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 $UPU^{1}$?

  • $\begingroup$ Related: physics.stackexchange.com/q/10068/2451 $\endgroup$
    – Qmechanic
    Mar 23, 2012 at 7:41
  • $\begingroup$ Doesn't seem to be an exact duplicate, but I'm not familiar with this stuff so im not sure. $\endgroup$ Mar 23, 2012 at 9:21
  • $\begingroup$ It is an essential duplicate. The collapse of the wavefunction is not required by anything, it is a feature of certain interpretations. $\endgroup$
    – ACuriousMind
    Nov 7, 2014 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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