Objective Probability in Everettian Quantum Mechanics

David Wallace has given a decision-theoretic argument for the Born Rule in the context of Everettian quantum mechanics (EQM). This approach promises to resolve some long-standing problems with probability in EQM, but it has faced plenty of resistance. One kind of objection (the ‘incoherence problem’) charges that the requisite notion of decision-theoretic uncertainty is unavailable in the Everettian picture, so that the argument cannot gain any traction; another kind of objection grants the proof’s applicability and targets the premises. In this article I propose some novel principles connecting the physics of EQM with the metaphysics of modality, and argue that in the resulting framework the incoherence problem does not arise. These principles also help to justify one of the most controversial premises of Wallace’s argument, ‘branching indifference’. Absent any a priori reason to align the metaphysics with the physics in some other way, the proposed principles can be adopted on grounds of theoretical utility. The upshot is that Everettians can, after all, make clear sense of objective probability. 1 Introduction2 Setup3 Individualism versus Collectivism4 The Ingredients of Indexicalism5 Indexicalism and Incoherence5.1 The trivialization problem5.2 The uncertainty problem6 Indexicalism and Branching Indifference6.1 Introducing branching indifference6.2 The pragmatic defence of branching indifference6.3 The non-existence defence of branching indifference6.4 The indexicalist defence of branching indifference7 Conclusion
Keywords probability  everett interpretation  quantum mechanics  modal metaphysics
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DOI 10.1093/bjps/axs022
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References found in this work BETA
Simon Saunders & D. Wallace (2008). Branching and Uncertainty. British Journal for the Philosophy of Science 59 (3):293-305.
Paul Tappenden (2011). Evidence and Uncertainty in Everett's Multiverse. British Journal for the Philosophy of Science 62 (1):99-123.

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Citations of this work BETA
Alastair Wilson (forthcoming). The Quantum Doomsday Argument. British Journal for the Philosophy of Science:axv035.
Emily Adlam (2014). The Problem of Confirmation in the Everett Interpretation. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 47:21-32.

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