1,483 research outputs found
Entropy and quantum gravity
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.
Application of linear hyperbolic PDE to linear quantum fields in curved spacetimes: especially black holes, time machines and a new semi-local vacuum concept
Several situations of physical importance may be modelled by linear quantum
fields propagating in fixed spacetime-dependent classical background fields.
For example, the quantum Dirac field in a strong and/or time-dependent external
electromagnetic field accounts for the creation of electron-positron pairs out
of the vacuum. Also, the theory of linear quantum fields propagating on a given
background curved spacetime is the appropriate framework for the derivation of
black-hole evaporation (Hawking effect) and for studying the question whether
or not it is possible in principle to manufacture a time-machine. It is a
well-established metatheorem that any question concerning such a linear quantum
field may be reduced to a definite question concerning the corresponding
classical field theory (i.e. linear hyperbolic PDE with non-constant
coefficients describing the background in question) -- albeit not necessarily a
question which would have arisen naturally in a purely classical context. The
focus in this talk will be on the covariant Klein-Gordon equation in a fixed
curved background, although we shall draw on analogies with other background
field problems and with the time-dependent harmonic oscillator. The aim is to
give a sketch-impression of the whole subject of Quantum Field Theory in Curved
Spacetime, focussing on work with which the author has been personally
involved, and also to mention some ideas and work-in-progress by the author and
collaborators towards a new "semi-local" vacuum construction for this subject.
A further aim is to introduce, and set into context, some recent advances in
our understanding of the general structure of quantum fields in curved
spacetimes which rely on classical results from microlocal analysis.Comment: Talk at Journe'es E'quations aux de'rive'es partielles, Nantes, 5-9
June, 2000. 19 pages, 6 eps figures. Style file edpa.cls supplied. Requires
latexsym, amssymb, eps
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