Entropy and quantum gravity

Abstract

We give a review, in the style of an essay, of the author's 1998 matter-gravity entanglement hypothesis which, unlike the standard approach to entropy based on coarse-graining, offers a definition for the entropy of a closed system as a real and objective quantity. We explain how this approach offers an explanation for the Second Law of Thermodynamics in general and a non-paradoxical understanding of information loss during black hole formation and evaporation in particular. It also involves a radically different from usual description of black hole equilibrium states in which the total state of a black hole in a box together with its atmosphere is a pure state -- entangled in just such a way that the reduced state of the black hole and of its atmosphere are each separately approximately thermal. We also briefly recall some recent work of the author which involves a reworking of the string-theory understanding of black hole entropy consistent with this alternative description of black hole equilibrium states and point out that this is free from some unsatisfactory features of the usual string theory understanding. We also recall the author's recent arguments based on this alternative description which suggest that the AdS/CFT correspondence is a bijection between the boundary CFT and just the matter degrees of freedom of the bulk theory.

15 pages. Considerably enlarged. 3 figures and many references added. Also published in the recent special issue "Entropy in Quantum Gravity and Quantum Cosmology" (ed. Remo Garattini) of the online journal "Entropy". (Note: that journal version will soon be replaced with an updated version where some typesetting errors are corrected. This arXiv version is also free from those errors.)

References in corpus (8)

• What is the entropy of the universe?
• Expectation values, experimental predictions, events and entropy in quantum gravitationally decohered quantum mechanics
• Non-existence of isometry-invariant Hadamard states for a Kruskal black hole in a box and for massless fields on 1+1 Minkowski spacetime with a uniformly accelerating mirror
• Modern foundations for thermodynamics and the stringy limit of black hole equilibria
• More about the stringy limit of black hole equilibria
• On the origin of thermality
• The robustness of a many-body decoherence formula of Kay under changes in graininess and shape of the bodies
• Limitations of holography

Cited by in corpus (5)

• Generally covariant dynamical reduction models and the Hadamard condition
• The information recovery problem
• The matter-gravity entanglement hypothesis
• Remarks on matter-gravity entanglement, entropy, information loss and events
• Quantum electrostatics, Gauss's law, and a product picture for quantum electrodynamics; or, the temporal gauge revised