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A question about QM "Observer effect"

  1. Feb 22, 2015 #1
    So if we "look" at a particle, then its wave function collapses. Does "look" mean, hitting that particle with another particle (a light particle?). A simple Yes/No answers first please.


    I mean, does its wave function collapse because we first need to hit that particle with an electron or a proton first, to observe it? not because it "knows" that we observe it?
     
  2. jcsd
  3. Feb 22, 2015 #2

    atyy

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    It cannot be a simple Yes/No answer. The wave function collapses when you get a measurement outcome, ie. a definite result, which is an "irreversible" "classical" or "macroscopic" mark. Whether you get a definite result is up to you to decide. You can delegate it to a machine, but you have to do the delegation. This is the "observer effect" and or the "Heisenberg cut", and is part of the orthodox Copenhagen-style interpretation of QM.

    However, you should also note that in that interpretation, the wave function is just a tool to calculate the probabilities of measurement outcomes.
     
    Last edited: Feb 22, 2015
  4. Feb 23, 2015 #3

    bhobba

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    A few comments.

    First collapse isn't actually part of QM - its part of some interpretations - but others like Bohmian Mechanics don't have it. There days the modern version is how does an improper mixed state become a proper one - colloquially its why we get any outcomes at all:
    http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

    Look does not mean that. Look means interacting with the system to produce decoherence.

    Thanks
    Bill
     
  5. Feb 23, 2015 #4
    how do we "Get a measurement"?

    Simpler the answer the better.
     
  6. Feb 23, 2015 #5
    How do we INTERACT?
     
  7. Feb 23, 2015 #6

    bhobba

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    Its the other way around in general - how do we not interact.

    In the everyday world systems are immersed in an environment they interact with all the time - in fact that's what gives things its classical properties. For example a few stray photons from the CBMR are enough to decohere a dust particle and give it a specific location. It turns out most interactions are of the radial type so that, for example, the closer two objects get the stronger the force they feel. Mathematically they have what's called an interaction Hamiltonian.

    Thanks
    Bill
     
  8. Feb 23, 2015 #7
    So am I correct If I tell that the reason for the wave function to collapse is this "interaction Hamiltonian" ?
     
  9. Feb 23, 2015 #8

    bhobba

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    Its a bit more subtle than that.

    As I said before collapse isn't really part of QM - only part of some interpretations.

    And secondly interaction (represented mathematically by the Hamiltonian) only explains what is called apparent collapse.

    But, basically, with those caveats, yes.

    Thanks
    Bill
     
  10. Feb 23, 2015 #9

    atyy

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    QM does not answer. You have to know when you get a measurement outcome.

    Only a deeper theory like Bohmian Mechanics can answer such a question.
     
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