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The detector itself contaminating double slit? How do we know?

  1. Jul 21, 2014 #1
    Just a simple question. How do we know the detector isn't messing up the double slit experiment when a single photon is shot out. When there is no detector we know the interference pattern is formed even with a single photon. If we add detector to see where the photon actually went through a defraction pattern appears. As if the wave form collapsed because it was being detected.

    To me my first logical thought was that the detector must be somehow messing up the experiment (plugging the hole) or interfering electromagnetically somehow, because the pattern is showing exactly like there is 1 slit again.

    Now I realize this experiment is over 100 years old and has been retested numerous times. I am assuming the detector isn't actually messing up the experiment, but I just want to understand how the detector works and how they know it isn't contaminating the results. I have looked all over the intertubes and can find no simple explanation of this.

    Thank you.
     
  2. jcsd
  3. Jul 21, 2014 #2

    atyy

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    Yes, of course the detector is messing up the experiment. Feynman made a mistake (I think) when he said the double slit is mysterious.
     
  4. Jul 21, 2014 #3
    Why answer with sarcasm. I just want to know how it is done.
     
  5. Jul 21, 2014 #4

    atyy

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    It wasn't sarcasm. I do think Feynman made an error.
     
  6. Jul 21, 2014 #5
    Oh... :D
     
  7. Jul 21, 2014 #6

    atyy

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    You can find a discussion of Feynman's error in the introduction of http://arxiv.org/abs/1301.3274 (published in Rev Mod Phys http://dx.doi.org/10.1103/RevModPhys.85.1693).

    In the above article, I am only recommending the introduction as a resource for references on a quite widely held view that the detector is messing up the interference pattern. The article goes on to propose an interpretation of QM, which I am not recommending, at least not for this thread. A major interpretational approach that sides with Feynman is the consistent histories approach, which again, I would say is still research and not consensus.

    Anyway, take a look at the discussion around this excerpt from p3 of the above:
    "Two systems are introduced for a good reason. Without the guarantee of arbitrarily distant parts within the experiment—so that one can conceive of measurements on one, and draw inferences about the other—what justification would one have to think that changing the conditions of the experiment (from one slit closed to both slits open) should not make a deep conceptual difference to its analysis? Without such a guarantee for underwriting a belief that some matter of fact stays constant in the consideration of two experiments, one—it might seem— would be quite justified in responding, “Of course, you change an experiment, and you get a different probability distribution arising from it. So what?”"

    I added the bolding above, since I think that's what the question in the OP was. Take also a look at footnote 4 (Ballentine! Maybe some of his many admirers here can add insight. I'm not a big Ballentine fan, but it looks like he got it right here.)
     
    Last edited: Jul 21, 2014
  8. Jul 22, 2014 #7

    bhobba

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    With photons that experiment can't be done because photons do not have an actual position or even a well defined path - certainly I don't know a way of detecting it without destroying it. The usual discussion of the double slit experiment with photons strictly speaking is a crock of the proverbial - but is useful as a starting point.

    Actually Feynman has a discussion of this sort of stuff in his Lectures On Physics. Many things in beginner treatments of physics are not really true - but are good for starting out and getting a feel for things.

    If you are worried about exactness, rather than simply getting a bit of a feel for what's going on, its much better to talk about the double slit with electrons.

    QM predicts it doesn't matter what kind of detector you use the same happens. As to whether it's been experimentally verified - blowed if I know - except if it was the case it would be BIG news leading to an immediate Nobel prize.

    The other thing to realize, is while popularisations, and even some beginning textbooks, do not point this out; the double slit experiment is not the basis of QM - its simply what some use as a pedagogical aid. The basis of QM actually lies in a generalisation of probabilty theory:
    http://www.scottaaronson.com/democritus/lec9.html

    From that QM to a large extent follows and because of its conceptual simplicity its pretty hard (but not impossible eg primary state diffusion) to violate without running into some well established experimental fact contradicting it.

    In fact QM explains the double slit experiment - not the other way around:
    http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    Its up to experimental guys to decide what might be interesting to check experimentally, but for my money that's a way way long shot.

    Thanks
    Bill
     
    Last edited: Jul 22, 2014
  9. Jul 22, 2014 #8
    Thanks Bill. So you are saying explanations like this one using photon in the link below are complete bs?



    He claims that the experimenter left the detector on and simply stopped recording the results and that still left an interference pattern. So it would seem that the recording of the result is what either collapses the photon into a particle or a wave? At least in this example. These types of accounts are all over the net.

