Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Double Slit Experiment - Question about states of observations.

  1. Aug 20, 2014 #1
    Hi, can smart people please assist me with understanding something: why doesn't the "final" observation at the back screen create the same kind of effect as an observation made before the particle hits the back screen?

    Simple term please; no PhD here. Thanks.
  2. jcsd
  3. Aug 20, 2014 #2
    An observation made before it hits the back screen (i.e. at the slits) is a which-way measurement. Thus, the quantum system in question acts like a classical particle and either goes through one slit or the other.

    In the case of observation at the back screen, we cannot find out, by noting where the system lands on the back screen, which slit the particle had gone through. Therefore interference will appear and no which way information is available to the experimenter.

    Does that answer your query?
  4. Aug 20, 2014 #3


    Staff: Mentor

    Exactly how much before?

    A bit before wont make any difference.

    Very close to the slits means you simply get blobs around the slits.

    In between you get sort of a mixture.


    Forget the stuff about wave-particle duality - that's wrong:

    Sorry - the jigs up. The correct explanation requires - horror of horrors - math.

    For electrons here is the why (photons are a much more difficult issue to do correctly - although most beginning texts treat them the same):

    See section 2.2 on the scattering from the double narrow slits.

    So if the distance to the screen is small then the slits will not scatter them enough before they are detected so its the sum of the states from both slits, which is how you get interference. That's why you get two blobs - basically its the geometry of the situation.

  5. Aug 20, 2014 #4
    Thanks, Stevie, but not really.

    I was trying to look at it this way: the back plate is, in essence, also an observation that asks the question "which way?", and if you ask the question when - or after, I suppose - the particle has reached the plate, then the answer is "both".

    However, It seems that if you make an observation to ask the same question anytime/anywhere before the plate, whether before or after the slits, then the answer is "either, or".

  6. Aug 20, 2014 #5
    Hi Bill.

    Thanks. I'll read the paper, but I assume that the intended audience is versed physicists, so it'll probably lose me at many point.

    I was hoping there might be a Feynman-like answer to go along with the mathematics.
  7. Aug 20, 2014 #6

    So, if I understand correctly, where you make the observation is important: there's something like a gradient blending of the apparent "either, or" and "both" results, dependent on where you make your observation/measurement. The closer to the slits, the closer to the single path result; the closer to the screen, the closer to the many path result.

    Is that right?
  8. Aug 20, 2014 #7


    Staff: Mentor

    Yea - its hard - but I will try.

    When the particle goes through the slits its state is the sum of the state at each slit. However the 'direction' of that state is random and the reason you get an interference pattern is if you go through the math that's how they sum.

    But if you have the screen really close to the slits the direction its scattered in will be intercepted by the screen before it has had a chance to sum and get the interference pattern.

    Hope it helps, because its the best I can do.

  9. Aug 20, 2014 #8


    User Avatar

    Staff: Mentor

    The black plate is an observation, but the question it is asking is "Where is the particle at the moment that it hits the plate". It tells us nothing definite (as opposed to a probabilistic statement about where might have found it if we had looked) about where the particle was at any time before that moment.

    You'll often see the double-slit experiment explained that way, but as far as the mathematical formalism of quantum mechanics is concerned, it is neither "either", nor "both", nor "or", nor "one, but we don't know which". It is undefined - when we aren't measuring the position of the particle, it has no position.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook