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Double Slit

  1. Feb 5, 2014 #1
    Hi all. I am just trying to get my head around the double slit experiment(I know im not the first)
    I am stuck trying to understand the initial state of the individual photons when they are being fired sequentially.(their direction and place of origin)(where the quantum confusion sets in)
    An analogy often used is to imagine a gun shooting through the slits(bullets representing photons)and the suprise one would get if they created interference bands despite being fired sequentially. The thing with this is that the gun would have an initial direction which could be used to infer which slit each bullet went through.
    Is the photon emitter not the same? light can be directed just like a machine gun(lasers for example)so how in this experiment is the emitter set up so that the individual photons are aimed at the two slits? how are the photons emitted such that we cannot infer their initial trajectory?
  2. jcsd
  3. Feb 5, 2014 #2


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    Staff: Mentor

    We're shining a coherent light source on a screen with two slits in it, illuminating both slits and forming an interference pattern on a piece of photographic film behind it. The number of photons emitted per second is proportional to the intensity of the light, and because we're illuminating the screen and both slits, we're talking about a flood of photons (it's worth calculating roughly how many photons are emitted from a 1 watt light source in a second - "flood" is an understatement).

    Note that we are NOT aiming the photons. At best, we're aiming the beam of light, and we're aiming it so that it illuminates both slits equally.

    Under these conditions, the only sensible thing that we can say about the trajectory of any single photon in the flood is that when the photon leaves a spot on the photographic film, that's where it was at that moment. We don't know anything about which slit it came through; all we know is that it was part of the flood of photons that came pouring through the two slits. It seems natural to assume that it passed through one slit or the other - but that's an assumption based on our experience with macroscopic objects like bullets, may or may may not be the way photons work - it's not something that has to be true.

    Now comes the fun part.... we reduce the intensity of the light while continuing to illuminate both slots equally. The number of photons per second is proportional to the intensity of the light, so as we turn the light intensity down we get fewer photons and fewer spots appearing on the photographic film every second. Turn the intensity down far enough, and we only have one photon in flight at a time (we can calculate this from the rate at which the spots appear on the film and the known travel time from light source to film). The interference pattern still appears, and we still have no way of knowing which slot any given photon passed through.
  4. Feb 5, 2014 #3
    do you know much about the emitting device which is used? i am trying to understand this intensity reduction process. which is works i assume by lowering electrical input energy which then excites fewer and fewer atoms which release less and less photons down to the point where single atoms are being excited.So these atoms would be random(to us) we dont know which one emits a photon.and when we put a detector at the slits the process is no longer random.and random means there is no observer to the process.so it seems that as soon as there is an observer in any process the pattern no longer appears? and is this the same for all quantum phenomenon, they cannot be witnessed so to speak, only their spooky manifestations?
  5. Feb 5, 2014 #4
    ps. thanks for the very detailed and informative reply.
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