The Philosophical Aspects of Spacetime Emergence
Group Organizer: Ali Barzegar (LMU)

According to many theories of quantum gravity, spacetime disappears at the fundamental ontological level. This gives rise to the problem of empirical incoherence: there are no local beables in a theory of quantum gravity and so the features necessary to confirm or disconfirm it through empirical evidence seem to be absent from reality. To avoid this threat of empirical incoherence, spacetime is regarded as emergent from the more fundamental non-spatiotemporal entities. One can understand this notion of spacetime emergence in two different ways. According to spacetime functionalism, the fundamental entities somehow realize the relevant spacetime functions or play the spacetime role. However, according to spacetime compositionalism, spacetime is composed of non-spatiotemporal entities or atoms of spacetime just like a chair is composed of atoms. Moreover, this idea of spacetime emergence seems to give rise to a kind of hard problem of spacetime. That is, there seems to be an explanatory gap between the notions of the spatiotemporal and the non-spatiotemporal. Whether this is a real problem or not needs to be addressed.

• First Session: We will discuss various aspects of the problem of empirical incoherence for theories of quantum gravity (Reading: Huggett and Wüthrich 2013).
• Second Session: There are two ways to understand the spacetime emergence: functionalism vs. compositionalism. The two solutions and their difficulties will be discussed (Reading: Baron 2019).
• Third Session: We will discuss the so-called hard problem of spacetime emergence and ways to solve or dissolve it (Reading: Le Bihan 2021).

Readings: (downloadable from Philpaper/Philsci-archive/arxiv)
– Baron, S. (2019). The Curious Case of Spacetime Emergence. Philosophical Studies 177(8):2207-2226.
– Baron, S. and B. Le Bihan (2022). Composing Spacetime. Journal of Philosophy 119 (1):33-54.
– Huggett, N. and C. Wüthrich (2013). Emergent spacetime and empirical (in)coherence. Studies in History and Philosophy of Modern Physics 44 (3), 276– 285.
– Lam, V. and M. Esfeld (2013). A dilemma for the emergence of spacetime in canonical quantum gravity. Studies in History and Philosophy of Modern Physics 44 (3), 286–293.
– Lam, V. and C. Wüthrich (2018). Spacetime is as spacetime does. Studies in History and Philosophy of Modern Physics 64, 39–51.
– Le Bihan, B. (2021). Spacetime emergence in quantum gravity: Functionalism and the hard problem. Synthese 199 (2):371-93.
– Wüthrich, C. (2018). The Emergence of Space and Time. Philsci-archive.pitt.edu/ 14535/
– Yates, D. (2021). Thinking about spacetime. In C. Wüthrich, B. Le Bihan, and N. Huggett (Eds.), Philosophy Beyond Spacetime. Oxford University Press.

Black Hole Thermodynamics, Information Loss, and the Nature of Entropy
Group Organizer: Bruno Arderucio (UNAM)

Hawking’s prediction for the temperature of a black hole
shows off four constants in a single formula: Boltzmann’s, Planck’s,
Newton’s, and the speed of light. Both quantum mechanics and
general relativity are manifestly indispensable for its obtention.
Following the birth of black hole thermodynamics, several conceptual
issues (re-)emerged.

• In our first encounter, we discuss the foundations
of the field and compare them to the thermodynamics of ordinary
systems’ (reading suggestion: sections 2-4 of Ref. [2]).
•Our second discussion session focuses on interpreting the nature of entropy (reading suggestions: Ref. [1] and section 5 of Ref. [2]).
• Our final meeting is devoted to information loss in black holes and how one can envision quantum gravity to modify the picture (reading suggestion: Ref. [3] and section 6 of Ref. [2]).

Reading List:
– E. T. Jaynes, Gibbs vs Boltzmann Entropies. American Journal of
Physics 33, 391 (1965)
– R. M. Wald, The Thermodynamics of Black Holes. Living Rev. in
Rel. 4, 6 (2001)
– A. Almheiri, T. Hartman, J. Maldacena, E. Shaghoulian, and A.
Tajdini, The entropy of Hawking radiation. Rev. Mod. Phys. 93,
035002 (2021)

Quantum Foundations and Gravity
Group Organizer: Emily Adlam (Western)

• Day 1: Indefinite Causal Structure
The framework of indefinite causal structure studies the possible correlations between agents who locally have free choice over all operations compatible with quantum mechanics but who globally are not subject to any restrictions of causal order. What does this framework teach us about causation in quantum gravity, and does it give us new insight into philosophical questions about causation? What do we learn from experimental implementations of indefinite causal structures?
– Flaminia Giacomini, Caslav Brukner: Quantum correlations with no causal order, https://arxiv.org/abs/1105.4464

