Virtual Seminars

Nick Huggett, with Tushar Menon and Fedele Lizzi
University of Illinois

Missing the Point in Noncommutative Geometry

Abstract: Noncommutative geometries generalize standard smooth geometries, parametrizing the noncommutativity of dimensions with a fundamental quantity with the dimensions of area. The question arises then of whether the concept of a region smaller than the scale makes sense in such a theory. We argue that it does not, in two interrelated ways. In the context of Connes’ spectral triple approach, we show that arbitrarily small regions are not definable in the formal sense. While in the scalar field Moyal-Weyl approach, we show that they cannot be given an operational definition. We conclude that points do not exist, and that continuous spacetime is an appearance, in such geometries.

Carlo Rovelli
Center for Theoretical Physics at Aix-Marseille University

Why can we influence the future?

Abstract: The two main components of the QISS community –quantum information and quantum gravity– have opposite views on the arrow of time. This is generally taken as foundational in the dominant instrumentalist approach of the fist; while is often considered to be only a contingent aspect of the macroworld in the second. I show how to reconcile the two perspectives. This requires two steps: a careful analysis of the arrow of implication; and an understanding of the physical source of the time orientation of the agent. A number of papers have recently addressed these issues offering a compelling solution to the apparent disagreement.

Sougato Bose
University College London

A Table-top Testing of the Quantum Nature of Gravity: Assumptions, Implications and Practicalities of a Proposal

Abstract: A lack of empirical evidence has lead to a debate on whether gravity is a quantum entity. Motivated by this, I will present a feasible idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. I will show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. A prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, is also provided and can be measured through simple spin correlations. Further, I clarify the assumptions underpinning the above proposal such as our reasonable definition of “classicality”, as well as the crucial aspect of the locality of physical interactions. The role of off-shell processes is also highlighted to clarify what the mediators actually are according to the standard theory of quantum gravity. How the experiment sits within relativistic quantum field theory is clarified. Lastly, a list of practical challenges are noted.

Eugenio Bianchi
Penn State University

Entanglement and the Architecture of Spacetime

Abstract: The quantum field vacuum is highly entangled, even in causally disconnected regions. In contrast, the state of a quantum geometry of space can be unentangled, resulting in an uncorrelated network of elementary quanta of space. In this talk I discuss how the architecture of spacetime emerges from entanglement between these elementary quanta. I will focus on loop quantum gravity, causal structures and the primordial universe.

Jeremy Butterfield and Henrique Gomes
University of Cambridge

On Reduction and Functionalism about Space and Time

Abstract: Various programmes and results in the philosophy/foundations of spacetime theories illustrate points about reduction and functionalism in general philosophy of science. I will focus on some programmes and results about how the physics of matter contributes to determining, or even determines, or even explains, chrono-geometry. I will say something about most of the following examples: the Helmholtz-Lie theorem on free mobility implying constant curvature; and in the philosophical literature, Robb (1914), and Mundy (1983). I also hope to mention from the physics literature: Barbour and Bertotti (1982), Hojman, Kuchar and Teitelboim (1976); Dull, Schuller et al. (2012, 2018); and Gomes & Shyam (2016: 1608.08236 = J. Math. Phys. 57, 112503).