Journal Club and Seminars

2024

June 5, 2024: Christopher Fewster
Local measurement theory for quantum fields
Abstract: Many presentations of quantum mechanics include a postulate that the state of a system undergoes an instantaneous change following a measurement. This is clearly incompatible with special and general relativity and raises questions concerning the description of measurement in quantum field theory (QFT). Attempts to extend measurement postulates to QFT by hand have produced pathologies, such as the "impossible measurements" described long ago by Sorkin. I will present a recent operational approach to these questions, which models measurement of one quantum field (the system) by coupling it to another (the probe). This is all accomplished in a model-independent way within algebraic quantum field theory (AQFT). The resulting framework provides a description of measurement in QFT that is causal, covariant and consistent, and includes state update rules that are derived from the formalism, and works equally well in flat or curved spacetimes. As well as covering the basics of the formalism I will touch on some more recent developments, including asymptotic measurement schemes, links to quantum reference frames, and how Bell inequality violation is described in this framework. The talk is mostly based on joint works with Rainer Verch, Henning Bostelmann, Maximilian Ruep and Ian Jubb (see https://arxiv.org/abs/2304.13356 for a survey). I will also mention recent joint work with Daan Janssen, Leon Loveridge, Kasia Rejzner, James Waldron (https://arxiv.org/abs/2403.11973)
May 22, 2024: Anna Biggs
Comparing the decoherence effects due to black holes versus ordinary matter
Abstract: Recently a certain thought experiment was discussed (arXiv: 2301.00026, arXiv: 2205.06279) which involves the decoherence of a quantum system due to a black hole. Here we show how this phenomenon is consistent with standard ideas about quantum black holes. In other words, modeling the black hole as a quantum system at finite temperature one obtains the same answer. We demonstrate this by analyzing the problem in terms of an effective theory that can apply both for the black hole case and for an ordinary matter system, showing that the same qualitative effect is present for ordinary matter at finite temperature. Based on arXiv: 2405.02227.
May 8, 2024: Marco Scalisi
Species Cosmology
Abstract: Towers of species can significantly affect the properties of gravitational effective field theories. They naturally appear in string theory, and one of their universal effects is a renormalization of the scale at which gravity becomes strongly coupled. In this talk, we will discuss some of the implications for cosmology arising from the presence of a large number of species. A major focus will be on cosmic inflation and its observational signatures in the cosmic microwave background. We will also discuss some of the implications for Starobinsky inflation.
April 24, 2024: Cynthia Keeler
Beyond the Holographic Entropy Cone via Cycle Flows
Abstract: TBA
April 10, 2024: Niayesh Afshordi
Quantum spacetime in the sky: From the horizon to the vacuum
Abstract: I will discuss two aspects of the quantum fluctuations of vacuum spacetime that can be probed in gravitational wave (GW) observatories. The first is the quantum fluctuations near the horizon that lead to (stimulated) Hawking emission (a.k.a. echoes) AND multipolar deformation of Kerr geometry. The second is an irreducible noise in the GW detectors due to the quantum fluctuations of spacetime geometry. These are both (potential) manifestations of a UV/IR coupling that is inevitable in any non-perturbative theory of quantum gravity.
March 6, 2024: Rodrigo Andrade E Silva
Quantizing non-linear phase spaces, causal diamonds and the Casimir matching principle
Abstract: In the quest to understand the fundamental structure of spacetime in quantum gravity, it is worth exploring the ultimate consequences of non-perturbative canonical quantization, carefully taking into account the constraints and gauge invariance of general relativity. As the reduced phase space (or even the pre-phase space) of gravity lacks a natural linear structure, a generalization of the standard method of quantization is required. One such generalization is Isham's method based on transitive groups of symplectomorphisms, which we test in some simple examples. In particular, considering a particle that lives on a sphere, in the presence of a magnetic monopole flux, we algebraically recover Dirac's charge quantization condition from a "Casimir matching principle", which we propose as an important tool in selecting natural representations. Finally, we develop the reduced phase space quantization of causal diamonds in (2+1)-dimensional gravity. By solving the constraints in a constant-mean-curvature time gauge and removing all the spatial gauge redundancy, we find that the phase space is the cotangent bundle of Diff^+(S^1)/PSL(2, R). Applying Isham's quantization we find that the Hilbert space of the associated quantum theory carries a (projective) unitary irreducible representation of the BMS_3 group. From the Casimir matching principle, we show that the states are realized as wavefunctions on the configuration space with internal indices in unitary irreps of SL(2, R). A surprising result is that the twist of the diamond boundary loop is quantized in terms of the ratio of the Planck length to the boundary length.
February 21, 2024: Maulik Parikh
Quantum Gravity Horizon Fluctuations
Abstract: Classically, the event horizon of a black hole is a precisely defined null hypersurface. But quantum-gravitational metric fluctuations should blur that picture. I’ll present a calculation in perturbative quantum gravity of the quantum variance in the area of a four-dimensional Schwarzschild black hole.
February 7, 2024: Vincent (Hong Zhe) Chen
Asymptotic entanglement and celestial holography
Abstract: This talk will deal neither directly with observational signatures nor directly with gravity. Instead, motivated by the fruitfulness of similar questions in AdS/CFT, I will present work [arXiv:2308.12341] addressing how subregions and their entanglement in asymptotically flat spacetime are described by celestial holography. For simplicity, we consider the entanglement of a Milne patch for Maxwell theory in Minkowski spacetime. In the Minkowski vacuum, we find that the Milne patch is thermally entangled. We interpret the thermal entangling operator that builds the Minkowski vacuum from the Milne vacuum as a coupling term between sectors of operators in the celestial CFT. We further examine the edge modes of the Milne patch, assigning them a physical interpretation as fluctuations in Milne asymptotic charge. Interestingly, we find that the constraint governing these edge modes includes sources that avoid the Minkowski interior. Altogether, by studying entanglement along the extra holographic direction present in celestial holography but absent in AdS/CFT, our work bridges a critical gap between our understanding of entanglement in the latter and the physically relevant setting of asymptotically flat spacetime.
January 24, 2024: Anthony Speranza
A covariant regulator for entanglement entropy
Abstract: Entanglement entropy in quantum field theory is UV divergent, but it is expected that entropy differences are well-defined in a wide class of states. In this talk, I will describe a regulator for entanglement entropy using a modular crossed product that leads to finite entropy differences in the continuum. This regulator satisfies nontrivial consistency conditions such as invariance of the entropy under local unitaries. As an application, I will describe proofs of the Bekenstein bound and quantum null energy condition directly in terms of regulated entropy differences.

