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How does a wave function collapse?

  1. May 20, 2010 #1
    Hi. I've asked the question many times (as I'm sure many others have) why does the particle behave differently once it has been observed? Does that not mean it knows it has been observed? How does it know?
    The only answer I get is: "observing destroys the wave function" , but that doesn't really answer anything, it just rephrases the question. I'm probably asking the unanswerable here, but seriously, how does a lifeless object know it is being watched?
  2. jcsd
  3. May 20, 2010 #2
    Collapse has been largely replaced by Decoherence.
  4. May 21, 2010 #3


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    This is a problem they way you describe only if you think that the wavefunction is an intrinsic property of the system you observe.

    This sure is interpretation dependent, but if you IMHO more rightly think of the wavefunction as the observer information of the system, then it's pretty obvious what the collapse is. It's simply an information update of part of the the observers state that encodes the information of the subsystem we study.

    In that view, though the only another "mystery" is that then the wavefunction of say and atom, seems to be "observer dependent" and thus somewhat subjective.

    But this is not a problem, because it just reflects that the action of each observer towards this atom can in general be different, but as these observers keep interacting, an agreement will quickly be establised, making the views of all observers "consistent" as there exists transformations between them at equilibrium.

    We already konw from SR and semiclassical GR+QM that the wavefcuntion IS observer dependent. It doesn't make sense to think of the wavefuction as a realistic objective property of the system we study, it depends on both the observer and the system in study.

    So the simple dechoerence view, that you consider the observer + subsystem we study as the new system' is IMO somewhat silly and is missing the entire point of observations. In the extreme case the "observer" is like a birds view observer, that has to observer the entire universe, and that simply doesn't make any reasonbble sense to me. It's a mathematical picture that lacks value and match of anything inferrable.

  5. May 21, 2010 #4


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    This is another angle of the issue, and the particle you study, can in some interpretations also qualifies as an "observer", which then of course observer it's environment, including the measurment apparatous :) So it's not really strange at all that when there is an interaction between apparatous and particle, also the particles internal state (think it's "cosomological wavefunction for the entire environment") also is changing. Therefor the particles action also is updated as beeing a function on this state.

    The only way to "observe" something is to "interact" and interactions is something mutual, so it's not possible for A and B to interact (make observations on each other) without if ever so slightly "perturbing" each other.

  6. May 21, 2010 #5


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    What I write about is as per my own "interpretation" which doesn't quite fit into any big camps, but it's a kind of intrinsic relational information interpreation in the context of evolving law. It's very very NON-realist, if you compare to some other peopl on here, where several are subscribing to different flavours of realist or structural-realist interpretations.

  7. May 21, 2010 #6
    There are questions, which simply don't have good answers. Your question is one of them.

    You should keep in mind that quantum mechanics with its wave functions and "collapses" is not a description of reality, but simply a calculational tool. Wavefunction has the same (distant) relationship to particle measurements as the Fourier series relates to drumbeats.

    The observable fact is that particles shot in the double-slit experiment always hit different places on the screen. The behavior of particles is apprently random. However, nobody has a clue where this randomness comes from. We simply cannot explain this randomness or suggest some "mechanistic" model for it. (By the way, I think that various "interpretations" of quantum mechanics do not offer any reasonable explanation.) The best we can do is to calculate the probabilities. Quantum mechanics is simly a mathematical tool for such calculations. Wavefunctions and their collapse is just a part of this mathematical tool. It would be ridiculous to think that some mysterious substance called "wavefunction" really propagates through the slits and collapses when it touches the screen, or that particles "know" when they are being watched.

  8. May 21, 2010 #7


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    It's IMO a common skewed picture to associate "observer" and "knowledge" with human qualities, and then the obvious questions is how the heck can an atom "think" or be consciously aware of thinks on par with a human observer.

    It can't of course, but that picture makes no sense. I think most people who take take informational view seriously, say something along the lines that

    "observation" is simply an informatin exchange or detection of an event, it has nothing to do with conscious observers.

    "knowledge" rather jsut referst to the "information" about the the system under observation, that is CODED physically in some kind of mictrostructure can simple can store information. Any microstructure that codes state can be used as a "memory device".

    So then your questions is easier to answer; how can an atom "know" things - I'd say the mictrostructure that defines the atom, is what codes it's knowledge about it's own environment. This is a physical code, not something that has anthing to do with human brains.

    Now, clearly also this state of the atom, is easilty disturbed. I think the atom knows it's beeing observed simply due to the feedback or backreaction it gets from it's own environment. note that any preparation and slits etc, are exactly what constitutes the nearast environment of the atom. I do not find it particularly strange that this is something the atom senses physically. After all "preparations of an experiment" is a very controlled thing, where there is a good extent of equilibration involved.

  9. Apr 19, 2011 #8
    That is allright. But IMO that our thinking ability and conciousness is equally as mysterious? We don't know for sure what the heck consciouness is?
  10. Apr 19, 2011 #9
    In this connection, I want to know what happens when the particle is not being observed although light is shone at it the same way as if it were being observed. Does the wave function collapse take place in such a case?
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