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So many physicists like Carroll use a variation of Wigner's friend to argue for the many worlds of many worlds. I shall stick to the Frauchiger and Renner version of the Wigner's-friend scenario which argues that quantum theory cannot be used to model physical systems that are themselves agents who use quantum theory.

Roughly speaking, those assumptions are

  • (Q): Quantum theory is correct.
  • (C): Agent's predictions are information-theoretically consistent.
  • (S): A measurement yields only one single outcome.

I don't see how (S) has anything to do with the paradox. I can reformulate the whole thing with density matrices and I still get a difference of descriptions in terms of density matrices.

What am I missing in my understanding?

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