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Double slit question

  1. Feb 11, 2015 #1
    this is probably a stupid question but is it not possible to know from which slit the electron or photon came when you time the impact on the detector screen? sometimes the electron can only have come from one slit because it must have gone faster than light otherwise no?
  2. jcsd
  3. Feb 11, 2015 #2


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    It's an interesting idea to be sure.

    In order to carry this experiment out, you would have to be capable of measuring precisely when the electron passes through the slits, and precisely when it hits the screen in back.

    The precision would have to be good enough to resolve the time difference between propagating from slit A or from slit B to the point on the screen. This time difference would be of the order of the time it takes the electron to travel a deBroglie wavelength (given its momentum).

    Unfortunately, here is where the uncertainty principle comes into play. In order to record the time that an electron passes through the slits to enough precision, that electron has to interact with some measurement device strongly enough, that the resulting back-action on the electron destroys the interference pattern you would otherwise see.

    So, hypothetically, you could measure which slit the electron passed through with timing information, but doing so runs into the same problems as other measurements devised to figure out which slit the electron went through.

    Good question, though.
  4. Feb 11, 2015 #3


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    In quantum mechanics, particles are not assigned trajectories, so it is not part of the theory to say which slit a particle went through.

    It is possible to know if one has a model for the trajectories, such as a form of Bohmian Mechanics. However, there is more than one such model consistent with quantum mechanics, so until there is experimental evidence for a particular one of the many possible models, the question cannot be answered uniquely.
    Last edited: Feb 11, 2015
  5. Feb 11, 2015 #4


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    To get an interference pattern at all, you need multiple particles. If you arrange the geometry in such a way that they all could only have passed through one slit on the way to the screen, then your setup is equivalent to having a detector at each slit.
  6. Feb 11, 2015 #5
    A stupid question? No, a very good question.

    First, the word and concept particle is drawn from our macro sensory experience and is not useful in describing micro quantum phenomena. Perhaps, we should make up and use a word like 'quanticle' to eliminate the confusion created by the use of 'particle' in the literature. To date, the best description of a photon is obtained using the Schrödinger wave equation.

    Now, to answer your question -- Probably not.

    The information we get from quanticle-detectors are measurements of incident energy vs. time, and at non-absolute zero these measurements are always accompanied by additive noise. We understand from the uncertainty principle that the more accurately we measure the time of occurrence of an energy event the less clear it becomes that there was an event to measure.

    Hope this response was informative,

  7. Feb 12, 2015 #6


    Staff: Mentor

    Indeed it was.

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