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I wonder if following thought experiment (which is most probably impossible to be put into practice) could have any implications concerning interpretations of QM.

Consider five parties A, B, C, D and E, lined up in that order and with no relevant relative motion. No pair of them have ever been in a common light cone (so we assume a universe that expanded quickly after the Big Bang). So they don't even know about each other, but they're all superb physicists with a huge optimism about the existence of the other parties.

At some point B and D decide to create many entangled photon pairs and send one photon of each pair to C, the other to A and E, respectively. Whenever C receives a photon from B and D, he performs a Bell measurement on them, entangling two photons on the way to A and E, respectively (entanglement swapping). A and E receive them and measure them in bases that are suitable for a Bell inequality test. At this point, B, C and D are in a common light cone, but A and E are still outside.

After enough measurements have been performed, A, C and E meet. C tells the others the outcomes of his Bell measurements. A and E throw everything out where their choices of measurement basis and C's results don't allow for a Bell inequality test (since they cannot perform the usual Bell rotation in entanglement swapping protocols after already having measured). Finally, they check if the remaining measurements violate a Bell inequality.

So what I'm basically trying to construct is an experiment where no past event could have an Einstein causal effect on the choice of measurement basis of both A and E. Would this perhaps rule out a local or causal form of superdeterminism?

If yes, maybe we should start sending out entangled photons into the universe, measure incoming photons or entangle them and hope for the best...

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# I Bell test where observers never were in a common light cone

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