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Grinkle

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Quote lifted from a thread in the cosmology forum.But quantum mechanics, as far as we know, has a property called unitarity: if I know the exact state of the system at time T, then I can, given enough computer power, calculate the precise state of the system at any other time, no matter what.

What does it mean to know the exact state of a QM system? QM predicts probabilities that particles will be in one of multiple states when the particles are observed, and when observed, not all properties of a particle are simultaneously knowable to an exact degree (eg position and momentum).

Does knowing the exact state mean I know the probability functions for each particle in a given system, or is it different than that?