Observables
(or the lack thereof) in Quantum Gravity
Any physical theory crucially relies on observables to understand its dynamics and make contact with observation. In quantum gravity this is a notoriously difficult task and a common challenge to all approaches. General covariance poses a conceptual difficulty to define any physically meaningful observables. Moreover, the Planck scale -- at which we expect quantum gravity effects to manifest -- is far out of reach for current experiments.
The first QG-AA seminar series will focus on the conceptual part of this challenge -- that is, how observables in a theory of quantum gravity should even be defined -- leaving actual observations and phenomenology for a future series.
Each talk will highlight key insights and pose crucial questions as a guideline to draw connections between the different seminars. The speakers, as well as all participants, are encouraged to discuss these questions before, after, and in between the seminar events on the Slack channel or elsewhere. The Slack channel can be joined by either registering using the link above or contacting the organizers directly.
Problem of time, relational observables and quantum covariance
by Philipp Höhn
07 October 2020, 17:00 CEST (GMT+2)
The diffeomorphism symmetry of general relativity leads to the infamous problem of time, haunting especially canonical approaches to quantum gravity. The core question it raises is how to define quantum dynamics in a diffeomorphism-invariant context (in the absence of external references such as boundaries) and is intimately tied with the more general question of what are observables in a background-independent quantum theory. Relational observables are one way of addressing these challenges. The underlying idea is to localize physical quantities of interest relative to other dynamical degrees of freedom, chosen as a dynamical reference system, rather than a spacetime background. A comprehensive relational picture then warrants covariance to be extended to changes of dynamical reference system. In this talk, I will offer an overview over achievements and challenges of the relational approach, as well as perspectives on quantum gravity arising therefrom.
The recorded talk can be found here.
Dressed observables in perturbative quantum gravity
by William Donnelly
02 November 2020, 18:00 CET (GMT +1)
Local observables are a central object of study in quantum field theory --- this leads to some tension with general relativity, where diffeomorphism invariance precludes the existence of local observables. I will show how to resolve this tension by introducing 'gravitationally dressed' observables: operators that are perturbatively diffeomorphism invariant but become local operators in the quantum field theory limit. Some explicit examples of dressed operators are constructed, and these exhibit corrections to microcausality at leading order in Newton's constant. In perturbation theory around a symmetric background, all dressed observables are shown to have a support which extends to the asymptotic boundary. I will sketch some implications for holography, cosmology and the formulation of local subsystems in quantum gravity.
The recorded talk can be found here.
Global symmetry, quantum error correction, and the black hole information problem
by Daniel Harlow
02 December 2020, 18:00 CET (GMT +1)
In this talk I will show how the essential ingredients of recent calculations of the Page curve of an evaporating black hole can be used to generalize a recent argument against global symmetries beyond the AdS/CFT correspondence to more realistic theories of quantum gravity, illustrating a close connection between the absence of global symmetries and a unitary resolution of the black hole information problem. I'll also give several low-dimensional examples of quantum gravity theories which do not have a unitary resolution of the black hole information problem, and which therefore can and do have global symmetries. Motivated by this discussion, I'll conjecture that in a certain sense Euclidean quantum gravity is equivalent to holography. Based on recent work with Edgar Shaghoulian.
The recorded talk can be found here.
Non-local observables in quantum gravity from the algebraic QFT perspective
by Kasia Rejzner
18 January 2021, 18:00 CET (GMT +1)
In this talk I will show how (perturbative) algebraic quantum field theory provides us with mathematical tools for studying observables in quantum gravity. These tools could be potentially applied in approaches to quantum gravity that use QFT as the basic tool (e.g. asymptotic safety program, effective QG approach). The two types of observables that I will focus on are relational observables and dressed observables. I will discuss the latter using the analogy with QED.
The recorded talk can be found here.
Observables for Quantum Gravity: Going Nonperturbative
by Renate Loll
17 February 2021, 18:00 CET (GMT +1)
The physical nature and explicit construction of observables in quantum gravity depend on length scale and context, and tend to be very different in perturbative and nonperturbative regimes. My talk will focus on the latter, which is characterised by the absence of a pre-existing background and where spacetime is not described by a smooth metric tensor. What constitutes appropriate (pre-)gravitational observables near the Planck scale is not known a priori, and their operational definition depends on how diffeomorphism symmetry is implemented nonperturbatively. Natural candidates are "quantum generalisations" of nonlocal classical observables, which in general will need to be renormalized and may display unexpected, anomalous behaviour. To compare with classical results, one also needs some control on how to coarse-grain. The construction and implementation of nonperturbative quantum observables poses creative and technical challenges that can be addressed successfully in sufficiently complete candidate theories, as I will illustrate in the context of Causal Dynamical Triangulations.
The recorded can be found here.
Panel discussion:
From Observables to Observations
08 April 2021, 15:00 CET (GMT +2)
The recorded discussion can be found here.