Entropy and quantum gravity

TL;DR

This paper reviews the author's 1998 matter-gravity entanglement hypothesis, proposing a novel, objective definition of entropy for closed systems that explains thermodynamics and black hole information loss without paradox.

Contribution

It introduces a new approach to entropy based on matter-gravity entanglement, offering a non-paradoxical view of black hole thermodynamics and a reinterpretation of black hole states in string theory.

Findings

Provides a new explanation for the Second Law of Thermodynamics.

Offers a non-paradoxical understanding of information loss in black holes.

Reinterprets black hole equilibrium states as pure, entangled states.

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.