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The Wave Function of a Godless Universe

Quentin Smith

Published May 4, 1995• 6 citations
Publisher

Abstract

Abstract This chapter discusses the wave function of a Godless universe. It analyses the concept of nothingness, the interpretation of quantum cosmology, and Stephen Hawking's quasi-instrumentalist interpretation of quantum mechanics. It comments on the concept of imaginary time in Hawking's quantum cosmology and his physical interpretations of the exponential part of the wave function, and suggests that Hawking's quantum cosmology is rationally preferable to theism. It could be argued that his cosmology has greater predictive power since it predicts that it is highly probable there is a universe with properties we observe and that the universe is near the critical density.

DOI
10.1093/acprof:oso/9780198263838.003.0012

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Abstract This chapter discusses the wave function of a Godless universe. It analyses the concept of nothingness, the interpretation of quantum cosmology, and Stephen Hawking's quasi-instrumentalist interpretation of quantum mechanics. It comments on the concept of imaginary time in Hawking's quantum cosmology and his physical interpretations of the exponential part of the wave function, and suggests that Hawking's quantum cosmology is rationally preferable to theism. It could be argued that his cosmology has greater predictive power since it predicts that it is highly probable there is a universe with properties we observe and that the universe is near the critical density.
A probabilistic interpretation of the wave function of the Universe is proposed in which ``probability'' and ``unitarity'' are inherently approximate concepts. Their validity is limited by the accuracy of the semiclassical approximation for the entire Universe. This approach (which is a logical extension of DeWitt's) defines a positive-semidefinite probability distribution and satisfies the correspondence principle with classical cosmology and with quantum mechanics. In particular, one recovers the standard probabilistic interpretation of the wave function for a small subsystem of the Universe.
This chapter discusses how a classical universe arises out of the quantum wave function of the universe. The process of decoherence is described, first in general and then applied to cosmology. The classicalization of the background spacetime (and the associated reduction in interference between saddle points) as well as the classicalization of long-wavelength perturbation modes is discussed, by studying an example of interactions between background and fluctuations, as well as interaction between perturbation modes of different wavelengths. Comments on the interpretation of the wave function are included.
We consider the problem of extracting physical predictions from the wave function of the universe in quantum cosmological models. We state the features of quantum cosmology an interpretational scheme should confront. We discuss the Everett interpretation, and extensions of it, and their application to quantum cosmology. We review the steps that are normally taken in the process of extracting predictions from solutions to the Wheeler-DeWitt equation for quantum cosmological models. Some difficulties and their possible resolution are discussed. We conclude that the usual wave function-based approach admits at best a rather heuristic interpretation, although it may in the future be justified by appeal to the decoherent histories approach.
A non-probabilistic interpretation of quantum mechanics asserts that we get a prediction only when a wave function has a peak. Taking this interpretation seriously, we discuss how to find a peak in the wave function of the universe, by using some minisuperspace models with homogeneous degrees of freedom and also a model with cosmological perturbations. Then we show how to recover our classical picture of the universe from the quantum theory, and comment on the physical meaning of the backreaction equation.