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Double slit

  1. May 17, 2006 #1
    I'm only a first year psychology student, so I appologize for my lack of understanding. But looking at the double slit expirament described in Feynmans book QED, for example, where two slits seperate a photon emitter and detector, where the seperation distance of the slits determines how many photons reach the detector.

    So lets say that 8% of the emitted photons are getting through to the detector. I suppose that we can assume that the other 92% are ending up elsewhere, hitting the boundry or something. Im also guessing that a wall could be interpretted as a detector, that when we say the particle must be observed that simply means that it must interact with something.

    So if you change the distance between the slits so that only 4% are getting through to the detector, this means that the percent of photons ending up elsewhere increases. So if we place a second detector close by the emitter why woulndt we be able to send signals faster than light (by recording the change in photon rate)?
  2. jcsd
  3. May 17, 2006 #2

    Doc Al

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    Not exactly sure what kind of scheme you are devising, but since the photon rate is due to photons (duh!), any change in photon rate will be transmitted to the second detector at the speed of light.

    I'm not seeing this as any different that having a light bulb shining onto a screen. You can change the rate at which photons hit the screen by blocking the bulb with your hand. But that change in photon rate--and any information it may carry--travels at the speed of light.
  4. May 17, 2006 #3
    Something like this:



    If you change the distance between the slits in the shield it should change the probability of photons reaching both detectors instantanously should it not?

    I dont suppose that if we separated the shield and the 2.detector from the emitter by one light year, and then altered the distance of the slits, that the change in rate of photons reaching the 2.detector would take 1 year to appear. If that were so then the photons arriving that that detector would seem to be violating our QM rules for that amount of time as they would be finding a way through the shield contrary to predicted.

    Would then the change at the second detector happen very quickly, but the change at the first take 1 year? So then we have a discrepancy in the amount of photons emitted and those actually ending up somewhere.
    Last edited: May 17, 2006
  5. May 17, 2006 #4

    Doc Al

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    Still not sure what you are doing with the multiple detectors.

    I don't see how. Changing the slit separation will affect the paths of any new photons passing through the slits, but I don't see how it would affect the paths of the photons that have already passed through the slits (with the original separation) and are already enroute to the screen.
  6. May 17, 2006 #5

    Doc Al

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    Why not?
    Why do you say that?
  7. May 17, 2006 #6
    Forgive me but I cannot tell if you have no clue at all what the double slit experiment is, or if I’m having a major vocabulary problem translating from your ‘psychology’ background to our physics jargon.

    On the off chance that you don’t, can you review and confirm you understand the point of the double slit. You seem to have be dealing with just one defector behind the barrier with two slits (the view screen area) and adding one in front of the slits. Unless the one in front is involved blocking the path from the point source to one of the slits this extra one should be totally meaningless.
    Plus, the two slit does really give meaningful data with just one detector in viewing area ether. Most useful is to visualize an infinite number of detectors as you would have using a wide roll of film aching around a 180 degree screen being exposed by the photons coming though over some period of time. That is capturing 100% of those going the double slits. By measuring the intensity (% of photons) across the light and dark pattern crated across the film strip you could plot the light and dark areas as shown in the red graph here.

    http://www.physics.northwestern.edu/vpl/optics/diffraction.html" [Broken] - (Try it with a impossable slit width of 1.)

    The point is the measured result is the same with a “normal” light source or one the has been proven to send just one photon at a time. Hence the need for long exposure time or detection methods. That is the perplexing issue in the double slit being addressed by various theories etc. How does the pattern survive when only one photon at a time is sent?

    Are you on the same page as above or were you somewhere else we are not getting?
    Last edited by a moderator: May 2, 2017
  8. May 20, 2006 #7
    What you seem to be missing is that the photons travel not just through space, but also time. Though in the present the slits have been pinched, in the photons past, the slits were wider, and thereby keep their own intererence patterns unchanged.

    Quite simply, unless you can travel back in time, and change the slits width in the photon's past, there is no way altering the slits in the present will affect photons that have already been sent on their way.
    Last edited: May 20, 2006
  9. May 20, 2006 #8


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    Are you basically saying that if you know what happens at one detector you know what will happen at the other thus you are somehow looking into the future?

    I mean I don't see how that's any different than a classical example: you have two marbles in a bag. One is red and one is blue. You pull out one marble at random and note its color. You can now predict with 100% certainty which marble you will pull out next.
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