Virtual Seminars

Abstract:
The formalism of quantum theory over discrete systems is extended in two significant ways. First, tensors and traceouts are generalized, so that systems can be partitioned according to almost arbitrary logical predicates. Second, quantum evolutions are generalized to act over network configurations, in such a way that nodes be allowed to merge, split and reconnect coherently in a superposition. The hereby presented mathematical framework is anchored on solid grounds through numerous lemmas. Indeed, one might have feared that the familiar interrelations between the notions of unitarity, complete positivity, trace-preservation, non-signalling causality, locality and localizability that are standard in quantum theory be jeopardized as the partitioning of systems becomes both logical and dynamical. Such interrelations in fact carry through.
(Joint work with Amélia Durbec and Matt Wilson, reference: https://arxiv.org/abs/2110.10587

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 »

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 »

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.

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.

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.

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.

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.