- #76

kith

Science Advisor

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- 452

From Wigner's point of view, there hasn't been an intermediate measurement in the sense of QM and thus there's no information which was available at time [itex]t_1[/itex]. In Wigner's description, the state at [itex]t_1[/itex] is a superposition which includes "friend has observed the particle to be in [itex]A_1[/itex]" and "friend has observed the particle to be not in [itex]A_1[/itex]".So let's say unitary evolution has just forgotten the position of some particle/pointer ##x_i## at time ##t_2##, even though it has been measured a while ago. How do I calculate from the final wave function ##\psi(\mathbf{X},t_2)## a probability for some information that once was available at ##t_1##, but has been forgotten in the meantime?

Just to be clear that we are talking about the same situation: the friend is the observer of your scenario and Wigner is an additional external observer.