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Double Slit experiment

  1. Nov 1, 2007 #1
    Double split creates a wave interference pattern.

    Closing up one slit creates a particle like pattern (all hitting one spot).

    Does the closing up of one slit (allowing the particle to go through in only one narrow spot) measure its position (by the powers of Heisenburg Uncertainty Principle), then only allow the momentum (wavelength) to be random? If so, then are there different colored photons striking the photographic plate when one slit is closed?

    ^^And is this guy entangling another particle with his particle counter at the slit? I went online looking for that apparatus on his presentation and didn't find anything.
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Nov 6, 2007 #2


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    Closing up one of the slits does not recreate any sort of observation.
    You're merely removing one of the possible paths for the electron (or whatever particle you're experimenting on) to traverse; as a result, there are no longer the two possible states (and thus, no superposition of the wavefunction - which is what causes the interference).
  4. Nov 7, 2007 #3
    You are missing a hugly significant point on the double slit.
    With one slit open the “particle like pattern” does not have the photons “all hitting one spot”.
    They are spread out over a wide area, bright in the center dim to the edges.
    HOW wide is that area?, - just as wide and almost the same position as with just the other slit is open. So close to the same position you would need to run a very large number of photons through to determine the center of the “one slit spot” accurately enough to guess which of the two slits was actually open.
    Also the width of both of these “spots” is just as wide as is covered when both slits are open. With both open it is also bright at the center dimming to the edges, only that width is broken up into a pattern of brighter and darker bands, instead of a smooth change from bright to dim. The area covered by the light does not change – only that it broken up into a bright & dark pattern when both slits are open vs. one slit.

    That is clearly not understood by guy in your Youtube Link.

    The poor folks at the church that received that YouTube Delayed Choice Quantum Erasure lecture got one of the worst explanations of Delayed Choice I have ever seen.

    The speaker there has no clue whatever. Run a search in this forum for “delayed choice” and find a PDF link to a real DCQE experiment.
  5. Nov 8, 2007 #4
    So the Heisenburg uncertainty principle isn't driving interference pattern? huh.

    Ok, the delayed choice experiment. In the REAL experiment, it looks like another entangled photon is created at the slit. If thats the case then wouldn't the deletion of the entangled photon kind of clash with the "concious observer" theory?
  6. Nov 8, 2007 #5
    I have no clue what your talking about or what “driving” means.

    On a screen behind a double slit with a long count of thousands of individual photon hits you find a ten inch wide collection of hits.

    Smoothly distributed from end to end bright in the middle of the ten inch span:
    one slit covered.
    Across the same ten inch span twice as many hits are found but displaying a pattern AKA interference pattern with the band in the center being the brightest:
    Both slits open

    Both distributions are well predicted by QM probabilistic formulas.
    And in both cases the expect location of any one photon hit is an “uncertainty” as defined by those probabilistic formulas. QM gives no certainty for the one.

    Entanglement comes from SPDC’s not double slits, extra photons are not created as one goes through a double slit.
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