    Thank you
     
    Last edited by a moderator: Sep 25, 2014
  10. Jul 22, 2014 #9

    bhobba

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    With photons - strictly speaking - yes. Since a photon travels at the speed of light there is no frame where it is at rest so can't have a position.

    But that's nothing new - many things in beginner treatments are like that eg its the same with the so called wave particle duality see our FAQ:
    https://www.physicsforums.com/showthread.php?t=511178

    I think what Zapper said sums it up pretty well:
    'We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn’t mean that in physics, or in the working of physicists, such a duality has any significance.'

    Same here - to get across the weirdness with laymen liberties are taken. You see it all the time in beginning texts on all sorts of issues. Another well known crock is the rubber sheet analogy to curved space-time - its nothing like that at all.

    Thanks
    Bill
     
    Last edited: Jul 22, 2014
  11. Jul 22, 2014 #10
    Last edited by a moderator: Sep 25, 2014
  12. Jul 22, 2014 #11
    Anyone care to provide a real answer instead of saying particle wave duality is just to confuse newbs?
     
  13. Jul 22, 2014 #12

    Cthugha

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    Bottomlayer is a well known crackpot site. Please avoid linking to it. So is Thomas Campbell whose youtube video you linked. If these were real experiments, there would be peer reviewed publications showing them. Anybody can claim sensational stuff and put it on Youtube (and make money from getting a lot of clicks). This is why discussions in these forums need to be based on peer reviewed publications. Peer review assures a minimum quality.

    You will not find peer reviewed publications claiming any influence of turning detectors on or off. There is a reason for that.
     
  14. Jul 22, 2014 #13

    bhobba

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    Mate its in the FAQ for a reason.

    Around here you get the truth - warts and all.

    BTW its not to confuse newbies - liberties are often taken for pedagogical reasons.

    Just this morning I was reacquainting myself with Zees book on Quantum Field Theory. He talks about quantizing a mattress connected by springs. No one seriously believes a quantum field is like that but it helps to start with. In fact its an interesting exercise I went through to derive the equations without such an aid. I wouldn't like to have done that though without going through the mattress analogy.

    Thanks
    Bill
     
    Last edited: Jul 22, 2014
  15. Jul 22, 2014 #14
  16. Jul 22, 2014 #15
    Bill I appreciate the truth and that's all I want, but I am getting a little upset that this so called "fake" information would be circling out there...
     
  17. Jul 22, 2014 #16
  18. Jul 22, 2014 #17

    Cthugha

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    Yes, it is and it is correct, but it does not claim any influence of any conscious observers or detectors being turned on or off. In fact, the "choice" is done randomly in terms of whether a photon gets transmitted or reflected at a beam splitter.

    In a nutshell the key to this experiment lies in understanding that you never see an interference pattern directly. You need to filter your detections. The "choice" is now whether you keep the necessary information to perform filtering in a way that leaves you with an interference pattern in the filtered state or not.

    What is not happening is that the detections change afterwards. It is not the case that an interference pattern appears out of nowhere which has not existed before.
     
  19. Jul 22, 2014 #18

    bhobba

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    Don't be - like I said liberties are taken for pedagogical reasons - its nothing to worry about.

    I am formally trained in applied math. When I came across analysis (that's calculus done rigorously without the the hand-wavy arguments like if delta x is small you can ignore delta x squared etc etc) I learnt many of the things from calculus were wrong - but you would have to have rocks in your head to teach it correctly from the start.

    And interestingly once you go through that you return to the hand-wavey explanation - its easier - but incorrect.

    Thanks
    Bill
     
  20. Jul 22, 2014 #19
    Let me paraphrase this...

    You are basically saying the detections don't change afterwards, however, if you keep the information to perform filtering in a specified way you have an interference pattern if you don't filter information in that way you do not... that is bizarre.
     
  21. Jul 22, 2014 #20

    atyy

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    Wave-particle duality is fine in its naive form for the double slit interference of a single particle. Here we imagine that the particle is a quantum wave, not too different from a classical light wave, and that it passes through the slit like a light wave. For a light wave, the interference pattern is different for the single slit and the double slit, and it is no different for the quantum wave. The only difference is that the meaning of a quantum wave (squared) gives the probability for the particle to be detected at a particular position. So that's the naive wave-particle duality - a quantum wave determines a position, which is a particle like quantity.

    For two particles, wave-particle duality still holds, but not in a naive form. For two particles, we cannot imagine that the wave is propagating in ordinary space. Instead it propagates it an abstract space called a Hilbert space. We still have wave-particle duality, because the number of dimensions of the Hilbert space is determined by the number of particles present in ordinary space.
     
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