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Double Slit Experiment

  1. Apr 17, 2005 #1
    I just posted this as a reply, but i figured Id open a new thread for discussion.

    correct me if im wrong here.

    according to the double slit experiment, a photon takes every possible path to it's destination, causing it to interfere with itself, right? My first question is, does each separate path have a different probability of occuring, or do they all have the same probability?
    and if they do have the same probability, how would a photon know it's destination if more than one path intersects with an object? Couldnt the photon just as easily have appeared on the double slit cover rather than the photosensitive sheet? This would sort of signify that a photon knows it's destination before it get's there. otherwise, we might observe the same photon hitting more than one object, and therefore being at multiple places at one time.

    am i looking at this completely wrong here?

    add-on question: since only one path can be a straight line, the other paths would be larger, whch would mean the photon would take longer on those paths. "Every possible path", to me, means even the one that travels in 9 loops across the galaxy just to go three feet away. Wouldnt we observe that a photon arrives someplace more than once?

    none of this makes sense to me.
  2. jcsd
  3. Apr 18, 2005 #2
    Each time you think the photon has a path you are wrong. QM just says that you can get a probability to detect a particle somewhere and not the particle has a path (in the classical view).

  4. Apr 18, 2005 #3


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    There are many different mental pictures of what is happening to photons, electrons, etc. during the time when we don't see them. The many paths idea (originating with Feynman, I believe) is one of those descriptions. Nobody knows what really happens and to many physicists it just can't be described. Quantum theory predicts results of experiments extremely accurately, but it doesn't try to say what is going on otherwise.
  5. Apr 19, 2005 #4
    The idea of many paths was due to Feynman. It is simply one explanation of single photon interference patterns (one which I happen to think is quite clever).
  6. Apr 21, 2005 #5
    They are all treated as being of equal importance but because of interference between the paths, all the numbers which could have been the probabilities of individual paths don't add together to equal one and so they are not probabilities, they are just numbers for the paths. :smile:

    It seems the modern way of looking at it is that ever possible history interferes with every other possible history to influence the final event we detect or see, so the histories in which the particle went through a single slit, bounced off the double slit cover, never made it out of the emittor, looped-the-loop for no reason, etc. all still have an influence on the final event even if the final event would appear to be quite different from almost all of the histories which influenced it.

    A particle can have paths in its sum-over-histories in which it travels faster than light, slower than light, backwards in time, in strange zig-zags across space and even loop around and around in time! It seems strange that a "particle of light" like the photon has countless possible paths in which it actually travels faster and slower than the "speed of light" but it does. The looping in time is a way to interpret the Casimir effect, so a particle actually can have its own detectable Groundhog Day. :biggrin:
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