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Double slit experimental method

  1. Mar 25, 2014 #1

    I've been trying to learn physics recently with the long term goal of understanding GR and cosmology. As a result, I have many questions, so I'll just get started..

    In the famous double slit experiment, there is a coherent light source aimed at a double slit which produces an interesting interference pattern behind the slit. I'm wondering about the width of the light beam relative to the space between the slits. I assume that the beam must be wider than the gap between the slits.. Is that right?

    My real confusion comes from the experiment where you limit the beam power so that it emits one photon at a time. Doesn't each photon have its own trajectory that is determined at the moment it's emitted? I know that you can't measure exactly where it is without "collapsing the wave function", but wouldn't that uncertainty represent a very small physical size (on the order of nanometers, maybe, like visible light wavelengths), much smaller than the space of just one of the gaps? And then how would any one photon "know" about the other gap to produce the bulk interference pattern?

    Hope that all makes sense - I'm still at the phase where I'm not sure how to phrase the questions. Cheers!
  2. jcsd
  3. Mar 25, 2014 #2


    Staff: Mentor


    One of the issues with the double slit experiment is its used to motivate the development of the full machinery of quantum mechanics but then don't go back and examine how that full blown theory explains the double slit experiment.

    Here is the proper analysis:

    IMHO such is not the best way to understand QM. I prefer first of all grasping its conceptual core then using it to explain experiments. Check out:

    Last edited: Mar 25, 2014
  4. Mar 26, 2014 #3
  5. Mar 26, 2014 #4


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    Science Advisor

    No, absolutely not. I know that intuition suggests things like that, but in quantum mechanics and especially in quantum optics following your intuition is a bad idea. It will lead you astray.

    This depends on the kind of light you use. Actually the uncertainty in the direction in which the light travels is higher for highly coherent light sources. As a comparison: unfiltered sunlight has a uncertainty of about 1 micrometer. A typical cw multimode helium neon laser already has an uncertainty of about 20 cm. Typical cw single mode lasers have an uncertainty of about 100m. Ultrastable lasers can give you uncertainties of about 100 km.
  6. Mar 26, 2014 #5
    Thanks for the responses - I appreciate the links, and the articles seem to have the answers I seek. I just need some time to digest the info. Goodbye, intuition.

    In general it seems that I was focused on the target in the experiment, only looking for quantum effects when the particle is detected at the wall. It seems that you actually need to consider quantum effects all the way back to the emission of the particle. And I guess it's not really "effects", but rather it's just what the particle is, and we don't have an intuitive way to understand it.

    It also sounds like I need to study up on lasers to understand what's special about a coherent light source for this experiment. One question that might get me started: does this uncertainty in direction for ultrastable lasers relate to the width of the beam or the ability to focus it?
  7. Mar 27, 2014 #6
    You can also use the Wave–particle duality, which is (partly) classical, so more intuitive. Then the photon-particle indeed follows its own quite accurate trajectory, but the photon-wave, goes through both slits.
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