One world versus many: the inadequacy of Everettian accounts of evolution, probability, and scientific confirmation (0905.0624v3)

Abstract: There is a compelling intellectual case for exploring whether purely unitary quantum theory defines a sensible and scientifically adequate theory, as Everett originally proposed. Many different and incompatible attempts to define a coherent Everettian quantum theory have been made over the past fifty years. However, no known version of the theory (unadorned by extra ad hoc postulates) can account for the appearance of probabilities and explain why the theory it was meant to replace, Copenhagen quantum theory, appears to be confirmed, or more generally why our evolutionary history appears to be Born-rule typical. This article reviews some ingenious and interesting recent attempts in this direction by Wallace, Greaves, Myrvold and others, and explains why they don't work. An account of one-world randomness, which appears scientifically satisfactory, and has no many-worlds analogue, is proposed. A fundamental obstacle to confirming many-worlds theories is illustrated by considering some toy many-worlds models. These models show that branch weights can exist without having any role in either rational decision-making or theory confirmation, and also that the latter two roles are logically separate. Wallace's proposed decision theoretic axioms for rational agents in a multiverse and claimed derivation of the Born rule are examined. It is argued that Wallace's strategy of axiomatizing a mathematically precise decision theory within a fuzzy Everettian quasiclassical ontology is incoherent. Moreover, Wallace's axioms are not constitutive of rationality either in Everettian quantum theory or in theories in which branchings and branch weights are precisely defined. In both cases, there exist coherent rational strategies that violate some of the axioms.

Critical Analysis of "One World versus Many: The Inadequacy of Everettian Accounts of Evolution, Probability, and Scientific Confirmation" by Adrian Kent

In the paper "One world versus many: the inadequacy of Everettian accounts of evolution, probability, and scientific confirmation," Adrian Kent presents a comprehensive critique of Everettian or "many-worlds" interpretations of quantum mechanics. He questions whether these interpretations can be both scientifically adequate and conceptually coherent, particularly in how they handle the concepts of probability, evolution, and scientific confirmation.

Kent's paper systematically addresses the limitations of Everettian interpretations by examining key components central to their scientific validity. Here, of particular interest is his rigorous analysis of decision theory, the Born rule, and how they are construed within the many-worlds framework. Kent is critical of recent attempts by proponents like David Wallace and Hilary Greaves to axiomatize decision theory within this ontology, questioning their assumptions and arguments.

Key Arguments and Analysis

1. Indeterminate Ontology: Kent argues that Everettean interpretations lack a well-defined realist ontology. Unlike Copenhagen quantum mechanics, which provides a coherent account of a single world outcome, many-worlds interpretations posit multiple realities without a consistent mechanism for how these realities are generated or observed. The fuzziness in defining quasiclassical branches complicates any coherent decision-making or probabilistic inference.
2. Probability and Born Rule: A fundamental problem, according to Kent, lies in the handling of probability within Everettian frameworks. Simply put, the Born rule – a cornerstone of quantum mechanics that assigns probabilities to different outcomes – does not naturally emerge from many-worlds interpretations. Attempts by Everettian theorists to derive the Born rule from decision-theoretic axioms are, in Kent's view, flawed. These derivations rely on presumptive rationality axioms that themselves hinge on the very probabilistic reasoning they attempt to justify.
3. Scientific Confirmation: Kent also challenges the possibility of empirically confirming many-worlds theories. Without a clear mechanism for distinguishing realized branches from unrealized ones, these theories struggle to interpret empirical data in a scientifically rigorous way. Essentially, if every outcome occurs in some branch, traditional notions of prediction and falsifiability become meaningless.
4. Rational Decision Making: The paper explores rational decision-making strategies in a multiverse, suggesting that existing Everettian decision-theoretic approaches do not achieve their goal of establishing a unique rational strategy. Alternative strategies, not reliant on Born weights, are shown to be logically consistent, casting doubt on the uniqueness of the proposed Everettian strategies.
5. Apparent Absence of Empirical Distinction: Kent discusses the empirical indistinguishability between many-worlds interpretations and traditional one-world theories. This lack of distinction raises substantial challenges regarding the motivation to adopt such theories, particularly without the potential for experimental validation.

Implications and Speculation

The critique posited by Kent implies significant theoretical and practical implications for the Everettian programme. The ambiguity surrounding probabilities and confirmatory mechanisms raises a fundamental question regarding the utility and necessity of maintaining many-worlds accounts. Practically, Kent's analysis suggests that researchers should refocus efforts on one-world interpretations, which might simplify the quest for a realistic quantum ontology without the requisite complexities injected by unobservable parallel branches.

Looking forward, Kent's paper hints at a deeper, possibly inherent limitation of quantum mechanics as it's currently understood. This perspective motivates further examination of quantum mechanics foundations and inspires experimental searches for phenomena that could decisively indicate the failure or limits of quantum mechanics in its conventional or Everettian forms.

Kent's meticulous dissection of these issues contributes to the philosophical and scientific dialogue surrounding quantum theory's interpretation. His paper serves as both a critique and a call to refocus the theoretical discourse to favor clarity, empirical verifiability, and philosophical coherence.