Decoherence in the density matrix describing quantum three-geometries and the emergence of classical spacetime
Abstract
We construct the quantum gravitational density matrix ρ(gαβ,g'αβ) for compact three-geometries by integrating out a set of unobserved matter degrees of freedom from a solution to the Wheeler-DeWitt equation Ψ[gαβ,qk(matter)]. In the adiabatic approximation, ρ can be expressed as exp(-l2) where l2(gαβ,g'αβ) is a specific ``distance'' measure in the space of three-geometries. This measure depends on the volumes of the three-geometries and the eigenvalues of the Laplacian constructed from the three-metrics. The three-geometries which are ``close together'' (l2<<1) interfere quantum mechanically; those which are ``far apart'' (l2>>1) are suppressed exponentially and hence contribute decoherently to ρ. Such a suppression of ``off-diagonal'' elements in the density matrix signals classical behavior of the system. In particular, three-geometries which have the same intrinsic metric but differ in size contribute decoherently to the density matrix. This analysis provides a possible interpretation for the semiclassical limit of the wave function of the Universe.
- Publication:
-
Physical Review D
- Pub Date:
- May 1989
- DOI:
- 10.1103/PhysRevD.39.2924
- Bibcode:
- 1989PhRvD..39.2924P
- Keywords:
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- 04.60.+n;
- 03.65.Bz;
- 12.25.+e