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    Entropy and quantum gravity

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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 AdS/CFT correspondence is a bijection between the boundary CFT and just the matter degrees of freedom of the bulk theory.Comment: 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.

    Technical Materials for an Inteview-Survey of the Aging

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    Contains materials for an inteview-survey of aging persons and housing design
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