# Weekly Papers on Quantum Foundations (33)

What Represents Space-time? And What Follows for Substantivalism \emph{vs.} Relationalism and Gravitational Energy?. (arXiv:2208.05946v1 [physics.hist-ph])

Authors: J. Brian Pitts

The questions of what represents space-time in GR, the status of gravitational energy, the substantivalist-relationalist issue, and the (non)exceptional status of gravity are interrelated. If space-time has energy-momentum, then space-time is substantival. Two extant ways to avoid the substantivalist conclusion deny that the energy-bearing metric is part of space-time or deny that gravitational energy exists. Feynman linked doubts about gravitational energy to GR-exceptionalism; particle physics egalitarianism encourages realism about gravitational energy.

This essay proposes a third view, involving a particle physics-inspired non-perturbative split that characterizes space-time with a constant background _matrix_ (not a metric), avoiding the inference from gravitational energy to substantivalism: space-time is (M, eta), where eta=diag(-1,1,1,1) is a spatio-temporally constant numerical signature matrix (already used in GR with spinors). The gravitational potential, bearing any gravitational energy, is g_(munu)(x)-eta (up to field redefinitions), an _affine_ geometric object with a tensorial Lie derivative and a vanishing covariant derivative. This non-perturbative split permits strong fields, arbitrary coordinates, and arbitrary topology, and hence is pure GR by almost any standard. This razor-thin background, unlike more familiar ones, involves no extra gauge freedom and so lacks their obscurities and carpet lump-moving.

After a discussion of Curiel’s GR exceptionalist rejection of energy conservation, the two traditional objections to pseudotensors, coordinate dependence and nonuniqueness, are explored. Both objections are inconclusive and getting weaker. A literal interpretation of Noether’s theorem (infinitely many energies) largely answers Schroedinger’s false-negative coordinate dependence problem. Bauer’s nonuniqueness (false positive) objection has several answers.

Branching States as The Emergent Structure of a Quantum Universe. (arXiv:2208.05497v1 [quant-ph])

Quantum Darwinism builds on decoherence theory to explain the emergence of classical behavior within a quantum universe. We demonstrate that the differential geometric underpinnings of quantum mechanics provide a uniquely informative window into the structure of correlations needed to validate Quantum Darwinism. This leads us to two crucial insights about the emergence of classical phenomenology, centered around the nullity of quantum discord. First, we show that the so-called branching structure of the joint state of system and environment is the only one compatible with zero discord. Second, we prove that for small, but nonzero discord, the structure of the globally pure state is arbitrarily close to the branching form. These provide strong evidence that this class of branching states is the only one compatible with the emergence of classical phenomenology, as described in Quantum Darwinism.

Schr\”odinger’s cat meets Occam’s razor. (arXiv:0905.2723v3 [quant-ph] UPDATED)

We discuss V.P. Belavkin’s (2007) approach to the Schr\”odinger cat problem and show its close relation to ideas based on superselection and interaction with the environment developed by N.P. Landsman (1995). The purpose of the paper is to explain these ideas in the most simple possible context, namely: discrete time and separable Hilbert spaces, in order to make them accessible to those coming from the philosophy of science and not too happy with idiosyncratic notation and terminology and sophisticated mathematical tools. Conventional elementary mathematical descriptions of quantum mechanics take “measurement” to be a primitive concept. Paradoxes arise when we choose to consider smaller or larger systems as measurement devices in their own right, by making different and apparently arbitrary choices of location of the “Heisenberg cut”. Various quantum interpretations have different resolutions of the paradox. In Belavkin’s approach, the classical world around us does really exist, and it evolves stochastically and dynamically in time according to probability laws following from successive applications of the Born law. It is a collapse theory, and necessarily it is non-local. The quantum/classical distinction is determined by the arrow of time. The underlying unitary evolution of the wave-function of the universe enables the designation of a collection of beables which grows as time evolves, and which therefore can be assigned random, classical trajectories. In a slogan: the past is particles, the future is a wave. We, living in the now, are located on the cutting edge between past and future.

Is instability near a black hole key for “thermalization” of its horizon?. (arXiv:2101.04458v4 [gr-qc] UPDATED)

We put forward an attempt towards building a possible theoretical model to understand the observer dependent thermalization of black hole horizon. The near horizon Hamiltonian for a massless, chargeless particle is $xp$ type. This is unstable in nature and so the horizon can induce instability in a system. The particle in turn finds the horizon thermal when it interacts with it. We explicitly show this in the Schrodinger as well as in Heisenberg pictures by taking into account the time evolution of the system under this Hamiltonian. Hence we postulate that existing instability near the horizon can be one of the potential candidates for explaining the black hole thermalization.

Observation of a symmetry-protected topological phase in external magnetic fields. (arXiv:2208.05357v2 [quant-ph] UPDATED)

Topological phases have greatly improved our understanding of modern conception of phases of matter that go beyond the paradigm of symmetry breaking and are not described by local order parameters. Instead, characterization of topological phases requires the robust topological invariants, whereas measuring these global quantities presents an outstanding challenge for experiments, especially in interacting systems. Here we report the real-space observation of a symmetry-protected topological phase with interacting nuclear spins, and probe the interaction-induced transition between two topologically distinct phases, both of which are classified by many-body Chern numbers obtained from the dynamical response to the external magnetic fields. The resulting value of Chern number ($\bar{\mathcal{C}h} = 1.958 \pm 0.070$) demonstrates the robust ground state degeneracy, and also determines the number of nontrivial edge states. Our findings enable direct characterization of topological features of quantum many-body states through gradually decreasing the strength of the introduced external fields.

On Bell’s Everett (?) theory

Gao, Shan (2022) On Bell’s Everett (?) theory. Foundations of Physics. ISSN 0015-9018

No Quantum Threat to Relativity

Le Bihan, Soazig (2020) No Quantum Threat to Relativity. UNSPECIFIED.

MOND and Meta-Empirical Theory Assessment

De Baerdemaeker, Siska and Dawid, Richard (2022) MOND and Meta-Empirical Theory Assessment. [Preprint]

For Whom the Bell Inequality Really Tolls

Silberstein, Michael and Stuckey, W. M. and McDevitt, Timothy (2022) For Whom the Bell Inequality Really Tolls. [Preprint]

Why We Should Be Suspicious of Conspiracy Theories. A Novel Demarcation Problem

Boudry, Maarten (2021) Why We Should Be Suspicious of Conspiracy Theories. A Novel Demarcation Problem. [Preprint]

Microscopes go molecular

Nature Physics, Published online: 11 August 2022; doi:10.1038/s41567-022-01712-x

Statistical correlations between particles play a central role in the study of complex quantum systems. A new study introduces microscopic detection of ultracold molecules and demonstrates the measurement of two-particle correlations.

Thermodynamic Signatures of Genuinely Multipartite Entanglement

Author(s): Samgeeth Puliyil, Manik Banik, and Mir Alimuddin

The theory of bipartite entanglement shares profound similarities with thermodynamics. In this Letter we extend this connection to multipartite quantum systems where entanglement appears in different forms with genuine entanglement being the most exotic one. We propose thermodynamic quantities that …

[Phys. Rev. Lett. 129, 070601] Published Tue Aug 09, 2022

The story of the tablecloth: deriving “before” from atemporal notions

Saudek, Daniel (2022) The story of the tablecloth: deriving “before” from atemporal notions. [Preprint]