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Question about electrons

  1. Jun 5, 2006 #1
    suppose we have a double slit and we shoot electrons throug hte slits, one at a time, we will see an interference pattern on a screen just as we see with light passing through two slits. Now suppose we placed a detector near the slits so we could find out which slit the eltron passed through, then the intereference pattern is ruined and we only see two bands on electrons on the screen. Why is this?

    Is it because that in order to detect which slit the electron passed through, we need to shoot a photon at the electron and as a result the photon ruins the trajectory, thus the probability wave (location where the electron will hit the screen) of the electron?

    Isn't an electron a charged particle? A moving particle creates a magnetic field. Suppose We measure the magnetic field right before the electron passes and then right as the electron passes we could pinpoint which slit the electron passed through. Wouldn't this work?
  2. jcsd
  3. Jun 6, 2006 #2
    I still have to read the remaining part of your post but according to quantum theory, you cannot say that the electron is going through a particular slit. In fact the electron goes through both slits. This is very nicely discussed in Feynman's lectures. Perhaps OP can enlighten you more....I am feeling sleepy :zzz:
  4. Jun 6, 2006 #3
    Lets see.Probably if you are confused that why a single electron gives a twwo band pattern...that has to do with the wave-partiicle duality ... a single electron moving with some velocity will have some wavelength associated with it in the form of matter-wave...so if you draw the wavefronts...you may get a better idea...you may refer a textbook.
  5. Jun 6, 2006 #4
    According to the http://en.wikipedia.org/wiki/Copenhagen_interpretation" [Broken] and it behaves as if it were a particle again. The important point here is that it's not because a photon hits it or because of some classically explainable consequence of the experiment, it's simply because the electron was observed.
    Last edited by a moderator: May 2, 2017
  6. Jun 7, 2006 #5
    Essentially, what this means is that every measurement disturbs the measured quantity due to a quantum-mechanical interaction between the measurement device and the quantity to be measured. This interaction cannot be understood properly. You can use terms like "wavefunction collapse" but the deeper implications are not completely understood yet. As I said earlier, Feynman discusses these issues in his lectures which you definitely must read up.

    Quote from http://math.ucr.edu/home/baez/open.questions.html#quantum

    Last edited by a moderator: May 2, 2017
  7. Jun 8, 2006 #6


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    Semi-classically, from a laboratory (not theoretical) perspective:
    The trouble with "measuring the magnetic field" before and after the slits
    is that the coils (say) that the B-field pierces will conduct current then,
    and the magnetic field produced by the coils (while they measure)
    will influence the electron as it passes the obstacle.

    Closely-spaced slits would require coils to be about the same distance away; at least one electron must be pushed around the coil to measure.

    It MIGHT be true that EVERY possible path is sampled ...
    but we'll never know, since only TWO have non-negligible amplitude.
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