Abstract
Bell’s theorem is sometimes taken to show that quantum mechanics undermines scientific realism. If so, this would be a striking empirical argument against realism. However, Maudlin has claimed that this is a mistake, since Bell’s theorem has precisely one conclusion—namely that quantum mechanics is non-local. I argue here that matters are more complicated than Maudlin acknowledges: quantum mechanics is not a unified theory, and what Bell’s theorem shows of it depends on which interpretation turns out to be tenable. I conclude that while the lesson of Bell’s theorem could be that quantum mechanics is non-local, it could equally be that measurements have multiple outcomes, or that effects can come before their causes, or even, as the anti-realist contends, that no description of the quantum world can be given.
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Notes
- 1.
Bohr’s position is subtle, and he wouldn’t qualify as anti-realist on every construal, but I think he would deny that quantum phenomena are explained via a description of the micro-world.
- 2.
Some versions of the underdetermination argument are also empirical, insofar as they appeal to actual underdetermination in the history of science rather than hypothetical underdetermination; indeed, quantum mechanics is arguably an excellent candidate for actual underdetermination (Barrett 2003, 1211). But again, this argument involves data about theories, not data about the world.
- 3.
This is a slight reworking of van Fraassen’s minimal formulation of scientific realism (1980, 8), designed to highlight the roles of description and explanation.
- 4.
Van Fraassen (1980, 23). Note that van Fraassen alludes to quantum mechanics here in his plea for limits on explanation.
- 5.
What he says is that an assignment of properties to a quantum mechanical system can only be made relative to a particular choice of measurements on the system, and hence no unique property ascription is possible (1935, 700). But it is hard to motivate this claim absent a proof like Bell’s.
- 6.
- 7.
In fact, this state takes the same form when the spins are expressed relative to any other choice of axis too.
- 8.
Note that if particle 1 has the spin properties (up, down, down) and particle 2 has the properties (down, up, up), then for measurements along different axes, the results agree 2/3 of the time. The same goes for all the other possible spin property assignments, except for the pair (up, up, up) and (down, down, down) for which the results never agree. So no assignment of spin properties to particles can produce agreement more than 2/3 of the time.
- 9.
I assume that the spin is measured by passing the particle through a Stern-Gerlach device.
- 10.
I’m not sure whether such an argument would really go through. If it is conceivable that every measurement outcome has its own sui generis physical explanation, then there might be no underlying causation, at least on a regularity view of causation. In which case the question of locality becomes moot.
- 11.
- 12.
In Davies and Brown (1986, 47).
- 13.
For a more detailed appraisal of this kind of theory, see Lewis (2006).
- 14.
Some of the potential problems for retrocausal theories are addressed in Price (1996).
- 15.
That is, each causal link is local, although the sum of a forwards-causal and a backwards-causal link can add up to instantaneous action at a distance. It is the former sense of locality that makes the theory compatible with special relativity.
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Lewis, P.J. (2019). Bell’s Theorem, Realism, and Locality. In: Cordero, A. (eds) Philosophers Look at Quantum Mechanics. Synthese Library, vol 406. Springer, Cham. https://doi.org/10.1007/978-3-030-15659-6_3
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