= Wigner's friend
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A thought experiment placing a friend's physical measurement record inside a larger isolated quantum laboratory. Conditioned on a record the friend has a definite outcome, while an external observer using universal unitary evolution can assign an entangled laboratory state. Its contrast with a collapsed mixture is an instance of the <quantum measurement problem>; it does not require giving consciousness a special dynamical law. <Eugene Wigner> used the observer-within-the-system construction to examine such tensions.
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