Virtual Seminars

David Wallace
Pittsburg University

Quantum gravity at low energies

I provide a conceptually-focused presentation of `low-energy quantum gravity’ (LEQG), the effective quantum field theory obtained from general relativity and which provides a well-defined theory of quantum gravity at energies well below the Planck scale. I emphasize the extent to which some such theory is required by the abundant observational evidence in astrophysics and cosmology …

David Wallace
Pittsburg University

Quantum gravity at low energies Read More »

Poster for Don Marolf's virtual seminar: Spacetime wormholes, superselection sectors, and ensembles in quantum gravity: An Overview

Don Marolf
University of California Santa Barbara

Spacetime wormholes, superselection sectors, and ensembles in quantum gravity: An Overview

Don Marolf will review and summarize recent developments regarding spacetime wormholes in the gravitational path integral and their implications for the existence of a certain notion of “superselection sectors” in quantum gravity.  The existence of such sectors implies that, in certain contexts, we can think of quantum gravity as describing a statistical ensemble of theories.  …

Don Marolf
University of California Santa Barbara

Spacetime wormholes, superselection sectors, and ensembles in quantum gravity: An Overview Read More »

Richard Howl
Oxford University

Testing quantum gravity with non-Gaussianity and a Bose-Einstein condensate

Due to rapid progress in experimental quantum information science, a table-top test of quantum gravity may soon be possible. A promising possibility is to place two micro-solids in a spatial superposition and separable state. If, after a short time, entanglement between the micro-solids is observed then this could provide evidence of a quantum theory of gravity, assuming all other interactions can be neglected and that gravity provides a local interaction. These proposals have raised a number of questions, such as whether entanglement generation would really provide a test of quantum gravity and whether the experiments are feasible in the near term. Here, we consider whether an alternative signature of quantum gravity to entanglement could be used for a table-top test, and an alternative experimental setting. Specifically, we consider non-Gaussianity rather than entanglement and how this could be searched for in a Bose-Einstein condensate (BEC) to evidence quantum gravity. We discuss whether using non-Gaussianity and a BEC could provide any advantages to entanglement and micro-solids.

Laurent Freidel
Perimeter Institute

The nature of quantum entanglement in gravity: a tale about Noether and subsystems

In this talk, I will present a new perspective about decomposing gravitational systems into subsystems. I will explain what is the nature of entanglement of gravitational subsystems and the importance of local symmetries. I will emphasize the central role of the corner symmetry group in capturing all the necessary data needed to glue back seamlessly quantum spacetime regions. I will explain some of the key results we have achieved in the construction of the representations of these groups. If time permits, I will present new results about the canonical description of open gravitational systems and what it teaches us about the nature of quantum gravitational radiation.

Emily Adlam
Western University, Rotman Institute of Philosophy

Contextuality, Fine-tuning and Teleological Explanation

In this talk I will assess various proposals for the source of the intuition that there is something problematic about contextuality, and argue that contextuality is best thought of in terms of fine-tuning. I will suggest that as with other fine-tuning problems in quantum mechanics, this behaviour can be understood as a manifestation of teleological features of physics. I will also introduce several formal mathematical frameworks that have been used to analyse contextuality and discuss how their results should be interpreted.

Fay Dowker
Imperial College London

Recovering General Relativity from a Planck scale discrete theory of quantum gravity

Abstract: An argument is presented that if a theory of quantum gravity is physically discrete at the Planck scale and the theory recovers General Relativity as an approximation, then, at the current stage of our knowledge, causal sets must arise within the theory, even if they are not its basis.
We show in particular that an apparent alternative to causal sets, viz. a certain sort of discrete Lorentzian simplicial complex, cannot recover General Relativistic spacetimes in the appropriately unique way. For it cannot discriminate between Minkowski spacetime and a spacetime with a certain sort of gravitational wave burst.

Ognyan Oreshkov
Université Libre de Bruxelles

On the arrow of time in quantum mechanics

Abstract: According to quantum mechanics, it is fundamentally impossible to predict with certainty the outcome of a future measurement on a system prepared in a pure state, unless the state is an eigenstate of the observable to be measured. The best prediction is probabilistic, given by the Born rule. This absolute limitation on our ability to predict certain future events constitutes a radical difference from classical mechanics. In the reverse time direction, however, the analogous limitation does not hold: it is in practice possible to know with certainty the outcome of any type of measurement on any type of state, since all such events can have records at present. What is the origin of this time-reversal asymmetry, and how should we think about quantum theory if we believe that a microscopic theory should be time-symmetric?

