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A Metaphysician Looks at the Everett Interpretation

John T. Hawthorne

Published Jun 24, 2010• 19 citations
Publisher

Abstract

Abstract This chapter compares the relation of emergent branching structure to the quantum state to the fundamental ontology with philosophically more familiar examples of emergence — of ordinary objects, in relation to classical atomism, and of mental properties or attributes, in relation to material bodies. It is argued that there remain ‘explanatory gaps’ in all such cases, and that these are likely to be particularly acute in the case of the Everett interpretation.

DOI
10.1093/acprof:oso/9780199560561.003.0006

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Abstract This chapter compares the relation of emergent branching structure to the quantum state to the fundamental ontology with philosophically more familiar examples of emergence — of ordinary objects, in relation to classical atomism, and of mental properties or attributes, in relation to material bodies. It is argued that there remain ‘explanatory gaps’ in all such cases, and that these are likely to be particularly acute in the case of the Everett interpretation.
Abstract This chapter argues for the view that the Everett (‘many worlds’) interpretation is nothing more or less than a literal reading of the formalism of unitary quantum mechanics. The central component of this story is decoherence: the process by which the evolution of the quantum state acquires a branching structure. The chapter gives a conceptual introduction to decoherence and its conceptual role, emphasizing the significance of its effects on the dynamics of degrees of freedom which are undergoing decoherence. It then argues that, although the branching structure generated by decoherence is ‘for all practical purposes’ in the sense that it is not part of the microscopic formalism of quantum theory, nonetheless it is a perfectly real, observer-independent feature of reality — it is emergent from that microscopic formalism in a way very familiar from many other parts of science.
According to the Everett interpretation of quantum mechanics, the apparent paradoxes of quantum theory require no change either in our philosophical attitude toward scientific theories or in the formalism of quantum mechanics itself. Rather, the unaltered theory can be understood as a literal description of physical reality. But the physical reality thus described is one of many noninteracting, approximately classical worlds. These worlds enter the theory not at the fundamental level—they are nowhere to be found in the basic formalism—but at a higher, emergent level; in this sense they are (ontologically speaking) on a par with the ontology of the special sciences.
This chapter considers some philosophical problems posed by the Everett interpretation: should someone expecting to undergo quantum-mechanical branching be uncertain about the result of that branching? And how are we to think about identity over time — for people, for objects, and for whole branches — in the face of continual, persistent branching? Using considerations from philosophy of language, the chapter argues for a conservative approach to these questions: properly understood, the Everett interpretation leaves these and other everyday concepts essentially untouched, even as it does violence to what philosophers have normally taken to be their metaphysical underpinnings.
The article continues the debate about whether classical “pure” metaphysics can explain scientific theories or science can generate its own metaphysical concepts. Two camps have emerged in modern analytic philosophy. The “metaphysician” camp defends the metaphysics of science by arguing that the fundamental con­cepts of classical metaphysics can be enriched, illustrated, or justified by exam­ples from scientific theories. The “philosophers of science” camp views this as yet another invasion of metaphysics into their scientific territory. They advocate a scientific or naturalized metaphysics that strictly corresponds to modern scien­tific theories. Since the concept of possible worlds is a key element of main­stream analytic metaphysics, proponents of the metaphysics of science find it natural to appeal to the many-worlds interpretation of quantum theory. In turn, proponents of naturalized metaphysics often use popular metaphysical systems to justify the many-worlds interpretation of quantum mechanics. I will argue that the interpretation originally proposed by H. Everett remains only one of the on­tologies of quantum theory, and attempts by philosophers of science to build a metaphysical construct on its basis encounter serious difficulties.
The aim of this chapter is to explain how the ‘many worlds’ of the Everett interpretation can emerge from the underlying physics of quantum theory, without being put in by hand at the microscopic level. The discussion is conceptually focussed: in the main the chapter is concerned with sketching the way in which higher-level ontology emerges, in general, from lower-level scientific theories. When this general framework is applied to quantum theory in particular, the chapter shows that it leads to a continual branching of physical reality at the emergent, macroscopic level.
There are two widely known interpretations of quantum theory: Bohr’s Copenhagen interpretation and Everett’s interpretation. The focus of Chapter 9 is to assess, within the context they provide, the interpretation-independent advances discussed in this book. We want to see whether the advances that include decoherence and quantum Darwinism fit these two established and widely known points of view. In fact, it is surprising that (with minor but significant adjustments) decoherence and quantum Darwinism fit very naturally, addressing questions that were recognized as open and important. We then discuss the existential interpretation. It can be seen as a continuation of the Copenhagen interpretation, with the Universe consisting of quantum and classical realms, but with classicality that is emergent, rather than preordained. It is also compatible with Everett’s interpretation, since quantum states and evolutions are all that is needed. However, unlike the Many Worlds interpretation (which regards the quantum state of the Universe as objectively existing, akin to a classical point in phase space or a classical electromagnetic field), the existential interpretation recognizes that quantum states combine information and existence—they are epiontic. The mix of existence and information they represent fits the relative states reading of Everett’s approach.