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kdv
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In the information loss paradox, people say that in quantum mechanics , time evolution is unitary. They usually do not say anything about the measurement process of if they do, they briefly say that the measurement process does not violate unitarity either if one takes into account the measuring device . I am guessing they mean by that that the measuring process amounts to entangling the observed state with the measuring device. But I thought that this was controversial, that it assumes a many-world type of interpretation of QM. If I measure the spin of a particle, say, and I get a certain result, surely I cannot deduce the state of the particle before the measurement (other than being able to say that the particle has a nonzero component of the eigenstate corresponding to my measurement).
So what do they mean by saying that in QM, time evolution is *always* unitary??
So what do they mean by saying that in QM, time evolution is *always* unitary??