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Does the environment cause wave function collapse

  1. Nov 28, 2013 #1
    I came across this statement by bhobba in another thread and it got me to thinking, if the "environment" itself is capable of collapsing the wave function, then how is it possible to produce an interference pattern in a double slit experiment? After all, the particle isn't traveling through a vacuum. It's traveling through, and doubtlessly interacting with, a vast multitude of air molecules on its path from the emitter to the screen, yet none of these interactions seem to be capable of collapsing the wave function. So why doesn't the environment collapse the wave function in a double slit experiment?

    Why is the environment capable of collapsing the wave function in some cases, but not in others?
     
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  3. Nov 28, 2013 #2

    naima

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    I think that it is because the information about which path on air molecules are erased by further scatterings with other molecules. When light decoheres a dust particules you can read the photons. they keep the information.
    If a double screen set up was in a wire chamber interference of electrons would not occur.
     
  4. Nov 28, 2013 #3

    bhobba

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    It doesn't collapse anything - it decoheres it - which causes 'apparent' collapse:
    http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]
    'So why does the electron in the double-slit experiment still show interference effects? Why does it not decohere? The answer is because it is not a macroscopic object, it is an isolated microscopic object. While decoherence happens extraordinarily fast for macroscopic objects, for an electron the decoherence time (the so-called coefficient fluctuation time) is about 107 seconds, or about a year - plenty of time to perform the double-slit experiment and see interference effects.'

    Thanks
    Bill
     
    Last edited by a moderator: May 6, 2017
  5. Nov 28, 2013 #4
    Bill, thanks for the link. I found it quite informative and bookmarked it so that I could go back and read the other linked articles. But it did raise a couple of further questions. In what way is the particle "isolated". Isn't it almost constantly interacting with the air molecules? Let's say that we performed the double slit experiment in water, instead of in air. Would the increased density of the water molecules cause a quicker decoherence than we see when we do the experiment in air? What about still denser mediums? Is there a correlation between the "density" of the environment and speed of decoherence? I ask these questions because it is part of my nature to ask questions, so no offense intended.

    Also, in the double slit experiment it seems that no matter how subtle we try to be in measuring the path of the particle, the act of measuring it will cause it to decohere. So one seemingly insignificant interaction will cause the particle to decohere, while the millions of interactions with the air molecules won't. It would seem that it's not just the quantity of the interactions, but the quality of the interactions, the type of interactions, that matter. So if one interaction can cause decoherence where millions of others can't, what is the nature of that specific interaction that causes it to have such a disproportionate effect? What is it about some interactions that cause them to have a greater effect than others? This would seem to show that interaction alone is not sufficient to cause decoherence, but some other characteristic is responsible for decoherence.


    Thanks
     
  6. Nov 28, 2013 #5

    atyy

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    http://arxiv.org/abs/0712.3703 section IV.B summarizes a bunch of decoherence experiments. Fig. 35 shows there's more decoherence if there is more uncontrolled environment.
     
  7. Nov 28, 2013 #6

    naima

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    Where did you read that?
     
  8. Nov 28, 2013 #7

    naima

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    If you have the photon in mind, Emitting just one photon is not enough to watch the electron.
     
  9. Nov 28, 2013 #8
    Keyword: apparent.

    Whether decoherence actually causes collapse or not is a question not answered yet.
     
  10. Nov 28, 2013 #9
    fully concur.


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  11. Nov 28, 2013 #10
    This may be of interest:

    P264 of the book “Decoherence and the Quantum to Classical Transition” by Maximilian Schlosshauer describes the effect of differing densities of background gas on “double slit experiments” using C70 molecules. The C70 molecule is not a microscopic particle (but neither is it a macroscopic object) so it doesn’t fully address your question concerning electrons, but the principle of the medium affecting the interference fringes perhaps applies to electrons as well, as discussed by d’Espagnant below.


    From “On Physics and Philosophy” by Bernard d’Espagnat.

     
  12. Nov 28, 2013 #11
    I thought that this experiment needed to be performed in a pretty robust vacuum or you would see nothing. Perhaps this was a false assumption!
     
