arXiv subjects

C. Kiefer

Publications and source records attributed to C. Kiefer.

27 records · Page 2Linked to original sources

arXiv1995

Decoherence of Black Holes by Hawking Radiation

We discuss in detail the semiclassical approximation for the CGHS model of two-dimensional dilatonic black holes. This is achieved by a formal expansion of the full Wheeler-DeWitt equation and the momentum constraint in powers of the gravitational constant. In highest order, the classical CGHS solution is recovered. The next order yields a functional Schrödinger equation for quantum fields propagating on this background. We show explicitly how the Hawking radiation is recovered from this equation. Although described by a pure quantum state, the expectation value of the number operator exhibits a Planckian distribution with respect to the Hawking temperature. We then show how this Hawking radiation can lead to the decoherence of black hole superpositions. The cases of a superposition of a black hole with a white hole, as well as of a black hole with no hole, are treated explicitly.

hep-th↗

arXiv1994

Arrow of time in a recollapsing quantum universe

We show that the Wheeler-DeWitt equation with a consistent boundary condition is only compatible with an arrow of time that formally reverses in a recollapsing universe. Consistency of these opposite arrows is facilitated by quantum effects in the region of the classical turning point. Since gravitational time dilation diverges at horizons, collapsing matter must then start re-expanding ``anticausally" (controlled by the reversed arrow) before horizons or singularities can form. We also discuss the meaning of the time-asymmetric expression used in the definition of ``consistent histories". We finally emphasize that there is no mass inflation nor any information loss paradox in this scenario.

gr-qc↗

arXiv1994

Symmetries, superselection rules, and decoherence

We discuss the applicability of the programme of decoherence -- emergence of approximate classical behaviour through interaction with the environment -- to cases where it was suggested that the presence of symmetries would lead to exact superselection rules. For this discussion it is useful to make a distinction between pure symmetries and redundancies, which results from an investigation into the constraint equations of the corresponding theories. We discuss, in particular, superpositions of states with different charges, as well as with different masses, and suggest how the corresponding interference terms, although they exist in principle, become inaccessible through decoherence.

gr-qc↗

arXiv1994

Wheeler-Dewitt Metric and the Attractivity of Gravity

We investigate the class of ultralocal metrics on the configuration space of canonical gravity. It is described by a parameter $α$, where $α=0.5$ corresponds to general relativity. For $α$ less than a critical value the signature is positive definite, while for all other values it is indefinite. We show that in the positive definite case gravity becomes repulsive. From the primordial helium abundance we find that $α$ must lie between $0.4$ and $0.55$.

gr-qc↗

arXiv1994

Consistency of Semiclassical Gravity

We discuss some subtleties which arise in the semiclassical approximation to quantum gravity. We show that integrability conditions prevent the existence of Tomonaga-Schwinger time functions on the space of three-metrics but admit them on superspace. The concept of semiclassical time is carefully examined. We point out that central charges in the matter sector spoil the consistency of the semiclassical approximation unless the full quantum theory of gravity and matter is anomaly-free. We finally discuss consequences of these considerations for quantum field theory in flat spacetime, but with arbitrary foliations.

gr-qc↗

arXiv1994

Semiclassical Gravity and the Problem of Time

I give a brief review of the recovery of semiclassical time from quantum gravity and discuss possible extrapolations of this concept to the full theory.

gr-qc↗

arXiv1993

The semiclassical approximation to quantum gravity

A detailed review is given of the semiclassical approximation to quantum gravity in the canonical framework. This includes in particular the derivation of the functional Schrödinger equation and a discussion of semiclassical time as well as the derivation of quantum gravitational correction terms to the Schrödinger equation. These terms are used to calculate energy shifts for fields in De~Sitter space and non-unitary contributions in black hole evaporation. Emphasis is also put on the relevance of decoherence and correlations in semiclassical gravity. The back reaction of non-gravitational quantum fields onto the semiclassical background and the emergence of a Berry connection on superspace is also discussed in this framework.

gr-qc↗

arXiv1993

Quantum cosmology and the emergence of a classical world

This paper gives an elementary introduction to some of the conceptual problems of quantum cosmology. Contents: 1. Why quantum cosmology? 2. Time in quantum gravity 3.Decoherence and the recovery of the Schrodinger equation 4. The direction of time

gr-qc↗

arXiv1993

Topology, Decoherence, and Semiclassical Gravity

We address the issue of recovering the time-dependent Schrödinger equation from quantum gravity in a natural way. To reach this aim it is necessary to understand the nonoccurrence of certain superpositions in quantum gravity. We explore various possible explanations and their relation. These are the delocalisation of interference terms through interaction with irrelevant degrees of freedom (decoherence), gravitational anomalies, and the possibility of $θ$ states. The discussion is carried out in both the geometrodynamical and connection representation of canonical quantum gravity.

gr-qc↗