• Day 2: Internal Quantum Reference Frames
We usually do physics relative to an external reference frame, but presumably there is no reference frame external to the universe as a whole, so at some point we will have to do physics relative to an internal subsystem of the universe instead. The internal quantum reference frame programme studies how to establish and switch between such reference frames. Are these reference frames really reference frames in an operationally meaningful sense? How should we expect them to be used in quantum gravity, and what is the significance of an Equivalence Principle defined using internal quantum reference frames?
– Ognyan Oreshkov, Fabio Costa, Caslav Brukner: Einstein’s Equivalence principle for superpositions of gravitational fields
https://arxiv.org/abs/2012.13754

• Day 3: Tabletop Gravity and the Measurement Problem
Recently there has been a great deal of interest in tabletop experiments intended to exhibit the quantum nature of gravity by demonstrating that it can induce entanglement, but the results of these experiments may also teach us something about the interpretation of quantum mechanics. Which interpretations naturally predict a positive result to these experiments, and which interpretations naturally predict a negative result? If the result is not as predicted by some particular interpretation, how would we have to change the interpretation to accommodate the result, and is that a reasonable cost to bear?
– Emily Adlam: Tabletop Experiments for Quantum Gravity Are Also Tests of the Interpretation of Quantum Mechanics
https://arxiv.org/abs/2204.08064

The Problem Of Time
Group Organizers: Eugene Chua (UCSD/UIC) and Lucy James (Bristol/UNIGE)

• This series of discussion groups explores the problem of time, along with some of its proposed solutions. Of course, there are many problems of time in philosophy and physics; for us, the focus is on the particular problem that arises when preparing a background independent classical theory, such as general relativity, for quantization. This involves an introduction to gauge symmetries, which are usually taken to imply unphysical degrees of freedom. In the case of a background independent theory, time evolution behaves like a gauge symmetry, leading to the idea that ‘time disappears’ in quantum gravity. The aim of the first session is to show how this problem arises, and to discuss what might be required from a possible solution. Reading for this session is Thebault’s ‘The Problem of Time’ (2019).

• Physicists have tried to resolve the problem of time in a multitude of ways. Claus Kiefer’s semiclassical time approach argues, roughly, that a variable which can play the time role in a Schrodinger-like equation emerges from an application of various approximation techniques to the canonical approach. We will read Chua and Callender’s ‘No Time for Time from No-Time’ (2021), which critiques this view by arguing that these approximation techniques are typically justified with respect to some background time variable, something we do not have access to in the canonical approach.

• Carlo Rovelli’s thermal time approach instead starts from the observation that thermodynamic equilibrium is classically defined in terms of time. He reverses this observation and argues that we can use the notion of equilibrium to define a time parameter. Time, on this view, emerges as a result of thermodynamical considerations. We will read Swanson’s ‘Can Quantum Thermodynamics Save Time?’ (2021), which raises a series of technical problems, alongside some open conceptual worries. Participants are invited to read Rovelli’s non-technical introduction / interpretive approach to thermal time in ‘The Order of Time’ (2018, end of Ch. 9 and beyond), though it is optional.

Quantum Gravity in the Early Universe
Group Organizer: Mike Schneider (UIC)

The standard Lambda-CDM model of cosmology correctly describes the past evolution of spatial structures in our observable universe over all cosmologically significant timescales, in terms of deviations exhibited within a history of uniform expansion of space. But the initial conditions assumed in the model have a very peculiar form. Taking the dynamics of the model as a hydrodynamic or effective field theory of the evolution of our entire cosmos, one would therefore like to also explain the necessary initial conditions, in terms of underlying fundamental physics. Moreover, the Lambda-CDM model exhibits a generic ‘Big Bang’ singularity within the ‘early universe’ epoch: a curvature and temperature blow-up everywhere in the vicinity of the moment in cosmic history, within the model, when one would like to supply a fundamental physical explanation to account for those initial conditions. This indicates that the fundamental physical explanation sought within the context of the early universe will draw on unknown particle physics of arbitrarily high energies, or even quantum gravity —possibly resolving the Big Bang singularity as well, with implications for large-scale cosmology beyond the standard Lambda-CDM model. Our discussion group will focus on all such topics to do with quantum gravity in the early universe. On the first day, we will rehearse and critique the dominant paradigm in ‘early universe’ cosmology: inflation. In this paradigm, the explanation for the initial conditions in the Lambda-CDM model is ostensibly separated from quantum gravity in the vicinity of the initial singularity, by means of positing a high-energy effective field theory dynamics shortly thereafter. But other paradigms differ on this point. On the second day, we will consider conceptual and operational issues in early universe cosmology concerning the fundamental physical explanation of ‘cosmic time’ familiar in the Lambda-CDM model. On the third day, we will discuss generally the relationship between research in early universe cosmology and research in quantum gravity on the topic of quantum cosmology.

