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In physics we have the CPT-theorem which guarantees time-reversal symmetry of dynamical evolution (although in some edge cases we will also have to reverse parity and charge).

It seems clear enough that the Schroedinger equation on its own determines past states as well as it determines future states. However, how does the so-called "Measurement Process" fit into this, where we place Psi discontinuously into an eigenstate? Do people ignore this when they say physics is reversible? Does CPT refer only to pure Schroedinger evolution? Or is "placing into an eigenstate" an idealization?

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A quantum system evolves one way between measurements and another way during measurements.

From the time a state is prepared and before a measurement is done, the system evolves deterministically, with time evolution given precisely by the time dependent Schroedinger equation. Then, if an observable is measured, the possible outcomes are probabilistic. Since what is relevant for the experiment - and therefore for our perception of the world - are the measurements of observables and not the states themselves, we say that Quantum Mechanics is not deterministic.

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    $\begingroup$ How is this reconciled with the CPT theorem? Does CPT relate only to Shroedinger (inter-measurement) evolution? $\endgroup$
    – DPatt
    Oct 15, 2017 at 0:03
  • $\begingroup$ The CPT symmetry is meant to be a symmetry of the laws of fundamental physics. In field theory it should be a symmetry of the lagrangian or the equations of motions of the theory. $\endgroup$
    – Diracology
    Oct 15, 2017 at 0:15

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