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Sniping slits in double slit experiment?

  1. Aug 28, 2014 #1
    Why can we not be more precise when shooting stuff through the double slit, so that we know if it goes in the direction of one slit or the other? By the way, is there any significant difference in workings between single and double slit interference?
     
  2. jcsd
  3. Aug 28, 2014 #2

    Dale

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    You can, but then it doesn't interfere.
     
  4. Aug 28, 2014 #3
    Do you know what keyword would help me find some more information about those "slit-sniping" experiments?
     
  5. Aug 28, 2014 #4

    Dale

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    You want to look for "which way" double slit experiments.
     
  6. Aug 28, 2014 #5
    I've seen those, but all of them we inserting some kind of detectors in one of the slits, rather than knowing which direction each photon went from the point of emission.
     
  7. Aug 28, 2014 #6

    fluidistic

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    The way I've been taught QM, there is no such thing as a trajectory of a particle.
    Therefore I fail to see any sense in trying to determinate through which slit did the particle pass.
     
  8. Aug 28, 2014 #7
    Not determine, but cause it. Lenses. I think we can produce very focused laser beams with precision range far below the size of the slits. So like they do in surgery or integrated circuit manufacturing procedures, we should be able to point our photons precisely in the direction where we want them to go, one slit or the other. Is there any technological limit here I am not aware of?
     
  9. Aug 28, 2014 #8

    fluidistic

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    Well if the slits are really large compared to the wavelength of the EM waves, the interference pattern disappears gradually as you enlarge the slits. So I am guessing that if we describe this effect at a quantum level, maybe we could replace the words EM waves by photons but I'm not 100% sure.
    I didn't learn the quantum mechanics of photons, these things are massless so I guess that they don't satisfy the Schrödinger's equation. I don't know how to describe them.
     
  10. Aug 28, 2014 #9

    Dale

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    That is a single slit experiment.
     
  11. Aug 28, 2014 #10

    DrChinese

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    Jabbu, you can't keep asking the same question over and over. You asked and it was answered by DaleSpam. I realize that you don't want to read and understand any more than the basics, but this will be necessary for you to get to the next level. I am trying to give you some helpful information, because at the current rate you will have managed to get more threads closed in a week than anyone in recent history here.

    The "mystery" of the double slit is that the particle goes through both slits, not just one. You can set it up so that you know which slit the particle goes through. And this can even be done WITHOUT pointing it to one slit or the other, and without blocking a specific slit (polarizers are used). When this happens, there is no interference.

    When there is no possibility of knowing the which-slit answer, there is interference. And only then. Molecules as large as buckyballs exhibit quantum interference (C60). Interference is explained by quantum concepts, but not classical ones (which fall apart when particle behavior is exhibited).
     
  12. Aug 28, 2014 #11
    I only know about experiments where detectors are placed in the slits. Can you point me where I can read more about those experiments which point particles to one slit or the other?
     
  13. Aug 28, 2014 #12

    Dale

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    Again, that is a single slit experiment.

    You certainly can do any number of things to aim the source so that you illuminate only a single slit. But then, by definition, it is not a double slit experiment. Is that not perfectly clear?
     
    Last edited: Aug 28, 2014
  14. Aug 28, 2014 #13
    I'm asking whether there is a double slit experiment in which particles are pointed towards one slit or the other.

    DrChinese said:
    - "And this can even be done WITHOUT pointing it to one slit or the other..."

    It's about a double slit experiment, suggesting they can be done by pointing particles to one slit or the other. I'd like to know about such double slit experiments because I never heard about them before. I thought no one has done anything like that, that's why I asked in the OP why not. But if someone has actually done it, then I'd like to read about it.
     
  15. Aug 28, 2014 #14

    Dale

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    You have heard about them. Everyone else calls them single slit experiments.

    You cannot illuminate a single slit and call your experiment a double slit experiment. It is a contradiction in terms.
     
  16. Aug 28, 2014 #15

    bhobba

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    What might help you is if you see a correct analysis of the single and double slit experiment:
    http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    Usually it is used to motivate the QM formalism, but then they do not go back as show how that new formalism explains it. The above corrects that.

    However I am with Dr Chinese - something seems to be holding you back.

    Maybe approaching QM a different way will help. Here's what it is from its conceptual core:
    http://www.scottaaronson.com/democritus/lec9.html

    But all of us have had to come to grips with QM - its a long hard slog with a lot of ah-ha moments over even the simplest of things. One such moment for me was the realisation an observation does not require an organic observer, then the realisation that the formalism was about observations that occur in a common sense classical world. For me, the foundations then fell into place.

    Hopefully you can sort out exactly what your primary issue is and come to grips with it.

    Thanks
    Bill
     
  17. Aug 28, 2014 #16
    They could point every odd photon in the direction of one slit and every even photon in the direction of the other slit. They could aim at only one of the two slits and see if the pattern matches single or double slit fringes. Has anyone done anything like that?
     
  18. Aug 28, 2014 #17

    bhobba

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    Look at the explanation of the double slit in the paper I linked to.

    We have a centrally located source whose position is known so the momentum of what it emits is unknown. Since it has a definite kinetic energy it means its direction is unknown. It can go through either slit. This means its state at the screen is the sum of the state of it going through both slits (a superposition) so you get interference.

    If you point it at a particular slit it cant go through both slits so you do not get interference.

    Thanks
    Bill
     
  19. Aug 28, 2014 #18

    bhobba

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    There is no need to do it.

    Its obvious what will happen.

    If you point it at one slit the state behind the slits is not a superposition of going through both slits and you will not get interference.

    Thanks
    Bill
     
  20. Aug 28, 2014 #19

    atyy

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    The strict answer is that you cannot aim an electron to go through one slit or the other. Basically, this is because a single electron is a wave. So even if there is only one slit, you will still get an interference pattern.

    Now let's think about classical light for the moment. Athough light is a wave, it can be treated to very good approximation as a ray in some circumstances, when the width of the slit is large compared to the wavelength of the light wave. Similarly, if the slits and the spacing between them is much larger than the electron's wavelength, then we can aim the electron at one of the slits.
     
  21. Aug 28, 2014 #20

    bhobba

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    You can.

    Imagine a long tube that exits close to one slit - and its easy to see such is really the single slit experiment.

    Thanks
    Bill
     
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