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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.

Qmechanic
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Dennis Weyland
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  • thank you for that link. haven't found it myself. guess that answers everything... – Dennis Weyland Oct 11 '15 at 11:40
  • if only it did......:) regards Dennis –  Oct 11 '15 at 11:42
  • 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. – Dennis Weyland Oct 11 '15 at 11:49
  • 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" – anna v Oct 11 '15 at 12:09
  • 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. – anna v Oct 11 '15 at 12:12
  • Rather than cats, the elephant in the room in this issue, personally speaking, and I really do appreciate it's not a physics topic, is human consciousness. Dennis, one book that gives, imo, a good summary of how we might view this, is Quantum Reality by Herbert. It's a bit out of date but if you havn't read it, maybe it's worth a look. This is a topic that starts more "robust" discussion, than anything else in my very limited knowledge of physics, so I will back away slowly now... –  Oct 11 '15 at 12:27
  • @count_to_10: I heard a lot of physicists talking about human consciousness. But for me this just makes the whole issue worse. To me it is not satisfactory for a physical theory to require a concept such as human consciousness, which is extremely fuzzy and might not even exist. In any case, I will try the book you mentioned. – Dennis Weyland Oct 12 '15 at 09:30
  • What I ultimately wanted to ask, was if there exist (or might exist) any quantum theories or interpretations which use the concept of an event or interaction. With that I mean, we use the description using wave functions as long as an event or interaction takes place. And taking place means that it really happened and leads to a collapse of the wave function. Does that make sense? Are there arguments for / against such a theory / interpretation? – Dennis Weyland Oct 12 '15 at 09:32
  • Two points, 1. I self study so I rely on professionals for deeper insights, (and they don't seem know as much as I hoped they might on this issue) and 2. imo, this is pretty much the same discussion that started when QM was initiated. Physics becomes philosophy and linguistics on this topic, for example when you say means that it really happened , I could ask you, what do you mean by real? Look at this slightly related question: http://physics.stackexchange.com/questions/211976/is-contemporary-physics-interpreted-in-an-instrumentalist-way. –  Oct 12 '15 at 10:29
  • Just to finish off, I am leaving this site for a few months but, if I were you,(unsolicited advice, no offence intended), I would keep refining my question until you get away from the discussion side, (or use the chat room) and phrase it in the concept Q and A format involved here, even if that takes a lot of small questions to build up an answer that is satisfactory to you, if there is indeed one. –  Oct 12 '15 at 10:35

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