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Correction published on 13 April 2016, see Entropy 2016, 18(4), 136.

Open AccessReview
Entropy 2015, 17(12), 8174-8186;

Entropy and Quantum Gravity

Department of Mathematics, University of York, York YO10 5DD, UK
Academic Editor: Remo Garattini
Received: 1 August 2015 / Revised: 16 November 2015 / Accepted: 26 November 2015 / Published: 15 December 2015
(This article belongs to the Special Issue Entropy in Quantum Gravity and Quantum Cosmology)
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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 Anti de Sitter space (AdS)/conformal field theory (CFT) correspondence is a bijection between the boundary CFT and just the matter degrees of freedom of the bulk theory. View Full-Text
Keywords: entropy; second law of thermodynamics; entanglement; gravity; black hole; information loss; string theory entropy; second law of thermodynamics; entanglement; gravity; black hole; information loss; string theory

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Kay, B.S. Entropy and Quantum Gravity. Entropy 2015, 17, 8174-8186.

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