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Oxford University Press eBooks

Everettian Statistical Inference

David Wallace

Published May 24, 2012• 0 citations
Publisher

Abstract

This chapter extends chapter 5's formal proof into a general discussion of how we should assess evidence — and, in particular, how we should test scientific theories — given the possibility that Everettian quantum mechanics is correct. The chapter proves — initially with informal arguments, but in due course with full mathematical rigor — that rational scientists should treat the branch weights assigned by quantum theory to quantum-mechanical branches exactly as if they were probabilities; as such, the chapter argues, probability is actually on firmer conceptual footing in quantum theory than in classical physics. After a brief digression to consider probability in approaches to quantum theory other than the Everett interpretation, the chapter concludes by placing the work in the broader context of recent work on Everett probability.

DOI
10.1093/acprof:oso/9780199546961.003.0008

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This chapter extends chapter 5's formal proof into a general discussion of how we should assess evidence — and, in particular, how we should test scientific theories — given the possibility that Everettian quantum mechanics is correct. The chapter proves — initially with informal arguments, but in due course with full mathematical rigor — that rational scientists should treat the branch weights assigned by quantum theory to quantum-mechanical branches exactly as if they were probabilities; as such, the chapter argues, probability is actually on firmer conceptual footing in quantum theory than in classical physics. After a brief digression to consider probability in approaches to quantum theory other than the Everett interpretation, the chapter concludes by placing the work in the broader context of recent work on Everett probability.
Abstract Much of the evidence for quantum mechanics is statistical in nature. Relative frequency data summarizing the results of repeated experiments is compared to probabilities calculated from the theory; close agreement between the observed relative frequencies and calculated probabilities is taken as evidence in favour of the theory. The Everett interpretation, if it is to be a candidate for serious consideration, must be capable of doing justice to this sort of reasoning. Since, on the Everett interpretation, all outcomes with non-zero amplitude are actualized on different branches, it is not obvious that sense can be made of ascribing probabilities to outcomes of experiments, and this poses a prima facie problem for statistical inference. It is incumbent on the Everettian either to make sense of ascribing probabilities to outcomes of experiments in the Everett interpretation, or to find a substitute on which the usual statistical analysis of experimental results continues to count as evidence for quantum mechanics, and, since it is the very evidence for quantum mechanics that is at stake, this must be done in a way that does not presuppose the correctness of Everettian quantum mechanics. This requires an account of theory confirmation that applies to branching-universe theories but does not presuppose the correctness of any such theory. This chapter supplies and defends such an account. The account has the consequence that statistical evidence can confirm a branching-universe theory such as Everettian quantum mechanics in the same way in which it can confirm a non-branching probabilistic theory.
This chapter begins my discussion of probability — generally held to be the most severe difficulty for the Everett interpretation. The chapter argues that since mathematically speaking probability has a natural place in the Everett interpretation, any ‘probability problem’ is philosophical in nature. Accordingly, the chapter's strategy is to review various different approaches to understanding probability — in particular, those based on long-run frequencies, on betting preferences, and on symmetries — and to argue that there are no problems with any of these approaches that are specific to Everettian quantum theory. The chapter concludes by arguing that, if probability is to be understood in terms of symmetry, it actually makes more sense in quantum than in classical physics. The chapter concludes with a sketch proof of the quantum probability rule on the basis of symmetry.
After an initial discussion of the general aims of scientific theories, the quantum measurement problem is presented. Non-Everettian solutions to the problem are briefly discussed; the aim of the discussion is to show that these solutions require significant change either to our philosophical approach to science or to quantum theory itself. In this context, the Everett interpretation is introduced and motivated as the result of simply applying our normal approach to scientific theories to the mathematical structure of quantum mechanics and following where it leads. The chapter ends with a introductory discussion of potential problems, and with references to subsequent chapters where they are discussed more fully.
How does it come about then, that great scientists such as Einstein, Schrödinger and De Broglie are nevertheless dissatisfied with the situation? Of course, all these objections are levelled not against the correctness of the formulae, but against their interpretation. […] The lesson to be learned from what I have told of the origin of quantum mechanics is that probable refinements of mathematical methods will not suffice to produce a satisfactory theory, but that somewhere in our doctrine is hidden a concept, unjustified by experience, which we must eliminate to open up the road. (Born [1954], pp. 8, 11) It is truly surprising how little difference all this makes. Most physicists use quantum mechanics every day in their working lives without needing to worry about the fundamental problem of its interpretation. (Weinberg [1992], p. 66) I endorse the view that it may be of no relevance to the acceptability of the Everett interpretation of quantum mechanics as a physical theory whether or not an informed observer can be uncertain about the outcome of a quantum measurement prior to its having occurred. However, I suggest that the very possibility of post-measurement, pre-observation uncertainty has an essential role to play in both confirmation theory and decision theory in a branching universe. This is supported by arguments which do not appeal to van Fraassen’s Reflection Principle. 1. Dendritic Unitary Evolution 2. The Born–Vaidman Rule 3. Greaves on Uncertainty 4. Retroactive Counterfactual Uncertainty 5. Odds and Ends
The main difficulty facing no-collapse theories of quantum mechanics in the Everettian tradition concerns the role of probability within a theory in which every possible outcome of a measurement actually occurs. The problem is two-fold: First, what do probability claims mean within such a theory? Second, what ensures that the probabilities attached to measurement outcomes match those of standard quantum mechanics? Deutsch has recently proposed a decision-theoretic solution to the second problem, according to which agents are rationally required to weight the outcomes of measurements according to the standard quantum-mechanical probability measure. I show that this argument admits counterexamples, and hence fails to establish the standard probability weighting as a rational requirement.
Abstract This chapter discusses what would need to be done to address the epistemic difficulties associated with the Everett interpretation, then considers a number of putative solutions to the Everettian probability problem and assesses whether these approaches succeed in solving the epistemic aspects of that problem. The discussion covers naïve branch counting and Saunders’ sophisticated branch counting; Vaidman’s ‘measure of existence’ approach; the Deutsch-Wallce decision-theoretic approach; and the Sebens-Carroll and Vaidman-McQueen self-locating uncertainty approaches.