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B Double Slit Experiment: Timing & Reflection Interference

  1. May 22, 2017 #1

    Andy_K

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    Dear All,

    I have a couple questions on the double slit experiment I hope you can help shed some light (or photons) on. =)

    Arrival Timing of Photons

    In a normal double-slit experiment like the above setup, do photons always arrive at the detector at a constant speed (basically, speed of light), or does the arrival time experience fluctuation (albeit an extremely minute one)?

    Since the interference pattern is a result of many troughs of (probability) waves, and troughs seem to have a certain order in its propagation (just like in waves, some troughs are in front and some behind), does that mean if a photon is detected at a position created by interference of troughs further back, it would actually arrive at the detector at a slightly later time?

    I understand that the "wave" is not physical, but if the interference resembles the characteristics of normal waves, wouldn't that also signify a correlation to the spatial and temporal sequence of troughs?


    Reflection Interference
    If we change some parts of P (please refer to photo below) to become a mirror, so that the photon is either reflected back or detected there, would the reflection's backward "wave" interfere with the forward "waves", or perhaps even cancel it out since it's an opposing "motion"?


    Thank you for your guidance.


    5nAW5an.png
     
  2. jcsd
  3. May 23, 2017 #2
    [Reflection Interference
    If we change some parts of P (please refer to photo below) to become a mirror, so that the photon is either reflected back or detected there, would the reflection's backward "wave" interfere with the forward "waves", or perhaps even cancel it out since it's an opposing "motion"? /QUOTE]
    EM waves form an interference pattern above the surface of a mirror.
     
  4. May 23, 2017 #3

    zonde

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    There are no probability waves in Quantum Mechanics. There is a term "probability amplitude", but it's phase does not describe amplitude oscillations. It's phase rather describes rotation in complex plane (mathematically speaking).
    You might give a try to Feynmann's (very layman-friendly) book "QED: The strange theory of light and matter".
     
  5. May 23, 2017 #4

    PeterDonis

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    We don't measure the time it takes for the photons to travel through the experiment, so there is no answer to this question. All we measure is the pattern of light and dark at the detector.
     
  6. May 24, 2017 #5

    Andy_K

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    May I know if we don't measure because it is not relevant, not possible, or that it will always be the constant speed of light?
     
  7. May 24, 2017 #6

    Andy_K

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    Does that mean the wave propagating back will interfere with the original forward wave, at spaces between the barrier/slit and detector?
     
  8. May 24, 2017 #7

    PeterDonis

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    Not relevant (since the point of the experiment is to measure the pattern of light and dark at the detector) and not possible (because we can't measure the times at which individual photons leave the source).
     
  9. May 24, 2017 #8
    In classical physics terms, which is all I know, yes. Light from the first double slit is interfered with with by light from its image, which is a second double slit located the same distance behind the mirror.
     
  10. May 24, 2017 #9

    PeterDonis

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    This is correct; the QM model of this is basically the same as the classical model as far as the wave interference is concerned.
     
  11. May 24, 2017 #10

    Andy_K

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    Thank you very much for your guidance, Zonde, Tech99 and PeterDonis! I have much to learn :)

     
  12. May 30, 2017 #11
    Of course you can measure the time of emission. You just turn the source on for a very small amount of time.

    Andrei
     
  13. May 30, 2017 #12

    PeterDonis

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    And then you won't get an interference pattern at the detector; you'll only get a single dot where the single photon that was emitted by the source landed. (Actually, you might not get a photon at all, or you might get two or three, since the source does not emit photons at precise time intervals, there is only a probability of photon emission per unit time.)
     
  14. May 30, 2017 #13

    vanhees71

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    You'll also not get an interference pattern by collecting a lot of photons in this way since if you turn on the source only for a very small amount of time, the energy of the produced photons is very uncertain. So you will get a structureless pattern on your photoplate at the end.
     
  15. May 30, 2017 #14
    Of course you can repeat the process until the pattern forms.

    You don't need to turn off the source, you could just block the light with something.
     
  16. May 30, 2017 #15

    PeterDonis

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    How accurately can you measure the timing of when the light is blocked or not blocked? (The same question would apply to turning the source on and off.)

    Also, how do you know you will still get an interference pattern in this way? The usual QM prediction of an interference pattern assumes that all photons from the source make it to the detector.
     
  17. May 30, 2017 #16
    I think you can measure the timing as accurately as you want. For example you can place a rotating disk with a small slit in front of the source. The rotation speed determines the time interval when the particle is emitted and I see no problem with making that interval as small as you want.

    I have never heard of such a requirement and I don't believe it can be true. As far as I can tell QM's prediction is about each particle. Each photon has a certain probability to be detected at each point on the screen. The number of photons that are detected is irrelevant, but, in order to "see" the pattern you need a certain number of them.
     
  18. May 30, 2017 #17

    PeterDonis

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    The requirement could be stated in an equivalent form that applies to a single photon: the wave function of the photon exiting the source must have no restriction in emission time. (The usual assumption is that the photon is a plane wave.) Your scheme for measuring the time of emission puts a restriction on the emission time, and that changes the wave function of photons exiting the "source" (which is now the original source plus your apparatus for measuring the emission time).

    In short, your apparatus changes the wave function of each photon in the experiment, and I think this change will affect the prediction of what will be observed at the detector. I understand that you disagree, but I would like to see something more to back that up than just your opinion. A mathematical treatment would be nice.
     
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