Abstract
In his 2013 Foundations of Physics paper Mathias Egg claims to show that my critical arguments toward the foundational significance of Leggett’s non-local theories are misguided. The main motivation is that my argument would connect too strongly the Leggett original motivation for introducing this new class of theories with the foundational significance of these theories per se. Egg basically aims to show that, although it can be conceded that the Leggett original motivation relies on a mistaken view of the original Bell theorem, the investigation on the Leggett theories does have a foundational meaning that can be disassociated from the view that Leggett himself has of of them. As a reply to Egg, I would like to argue here that, even if we assume to disentangle the Leggett view from the fate of the Leggett theories, there is still room to dispute the foundational significance of the Leggett ‘non-local realistic’ research program.
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Notes
The limitations I will refer to are analogous to several other limitations occurring in the now long sequence of no-go results for non-relativistic quantum mechanics: for a sample of related, recent work I refer to [11].
It should be noted, however, that it would seem highly plausible—in the case of both the abstract setting of the Leggett theories in [12] and the experimental test of the Leggett inequality of [9]—to motivate the realism assumption at the subsensemble level with the validity of realism also at the individual level. This point has been raised recently in an exchange between [13] and [3] that appears to be relevant here. According to Navascués, the Leggett theories in fact assume what he calls the realistic polarization principle, on the basis of which individual photons have a definite polarization state. Navascués shows then that, if something like the realistic polarization principle is assumed, then the statistics for the polarization measurements coincide necessarily with the correlations obtained when measuring separable states: in turn, this implies that the Leggett theories are in fact local realistic (since any quantum experiment verifying entanglement leads to a refutation of these theories). Branciard objects that assuming a realistic polarization principle is a matter of interpretation and in principle is not directly required by the strictly mathematical formulation of crypto-nonlocal theories (that there is no necessity, anyway, is something that is already acknowledged by Navascués in the first lines of the above quotation). However, it is Branciard himself ([3], p. 3) who stresses that the Navascués analysis shows how physically unreasonable crypto-nonlocality turns out to be when not supplemented with a physical interpretation along the lines highlighted by Navascués (and consistent with my 2008 paper).
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Laudisa, F. On Leggett Theories: A Reply. Found Phys 44, 296–304 (2014). https://doi.org/10.1007/s10701-014-9787-z
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DOI: https://doi.org/10.1007/s10701-014-9787-z