This question already has an answer here:

At what point do quantum wave functions collapse? Let me give you two examples to make that question more clear:

  1. Let us think of the double slit experiment. We humans see the light at certain positions. That means the wave function has to collapse somewhere between having passed the slits and being registered by our brain. At which point does this collapse occur?

  2. Let us think of Schrodinger's famous cat. As far as I understood, the decay process of the radioactive particle can be described as a wave function. When we open the box, we see either a living or a dead cat. At which point does the wave function actually collapse?

I asked this question to many people, but the answers were not very satisfactory. The most common answer is that a measurement makes the wave functions collapse, but nobody could ever give me a clean definition of what such a measurement actually is. Others say that this is still not very well understood and an open problem. I would be happy about any comments, suggestions, or even pointers to literature.


marked as duplicate by user81619, Steeven, Hritik Narayan, Danu, Community Oct 11 '15 at 11:49

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.

  • $\begingroup$ thank you for that link. haven't found it myself. guess that answers everything... $\endgroup$ – Dennis Weyland Oct 11 '15 at 11:40
  • $\begingroup$ if only it did......:) regards Dennis $\endgroup$ – user81619 Oct 11 '15 at 11:42
  • $\begingroup$ after reading it i have to say it does not answer a lot, but the same would have happened here with my post as well, i guess. $\endgroup$ – Dennis Weyland Oct 11 '15 at 11:49
  • $\begingroup$ Here is what I would have said if the question were not closed: The wavefunction is a mathematical model for the probability distribution of many measurements of a set up with the exact boundary conditions. It is the probability that waves, i.e. has a sinusoidal shape, instead of a gaussian or other classical shape. One has to do many measurements of the cat situation, throw many electrons at the two slits etc, then the probability distribution appears. Each measurement adds a point to building up the wavefunction shape, and a probability can be given. Each measurement is a "collapse" $\endgroup$ – anna v Oct 11 '15 at 12:09
  • $\begingroup$ since no wave nature appears with one measurement/interaction. All measurements are interactions, some interactions are nested with others until a measurement is made, but an interaction picks up an instance of the probability distribution as calculated by the QM equations and boundary conditions. Same as throwing 5 at dice, one has "collapsed" the flat probability distribution to one point, 5. $\endgroup$ – anna v Oct 11 '15 at 12:12