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The Real Double-Slit

  1. Dec 24, 2005 #1
    Central to the mystery of the quantum world are the ramifications of the famed 'double-slit experiment', in which the same electron, it turns out, can be in two places at the same time. Alot of the 'strangeness', mysteriousness etc stems from this experiment, for as Feynman put it in the Character of Physical Law, "any other situation in quantum mechanics...can always be explained by saying, 'You remember the case of the experiment with the two holes? It's the same thing.'"

    It was only recently that I found out that the real experiment doesn't involve shooting an electron gun (or some such thing) through a wall with two slits towards a detector on the other side, that this is all an analogy for the real experiment, which involves shocking quartz crystals (or something like that). Does anybody have the details of the real experiment, or know where I can look at it?
  2. jcsd
  3. Dec 24, 2005 #2
    Where did you find that out? There are a gazillion demostrations of the real real double slit experiments (the one with the electron gun).
  4. Dec 24, 2005 #3
    What you are talking about is doing electron diffraction experiments using a crystal lattice as your grating, rather than two (or more) slits in a wall. You can do the same thing with graphite, or any other solids with a regular structure.
  5. Dec 24, 2005 #4
    Forgive me, I got confused. I came across the following quote from Feynman and I mistakenly thought it still held true:

    "We should say right away that you should not try to set up this experiment. This experiment has never been done in just this way. The trouble is that the apparatus would have to be made on an impossibly small scale to show the effects we are interested in. We are doing a "thought experiment", which we have chosen because it is easy to think about..."
    The following article helped to shed some light on the matter for me:

    In that article it states that a fella named Jönsson did the actual experiment using up to five slits in a wall, shortly after Feynman made the above quote.

    So, not only can the electron be in two places at the same time, but five? Is there then any limit to the number of holes through which an electron can be at the same time, then? What about a range between these slits, has that been investigated? At what distance - between slits - is an electron unable to travel through two or more slits at the same time?

    Any links or knowledge much appreciated.
  6. Dec 25, 2005 #5


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    I think you're missing the "punch line" here. Don't start with "electrons can be at many places at one time". Start with "electrons are described by a deBroglie wavelength".

    Once you have understood what that is, then you will no longer feel the need to want to know if an electron passes through 2 slits, 3 slits, etc... because you would have associated that with an analogy to light.

    Note that the "superposition in space", which is essentially the effect you're asking here, is VERY apparent in chemistry. H2 molecules, NH3 molecules, etc. exhibit properties such as bonding-antibonding bonds that can only be explained when an electron occupies several different locations simultaneously.

  7. Dec 27, 2005 #6
    So, in regards to my question about the range between slits through which the same electron can appear, would the answer be one deBroglie wavelength?
  8. Dec 27, 2005 #7
    You're thinking about this the wrong way. Unless you are attempting to measure which slit the electron is passing through at a given time, it doesn't make sense to ask the question "which slit does the electron go through?" because there's no possible way of knowing. It behaves as if it were a wave that hit the slits, but this does not imply that the electron is behaving like a classical wave part of the time, and then we measure it and it becomes a classical particle. It's some quantum object in between that is neither.
  9. Jan 4, 2006 #8
    Exactly. I believe we must think of the electron as a wave of probability. One cant measure the electron at either of the slits as this will destroy the pattern. This suggests that it goes through both slits. However, we must also think about the fact that if it does go through both slits, where will it appear (register in a detector) on the screen? This is where quantum jumps are involved. This is where the electron changes from being a wave of probability, to an electron again (hence it is detected).

    So as said, It is some quantum object that we do not know about yet.
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