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The British Journal for the Philosophy of Science

The Quantum Doomsday Argument

Alastair Wilson

Published Aug 20, 2015• 17 citations

Abstract

If the most familiar overlapping (branching universe) interpretation of Everettian quantum mechanics (EQM) is correct, then each of us is constantly splitting into multiple people. This consequence gives rise to the quantum doomsday argument, which threatens to draw crippling epistemic consequences from EQM. However, a diverging (parallel universe) interpretation of EQM undermines the quantum doomsday argument completely. This appears to tell in favour of the diverging interpretation. But it is surprising that a metaphysical question that is apparently underdetermined by the physics should be settled by purely epistemological considerations; and I argue that the positive case for divergence based on the quantum doomsday effect is ultimately unsuccessful. I discuss how some influential treatments of Everettian confirmation handle the quantum doomsday puzzle, and suggest that it can most satisfyingly be resolved via a naturalistic approach to the metaphysics of modality. 1. Introduction2. The Doomsday Argument3. The Quantum Doomsday Argument4. Doomsday and Divergence5. Solutions to the Quantitative Problem6. Many Worlds and Modal Realism7. Conclusion

DOI: 10.1093/bjps/axv035

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If the most familiar overlapping (branching universe) interpretation of Everettian quantum mechanics (EQM) is correct, then each of us is constantly splitting into multiple people. This consequence gives rise to the quantum doomsday argument, which threatens to draw crippling epistemic consequences from EQM. However, a diverging (parallel universe) interpretation of EQM undermines the quantum doomsday argument completely. This appears to tell in favour of the diverging interpretation. But it is surprising that a metaphysical question that is apparently underdetermined by the physics should be settled by purely epistemological considerations; and I argue that the positive case for divergence based on the quantum doomsday effect is ultimately unsuccessful. I discuss how some influential treatments of Everettian confirmation handle the quantum doomsday puzzle, and suggest that it can most satisfyingly be resolved via a naturalistic approach to the metaphysics of modality. 1. Introduction2. The Doomsday Argument3. The Quantum Doomsday Argument4. Doomsday and Divergence5. Solutions to the Quantitative Problem6. Many Worlds and Modal Realism7. Conclusion

Abstract This chapter sets out the version of quantum mechanics—diverging EQM—that constitutes the physics component of quantum modal realism. After a potted history of Everettian quantum mechanics, and an introduction to recent decoherence-based versions of the approach, I sketch a distinctive metaphysical understanding of Everettian quantum mechanics that involves diverging ‘parallel worlds’ instead of overlapping ‘splitting worlds’. I argue that the choice between divergence and overlap is underdetermined by the physics itself; I then adapt an argument of David Lewis’s that supports divergence over overlap, emphasizing the need to make room for objective probabilities in the Everettian picture. The chapter concludes by rejecting some unpersuasive motivations for overlap.

It is sometimes said that a reason to prefer Everettian quantum mechanics is that it allows one to avoid the kind of spooky action at a distance that is a consequence of quantum entanglement according to other approaches. There is a straightforward argument to show EQM has this virtue, however some advocates argue that this requires we see branching as a local causal process. I explain this reasoning and then show why, given the way Everettians standardly conceive of worlds and branching as emergent phenomena, branching need not be viewed as a causal process for EQM to be relativistically local. Forthcoming in Local Quantum Mechanics, OUP 2026.

The question of whether Everettian quantum mechanics (EQM) justifies the existence of metaphysical indeterminacy has recently come to the fore. Metaphysical indeterminacy has been argued to emerge from three sources: coherent superpositions, the indefinite number of branches in the quantum multiverse and the nature of these branches. This paper reviews the evidence and concludes that those arguments don't rely on EQM alone and rest on metaphysical auxiliary assumptions that transcend the physics of EQM. We show how EQM can be ontologically interpreted without positing metaphysical indeterminacy by adopting a deflationary attitude towards branches. Two ways of developing the deflationary view are then proposed: one where branches are eliminated, and another where they are reduced to the universal quantum state.

Do the worlds described by the Many Worlds Interpretation (MWI)—the most prominent Everettian interpretation of quantum mechanics—genuinely ‘branch’? According to the overlap view, the relationship between branching worlds is described in terms of mereological overlap: worlds share parts with one another through some temporal intervals. In recent years, an alternative known as the divergence view has gained prominence. The divergence view eschews ‘splitting’ and instead holds that Everettian worlds are mereologically disjoint parallel worlds whose histories diverge. I introduce a dilemma for the divergence view to demonstrate that it is metaphysically unsuitable for the MWI.

Abstract In Everettian quantum mechanics, the universal quantum state is fundamental, non-contingent, and wholly determinate. By contrast, the parallel worlds of diverging EQM, and the contingency constituted by self-location amongst those worlds, are emergent and partly indeterminate. In particular, it is indeterminate both how many worlds there are, and what microscopic qualitative features those worlds have. This chapter discusses various ways to understand indeterminacy in the Everettian multiverse, and argues that the indeterminacies of EQM present no obstacle to the analytic ambitions of quantum modal realism. Everettians can understand quantum indeterminacy using models of indeterminacy that are familiar from the philosophical literature on vagueness.

This article sits at a point of intersection between the philosophy of physics and the metaphysics of modality. There are clear similarities between Everettian quantum mechanics and various modal metaphysical theories, but there have hitherto been few attempts at exploring how the two topics relate. In this article, I build on a series of recent papers by Wilson ([2011], [2012], [2013]), who argues that Everettian quantum mechanics’ connections with traditional modal metaphysics are vital in defending it against objections. I show that Wilson’s preferred version of Everettian quantum mechanics has two problems. First, it is unable to account for the contingency of various intuitively contingent modal claims. Second, it fails to yield intuitive truth values on modal claims about the number of branches in a given Everettian multiverse. Since modal claims about branch number are instrumental in decision-theoretic solutions to Everettian quantum mechanics’ problem(s) with probability, this second problem has wider dialectical implications. I suggest amendments to the underlying metaphysics that overcome these problems. The result is a more robust version of Everettian quantum mechanics.