It has been suggested that quantum theory in its usual predictive form is already time symmetric, if suitably applied back in time, while the observed asymmetry in the information we have about the past and the future can be traced to the thermodynamic irreversibility of macroscopic phenomena. In this talk, adopting a specific operational way of thinking about quantum theory, I will argue that the above asymmetry can be understood as a consequence of a special form of a joint past-future boundary condition at the level of quantum theory itself, without invoking considerations of macroscopic coarse-graining. Improving on an argument originally suggested in [O. Oreshkov and N. J. Cerf, Nature Phys. 11, 853-858 (2015)], I will explain how such a boundary condition implies the inability of a local observer in spacetime to predict future events better than the Born rule, in contrast to past events. I will argue that this can accounts for our perceived ability to influence the future and not the past, as well as to remember the past but not the future, and will speculate on the link between this arrow of time and the thermodynamic arrow. I will argue that a meaningful time-symmetric formulation of quantum theory requires rules that work for all physically admissible situations, hence the Born should be regarded as a special case of a more general rule. Adopting this generalization allows us to reformulate quantum theory in a way that makes sense without predefined time, which may be important for quantum gravity.

James Hartle
University of California

Living in a superposition

This talk will describe a model quantum universe consisting of a very large box containing a screen with two slits and an observer (us) that can pass through the slits. We apply the modern quantum mechanics of closed systems to calculate the probabilities for alternative histories of how we move through the universe and what we see. After passing through the screen with the slits, the quantum state of the universe is a superposition of classically distinguishable histories. We are then living in a superposition. Some frequently asked questions about such situations are answered using this model. In particular we will discuss whether or not if we are living in a superposition we would in some way feel it.

Giulio Chiribella
The University of Hong Kong

Quantum operations with indefinite time direction

The standard operational framework of quantum theory is time-asymmetric. This asymmetry reflects the capabilities of ordinary agents, who are able to deterministically pre-select the states of quantum systems, but not to deterministically post-select the outcomes of quantum measurements. However, the fundamental dynamics of quantum particles is time-symmetric, and is compatible with a broader class of operations where pre-selections and post-selections are combined in general ways that do not presuppose a definite direction of time. In this talk I introduce a framework for quantum operations with indefinite time direction, providing an example, called the quantum time flip, where an unknown, time-symmetric process is accessed in a coherent superposition of two alternative time directions. To highlight the potential of quantum operations with indefinite time direction, I will show a game where a hypothetical agent with access to the quantum flip can in principle outperform all agents who operate in a definite time direction.

Bob Coecke
Cambridge Quantum Computing

From Quantum Linguistics to Spacetime Linguistics, and Cognition

In earlier work, sometimes referred to as quantum linguistics [NewScientist], or as grammatical quantum field theory [by quantum gravity specialist Louis Crane], we for the first time combined grammatical structure with the distributional meanings of machine learning [CSC], which are typically represented in an inner-product space. The key insight was that grammar as well as more general linguistic structure perfectly matches the diagrams of categorical quantum mechanics [CKbook]. Our recipe was not restricted to inner-product space representations, for example, we also used density matrices, and we can also use spacetime as a representation of meanings. In fact, much in language has direct spatio-temporal connotations, either direct or metaphorical, e.g. prepositions like in, after, above etc. In joint work with Vincent Wang we constructed a linguistic model of spacetime, and how it extends to account for many more cognitive features [ConcSpacI] e.g. shape, taste, colour etc. We will argue that linguistic structure is really an interaction/process logic of things happening in the world out there. Hence this work could be a stepping stone to an alternative formalism for combining quantum structure with spacetime, at the crossroads of AI. This is joint work With Vincent Wang.

REFERENCES: [NewScientist] J. Aron. Quantum links let computers read. New Scientist nr 2790, pages 10-11. [CSC] B. Coecke, M. Sadrzadeh, and S. Clark. Mathematical foundations for a compositional distributional model of meaning. In: A Festschrift for Jim Lambek, volume 36 of Linguistic Analysis, pages 345–384. 2010. arxiv:1003.4394. [CKbook] B. Coecke and A. Kissinger. Picturing Quantum Processes. A First Course in Quantum Theory and Diagrammatic Reasoning. Cambridge University Press, 2017. [ConcSpacI] J. Bolt, B. Coecke, F. Genovese, M. Lewis, D. Marsden, and R. Piedeleu. Interacting conceptual spaces I: Grammatical composition of concepts. In: Concepts and their Applications, Synthese Library. 2018. arXiv:1703.08314