- #1
iste
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- TL;DR
- Question of the plausibility of a local explanation of bell violating entanglement specifically in the Kocher-Commins experiment using quantum retrodiction.
These papers by Pegg et al. (doi: 10.1016/j.shpsb.2008.02.003 [section 4]; https://www.researchgate.net/publication/230928426_Retrodiction_in_quantum_optics [section 3.2]) seem to show that photon Bell correlations can be inferred using quantum theory in a manner that is compatible with locality by performing quantum retrodiction (i.e. inferring information about the past: e.g. https://doi.org/10.3390/sym13040586; more papers at end) where they evolve backward from Alice's measured outcome, and then forward again to Bob's. They only specifically model the Kocher-Commins experiment, where retrodicting about an atom-field state in the past tells you the state that Bob's photon was emitted as, and do not seem to have generalized beyond this specific example. Nonetheless, I think there is a general force to the argument that if quantum theory itself actually allows you to retrodict a definite state at the time of a locally-mediated correlating interaction, then non-local influences arguably seem redundant for producing the correct probabilities in entanglement scenarios similar to the one in Pegg et al.
If valid, then clearly the retrodiction from a measurement outcome assigns different states in the backward-in-time direction compared to what the corresponding regular forward-in-time description would start out with, and this looks retrocausal. But at the same time, the quantum retrodiction as described above is just another equivalent way of formulating the same empirical content of quantum theory, and related to regular quantum prediction by Bayes' theorem such that it would always be possible to derive or express backward-in-time probabilities entirely in terms of regular forward-in-time ones. One might then ask whether interpretations in which the wavefunction isn't real avoid any purported need for retrocausation (for instance, in Barandes' indivisible approach, the wavefunction is more or less reducible to a stochastic process description which then doesn't really leave any explicit indicators of retrocausality without importing additional ontological interpretation into the description). It might be worth noting that though the 2008 Pegg et al. paper uses explicit retrocausal language, the later review https://doi.org/10.3390/sym13040586 seems to be much more open to the non-reality of the wavefunction, and at the very least explicitly suggests that collapse should not be seen as a physical process.
I'm aware that this "Parisian zig-zag" explanation has been described by various others; but from what I have skimmed, others don't seem to be explicitly suggesting that it is actually inherent in quantum theory in the same way as these papers. I guess the most important criticisms of this would be whether their description is actually truly a valid way of using quantum theory to explain those correlations, and whether their type of description broadly-speaking can work for any entanglement experiment. Again, I'm inclined to think that if it is valid, it may not be strictly necessary that the explanation is retrocausal, at least with regard to the probabilities. But obviously maybe there are other arguments I'm not thinking of for retrocausality here.
More quantum retrodiction papers:
https://eprints.gla.ac.uk/334605/;
arXiv:1107.5849v4 [e.g. section IVC];
arXiv:2010.05734v2 [e.g. section 2];
https://doi.org/10.1098/rsta.2023.0338; https://strathprints.strath.ac.uk/5854/;
arXiv:quant-ph/0106139;
arXiv:quant-ph/0207086v1
If valid, then clearly the retrodiction from a measurement outcome assigns different states in the backward-in-time direction compared to what the corresponding regular forward-in-time description would start out with, and this looks retrocausal. But at the same time, the quantum retrodiction as described above is just another equivalent way of formulating the same empirical content of quantum theory, and related to regular quantum prediction by Bayes' theorem such that it would always be possible to derive or express backward-in-time probabilities entirely in terms of regular forward-in-time ones. One might then ask whether interpretations in which the wavefunction isn't real avoid any purported need for retrocausation (for instance, in Barandes' indivisible approach, the wavefunction is more or less reducible to a stochastic process description which then doesn't really leave any explicit indicators of retrocausality without importing additional ontological interpretation into the description). It might be worth noting that though the 2008 Pegg et al. paper uses explicit retrocausal language, the later review https://doi.org/10.3390/sym13040586 seems to be much more open to the non-reality of the wavefunction, and at the very least explicitly suggests that collapse should not be seen as a physical process.
I'm aware that this "Parisian zig-zag" explanation has been described by various others; but from what I have skimmed, others don't seem to be explicitly suggesting that it is actually inherent in quantum theory in the same way as these papers. I guess the most important criticisms of this would be whether their description is actually truly a valid way of using quantum theory to explain those correlations, and whether their type of description broadly-speaking can work for any entanglement experiment. Again, I'm inclined to think that if it is valid, it may not be strictly necessary that the explanation is retrocausal, at least with regard to the probabilities. But obviously maybe there are other arguments I'm not thinking of for retrocausality here.
More quantum retrodiction papers:
https://eprints.gla.ac.uk/334605/;
arXiv:1107.5849v4 [e.g. section IVC];
arXiv:2010.05734v2 [e.g. section 2];
https://doi.org/10.1098/rsta.2023.0338; https://strathprints.strath.ac.uk/5854/;
arXiv:quant-ph/0106139;
arXiv:quant-ph/0207086v1