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Double-slit experiment questions

  1. Sep 20, 2012 #1
    I recently been reading up quantum physics including the double-slit experiments and I got two basic questions:

    1# Why does the electron not act like a wave when there is only one slit? Shouldn't a single slit still result in a wave like pattern?

    2# 'Measuring' the wave seems to turn it into a particle. Even if this is done after the slits. Yet some scientist are calling this potential time travel. How do they know its not simple a wave that instantly turns into a particle at the point of 'measurement' without any 'time travel'?
     
  2. jcsd
  3. Sep 20, 2012 #2

    Drakkith

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    Staff: Mentor

    It does. If you map out the pattern from thousands of electrons on the detector, you will find that it matches that of a wave passing through a single slit. See here: http://physics.bgsu.edu/~stoner/P202/quantum2/sld012.htm

    The pattern matches that of light that passes through a single slit as well.

    First, let me clarify that when we detect any particle, we are detecting it as a particle. It is only when we look at the combined pattern of many many particles do we see the wave-like effects such as interference. As for time travel, I don't see it that way.
     
  4. Sep 21, 2012 #3
    But that still leaves me with the same question... What if we 'measure' every single one these electrons/photons before during or after the slits, we wouldn't see interface in any of these situations right?. So why isnt the conclusion that the measures are simple 'converting' it into particles?
     
  5. Sep 21, 2012 #4

    DrChinese

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    When this test is done with polarized light, you can see why this view can be eliminated. Have the source light polarized at 45 degrees. Place a polarizer in front of the left slit oriented at 0 degrees. Place another polarizer in front of the right slit also oriented at 0 degrees. RESULT: no information is gained when light comes through the slits as to which slit the photon passed through. Therefore, an interference pattern IS seen.

    Change the polarizer in front of the right slit to be oriented at 90 degrees. The photon can now only go through the left slit OR the right slit, but NOT BOTH. Further, if you were to measure the polarization of the photon when it hits the screen, you would know which slit the photon went through. You don't actually need to do this, it is enough that you could. RESULT: information IS gained when light comes through the slits as to which slit the photon passed through. Therefore, NO interference pattern is seen.

    Obviously, the only change is the orientation of a polarizer and that is sufficient to change the results from interference to no interference. So that does not correspond to your hypothesis. Because the same kinds of measurements are happening regardless of polarizer orientation!
     
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