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What interference pattern of double-slit experiment does mean

  1. Jan 24, 2014 #1
    I have a doubt about the interference wave pattern of double slit experiment with light beam.
    does it mean the interference of electromagnetic wave (which tells about the frequency causing color) or Spatial probability wave (which was explained by Max Born).

    I am not good at math. can someone please explain with math-free manner.
    Last edited: Jan 24, 2014
  2. jcsd
  3. Jan 24, 2014 #2

    Simon Bridge

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    Welcome to PF;
    There is no totally math-free manner - the short, non-math, answer is "both" - sort-of.

    The distribution of photons is described using the spacial wavefunction.
    The spacial probability amplitudes and EM waves are closely related to each other - so either approach is valid.
  4. Jan 24, 2014 #3
    Thanks for the information.

    double slit with single photon takes long time to get accumulated and shows interference pattern. but stream of continues photons shows interference immediately. Are both different phenomenon? or how both can be related if both are same phenomenon?.
  5. Jan 24, 2014 #4

    Simon Bridge

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    Both are the same phenomenon.
    The fact that it works for the single photon indicates that the result is not caused by EM field interference.
    Rather the EM field is an emergent phenomenon of the statistics.

    On the scale of individual particles, Nature is statistical.
    Over many particles, individual effects get averaged out to give you the classical behavior.
  6. Jan 24, 2014 #5
    ok. so the interference is caused by two waves regardless of intensity and time, fine.

    light has photons of so many different polarizations. but we are seeing the interference with only two wave. can we consider that polarization doesn't matter in this interference pattern?
  7. Jan 24, 2014 #6

    Simon Bridge

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    Nope - not what I said.
    There is only one wavefunction - this is why the better descriptions have math.
    But the wavefunction does not depend on intensity.

    The effect of different polarizations on the interference pattern is well documented - it's something you can look up.

    But the statistics works at an individual-particle level.
    You should probably see these lectures: http://vega.org.uk/video/subseries/8
  8. Jan 25, 2014 #7
    What is classical behaviour how it differs from statistical?
  9. Jan 25, 2014 #8

    Simon Bridge

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    Classical behavior is basically anything that follows Newton's Laws and the rules in his Principia.
    Statistical behavior is anything that needs probability math to describe.

    For examples - watch the lecture series I gave you in post #6.

    But: if you really don't know what "classical physics" or "statistical" means, then I cannot help you because the answers require a much higher education level than you have in order to understand them. It would take years.
  10. Jan 25, 2014 #9
    I know something about Classical and quantum physics

    So, if the light has very low intensity (1 photon or 2 photons or 3 photons...) then the double-slit will show probability wave interference. on the other hand the high-intensity light is not probability wave under double slit and it is the EM wave (of classical) interference.

    Double slit exhibits different wave behaviour based on the light intensity.

    is that correct?
    Last edited: Jan 25, 2014
  11. Jan 25, 2014 #10


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    No. There is no appreciable difference between the two.

    Higher intensity simply means that there are more photons present at once. The pattern is always a build-up (sum) of the individual photons over time. With higher intensity, the number of photons needed to see the pattern (perhaps around 200* on the low side) will arrive in a much shorter time span. The pattern becomes more clear as the number of photons detected goes up.

    *There is no exact minimum. Some people see the pattern earlier, some later.
  12. Jan 25, 2014 #11
    so did you mean that both the single-photon and stream-of-photons (Light beam) shows only the interference of spatial-probability-wave(of Max Born) and Not EM wave (of Maxwell, which tells wave frequency causing color)?
    Last edited: Jan 25, 2014
  13. Jan 25, 2014 #12

    Simon Bridge

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    No. He is saying that there is no difference between the two.
    The interference pattern you calculate using probability-amplitude waves is exactly the same as the pattern you calculate using EM waves, and both agree staggeringly well with the results of experiments. When it comes to the interference pattern, there is no difference between the theories.

    Have you watched the lecture series in post #6 yet?
    It is essential viewing for anyone contemplating this stuff.

    ... what does that mean?
    What education level are you doing this at?

    If we don't have a good idea of your current state of knowledge, we cannot help you very well.
    If you won't follow suggestions, we cannot help you at all.
  14. Jan 25, 2014 #13
    I just saw some videos about Quantum Mechanics and I am not Physics student.

    I saw that video, and hard to understand.

    This is the place I get confused.

    if both are same then why are you saying that low intensity light(1, photon , 2 photos 3 photons) is statistical and high intensity light (multiple-particles stream) are classical.

    I meant statistical that the interference pattern is caused by two Spatial-probability wave. and I meant classical that the interference patterns is caused by two electromagnetic wave.

    does intensity determine whether it is statistical or classical?.
  15. Jan 25, 2014 #14


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    Its just that by the law of large numbers probabilistic things can behave in a deterministic way if the number of observations is so large you don't even notice that are separate observations. In this case it means the photons strike the screen so frequently it looks predictable and continuous.

  16. Jan 25, 2014 #15

    Simon Bridge

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    Ahah ... I had a feeling this would be the case.
    It would still help to know where your understanding is at - you realize that the questions you are asking involve some quite deep concepts?

    The trouble is, there is a limit to what we can get you to appreciate if you don't have the background.
    It's like trying to show the beauty of fibonnachi numbers to someone who has yet to learn to add or subtract.

    So where is your science and math at generally - besides what you see in videos (this was online right?).
    Don't be imbarrassed if it's quite basic - that's just normal. It's us uber-educated folks who are the weird ones.

    I did not intend to give that impression.
    I was making a generalization between quantum mechanics and classical physics ... classical physics is what happens on average.

    In the interference example - while there is no difference in the pattern, there is a difference in the details of how we do the predictions at different scales.

    i.e. for many particles, we can work out the intensity of different parts of the pattern by multiplying the probability by the total number of particles. This does not make sense for just one particle - which either gets detected in it's entirety or not at all.

    The first approach is really just an average - but it works out so well because there are a great many particles so the variation from the average is usually too small to measure - like you cannot hear someone shouting backstage during a rock concert. It is this first approach that is indistinguishable from the EM approach.

    So you can think of it like this: it's all statistics and probability.
    Last edited: Jan 25, 2014
  17. Jan 25, 2014 #16
    as we are seeing wave in both single-photon and photon-stream scenarios how light can be a deterministic?

    can you agree that both are probability waves.

    Sorry I am a non physics student, I am a software engineer. somehow get interested in quantum mechanics. it is very hard to me. please dont mind.
  18. Jan 25, 2014 #17
    Ok. thanks. then my assumed logic will be matched. clear now.
  19. Jan 25, 2014 #18
    Simon Bridge.
    I need to clarify some doubts in Quantum mechanics. can I use this amed thread?
  20. Jan 25, 2014 #19


    Staff: Mentor

    Yes indeed they are probability waves - but a funny sort of probability:

    It will take a while to get the gist - but persevere.

  21. Jan 25, 2014 #20
    clear now. thanks
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