentum.

### Anne-Catherine de la Hamette (Austrian Academy of Science): Quantum Reference Frames for Superpositions of Spacetimes

The current theories of quantum physics and general relativity on their own do not allow us to study situations in which spacetime is in a quantum superposition. Despite the existing approaches to quantum gravity, we still lack a generally accepted answer to the question of how probe particles or fields behave in a spacetime featuring genuine quantum effects. In this talk, I propose a general strategy to determine the dynamics of objects on an indefinite spacetime metric, using an extended symmetry principle that relies on the covariance of physical laws under changes of quantum reference frames. Combining the validity of our most well-tested theory of gravity with the linearity of quantum theory, this bottom-up approach provides concrete predictions for the motion of test particles or fields in a superposition of spacetimes, without relying on perturbative methods. We study two concrete situations: a clock in the vicinity of a gravitational source mass in spatial superposition, and a superposition of conformally related spacetime metrics inhabited by a massive quantised Klein-Gordon field. We construct the explicit quantum frame changes and, by invoking the extended symmetry principle, show how to transform to a quantum frame in which the standard theories of general relativity and quantum field theory allow to determine the dynamics. The proposed approach can be used to study concrete physical phenomena such as time dilation and cosmological particle production in different quantum frames.

### Stefan Hollands (University of Leipzig): TBA

### Vincent Lam (University of Bern): Laws of nature and spacetime

This talk reviews the different ways in which the standard philosophical accounts of laws rely on spacetime. We then discuss the extent to which these conceptions can be adapted to a context where central spacetime features (or even spacetime itself) may not be fundamental but only emergent, such as within certain approaches to quantum gravity. Some of the difficulties at this level can be traced back to the tension between, on the one hand, central physical features of quantum theory and general relativity––the ingredient theories of most approaches to quantum gravity––and, on the other hand, the spacetime characterization underlying the standard analyses of laws.

### Luca Marchetti (University of New Brunswick): Emergent cosmological physics from Quantum Gravity

In this talk, I discuss the emergence of semi-classical, continuum and local physics from quantum gravity. In particular, I highlight the technical and conceptual difficulties associated with the extraction of a continuum description via an appropriate coarse-graining, and the implementation of physical localization via the relational strategy.

These difficulties naturally arise in the (Tensorial) Group Field Theory approach to quantum gravity, which I introduce next. I also show, however, how effective techniques can be used to bypass them in order to obtain continuum cosmological physics.

I then describe the main properties of such emergent cosmology and conclude by commenting on possible generalizations and future perspectives.

### Niels Martens (University of Copenhagen): Taking the long view: historical, philosophical & cultural aspects of the ngEHT

The Next Generation Event Horizon Telescope Collaboration (ngEHT) aims to use a virtual, Earth-sized telescope to make high quality images and videos of black holes. The History, Philosophy & Culture Working Group (HPC) constitutes one of its 11 working groups, consisting primarily of scholars from the humanities and social sciences, thereby providing an unprecedented opportunity for these fields to contribute in real-time to a large (astro)physics collaboration. Conversely, it provides a contemporary case study for various approaches, methods and tools within these fields. The HPC has prioritized four topical areas of contribution: 1) responsible telescope siting, 2) foundations, 3) algorithms, inference & visualization, and 4) collaborations (social epistemology & governance structure). After a

short overview of the HPC, this talk will zoom in on the collaborations focus area.

The process and results of knowledge formation within a scientific collaboration, i.e. its social epistemology, depend on its governance and social structure. Large scale scientific collaboration can take place within a variety of governance/organisational structures, ranging from top-down hierarchical structures to more loosely organised bottom-up collaboration in the absence of a formal governing structure. Extreme top-down collaborations prioritise the aims of the collaboration as a whole over the interests of its individual members, implicitly and incorrectly assuming that these are in tension with one another. On the other hand, too loose a bottom-up approach will be insufficient as sustained collaboration is required for the ngEHT to build the additional telescopes and coordinate the whole process of observing and data analysis to arrive at high-quality black hole videos. Moreover, utilising the epistemic value of diverse opinions, optimising individual creativity and enabling individual agency is not a mere matter of the absence of top-down governance structure, but also the implementation of positive measures that bring out these epistemic advantages. The main goal of the collaborations focus group is to provide the ngEHT collaboration with advice on optimising its social epistemology by developing a governance structure that is located somewhere in the middle of this spectrum, in a way that is the best of both worlds. Our main starting point is the realisation that supporting individual agency―via a collaboration structure that enables, encourages, supports and emphasizes transparent decision-making, diversity, fair credit assignment and accountability—is to the ultimate benefit of the collaboration as a whole, as well as its epistemic outputs. Keeping this in mind, I will discuss our current efforts with regards to 1) governance and team structures, 2) consensus vs. dissenting opinions, 3) a forecasting tournament, and 4) authorship practices.

### Siddharth Muthukrishnan (University of Southern Caifornia): Unpacking Black Hole Complementarity

To what extent does the black hole information paradox lead to violations of quantum

mechanics? I explain how black hole complementarity provides a framework to articulate how quantum characterizations of black holes can remain consistent despite the informa- tion paradox. I point out that there are two ways to cash out the notion of consistency in play here: an operational notion and a descriptive notion. These two ways of thinking about consistency lead to (at least) two principles of black hole complementarity: an operational principle and a descriptive principle. Our background philosophy of science regarding realism/instrumentalism might initially lead us to prefer one principle over the other. However, the recent physics literature, which applies tools from quantum information theory and quantum computational complexity theory to various thought experiments involving quantum systems in or around black holes, imply that the operational principle is successful where the descriptive principle is not. This then lets us see that for operationalists the black hole information paradox might no longer be pressing.

