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Double Slit Question

  1. Sep 26, 2006 #1
    If an electron moves through a double slit without being subjected to any measurement device it will act as a wave and cause
    interference patterns on a screen. If the electron it measured before or after the slits it will act as a particle and no interference patterns
    will be be seen on the screen. What if the screen where to act as both screen and measurement device (to measure spin for example).
    Is this possible? Would the screen act as screen and measurement device at the same time "fooling" the electron ?
    Can it be done? and what would be on the screen: interference patterns or no interference patterns?
  2. jcsd
  3. Sep 26, 2006 #2
    ‘Fooling’ the electron not needed & it does not need to be detected at the slits.
    The point is when they set up a detection field at just one of the two slits, sufficient to detect any electron going through that one slit. And then they eliminated the screen detections when the slit detector did detect an electron going though that slit. Therefore the remaining screen detections we know are for electrons going though the other slit where there is no detection at the slit. But still no interference pattern is produced for that undetected group at the screen, just the sure knowledge of which way seems to be enough to destroy pattern production – THAT IS THE PARADOX.
    Only solved by non-local assumptions like QM.
  4. Sep 26, 2006 #3
    What do you mean by knowledge is enough? :confused:
  5. Sep 26, 2006 #4
    By placing the detector at slit A you will know when the electron passes through this slit. Since slit A is blocked, only those electrons that go through B will strike the detector at the back.

    Since you've blocked the detector for electrons going through A, you know that the pattern produced on the back detector is from slit B - hence you have knowledge of which path the electron travelled, this knowledge is enough to destroy the interference pattern.
  6. Sep 26, 2006 #5
    But to assert that it is our knowledge of the situation that causes the "destruction" of the interference pattern is rather unscientific IMHO, it is more like some anthropomorphic view of reality.

    I think that "our knowledge" has absolutely nothing to do it.

    But since you make the assertion, would you care to demonstrate or explpain that it is in fact our knowledge that causes the "destruction" of the interference pattern?
  7. Sep 26, 2006 #6


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    Well, what exactly _is_ the measurement process?

    The Wikipedia page on "[URL [Broken] cat[/url] sais:

    Apparently, the act of observation doesn't have anything to do with knowledge (or counsciousness) of the effect, but rather what matters is whether the particle produces an irreversible thermodinamic event or not.

    Can this be generalised, to say that :
    thermodinamically irreversible efect => no interference patterns
    else => interference patterns

    The current way of looking at things leaves lots of room for New Age theories who say that you can counsciously change/create your own reality simply by observing the world around you .. which is too "Matrix" for me to accept.
    Last edited by a moderator: May 2, 2017
  8. Sep 26, 2006 #7


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    Staff Emeritus
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    Hear, hear. But then what is it that can be "collapsed" by an irreversible thermodynamic process? If we regard the state function as a kind of library or database of states possible after the next I.T.P. then where is it kept?
    Last edited by a moderator: May 2, 2017
  9. Sep 26, 2006 #8


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    I don't think that's something anyone can answer, but then again, i'm not an expert, so I'm just quoting some guy:
  10. Sep 26, 2006 #9
    Back to the OP, I think the answer is no. QM implies that some measurements fundamentally are mutually exclusive (which is difficult to grasp classically).

    Unfortunately, just like perpetual motion machines, it often takes less time to propose a method of "circumventing the known laws of physics" than what it does to understand why that particular method will fail.
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