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Many Worlds and Such

Jeffrey Alan Barrett

Published Dec 26, 2019• 0 citations
Publisher

Abstract

Abstract We consider a number of radically different ways that Everett’s pure wave mechanics has been understood. Each of these reconstructions aims to provide a stronger variety of empirical adequacy than Everett’s own formulation of the theory. Among the alternative formulations of quantum mechanics we consider are splitting worlds, decohering worlds, many minds, many threads, and many maps. Each of these differs in its metaphysical commitments and, hence, in how it explains determinate measurement records and probabilities. We focus, in particular, on the problem of accounting for the standard quantum probabilities. To this end, we consider the relationship between typicality and probability and contrast synchronic and forward-looking probabilities. We conclude with a brief discussion of epistemological, pragmatic, and information-theoretic formulations of quantum mechanics. A recurring issue in this chapter concerns what it should mean for a physical theory to be empirically adequate.

DOI
10.1093/oso/9780198844686.003.0010

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Abstract We consider a number of radically different ways that Everett’s pure wave mechanics has been understood. Each of these reconstructions aims to provide a stronger variety of empirical adequacy than Everett’s own formulation of the theory. Among the alternative formulations of quantum mechanics we consider are splitting worlds, decohering worlds, many minds, many threads, and many maps. Each of these differs in its metaphysical commitments and, hence, in how it explains determinate measurement records and probabilities. We focus, in particular, on the problem of accounting for the standard quantum probabilities. To this end, we consider the relationship between typicality and probability and contrast synchronic and forward-looking probabilities. We conclude with a brief discussion of epistemological, pragmatic, and information-theoretic formulations of quantum mechanics. A recurring issue in this chapter concerns what it should mean for a physical theory to be empirically adequate.
http://dx.doi.org/10.5007/1808-1711.2016v20n1p45Because of the conceptual difficulties it faces, quantum mechanics provides a salient example of how alternative metaphysical commitments may clarify our understanding of a physical theory and the explanations it provides. Here we will consider how postulating alternative quantum worlds in the context of Hugh Everett III’s pure wave mechanics may serve to explain determinate measurement records and the standard quantum statistics. We will focus on the properties of such worlds, then briefly consider other metaphysical options available for interpreting pure wave mechanics. These reflections will serve to illustrate both the nature and the limits of naturalized metaphysics.
Everett ( 1957a , b , 1973 ) relative-state formulation of quantum mechanics has often been taken to involve a metaphysical commitment to the existence of many splitting worlds each containing physical copies of observers and the objects they observe. While there was earlier talk of splitting worlds in connection with Everett, this is largely due to DeWitt’s (Phys Today 23:30–35, 1970 ) popular presentation of the theory. While the thought of splitting worlds or parallel universes has captured the popular imagination, Everett himself favored the language of elements, branches, or relative states in describing his theory. The result is that there is no mention of splitting worlds or parallel universes in any of Everett’s published work. Everett, however, did write of splitting observers and was willing to adopt the language of many worlds in conversation with people who were themselves using such language. While there is evidence that Everett was not entirely comfortable with talk of many worlds, it does not seem to have mattered much to him what language one used to describe pure wave mechanics. This was in part a result of Everett’s empirical understanding of the cognitive status of his theory.
Abstract Each formulation of quantum mechanics we consider in the book seeks to explain why we have determinate measurement records distributed according to the standard quantum statistics. The explanation we get relies on the metaphysical assumptions that underlie the particular theory. Metaphysics here does real explanatory work in accounting for our experience. While it is not at all clear what specific metaphysical commitments best explain our experience, one can distinguish between different types and degrees of empirical adequacy each formulation provides. Stronger notions of empirical adequacy might be characterized in terms of such things as the plausibility that one’s experience in fact supervenes on the sort of records a particular theory provides and the richness of the account of experience provided by the theory.
Abstract Everett thought of the quantum measurement problem as one of providing a consistent description of nested measurement. He proposed solving the measurement problem by simply supposing that all physical systems whatsoever always obey the linear dynamics and hence never collapse. Dropping the collapse dynamics immediately solves the measurement problem, but it introduces two new problems: explaining determinate measurement records and explaining quantum probabilities. In addition to these, we also consider the problem of empirical coherence in the context of pure wave mechanics. We then discuss how Everett himself understood determinate records and probabilities in his relative-state formulation of pure wave mechanics. What he ultimately provided was an argument that his formulation of quantum mechanics was consistent and empirically faithful. We will see why this is a relative weak standard by which to judge the empirical adequacy of a physical theory.
Abstract Accounts of quantum theory that bring to the fore the relationship between the system and the observer, or measuring device, include Everett’s ‘Many Worlds’ interpretation and Rovelli’s ‘Relational Quantum Mechanics’, both of which are considered in Chapter 9. The former understands the state of a system as relative to that of the observer and in Everett’s original formulation, the definite outcomes obtained through measurement were manifested via the observer’s memory sequences. The subjectivist aspect of this interpretation is made even more explicit in the so-called ‘Many Minds’ variant. Relational Quantum Mechanics likewise adopts a relational or perspectival approach to the states of the system under observation. In this chapter both approaches are considered as offering a way of aligning quantum theory with the correlationist feature of phenomenology. Both are then compared with QBism and the pros and cons of all three are considered in terms of possible avenues for developing a phenomenological form of quantum theory.
Abstract What does realism about the quantum state imply? What follows when quantum theory is applied without restriction, if need be, to the whole universe? These are the questions which are debated in this book. All the chapters start from the point of realism, and based on the need, or the aspiration, for a theory that unites micro- and macroworlds, at least in principle. But the further claim argued by some is that if you allow the Schrödinger equation unrestricted application, supposing the quantum state to be something physically real, then this universe is one of countlessly many others, constantly branching in time, all of which are real. The result is the many worlds theory, also known as the Everett interpretation of quantum mechanics. The contrary claim sees this picture of many worlds as in no sense inherent in quantum mechanics, even when the latter is allowed unrestricted scope and even given that the quantum state itself is something physically real. For this picture of branching worlds fails to make physical sense, let alone common sense, even on its own terms. The status of these worlds, what they are made of, is never adequately explained. Ordinary ideas about time and identity over time become hopelessly compromised. The concept of probability itself is brought into question. This picture of many branching worlds is inchoate, it is a vision, an error. There are realist alternatives to many worlds, some even that preserve the Schrödinger equation unchanged. Twenty chapters, accompanied by commentaries and discussions, examine these claims and counterclaims in depth. They focus first on the question of ontology, the existence of worlds, second on the interpretation of probability, and third on alternatives or additions to many worlds. The introduction offers a helpful guide to the arguments for the Everett interpretation, particularly as they have been formulated in the last two decades.