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Double Slit Interference.

  1. Apr 8, 2008 #1
    This might sound very basic but...
    when the electron passes through the two slits and we see the 'pretty' interference pattern on the opposite side what causes the dark fringes to be seen,
    how does an electron, after acting as a wave and then as an electron when it is recieved, interfere with itself.
    What happens in these dark fringes, where there is destructive interference, if one electron is fired and interferes with itself, will it be detected??
    does that make sense?
  2. jcsd
  3. Apr 8, 2008 #2


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    When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern. If you continue firing electrons at the slits, the dots will build up into an interference pattern.

    Think of the dark bands as places where no ( or few ) electrons end up.
  4. Apr 8, 2008 #3


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    For each electron there is a probability of going through each slit. After many passes of single electrons through the apparatus, you begin to see the probability distribution i.e the frequency (number of occurances) of each possible path the lectron can take.

    experimentally the observated distribution turns out to be equivalent to that made by a wave.

    (t's been a while since I did this so I may have got this wrong.)
  5. Apr 8, 2008 #4
    The experiment is easy to understand, the implications are what makes it interesting. Ie the fact that a single warticle (wave/particle) can interfere with itself, but if the warticle is measured after it passes through the slits then no interference pattern builds and a pattern on the back screen appears as it would if a particle had travelled through either the top or bottom slit, with a 1/1 distribution. If you can get your head round why that is you're laughing. :smile:
  6. Apr 8, 2008 #5
    Are you sure?
  7. Apr 8, 2008 #6


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    Do you think Mentz's statement is not correct? The interference pattern is in the probability distribution for finding a given electron at a single definite location, but you do always find it at a single definite location when you measure its position. Only by looking at the distribution of large numbers of electrons can you actually see an interference pattern.
  8. Apr 8, 2008 #7
    So are you saying that a single electron does not interfere with itself?
  9. Apr 8, 2008 #8


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    No, why would you think I was saying that? As I said, the probability distribution does show interference when both slits are open and you don't measure which slit the electron went through. But you can't see a probability distribution if you only measure a single electron--you only find it at one single location, with the probability of finding it at one location vs. another location given by the probability distribution.
  10. Apr 8, 2008 #9
    As you said? Where?

    At any rate, if you now say so you clearly must disagree with:

    "When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern. "
  11. Apr 8, 2008 #10


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    In post #6, where I said "The interference pattern is in the probability distribution for finding a given electron at a single definite location"
    Your logic is completely inscrutable. Why would I "disagree" with this, when I just said exactly the same thing in my last post? i.e.:
  12. Apr 8, 2008 #11
    What part of ""When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern. " talks about measuring which slit the electron went through? Did you read the OP? Where does it question the case when a measurement at the slits take place?
  13. Apr 8, 2008 #12


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    It doesn't, but the statement is true regardless of whether you measure which slit it went through or not. Whether you measure or not just changes the probability distribution (the probability distribution is an interference pattern if you don't measure, a non-interference pattern if you do), but you never see the probability distribution experimentally in a measurement of a single electron, you deduce the probability distribution either by sending a lot of electrons through with the same experimental setup, or by doing a theoretical calculation using the equations of QM. In a measurement of a single electron, all you see experimentally is the electron being detected at a single definite location on the screen.

    Anyway, I really don't understand why you're asking me that question. Are you implying that I have claimed the statement does talk about measuring which slit the electron went through? If so you have misunderstood me.
    It doesn't, it seems to just be talking about the normal version of the experiment where there is no measurement at the slits. Again, I don't understand the point of this question.
  14. Apr 8, 2008 #13
    Patently JesseM a lot of people can't get their heads round what is happening in a consistent manner. And you really do often need a visual representation to get this across. The number of times this thread comes up demonstrates this more than adequately. But if they want an answer try the FAQ.

    I think it might need a rewrite to include a good "visual" model but here it is:

    And here a good "visual" model is, if only copyright laws weren't such a beast.

    Feynman double slit experiment

    Don't worry it does photons and electrons although the issues are the same for all intents and purposes here.

    Warticle, not particle not wave, not neither not never. :smile:
    Last edited: Apr 8, 2008
  15. Apr 8, 2008 #14


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    Yes. The electron will be detected at one place and time and make one spot. Are you suggesting that the electron splits ?

    Here's a link that shows a pic of the DeB-B trajectories in the double-slit experiment -

    Last edited: Apr 8, 2008
  16. Apr 9, 2008 #15

    is there a chance that with the first electron traveling through this hole that it interferes with itself and ends up on the furthest left point on a light fringe not straight through the hole, or does this interference only work with multiple electrons?
  17. Apr 9, 2008 #16
    do any of the electrons that are fired when they become waves interfere and dissapear and do some hit the space between the two slits before travelling through(that sounds basic i am just tryin to build up a picture in my mind)
  18. Apr 13, 2008 #17
    After my little knowledge I really do think there is some photons that hit the wall inbetween the slits.

    I suggest you check out deBrogile bohm's pilot wave explanation of the double slit experiment.
    Really layman easy to understand
  19. Apr 13, 2008 #18
    Electrons don't go from being particles to waves and vice versa. Pick a description (both are correct/equivalent), and stick with it. The electron goes through the slits, however it does that, and will excite the detector at exactly one point. That point can be anywhere on the detector, so it's not good to talk about "interfering with itself".

    However, if you performed the experiment over and over with many electrons, one at a time, then fewer electrons will end up in some places, and more will end up in other places. The interference pattern you see resembles that of a wave, and so we postulate the existence of a wavefunction that governs the electron's probability amplitude. Here's a nice picture: http://en.wikipedia.org/wiki/Image:Double-slit_experiment_results_Tanamura_2.jpg

    Also, be ware of explanations that are "easy to understand" but which are either wrong, kludgy or both. The postulates of quantum mechanics may not be easy to understand the first time around, but they are very elegant and very simple. Stuff like pilot waves might sound appealing now but they're god-awful when you want to do quantum field theory, and you end up having to sweep a lot of ugliness under the rug.
  20. Apr 13, 2008 #19
    oh wow i think i get it now, thanks guys!
  21. Apr 13, 2008 #20
    To me, all of the following descriptions of the interference patterns are basically the same and tries to explain the wave-nature of photon/electron or other particles used in the experiment:

    - Photon/Electron interfered with itself.
    - Probability Wave interfered.
    - Pilot Wave interfered.

    Also, the moment we make an observation to detect which slit the particle went through, the wave function "collapses." I'm not exactly sure what this means, but it seems to imply that the source of interference are no longer compatible to cause the interference (due to the act of measurement), so for all intents and purposes, the Photon/Electron can now be considered a Particle.

    One question I have is, are the Probability Waves and/or Pilot Waves real or are they simply a proxy of something that the scientists think may exist in an effort to explain these phenomenons?
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