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Double-Slit Experiment and Interference

  1. Jan 28, 2006 #1
    I found some very entertaining presentations of the Double-Slit Experiments at ... University of Colorado ... and it made me start thinking. The "professor" presents an example of someone shooting a machine gun through the two slits and the "student" asks why the interference can't come from the bullets hitting each other as they travel through the slits. The professor responds by reminding the student that only a single bullet can come out of the machine gun at a time preventing the bullets from interacting with one another as they travel through the slits. But what about bullets bouncing off the slit apparatus and interfering with incoming bullets. Another website ... Influence of Measurement ... explains that, if the distance between the slits is further than the distance to the machine gun, the bullets near Slit A should not be able to travel fast enough over to interfere with incoming bullets at Slit B. However, what wasn't discussed was what prevents the bullets from bouncing off the detector wall and "interfering" with incoming bullets from the other slit. Can't the interference pattern come from photons (or something similar) bouncing off the detector and interfering with the incoming photons?
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  3. Jan 28, 2006 #2


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    Well, you can always lower the light intensity to the point where you only see one photon hitting the screen at a time, with the time between photon detections on the screen being larger than the time it would take a photon to get from the emitter to the screen. So if you think of photons taking a definite path, in this case they should be going through the double-slit apparatus one at a time. You still get the same interference pattern in this case.
  4. Jan 28, 2006 #3


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    Before one can use the argument that the "bounced" photon interferes with the incoming photon, one must FIRST show that there is a such a thing as a "bounded" photon off the detector. A "detector" in this case, typically a CCD, or crudely a semiconductor or even a light-sensitive film, is NOT a good reflector, unlike a metal. So already you are in deep doo-doo in trying to show that such a thing actually occurs. The detector, by definition, absorbs almost all of the photons that hit it within a certain energy range. Very little gets reflected, and even if they do, the reflected light is NOT COHERENT, which is a strong requirement for a clear interference pattern.

    Secondly, one certainly will be at a lost as to what would explain the pattern when very low light intensity is used, as in the single-photon case. There's no other photon to interfere with the bounced photon. Yet, we know that can generate the identical interference effect.

    Finally, such interference pattern are seen elsewhere, not just from photons being detected by such detector. We see the SAME effect for electrons, neutrons, Cooper Pairs, buckyballs, etc. with a large variety of measuring devices. Because they all have the identical effects, we know that they are all based on the identical principle that describes the interference pattern. So what would be the source of "reflection" in all of these?

    The lesson here is that in physics, before on can make the leap into proposing an explanation for something (photon interference), one must first show an indication that the source of the explanation (photon bouncing off detector) has already been established to be valid. Show that there's already experimental verification of it, it has occured in other situations, etc.. etc. You work in systematic steps to show that the foundation of your explanation has already been established and shown to be valid. Only THEN can you apply it to explain other things. If not, you are basing a hypothetical explanation on a non-existing foundation. That is a double-whammy of errors there.

  5. Jan 28, 2006 #4
    In the small amount of reading that I've done so far and in watching those on-line videos of Feyman giving a simplistic explanation of QED it is still very confusing, particularly when you say that this effect occurs with objects that are as large as buckyballs.

    It sounds like Feyman is saying that an individual photon, (electron, buckeyball, etc.), goes in all possible directions and that the image that we see as diffraction is the end result of the mathmatically described interactions of a photon with itself. I am obviously not up on the math describing this and my question is not about the details in the math, but whether this decription is how people who work in this field actually "see" this as happening ? Do you "view" a photon, an electron or a buckyball as "actually" being spread out in space and then interfereing with itself or do you just ignore that sort of speculation and use the numbers to predict an outcome to an experiment ? An even more general question would be whether you view the math as having a distinct correlation to what is "really" happening. Does the photon actually take all possible paths, or is this a mathematical convenience to arrive at conclusions that match the end results of experiments and has limited use in gaining a deeper understanding of the processes involved in those results ? Thanks for your views Zz.

    Michael E.
  6. Jan 28, 2006 #5


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    The problem here is that our classical work has forced us to view such things as having (i) a definite boundary in space (i.e. a ping pong ball has an abrupt boundary in space that separates it and "space" that isn't a ping pong ball) and (ii) having definite locations. When we realize that we are trying to force a square object through a round hole, we will also realize that the rules that we are familiar with just simply do not work.

    Here's what we know based on the mathematical formalism:

    1. Single photon (or electron, or neutron, or buckyball) interference is the interference effect that we know and love.

    2. Two photon, or multiphoton interference almost never happen (Dirac didn't even think it was possible). Even if it does happen, it produces a very different effect than what we know and love (see Mandel's paper that I've cited elsewhere in PF).

    Now one can try to describe such a thing as a photon passing through BOTH slits and interfering with itself. Feynman would say that this is nothing more than a superpostion of ALL the possible paths that the photon (or electron etc..) can take without us knowing which. Some people are more comfortable with saying one over the other. To me, being pragmatic, I'd say "whatever turns you on". If there's no measurable differences between one over the other, let's move on to the next problem, because I don't see the beneficial point of hovering over such issues (can't you tell why I became an experimentalist?).

    I probably didn't answer your question, but my view on this has always been "What's the big deal?" Maybe I have no difficulties in letting go of all the classical inhibitions that we were born with, especially after I've seen so many things personally that simply defy classical explanations. Remember, you tend to not view certain things to be strange if you keep seeing them as common as the grass on your lawn.

  7. Jan 28, 2006 #6
    Thanks Zz,
    You did answer my question quite well. Even having "let go of your classical inhibitions" you must still think of prying into the way that things work as a big deal. It must be difficult to develop and express imaginative ideas and experiments when the physical tools and spoken language are so far away from that quantum realm. Thanks again Zz!

    Michael E.
  8. Jan 30, 2006 #7
    Thanks for the input ... I didn't realize buckyballs produce the same effect.

    I'd like to see some videos of this experiment, maybe someone could provide some links. What happens when the detector and the slit-apparatus are moved closer and further from each other? Does that provide any more useful information?
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