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What actually triggers the collapse of the wave function ?

  1. Jun 26, 2015 #1
    I am wondering if there are some views on this. One of the key mysteries of the double slit experiment boils down to when the observation mechanism is placed by one of the slits - and switched on it triggers the change from wave-like outcomes to particle-like outcomes. Is it the observation or the interference of the interaction that causes the collapse of the wave function ? I am sure one could configure the experiment to be such that the observation machine is switched on, "looks" at the slit, but that its result is made unobservable. What happens ?
     
  2. jcsd
  3. Jun 26, 2015 #2
    The answer is: the measurement problem has not been solved. We don't know where collapse of the wave function occurs; or whether we modify QM to allow for collapse eg. by gravity.

    The answer to your experimental question: we would see two hidden interference patterns on the back-screen. I say hidden, because if you conduct quantum eraser of which way information you correlate the data with the points and two interference patterns emerge. The combination of those interference patterns looks like the back-screen you would get if the system went through one slit or the other.
     
  4. Jun 26, 2015 #3

    atyy

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    If the result is made unobservable, then there is only apparent collapse.

    Collapse occurs when one observes the result and retains the result.
     
  5. Jun 26, 2015 #4

    ShayanJ

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    The current view is that its not caused by observation, but by interaction with the environment. Also this is not an abrupt process, but a continuous one, although it happens very very fast.
    For example in the double slit experiment, it is observed that in the vacuum, the result is a interference pattern. As they increased the density of air, the interference pattern gradually disappeared.
    But its not just scattering from air molecules. Even photons bouncing off the objects can cause decoherence. And even in the darkest room you can find, the thermal radiation from the walls and also the CMB is enough to induce decoherence in a very efficient way.
    I suggest you read this book, of course if you have a good knowledge of QM.
    At the end, I should mention that it doesn't solve the measurement problem completely, but only some parts of it.
     
  6. Jun 27, 2015 #5

    atyy

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    The modern point of view is the same as the ancient point of view: collapse occurs when an observation is made and the result is known. Decoherence only causes apparent collapse.
     
  7. Jun 27, 2015 #6

    ShayanJ

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    Yes, I wasn't careful. But it doesn't mean the two viewpoints are the same. In the "ancient" point of view, the collapse is a blackbox. The "modern" point of view opens up the blackbox and explains some parts of it but encounters a smaller blackbox(the problem of outcomes).
     
  8. Jun 27, 2015 #7

    atyy

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    Yes, people say that and I'm not sure whether I agree. But what is nice about the modern point of view for the ancient point of view is that decoherence is implicit in the ancient point of view, even if the ancients perhaps didn't understand it so well (though I wouldn't put it past von Neumann to have seen it and Bohm sketches it out for Bohmian Mechanics). Decoherence ensures the consistency of the ancient point of view - if we measure and throw away the results, then we can just have decoherence and apparent collapse - so collapse is really dependent on keeping the information from the measurement.
     
  9. Jun 27, 2015 #8
    The exact meaning of "measure and throw away" is sometimes not clear to someone who is new to this subject. This leads to questions like, "what if I burn the printout without anyone ever looking at it"?
     
  10. Jun 28, 2015 #9

    atyy

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    If you burn the printout without anyone ever looking at it, then you don't need collapse, you only need decoherence and apparent collapse. Of course if you want to say it collapsed and you threw away the results, it's fine too. But you have the option of saying it only decohered with apparent collapse, but no true collapse.

    Remember, the wave function and collapse are not necessarily real, so this subjectivity is fine. We only care that we make correct predictions.
     
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