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Michaelson-Morley expt. problem

  1. Jun 8, 2013 #1
    The M-M expt. concluded the constancy of speed of light, by emitting a beam of light, one horizontally and other vertically to the direction of the motion of the earth. Since they returned from the mirror to the source at the same time (having no effect of earth's motion on them) it was concluded that light speed is constant and independent of the source.

    The problem:
    My argument is, even if we replace the beam of light by two ordinary balls, the result is the same, i.e. they will return to the source at the same time (considering equal distance traveled and common speed). To explain in detail (refer to attachment):

    If the two balls start at pt. A, one horizontally and second perpendicular to other and if they return from two reflectors R equidistant from A, they reach the source A, at the same time. This is true even if this set up is on a uniform moving train as shown.

    Here we won't conclude that the speed of the balls is constant irrespective of the motion, as we have concluded for light. Kindly explain.

    Attached Files:

  2. jcsd
  3. Jun 8, 2013 #2


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    I think you have misunderstood the conclusions of MMX. It did not measure the speed of light or conclude that it was a constant. What it did was determine that the round trip speed of light along two perpendicular paths were equal to each other no matter when then did the experiment or in what orientation they did it and since they carried the light source with them, they drew no conclusion with regard to the speed of light being independent of the source.

    So I think you have correctly figured out from your analogy with the balls that if they had come to those conclusions, they would have been illogical conclusions.

    By the way, why did you think those were conclusions of MMX? Can you point to a link or reference that makes that claim?
  4. Jun 8, 2013 #3
    I think MMX tried to find if the speed of light C was relative to ether. Since it could not see any change in speed of light relative to ether, it concludes that it's speed is independent of source.
    Correct? Or are there any other expts. proving constant speed of light?
  5. Jun 8, 2013 #4


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    M&M believed that the speed of light was independent of its source because they believed light propagated relative to the ether. So I don't know why you would think that the null result of MMX would also conclude that the speed of light was independent of its source. Ether or not, MMX or not, M&M believed that light is independent of its source. And that's because they believed in a wave model for light.

    But, yes, the definitive experiment that light is independent of its source is de Sitter's.
  6. Jun 8, 2013 #5


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    Staff: Mentor

    Balls don't have a constant speed relative to a medium. If you did the experiment with sound you'd get a positive result.
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