Compatibility of State Assignments – Niels Bohr Institute - University of Copenhagen

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Compatibility of State Assignments

If the quantum state of a system is not an objective property of that system, but an encapsulation of our knowledge of the system (as Heisenberg explicitly maintained) then different people with different degrees of knowledge about one and the same system, ought to assign different states to it.  This raises the question of whether there are any constraints on the states that different people can assign to one and the same system.  As far as I know the only person to explicitly raise and answer this question in the 20th century was Rudolf Peierls.

I shall review Peierls's answer, argue that he was wrong, argue that Peierls would have agreed with me that he was wrong, suggest what I believe is the right answer, and mention subsequent arguments that I am wrong.