  13. Nov 28, 2013 #12
    Thank you Len M, this is indeed helpful, and what atyy was perhaps alluding to in his/her linked article, but I was having trouble deciphering. However I'm not sure as to whether this is evidence of decoherence, or diffraction, or some other phenomena. Is it evidence that the environment induced decoherence, or that the environment altered the wave function so as to eliminate interference? In other words, does the particle still take both paths, but the effect of the medium on the paths is such as to obscure the interference pattern? I would assume that a careful examination of the resulting interference pattern would shed some light on this question, but I lack the information and intelligence to reach a definite conclusion. Maybe I just need to give it some more thought, or perhaps someone else can shed some further light on this subject.

    Thanks for the input everyone.
     
  14. Nov 28, 2013 #13

    another big problem, at the begginig of the universe, which environment ?



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  15. Nov 28, 2013 #14

    atyy

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    Yes, Len M's referring to the same stuff I was alluding to.

    The rough idea of uncertainty/complementarity is that the particle does not have definite position and wavelength and that position and wavelength cannot be simultaneously measured with complete precision. Decoherence takes the environment and the apparatus and particles, and the interaction with the environment causes information about the path to be in the environment. Since someone could (in principle) measure the environment perfectly to find out the path, decoherence causes the interference to disappear. The more precise way of calculating it is by taking the state of the environment and system, and tracing out the environment (which you are in practice ignorant about) leaving the effective state or "reduced density matrix" of the system - which is no longer pure, but mixed, so that when a measurement is made on the system, the results are the same as if the environment had made a "measurement" on the system.

    One way to test the idea that it's the uncontrolled environment that is causing the loss of coherence is to see whether there is more decoherence when there is more uncontrolled environment. Here is an experiment that shows the degree to which the interference disappears depends on how much "uncontrolled environment" there is. Take a look at Fig. 3 of http://www.physics.arizona.edu/~cronin/Research/Publications/multi-photonPRL.pdf
     
    Last edited: Nov 28, 2013
  16. Nov 29, 2013 #15

    bhobba

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    Its been answered - it doesn't.

    That's not, nor ever has been, the issue.

    The issue is, is APPARENT collapse good enough - that's the key point, that's where the argument lies.

    Different interpretations have different takes.

    As I have posted many times I will not get into a discussion about it. I will simply point people to a paper I think presents the issue fairly and people can make up their own mind:
    http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

    If anything that paper is slighly pessimistic in my view - but I rather like that because people are not getting what I think, and will not be 'contaminated' by that.

    Thanks
    Bill
     
  17. Nov 29, 2013 #16

    bhobba

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    Glad others are reading that book - it my go-to book on such things.

    I like the essay I link to on decoherence, but that book is a few steps above in comprehensiveness, and careful explaining.

    Thanks
    Bill
     
  18. Nov 29, 2013 #17

    bhobba

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    Well, as far as I am aware, no one is claiming any kind of observation then, so its a total non issue.

    Thanks
    Bill
     
  19. Nov 29, 2013 #18

    bhobba

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    One thing you need to understand about Feynman's sum over histories approach is viewing particles as actually taking all the paths is really a hidden variable theory of a rather non trivial type.

    Mathematically that the wave function behaves LIKE that is beyond question, but if it really does is an interpretive assumption. Nice in understanding certain problems like the double slit experiment - but still its not strictly implied by the formalism.

    In this context its important to realize that while the double slit experiment is usually discussed as an aid, and motivation for, discussing quantum principles, it in fact can be analysed the other way around.

    A link that does just that has been posted in the past and its probably worthwhile those interested in the issue seeing if they can dig it up, or those that know it post it in this thread.

    Regarding why photons in the double slit experiment have very long decoherence times, its like many things in physics, for the details you need to consult the technical tomes. I don't know a reference off hand for that, but again if anyone is interested I am sure a bit of investigation at a university library will yield the details.

    Thanks
    Bill
     
  20. Nov 29, 2013 #19

    bhobba

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    For objects other than photons - yes - for photons its probably related to they travel so quickly and there is so many of them, since their decoherence time is so long, very few interact with objects on the way through enough to decohere them - although it may decohere other objects. Of course those that are decohered and given an actual position will not participate in the interference effect.

    Thanks
    Bill
     
  21. Nov 29, 2013 #20
    who said observation ?


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