Abstract
In a recent paper on Foundations of Physics, Stephen Boughn reinforces a view that is more shared in the area of the foundations of quantum mechanics than it would deserve, a view according to which quantum mechanics does not require nonlocality of any kind and the common interpretation of Bell theorem as a nonlocality result is based on a misunderstanding. In the present paper I argue that this view is based on an incorrect reading of the presuppositions of the EPR argument and the Bell theorem and, as a consequence, is unfounded.
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Notes
For a recent review see Goldstein 2017. As is well-known, Bell himself disliked the ‘hidden-variables’ jargon and (rightly) considered the hidden-variables terminology seriously misleading. Moreover, needless to say, Bohmian mechanics is a non-local theory, consistently with Bell’s theorem.
A recent example out of many similar is the nonlocality results concerning the so-called hypergraph states, a family of multiparticle quantum states that generalizes the well-known concept of Greenberger-Horne-Zeilinger states and that turns out to be interesting resources for quantum metrology and measurement-based quantum computation. Gachechiladze et al. 2016 show that correlations in hypergraph states can be used to derive various types of nonlocality proofs: the term ‘realism’ occurs in the title but it plays no role whatsoever in the result.
Curiously enough, an interpretation which is light-years remote from the “local-realistic” one, namely Bohmian mechanics, assumes not only that physical systems have at least some pre-existing properties, but also that such properties – jointly with (suitable properties of) the measurement context – determine the measurement outcomes. Clearly Bohmian mechanics does not collapse into a ‘local-realistic’ interpretation because it is non-local by construction and takes the properties of the physical system to strongly depend on measurement interactions: but a symptom of how confused is the ‘local-realistic’ interpretation is just the circumstance that those who endorse it wish to free quantum theory from ‘realism’ without realizing how close some of their assumptions may be to a thoroughly realistic interpretation!
The story of this misunderstanding is long: see for instance the recollection in Laudisa 2008, pp. 1113–1116.
In the same sense the term Classicality was also adopted by Werner in his critical comment of the Maudlin contribution to the 2014 issue of Journal of Physics A dedicated to 50 years since the Bell theorem (see Maudlin 2014; Werner 2014): this exchange is also illuminating on the issues at stake when discussing the general meaning of Bell’s theorem.
The details can be found in Laudisa 2008, pp. 1118–1122.
In order to make the this intuitive idea more formally precise, we might employ the notion of quantum-mechanical non-selective measurement. I am grateful to the referee for pressing this clarification.
Finally, as far as the PI/OI distinction is concerned, we should recall that standard quantum mechanics – although violating OI – does satisfy PI (Ghirardi et al. 1980). On the other hand, Bohmian mechanics displays a reverse behaviour with respect to standard quantum mechanics concerning the distinction PI/OI. PI is clearly violated in principle, due to the very construction of the theory, whereas OI is preserved, since in the deterministic framework of Bohmian mechanics the outcomes are fixed and cannot depend on each other. As is well-known, the role of this distinction within standard quantum mechanics provided in the past some ground for the so-called peaceful coexistence thesis, recalled above (Shimony 1984). According to this thesis, quantum-mechanical non-locality would not be so harmful: the outcomes are somehow non-locally affecting each other and this seems to threaten the prescriptions of special relativity, but such outomes are uncontrollable and thus we cannot exploit them to produce any robust action-at-a-distance. The effectiveness of the PI/OI distinction in carrying the burden of such an ambitious coexistence has been often questioned: see Maudlin 1994, 2011 3 pp. 85–90, for a critical analysis.
See Laudisa and Rovelli 2013 for a review of this interpretation.
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Laudisa, F. Stop making sense of Bell’s theorem and nonlocality?. Euro Jnl Phil Sci 8, 293–306 (2018). https://doi.org/10.1007/s13194-017-0187-z
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DOI: https://doi.org/10.1007/s13194-017-0187-z