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