### Ramesh Narayan (Harvard University): Is Sagittarius A* Really a Black Hole?

At the centre of our Milky Way Galaxy lives an object called Sagittarius A* (Sgr A*) with a mass of 4 million solar masses. This object has a radius less than a few Schwarzschild radii and is thus ultra-compact. But is it a black hole? Does it necessarily have an event horizon, or could it be some unusual object with a physical surface? The talk will describe efforts based on astronomical observations to eliminate models of Sgr A* with a surface.

### Daniele Pranzetti (University of Udine): Local holography and celestial symmetries

In this talk I will introduce a new program for quantum gravity, grounded in the notion of corner symmetry algebra in gravitational systems and a proposal for states of quantum geometry as representation states of this algebra. In the first part, I will review how the Noether analysis can be performed in an ambiguity-free manner within the covariant phase space formalism. I will then use this to explain how different formulations of gravity, in both metric and tetrad variables, share the same bulk symplectic structure but differ at the corner, and in turn lead to inequivalent representations of the corner symmetry algebra. In the second part, I will show how these symmetry groups reduce to asymptotic symmetry group at scri and control gravitational dynamics in a large-r expansion around null infinity, allowing to derive the leading Einstein equations in a compact form. In addition to the to spin-0 and spin-1 gravitational charges related to the Bondi mass and angular momentum, this analysis reveals the existence of a spin-2 charge. I will review how these charge Ward identities are equivalent to the soft graviton theorems.

### Renato Renner (ETH Zürich): The information loss paradox and reference frames

Recent calculations based on gravitational path integrals provide evidence that the Page curve accurately describes the entropy of Hawking radiation. They thus support the hypothesis that the time evolution of a radiating black hole as viewed from the outside is information-conserving, hence contradicting Hawking’s original result. In my talk, I will revisit these calculations from an information-theoretic perspective and explain why the choice of reference frames may play a crucial role in the discussion of the discrepancy between the new calculations and Hawking’s result.

This work is based on a preprint with Jinzhao Wang, arXiv:2110.14653.

### Ana Alonso Serrano (Max Planck Institute for Gravitational Physics): Thermodynamics of Spacetime and Quantum Gravity Phenomenology

I present a review of concepts of thermodynamics of spacetime and the gravitational dynamics encoding in it, discussing the connection between thermodynamics and gravitational dynamics in the light of the derivation of Einstein equations of motion from the proportionality of entropy with an area, and analyze the entropies involved in the process. Then, I present a formalism to analyze low-energy quantum gravity modifications in a completely general framework based on the thermodynamics of spacetime. For that purpose, I consider quantum gravity effects via modification of entropy by an extra logarithmic term in the area. This modification is predicted by several approaches to quantum gravity, including loop quantum gravity, string theory, AdS/CFT correspondence and phenomenological models, giving our result a general character but allowing a parametrization in terms on the underlying theory. I show the derivation of the quantum modified gravitational dynamics from this modified entropy expression and discuss its main features. These results provide a general expression of quantum phenomenological equations of gravitational dynamics. Furthermore, I outline the application of the modified dynamics to particular models, such as cosmology, suggesting the replacement of the Big Bang singularity with a regular bounce.

### Ronak Soni (University of Cambridge): The Crossed Product Construction from Holographic Quantum

Error-Correcting Codes

Recently, the old observation that the generalised entropy of a horizon — the sum of the area and the entropy of exterior fields --- is better defined than either of the objects that make it up has been put on firmer footing by Witten et al. The main observation is that including fluctuations in the ADM charges in the algebra of matter fields changes the type of algebra from type III_1 --- which doesn't have an entropy function --- to type II --- which does --- using the crossed product construction. We show that a holographic quantum error-correcting (QEC) code naturally has a structure analogous to the crossed product. When the encoded algebra is type I, we show that ‘boundary modular flow’ in the code makes it into a direct sum of type I factors. In the limit where we take the encoded algebra towards a type III_1 algebra, a certain recasting of the direct sum becomes a type II algebra. We are able to recover both type II_\infty ---relevant for black holes --- as well as type II_1 algebras -- relevant for cosmological horizons in de Sitter space --- by imposing different constraints on the area operator in the code. These results indicate that the holographic QEC code might be useful in contexts where the 'boundary' is actually an observer, like the static patch of de Sitter.

### Marika Taylor (University of Southampton): Recovering information from black holes: microstates and islands

The recovery of information from black holes as they evaporate has puzzled the physics community since the discovery of Hawking radiation in 1974. In recent years the theory community has attempted to explain how information is recovered using approaches from quantum information. This work on entanglement islands has been developed in parallel with studies on the realisation and characterisations of black hole microstates. In this talk we will begin by reviewing the key ideas from both approaches and then report on recent work that brings together the two perspectives, i.e. explains how entanglement islands capture features of individual microstates.

### Jos Uffink (University of Minnesota): 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 thermodynamic 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 has remained open to 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.

### Aron Wall (University of Cambridge): Cauchy Slice Holography

I will explain how to use the T^2 deformation to reformulate the holographic principle in terms of a dual theory which lives on Cauchy slices, so that time is the emergent dimension. This new formulation interfaces with the usual AdS/CFT duality, and defines a dictionary mapping between boundary CFT states and bulk Wheeler-DeWitt states.

Based on arXiv:2204.00591.