2023

November 29, 2023: Markus Aspelmeyer
November 15, 2023: Bob Wald
Black Holes Decohere Quantum Superpositions
Abstract: We show that if a massive body is put in a quantum superposition of spatially separated states, the mere presence of a black hole in the vicinity of the body will eventually destroy the coherence of the superposition. This occurs because, in effect, the gravitational field of the body radiates soft gravitons into the black hole, allowing the black hole to harvest "which path'' information about the superposition. A similar effect occurs for quantum superpositions of electrically charged bodies. The effect is very closely related to the memory effect and infrared divergences at null infinity.
November 1, 2023: Nava Gaddam
Black hole scattering
Abstract: I will describe scattering processes near a black hole horizon mediated by graviton fluctuations of the background. I will give an overview of what physics these processes capture and explain why they are important from three different perspectives: i) a relativist's point of view in terms of classical solutions and asymptotic symmetries ii) high energy scattering involving black holes (including new emergent soft physics in the infrared and aspects of ultraviolet gravitational physics) from a particle physicist's perspective iii) observational consequences from an astronomer's lens.
October 18, 2023: Astrid Eichhorn
Black hole shadows as a probe of quantum gravity
Abstract: Quantum gravity is notoriously hard to test observationally. The reason is that the Planck-scale, at which quantum gravity effects are usually expected to set in, is much smaller than length scales that can be resolved in observations. In this talk, I will present two potential strategies to test quantum gravity. The first strategy is to challenge the expectation that the quantum gravity scale is the Planck scale. The second strategy is to find leverarms that can translate tiny Planck-scale-effects into larger, observable effects. I will focus on black-hole shadows as a testing ground to apply the two strategies.
October 4, 2023: Sougato Bose
Quantum Nature of Gravity through a Table-Top Experiment
Abstract: We will discuss a laboratory experiment to reveal the underlying quantum nature of gravity. Along with justifications, we will explore a methodology and the main challenges to meet in order to make such an experiment a reality. Time permitting, the applications of the same technology to deliver extreme quantum sensors will also be discussed.
September 20, 2023: Flaminia Giacomini
Quantum systems as gravitational sources: which quantum aspects of gravity can we test?
Abstract: Understanding the fundamental nature of gravity at the interface with quantum theory is a major open question in theoretical physics. Recently, the study of gravitating quantum systems, for instance a massive quantum system prepared in a quantum superposition of positions and sourcing a gravitational field, has attracted a lot of attention: experiments are working towards realising such a scenario in the laboratory, and measuring the gravitational field associated to a quantum source is expected to give some information about quantum aspects of gravity. However, there are still open questions concerning the precise conclusions that these experiments could draw on the nature of gravity, such as whether experiments in this regime be able to test more than the Newtonian part of the gravitational field. In my talk, I will argue that a full answer to this question requires a combination of different techniques and tools derived from quantum information. On the one hand, I will show how a careful analysis of physical situations using a weak-field description of the gravitational field allows us to identify specific quantum features of gravity. On the other hand, a theory-independent approach allows us to derive no-go theorems which help us to constrain the possible theories compatible with a certain experimental outcome. A solid theoretical investigation of these aspects is necessary to identify quantum properties of gravity that could be observed in the future.