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Feynman's detectors at double slit

  1. May 28, 2014 #1
    As far as I know Feynman was the first who told that if you would put detectors at the double slits the interference pattern would disappear, because of "knowing with path". But I think this specific experiment has never been done because a photon cannot be detected without absorption. So if Feynman could not have done the experiment, why did he stated in "QED, The strange theory of light and matter" that this is how nature works? Or is it the result of his calculation of paths, so he expects that nature would work so.
  2. jcsd
  3. May 28, 2014 #2
    I honestly don't know exactly what Feynman said in that context, however the double slit experiment also works with electrons. In that case you can detect their position without involving any absorption.
  4. May 28, 2014 #3


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    This describes a recent "which way" or "Welcher Weg" experiment with electrons:

    Or this one:
  5. May 29, 2014 #4
    But experiments which electrons has also not be done in Feynman's time. So what was the theoretical base of his statement?
  6. May 29, 2014 #5
    As far as I know, Feynman never attempted the actual experiment with observation for photons. I do not believe such equipment was even available. What he could have done, however, was carry the experiment out with high-energy photons in a cloud chamber, thus revealing their tracks. This should (at least in principle) destroy the interference pattern.
  7. May 29, 2014 #6


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    It's a fundamental result of QM; Feynman goes into the details in "Lectures on Physics", the third volume.
  8. May 29, 2014 #7


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    You can know which way of photons without absorption. You simply place a polarizer in front of each slit oriented 45° relative to the source. When those are parallel, there is interference. When they are crossed, there is no interference.
  9. May 29, 2014 #8
    There Feynman says:

    "If an experiment is performed which is capable of determining whether one or another alternative is actually taken, the probability of the event is the sum of the probabilities for each alternative. The interference is lost"

    It would be jumping into conclusions to say that the interference is lost "because of knowing which path". Does he actually say that in the QED book ?
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