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The laws of physics are invariant under CPT transformations reversing time, inverting space and flipping charges. Almost so. The collapse of the wave function is the odd man out. Can the time direction of the collapse of the wave function be reversed so that the collapse happened in the past?

The collapse presupposed the Schroedinger picture. Can a collapse occur in the Heisenberg picture, with the projection operator $P$ replaced with $UPU^{1}$?

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marked as duplicate by ACuriousMind, Brandon Enright, Kyle Kanos, John Rennie, JamalS Nov 8 '14 at 16:27

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Related: – Qmechanic Mar 23 '12 at 7:41
Doesn't seem to be an exact duplicate, but I'm not familiar with this stuff so im not sure. – Manishearth Mar 23 '12 at 9:21
It is an essential duplicate. The collapse of the wavefunction is not required by anything, it is a feature of certain interpretations. – ACuriousMind Nov 7 '14 at 23:42

1 Answer 1

The collapse is a non-unitary operation. It cannot be reversed (undone). Typically, we establish contact between the studied system and a macroscopic body (apparatus) that we are unable to describe by a wave-function. Also, we are not able to describe rigorously the process that follows this contact. The projection transformation done by such an apparatus on the wave-function is an irreversible truncation of the wave-function. Therefore the expression $UPU^{-1}$ is not correct because U is a unitary transformation, and this is not our case.

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