Dulani, Saakshi and Ney, Alyssa (2025) Is the Universe Fundamentally a Density Matrix? British Journal for the Philosophy of Science. ISSN 1464-3537
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Abstract
This article examines the case for preferring density matrix realism over wave function realism, as an approach to the fundamental ontology of our world. To date, there are two arguments that have been used to motivate density matrix realism. One is that we get a simpler and more appealing metaphysics if we move from wave function realism to density matrix realism. The second is that density matrices are more general than wave functions. In both cases, a key purported advantage is that density matrix realists can allow that the universe could be in a mixed state. To be convincing, however, these arguments can’t simply assume that our universe could be in a mixed state, since standard definitions of mixedness seem to preclude such a possibility. Thus, to make density matrix realism plausible, one would need to appeal to some scientifically backed argument that standard definitions of mixedness need revision, that what seemed to be perplexing before represents a genuine way that our world could fundamentally be. Here, Hawking’s model of black hole evaporation seems poised to come to the rescue, and so, we develop a novel argument to this effect. Although we find this way of arguing for density matrix realism to be intriguing and worthy of investigation, we show that there is substantial reason to be cautious. Most physicists, including Hawking, believe that this model will be superseded by a more fundamental theory of quantum gravity. Despite a lack of agreement on the details of such a theory, the guiding principles that do enjoy some consensus point away from an interpretation of black hole evaporation in which the universe as a whole evolves into an (objectively) mixed state. We conclude that there do not seem to be compelling motivations coming from fundamental physics to favour a metaphysics in which the universe is fundamentally a density matrix rather than a wave function.
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| Item Type: | Published Article or Volume | |||||||||
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| Keywords: | quantum metaphysics, wave function realism, density matrix realism, black hole information | |||||||||
| Subjects: | Specific Sciences > Physics > Quantum Gravity Specific Sciences > Physics > Quantum Mechanics Specific Sciences > Physics > Statistical Mechanics/Thermodynamics |
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| Depositing User: | Dr. Saakshi Dulani | |||||||||
| Date Deposited: | 31 May 2026 12:44 | |||||||||
| Last Modified: | 31 May 2026 12:44 | |||||||||
| Item ID: | 29820 | |||||||||
| Journal or Publication Title: | British Journal for the Philosophy of Science | |||||||||
| Official URL: | https://www.journals.uchicago.edu/doi/10.1086/7377... | |||||||||
| DOI or Unique Handle: | 10.1086/737744 | |||||||||
| Subjects: | Specific Sciences > Physics > Quantum Gravity Specific Sciences > Physics > Quantum Mechanics Specific Sciences > Physics > Statistical Mechanics/Thermodynamics |
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| Date: | 2025 | |||||||||
| ISSN: | 1464-3537 | |||||||||
| URI: | https://philsci-archive.pitt.edu/id/eprint/29820 |
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