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Cambridge University Press eBooks

Interpreting the wave function

Jean-Luc Lehners

Published Feb 2, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1017/9781009516815.009

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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.
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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.
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