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Something's wrong in < Intro to QM >, by Griffiths

  1. Feb 5, 2012 #1
    Hi, I am Liu, a undergraduate in mainland China, I post out this idea and want to
    see how you gentleman think, here it is:

    Griffiths says, in his book < Introduction to Quantum Mechanics >, if an insect fly
    in a path perpendicular to the direction in which lightbeam goes, the velocity of
    its shadow is proportional to the distance between light source and shadow.

    I think the problem is, the velocity of insect's shadow is same as insect's v.

    From perspective of wave theory, you can draw a picture below:

    --Goal plane---
    |||\ \|||||||||
    ||||\ \||||||||
    |||||\ \|||||||
    |||||||\ \|||||
    ||||||||\ \||||
    Two parts of the space is divided, 1 is for in which light doesn't exist, opposite the 2,
    and is the insect.
    Now think of the move of "slash tunnel", it is like "be pushed by light", hence the end of
    the tunnel, or the shadow, is moving at the insect's speed on the goal plane.
    Am I right?

    Sorry for grammar error, if any.
    Last edited: Feb 5, 2012
  2. jcsd
  3. Feb 5, 2012 #2


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    It would be true for Griffith if he is talking about a central light source that radiates spherically - the rays are not parallel.

    In your case you have parallel light rays, so in your case your version is true.
  4. Feb 5, 2012 #3
    Thank you for replying, I get it.

    So further question, once you have a shadow travelling with v larger than
    speed of light, we now set a negative film far far away, rolling like a tape,
    and shot bullets in front of the source in certain frequencies, meanwhile we
    are very lucky that there's a supercomputer interpreting those shadow
    frequencies into human language, didn't we just give information over the
    speed of light ?
  5. Feb 5, 2012 #4
    What you think the answer is to this question may help: A radio station is transmitting information in all directions. If houses A, B, and C are all 50 km from the station and are the vertices of an equilateral triangle with the station at its center, can one say that information received at A was transmitted faster than the speed of light to B and C?
  6. Feb 6, 2012 #5
    Right, that is much clearer. Thanks.
  7. Feb 6, 2012 #6


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    2017 Award

    The shadow will not be a straight like but, at great distances, a spiral shape, spreading outward at the speed of light. A distant receiver will see flashes (as from a lighthouse) and there timing will be delayed due to the transit time for the light.
  8. Feb 6, 2012 #7
    Clearer in that I've restated your problem, or in that you saw my point? :-)
  9. Feb 6, 2012 #8
    I saw your point. Thank you, everyone.
    Last edited: Feb 6, 2012
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