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Conditional probabilities and entropy in (minisuperspace) quantum cosmology

H E Kandrup

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This paper exploits an analogy with a time-independent Schrodinger equation to propose a concrete interpretation of a 'wavefunction of the universe' psi which solves the Wheeler-DeWitt equation for a minisuperspace quantum cosmology, and then uses this interpretation to define information theoretic notions of entropy. Given a wavefunction psi depending, in a configuration space representation, on some set of variables q0, . . ., qN, one is instructed to pick one of the qA, say q0 identical to a, as a 'time' and then view psi (Q; a) as an unnormalised conditional probability amplitude for finding the values Q identical to (q1, . . ., qN) at 'time' a. By identifying the 'natural' measure dQ on an a=constant hypersurface in minisuperspace, rho (Q, Q'; a)dQ varies as psi *(Q; a) psi (Q'; a)dQ is interpreted as representing a density matrix in the Q-representation; and, by viewing the Q as abstract operators for fixed a, rho is interpreted as an abstract density matrix. This interpretation admits no a priori notion of unitary evolution with respect to a 'translation' da, unitarity being viewed instead as a derived concept arising only in a semiclassical limit. The object then is to use rho to define reasonable measures of 'coarse-grained entropy' in accord with physical intuition which could serve as a diagnostic for an 'arrow of time'. Thus, e.g., by interpreting the diagonal components rho ( lambda i, lambda i; a) in some Oa-representation as conditional probabilities pi for measuring the eigenvalues lambda i of some operator Oa, one is led to consider an observable dependent entropy S0(a)=- Sigma ipi log pi. And similarly, at least when the a=constant hypersurfaces are flat, one can map psi into a 'Wigner function' fw and then use that fw to define a 'Wigner function entropy' Sw.


PACS

98.80.Qc Quantum cosmology

04.60.Kz Lower dimensional models; minisuperspace models

MSC

83F05 Cosmology

Subjects

Gravitation and cosmology

Astrophysics and astroparticles

Dates

Issue 6 (June 1988)



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