The paper tries to make some hay out of that the non-locality that show up already in Bell tests experiments is more severe in these entanglement experiments. But there is still no relativistic light cone causality destroying signal mediated here but the same type of correlations that still needs observers comparing observations to recognize them.
As one may suspect, it seems like quantum field theory has non-local states entanglement as an explicit feature of vacuum states [
https://en.wikipedia.org/wiki/Reeh–Schlieder_theorem ]. (String physicist Witten goes as far as to claim "it is a property of the algebra of observables and not just of the states" [
https://arxiv.org/abs/1803.04993 ].)
C.f. the idea of "locality" as used in the paper, which is not what we see. Relativistic light cone causality depend on locality for light cone signals, no more and no less, and that is what quantum field theory express (however pesky it is to tease out on a background of obscuring non-local "algebra" properties).
If you are interested in the general misappropriation of Wigner's Friend models, computer scientist Scott Aaronson is your friend [
https://www.scottaaronson.com/blog/?p=3975 ].
Quote:
It's hard to think when someone Hadamards your brain
"Unperformed measurements have no results." —Asher Peres
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I was sufficiently interested (or annoyed?) that I pulled an all-nighter working through the argument, then discussed it at lunch with Renner as well as John Preskill. I enjoyed figuring out exactly where I get off Frauchiger and Renner's train—since I do get off their train. While I found their paper thought-provoking, I reject the contention that there's any new problem with QM's logical consistency: for reasons I'll explain, I think there's only the same quantum weirdness that (to put it mildly) we've known about for quite some time.
In more detail, the paper makes a big deal about how the new argument rests on just three assumptions (briefly, QM works, measurements have definite outcomes, and the "transitivity of knowledge"); and how if you reject the argument, then you must reject at least one of the three assumptions; and how different interpretations (Copenhagen, Many-Worlds, Bohmian mechanics, etc.) make different choices about what to reject.
But I reject an assumption that Frauchiger and Renner never formalize. That assumption is, basically: "it makes sense to chain together statements that involve superposed agents measuring each other's brains in different incompatible bases, as if the statements still referred to a world where these measurements weren't being done."
I don't know if the new paper makes the same assumption, but I would guess it may do. For me it doesn't matter (so I didn't check) since there doesn't seem to be any problem with quantum field theory relativity, as expected.