Lessons from realistic physics for the metaphysics of quantum theory

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Lessons from realistic physics for the metaphysics of quantum theory David Wallace1

Received: 7 November 2016 / Accepted: 29 January 2018 © Springer Science+Business Media B.V., part of Springer Nature 2018

Abstract Quantum mechanics, and classical mechanics, are framework theories that incorporate many different concrete theories which in general cannot be arranged in a neat hierarchy, but discussion of ‘the ontology of quantum mechanics’ tends to proceed as if quantum mechanics were a single concrete theory, specifically the physics of nonrelativistically moving point particles interacting by long-range forces. I survey the problems this causes and make some suggestions for how a more physically realistic perspective ought to influence the metaphysics of quantum mechanics. Keywords Quantum mechanics · Quantum field theory · Classical mechanics

1 Introduction Recent years have seen a sharp increase in interest by metaphysicians in the ontology of quantum mechanics, our current best theory of physics. But this welcome development has an unwelcome feature: the bulk of this work has taken a particular example of a quantum theory (the theory of finitely many nonrelativistic particles interacting by long-distance forces), in a particular formulation (the position representation) and treated it as if it were quantum mechanics in general.1 This leads to problems: first, because it confuses quantum mechanics, which is essentially a framework theory covering a huge number of particular theories, with one particular theory falling under that framework, and thus falls victim to a sort of

1 For examples, see Albert (1996), Esfeld et al. (2017), and most of the papers in Ney and Albert

(2013).

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David Wallace [email protected] School of Philosophy, University of Southern California, Los Angeles, CA, USA

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category error; second, because the discussions of the ontology of quantum theory mostly seem to aim at ‘fundamental’ ontology, while those quantum theories which are more plausible candidates for ‘fundamental’ physics (specifically, the quantum field theories underlying modern particle physics) differ radically from nonrelativistic particle mechanics. In this paper, I try to provide a more realistic picture of what quantum theory looks like, in the hope of warding off these problems. I begin (Sects. 2–4) with classical mechanics as a sort of warm-up case, proceed to quantum mechanics (Sect. 5–6), and quantum field theory (Sect. 7). I also briefly consider modifications to quantum mechanics made to solve the quantum measurement problem (Sect. 8). The physics I describe in this article is standard textbook material, and I don’t attempt to give original references; readers can consult, e.g., Arnol’d (1989) or Goldstein et al. (2013) for classical mechanics, Cohen-Tannoudji et al. (1977), Sakurai (1994), or Weinberg (2013) for quantum mechanics, and Banks (2008), Duncan (2012), Peskin and Schroeder (1995), or Zee (2003) for quantum field theory.

2 N-particle classical particle mechanics One of the most im