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Double Slit Experiment Questions

  1. May 13, 2014 #1
    Ok, so I've just started studying quantum physics with the introductory double slit experiment. I thought I had a pretty good grasp on the results of the experiment but my tutor says I don't.

    This is what I interpreted from the experiment:

    Both slits are open.

    We place a detector along one of the slits. The fact that we know which slit the electron went through makes it act as a particle.

    Now, we remove the detector. We don't know which slit the electron went through. Because of this, the electron acts as a wave and we observe an interference pattern.

    My tutor says that I'm missing the point and this is all bs. He said Feynman's experiment was theoretical, and has nothing to do with the main point. My tutor didn't elaborate beyond this! I'm left wondering if he's correct that I'm missing the point, and if so, what the point is?!

    Another person told me that the reason detection changes the result of the two slit experiment is because there are no tools capable of not interfering with the experiment. Is this true?

    One more question: Can we say the electron goes through one slit or does it go through both if it acts as a wave?
  2. jcsd
  3. May 13, 2014 #2


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    The electron does not magically decide whether to act as a wave or as a particle because it is neither, it is an electron. What seems to confuse people is that the electron has some properties that we would typically associate with particles and some that we would associate with waves.

    What you are doing when you are measuring which slit the electron goes through is to break the coherence of the system and the coherence between the two paths is what creates the interference pattern. Once you have broken the coherence, you will not get an interference pattern.

    To try to address your last question: Unless you measure which slit it goes through, you cannot say that it went through a particular slit. If you measure it, you break coherence and lose the interference pattern.
  4. May 13, 2014 #3


    Staff: Mentor

    One of the issues here is the double slit experiment is often used to motivate the quantum formalism, but the reality is that formalism explains it and is really what's going on for want of a better way of expressing it.

    What I suggest you do is put aside what you have read about it for now and instead concentrate on the conceptual core of QM. There are a number of ways of approaching that depending on your mathematical sophistication, but at your level the following is probably a good place to start:

    BTW that's probably so different from what you have read you may think its crankish. It isn't - its from an actual lecture at MIT - rest assured its as solid as you can get - just a bit different.

    Now with that core in mind you can see a correct analysis of the experiment:

    I also need to point out that paper is slightly simplified, but for getting the gist is fine - here is a paper explaining some of those fine points - but don't worry about it for now - I just give it for completeness:

    As you can see its not wave particle duality, particles going through both holes, changing from wave to particle on observation etc etc - its simply the working out of an extension of probability theory. That however often gets obscured by populist 'half truths'.

    Last edited: May 13, 2014
  5. May 14, 2014 #4
    Feynman's experiment was with photons so better to keep with that. Indeed you cannot measure photons without absorbing them, so as he described it cannot be practically done. I don't know what point you tutor means, but Feynman saw every path as a possible history of the photon. The final result was the sum of all possible histories. But if a slit-detector detects a photon, it is sure that the particle did not take the path of the other slit. Then those histories (which cause interference) do not exist anymore, so should not be added to the final result.

    The particle by definition can only go through one slit. But as Orodruin mentions, a photon is no particle nor wave. It has properties of a particle and wave. We don't know (yet) what is really is, so cannot conclude beyond what we measure.
  6. May 14, 2014 #5


    Staff: Mentor

    Well actually we do.

    It obeys the laws of QM. You just need to face them head on without 'irrelevancies' like what is it doing when not observed.

    Just take as fact its axioms as found in a good text like Ballentine and accept that's it without trying to read more into it and things will be a lot clearer.

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