Readings:
• Day 1 — Inflation and its alternatives:
– “Beyond Standard Inflationary Cosmology” by Robert H. Brandenberger, in Beyond Spacetime, 2020
•Day 2 — The physical origins of cosmic time
– “Limits of Time in Cosmology” by Svend E. Rugh and Henrik Zinkernagel, in The Philosophy of Cosmology, 2017
• Day 3 — Quantum cosmology:
– “Relational Quantum Cosmology” by Francesca Vidotto, in The Philosophy of Cosmology, 2017

Duality
Group Organizer: Rasmus Jaksland (NTNU)

This discussion group concerns dualities in string theory. A duality
obtains when two apparently very different theories or models
nevertheless prove to be physically equivalent.

• In the first session, we will introduce various string dualities and discuss whether they generate a new kind of underdetermination that could challenge scientific realism.
– Underdetermination: The Empirical Under-Determination Argument
Against Scientific Realism for Dual Theories | SpringerLink

• The second session focuses on T-duality (a duality between spaces
with large radius and spaces with small radius) and on the implications
this duality appears to have for the nature of spacetime in string theory.
– T-duality: Target space ≠ space – ScienceDirect

• Finally, the third session investigates the intriguing AdS/CFT duality
which relates semi-classical gravity in five dimensions (AdS side) to
non-gravitational quantum field theory in four dimension (CFT side). We will in particular discuss whether this duality shows that gravity and (dynamical) spacetime emerges from quantum degrees of freedom.
– AdS/CFT: Emergence in holographic scenarios for gravity –
ScienceDirect (we won’t discuss section 4, so only read it out of
interest).

Epistemology of cosmology
Group Organizer: Siska de Baerdemaeker (Stockholm)

Leveraging observational cosmology to test theories of (quantum) gravity ΛCDM, the concordance model of the evolution of the universe, is hugely successful. However, the model faces challenges when being extended to (sub-)galactic scales, as well as to the very early universe. This has opened up the possibility to leverage observational cosmology to text various speculative theories—including theories of quantum gravity or hybrid dark matter/modified gravity theories. We’ll investigate how observational cosmology can be leveraged to test theories of (quantum) gravity and other introductions of novel physical phenomenology. Specifically, we will discuss (i) the structure of justification for ΛCDM itself, including the application of the FLRW-metric to the universe; (ii) epistemology of cosmological simulations; and, (iii) gravitational wave tests of modified theories of gravity.

• Day 1: Our starting point will be a discussion of the epistemic foundations of ΛCDM itself. Smeenk uses Howard Stein’s philosophy of science to illuminate the justification for cosmological knowledge. The central question of the paper is “whether [ΛCDM] has a clear physical status, such that any systematic discrepancy between the model and observations can be reliably taken to indicate the need to include a new physical feature” (230).
– Smeenk, Chris (2020). Some reflections on the structure of cosmological knowledge. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 71:220-231.

• Day 2: Leveraging observational cosmology to test alternative theories of gravity in part depends on being able to derive predictions from ΛCDM. Such predictions depend largely on numerical simulations. But how is the reliability of these simulation results established? Gueguen shows that two common strategies—convergence studies and code comparisons—have inherent shortcomings.
– Gueguen, Marie (2021). A Tension Within Code Comparisons. [Preprint] URL: http://philsci-archive.pitt.edu/id/eprint/19227 (accessed 2022-05-09).

• Day 3: Patton uses LIGO as a case study to argue that formal reasoning can extend GR’s empirical reach. Specifically, she investigates the parametrized post-Einsteinian framework to allow a broader, more flexible testing of assumptions underlying theories of gravity. Patton ties the ppE-testing from multi-messenger astronomy explicitly into views on theory testing from Carnap, Hempel, and Stein.
– Patton, L. (2020). Expanding theory testing in general relativity: LIGO and parametrized theories. Studies in History and Philosophy of Modern Physics, 69, 142–153.