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B How does the double slit experiment prove super position?

  1. Jun 9, 2016 #1
    I know this question has been asked before but I have only found threads for it at an undergraduate's level of understanding and I am just finishing ninth grade. How does the double slit experiment prove superposition? What led me to ask this question, was that, since when you close one slit the pattern you would otherwise see if both slits were open disappears, it must mean that the electrons are going through both slits simultaneously(In a ted talk about Schrodinger's cat). But, the diagram showed that electrons were being sent individually through each slit. Couldn't this mean that the reason the pattern isn't being seen is because the electrons are only being sent through one slit instead of two? Maybe, now that there is only one slit, it is harder to identify the wave pattern.
    It just seems like a real jump to say that, pattern disappears, therefore superposition. I tried to read this paper on the double split experiment, but I can't comprehend the Math yet, so it's essentially useless.
    Can you please give me an explanation that a high schooler can understand?
    P.S
    I am not debating whether or not the double split experiment shows an example of superposition. Based on the number of times this test has been done with the same conclusions again and again I can guess it is almost definitely correct. But I just can't seem to understand it.
     
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  3. Jun 9, 2016 #2

    Zafa Pi

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    The double slit experiment, that when unobserved individual electrons form an interference pattern, but when observed which slit they pass thru there is no interference pattern, is a weird aspect of nature. Superposition is a mathematical construct (part of QM) that provides an aid in calculations and cannot be claimed to be a "part of reality" .
     
  4. Jun 10, 2016 #3

    bhobba

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    Last edited by a moderator: May 7, 2017
  5. Jun 10, 2016 #4
    The double slit experiment can be explained using classical electromagnetism, without any quantum mechanics.
    Superposition is a principle in electromagnetism where the electromagnetic field from two sources is just the sum of the electromagnetic field from one source. Interference occurs because in some places the electromagnetic field from each source are pointing mostly in the same direction and add together, and pointing in opposite directions in other places and subtract.
     
  6. Jun 10, 2016 #5

    ogg

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    There are different points of view about the physics of the double slit. One problem you as a beginner have is that while a (good) textbook will give you one explanation, asking on a forum such as Phys.org will probably give you several different but equally correct points of view. Another way to put this is that the math can be done in various different ways - just like solving a polynomial equation can be done several different ways - all can give you the correct answer, none are necessarily "best" in all circumstances. The KEY point about the double slit experiment, is that you get an interference pattern when you are sending ONE ELECTRON AT A TIME through the slits. A simple example is setting up an electron gun (you could do the experiment with photons (light) too, but the detector would have to be different - and actually using a sheet of film along with a photon "gun" would be a much easier experiment to do, but I digress...) So one at a time, the photon should either go thru slit A or slit B and the resulting 'image' should be the same (and take just as long to create) as having only one slit open and then closing it and opening the other. You should get two "humps" (or if the source were really colimated (sharp) you'd get two lines, but this isn't really possible, the electrons will spread out unless somehow focused). Classically having two open at the same time should be no different than having one, then the other open. BUT!! this isn't what happens! What happens, even if you're sending 1 electron per second, is that you get an interference pattern. You get a bunch of peaks! This demonstrates the wave nature of electrons (they aren't "just" particles)...and you can do the same thing with atoms or protons or any sub-microscopic particle of your choice. You WILL get an interference pattern. Only with big stuff (stuff we can see) will the interference be too small to be seen. If you tried the same type of experiment with BBs, you wouldn't get a interference pattern (of course, this experiment could only be set up if you could build a gun which had a random aim, which would lead down a rabbit hole of cause and effect, because if you design in a randomness generator, then your critics would say that was ruining the experiment ...). Anyway, the double-slit experiment is only "useful" if you start from the POV that the sub-atomic particles do NOT have wave-like character. It proves they (all) do. [meaning the experiment is meaningless (or obvious) if you use electromagnetic waves - and here by "use" I mean describe what you are sending through the slits. Your results will be boring and obvious for any wave propagation thru the slits (like doing it with channels in a trough of water). So, someone telling you that you should do the analysis using electromagnetics (Maxwell's equations) is begging the question. The d.s. expt. proves that sub-microscopic particles are wave-like. It doesn't speak (by itself) to whether waves have particle-like characteristics (they do, but that's a different kettle of fish).
     
  7. Jun 10, 2016 #6

    ogg

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    It might be worthy of noting that if you really did such experiment with electrons and compared it to the same experiment with a laser (say), then the results would NOT look exactly the same. The electron result would be speckled, the photon result would be smooth. Both would show (for two slits open simultaneously) interference, with the intensity in the light picture smoothly increasing and decreasing and in the electron detector the result would show more and less speckles (spots). So, even though the electrons are interfering 'like waves' they are still hitting the target detector 'like particles'.
     
  8. Jun 10, 2016 #7

    Nugatory

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    A beam of light delivers an enormous number of photons to the surface it illuminates, so we're accustomed to seeing smooth patterns from light. However, if the number of particles involved is roughly the same, then the graininess of the pattern will be similar.
    An individual photon also leaves only a single dot at a single point on the screen - they're no less particle-like than the electron in this respect.
     
  9. Jun 10, 2016 #8

    Nugatory

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    This is a good point, one that bears repeating: the double-slit experiment best demonstrates quantum superposition when you do it with both slits open and send a single particle at a time so individual dots appear at the detector, one at a time. It is very difficult to explain the interference pattern that gradually appears under these conditions as anything except quantum superposition.

    The lack of interference pattern when one slit is closed is a fascinating example of how quantum mechanics works (weird that when we're sending one particle at a time it matters how many slits are open) but is a less convincing argument for superposition. With one slit open we don't expect an interference pattern and we don't get one. However, a sensitive enough experiment will find a diffraction pattern, so the wave nature of the particles is still demonstrated even though superposition is not.
     
  10. Jun 10, 2016 #9

    bhobba

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    Indeed. As the paper I keep linking to explains one slit open is explained by the uncertainty principle. Two slits is the uncertainty principle plus superposition.

    Thanks
    Bill
     
  11. Jun 10, 2016 #10
    Single slit diffraction happens when the slit is not very narrow. It is interference between wave fronts that have passed through different parts of the slit. So perhaps in quantum terms we can think of it as a superposition of going through many different parts of the same slit...?
     
  12. Jun 11, 2016 #11

    bhobba

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    Sure. Again as the paper I keep linking to explains a slit of non infinitesimal width is broken into a lot of slits of infinitesimal width and the superposition taken:
    http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    It's all there. Read it, re-read it, think about it and everything will be clear.

    Note - although much better than the usual half truth explanations in popularizations, even it is not the full truth:
    http://arxiv.org/abs/1009.2408

    To make matter worse even the above is not the last word.

    Physics unfortunately can be like that. You need to modify your understanding as you progress.

    Thanks
    Bill
     
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