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Lessons from realistic physics for the metaphysics of quantum theory

  • S.I. The Metaphysics of Entanglement
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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.

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Notes

  1. For examples, see Albert (1996), Esfeld et al. (2017), and most of the papers in Ney and Albert (2013).

  2. In this paper I use “mechanics” (as in ‘classical mechanics’ or ‘quantum mechanics’) to refer to the general framework of classical or quantum theories. This accords with current usage in theoretical physics (as seen in the titles of the references by Arnol’d, Cohen-Tannoudji et al., Goldstein et al. and Sakurai in the bibliography, each of which is concerned at least in large part with the framework theory) but conflicts with an older usage (still often seen in philosophy of physics) where the framework is called “classical/quantum theory” or “classical/quantum physics”, and “mechanics” is reserved for particle mechanics and to some degree for other more-or-less “mechanical” systems, like rigid bodies, springs and perhaps fluids. (The difficulty of saying exactly where “mechanics” leaves off is one reason I’ve adopted the more modern convention.) Where I have in mind the mechanics of point particles, I say “classical/quantum particle mechanics” explicitly.

  3. Albeit with some subtleties in the last two cases, due to various aspects of gauge invariance (see, e.g., Matschull 1996).

  4. This disanalogy is largely absent if we formulate quantum mechanics in path-integral form, but in this paper I confine my attention to the more familiar Hilbert-space formalism.

  5. That is: whenever the state is not an eigenstate of the observable being measured.

  6. Readers familiar with the mathematics of quantum theory will recognise that linear sums of mixed states are not the same sort of thing as superpositions of pure states; further consideration of these subtleties lies outside the scope of this paper.

  7. See, for instance, pretty much all the papers in Ney and Albert (2013).

  8. Albert (1996), in his discussion of the ‘marvellous point’, flirts with but does not commit to the position.

  9. An anonymous referee makes another suggestion: that given that the ontology of particular classical theories is often relatively transparent, while the ontology of particular quantum theories is invariably obscure, one virtue of studying a quantum theory of (say) particles is to clarify the relation it holds to a classical theory of particles—and this can be pursued whether or not we are treating the theory as fundamental. Extant metaphysics of quantum theory mostly has not taken this route, but it seems worth exploring further.

  10. This is not to say that the standard model in any way rules out more disunified approaches to physics, like Nancy Cartwright’s (1983,1999)—only that prior to the standard model, there was no single physical theory that was even a plausible candidate for unifying the physics of the observed world.

  11. I should acknowledge that this position on quantum field theory, though standard in theoretical physics, is contested in philosophy of physics; see Wallace (2011, 2018b) and, for the opposing view, Fraser (2009, 2011).

  12. There are a variety of proposals both for relativistic hidden-variable theories (Colin 2003; Dürr et al. 2005; Colin and Struyve 2007) and for relativistic collapse theories (Tumulka 2006; Bedingham 2010) but to my knowledge no full working through of any such theories to cover realistic interacting, renormalised theories.

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  1. School of Philosophy, University of Southern California, Los Angeles, CA, USA

    David Wallace

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Wallace, D. Lessons from realistic physics for the metaphysics of quantum theory. Synthese 197, 4303–4318 (2020). https://doi.org/10.1007/s11229-018-1706-y

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