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2 slit questions

  1. Jul 26, 2010 #1


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    My first post. I'm an interested layman.

    1. There was a thread about this a few weeks ago but it didn't have an explanation that I could understand. Suppose my apparatus has a source that is not centered between the slits. Suppose I can measure the travel time for each particle from source to screen. With detectors at the slits I measure travel times of each clump on the screen and find that the travel times of each pile is different and agrees with what I expect. Now I shut off the slit detectors and start measuring travel time for each particle. Do I get interference? Can this experiment be done? Has it? Does being able to measure these times collapse the pattern because it gives me which way information?

    2. Suppose I have a typical apparatus. I find that with detectors at the slits 1% of the particles get to the screen and 99% hit the barrier and are stopped. Now I shut off the slit detectors. Do I still see 1% getting to the screen? If so, does it mean that even though the particles are now ghosts, 99% of them are still banging their heads on the barrier? If they're spooky enough to go through both slits why does the barrier stop them? Does this mean that the "real" particle has to go through one of the slits and then it's ghost goes through the other one.
  2. jcsd
  3. Jul 27, 2010 #2

    That really depends on the design of detectors. The point is that active detectors interfere with the particles strongly enough to disrupt the interference pattern, otherwise they won't detect anything.

    In my opinion, the double slit experiment is not the best way to approach the problem of understanding QM ... unless you understand the underlying principles, it's an almost infinite source of questions and "paradoxes" (as evidenced by this forum!)
  4. Jul 27, 2010 #3
    There's a discussion thread about your very proposal over here: http://bb.nightskylive.net/asterisk/viewtopic.php?f=30&t=19826

    It's perhaps a bit confused though. IMO the best answer is:

    The particles travel as wave packets that have a finite length. In the experiment, you can imagine one wave packet that goes through the left slit, then spreads out and hits the screen, and a similar wave packet that goes through the right slit, then spreads out and hits the screen. The two packets overlap when they spread out behind the slits, so you get interference. Now, in order to use timing to determine which wave packet you're detecting (left slit or right slit) you need to set things up so that the difference in path length is longer than the length of a wave packet, so that one wave packet finishes arriving before the other one starts arriving, and they won't overlap. But if they don't overlap you don't get interference, period. No matter what your detectors are doing, if the geometry prevents the wave packets from overlapping, they can't interfere with each other. They're like two ripples on a pond. If they overlap each other they form a complex interference pattern and you can't say which bump on the surface of the water came from which ripple. But if they pass the same point at different times nothing interesting happens.
  5. Jul 28, 2010 #4
    I shall now quote page 304 of 'The Emperor's New Mind' by 'Roger Penrose' (1999 publication), he is commenting on the double slit experiment:

    Individual particles DO act in accordance with the probabilities of interference, which is one of the key aspects of the quantum world.
  6. Jul 28, 2010 #5
    Yes, sorry, I wasn't clear: I meant that you can imagine the single particle splits into two probability wave packets, each of which passes through one slit and interfere with each other (so the particle is interfering with itself) behind the slit.
  7. Jul 28, 2010 #6


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