
Quantum paradox puts objectivity on shaky footing - bookofjoe
https://science.sciencemag.org/content/369/6506/889
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bookofjoe
>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 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](https://www.nature.com/articles/s41567-020-0990-x)

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bookofjoe
>Whereas some researchers believed resolving the paradox requires quantum
mechanics to break down for large systems such as human observers, some of the
new study's authors believe something just as fundamental is on thin ice:
objectivity. The puzzle could mean there is no such thing as an absolute fact,
one that is as true for me as it is for you.

