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For those who can't get past the paywall on the actual paper by Cavalcanti et al., the arxiv preprint is here: https://arxiv.org/abs/1907.05607
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A strong no-go theorem on the Wigner's friend paradox
Kok-Wei Bong, Aníbal Utreras-Alarcón, Farzad Ghafari
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Yeong-Cherng Liang, Nora Tischler, Eric G. Cavalcanti, Geoff J. Pryde, Howard M. Wiseman
2019 · arXiv
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From the abstract
Does quantum theory apply at all scales, including that of observers? New light on this fundamental question has recently been shed through a resurgence of interest in the long-standing Wigner's friend paradox. This is a thought experiment addressing the quantum measurement problem -- the difficulty of reconciling the (unitary, deterministic) evolution of isolated systems and the (non-unitary, probabilistic) state update after a measurement. Here, by building on a scenario with two separated but entangled friends introduced by Brukner, we prove that if quantum evolution is controllable on the scale of an observer, then one of 'No-Superdeterminism', 'Locality' or 'Absoluteness of Observed Events' -- that every observed event exists absolutely, not relatively -- must be false. We show that although the violation of Bell-type inequalities in such scenarios is not in general sufficient to demonstrate the contradiction between those three assumptions, new inequalities can be derived in a theory-independent manner, that are violated by quantum correlations. This is demonstrated in a proof-of-principle experiment where a photon's path is deemed an observer. We discuss how this new theorem places strictly stronger constraints on physical reality than Bell's theorem.
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Neutral mention · rule
There are other possible choices that reject superdeterminism but are still ok: https://arxiv.org/abs/1907.05607 -- you have to abandon either locality or the assumption that observed events exist absolutely Personally I think it's easiest to reject the "absoluteness" of events and subscribe to RQM or QBism. Superdeterminism is a little boring if ultimately plausible.
Neutral mention · rule
Science magazine [0] and Ars Technica [1] published readable overviews. The paper on the experiment regarding Wigner's friend paradox ("A strong no-go theorem on the Wigner’s friend paradox") was published in Nature Physics. [2] [3] [0] https://www.sciencemag.org/news/2020/08/quantum-paradox-poin... [1] https://arstechnica.com/science/2020/08/quantum-reality-is-e... [2] https://doi.org/10.1038/s41567-020-0990-x [3] https://www.nature.com/articles/s41567-020-0990-x
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Neutral mention · rule
The original Wigner's friend paradox: https://en.wikipedia.org/wiki/Wigner%27s_friend The paradox seems to rest on magical thinking about consciousness, and if one simply accepts that a conscious observer can be in a superposition like any other piece of matter, the paradox is resolved. And, from the article: For Wigner, this was an absurd conclusion. Instead, he believed that once the consciousness of an observer becomes involved, the entanglement would “collapse” to make the friend’s observation definite. But what if Wigner was wrong? Well, obviously Wigner is wrong, sorry for being flippant. In this paper (preprint here: https://arxiv.org/pdf/1907.05607.pdf), the…
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magical thinking about consciousness seems to be transmuted into "“Absoluteness of Observed Events” (i.e. that every observed event exists absolutely, not relatively)". The authors (it appears to me) mean by this that if an observer performs an experiment, while this observer is himself in a superposition, we have to regard the outcome of the experiment as absolute (i.e. not in a superposition) because it was made by a (conscious) observer. In my opinion, both "Absoluteness of Observed Events", and the equivalent from the layman's article, "When someone observes an event happening, it really happened", is a disingenuous and confusing way of talking about observers who are in a superposition. We have crossed over from "quantum mechanics is weird" to "these superficially intuitive but clearly false assumptions about quantum mechanics are weird". (by the way, I read the article here: https://theconversation.com/a-new-quantum-paradox-throws-the... because that site doesn't think I'm a robot and then redirects me to the homepage after filling out the captcha)
Neutral mention · rule
>A strong no-go theorem on the Wigner’s friend paradox Does quantum theory apply at all scales, including that of observers? New light on this fundamental question has recently been shed through a resurgence of interest in the long-standing Wigner’s friend paradox. This is a thought experiment addressing the quantum measurement problem—the difficulty of reconciling the (unitary, deterministic) evolution of isolated systems and the (non-unitary, probabilistic) state update after a measurement. Here, by building on a scenario with two separated but entangled friends introduced by Brukner, we prove that if quantum evolution is controllable on the scale of an observer, then…
Read the full comment
one of ‘No-Superdeterminism’, ‘Locality’ or ‘Absoluteness of Observed Events’—that every observed event exists absolutely, not relatively—must be false. We show that although the violation of Bell-type inequalities in such scenarios is not in general sufficient to demonstrate the contradiction between those three assumptions, new inequalities can be derived, in a theory-independent manner, that are violated by quantum correlations. This is demonstrated in a proof-of-principle experiment where a photon’s path is deemed an observer. We discuss how this new theorem places strictly stronger constraints on physical reality than Bell’s theorem. https://www.nature.com/articles/s41567-020-0990-x
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