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Michelson and morley interposition of the experiment

  1. Dec 25, 2012 #1
    Michelson and Morley conducted their famous aether wind experiment and then after finding their first result they interposed the experiment and started it all over again. Why would they do this?
  2. jcsd
  3. Dec 25, 2012 #2


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    Staff: Mentor

    I'd guess that they devised a more accurate experiment and wanted to make sure.
  4. Dec 25, 2012 #3


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    Gold Member

    I cannot understand your question. I looked up the meaning of "interposed":
    in•ter•posed in•ter•pos•ing
    Definition of INTERPOSE
    transitive verb
    1 : to place in an intervening position
    2 : to put forth by way of interference or intervention
    3 : to introduce or throw in between the parts of a conversation or argument

    These definitions do not help explain your meaning for me. Will you please describe more specifically the Michelson and Morely experiment you are referring to? Will you please post a reference document? Thank you.

  5. Dec 26, 2012 #4
    I'm sorry Bobbywhy thats how my text books have all stated it, the meaning of interpose is to turn the whole experiment around, 90degrees.

    So for this experiment you can image a + sign to visualise how the light travelled. The Bottom end would be where the light went into the interferometer. After the experiment was finished you can imagine turning the whole experiment 90 degrees clockwise. The new location for the interferometer would now be on the left side of the + sign.

    I hope that makes sense?
  6. Dec 26, 2012 #5
    O.k., I found how they are using the term "interpose" here...


    “… they raced two light rays over two courses, one into the supposed aether wind and one across it, then swung the apparatus through 90 degrees to interpose the rays.”

    Michelson and Morley were trying to detect an aether wind, and their apparatus was composed of two beams of light which had been split at 90 degrees from each other. They supposed that the direction of one beam might be heading perpendicular to the aether wind and one beam might heading parallel to it.

    However, if they did measure a difference between the speed of the two light rays as they were hoping, it might be because their apparatus had some flaw in which one path of light was different from the other. In other words, recording a different speed might be because the path lengths that the two beams travelled were slightly different form each other. So what they did was rotate the apparatus 90 degrees so that they would be swapped with respect to which one was heading parallel to the aether wind and which one would be perpendicular to it.

    Note: Having said all that (and I hope I got it right), I think what they actually did was rotate the apparatus gradually, and take measurements from all angles.
  7. Dec 26, 2012 #6


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    MikeGomez, Thanks, you have discovered one source that uses "interpose" when describing the M-M experiment. Note that they "interposed the light rays". Not what OP posted: "interposition of the experiment". Well done!

  8. Dec 26, 2012 #7
    Nicely said Mike, so what your saying is that by interposing the rays the scientists could check that the path lengths that the light travelled were the exact same, making the experiment valid?
  9. Dec 27, 2012 #8
    Yes. They did it for error correction. This measurement precision issue gets quite tricky though. You see, if space changes such that light travels a shorter or longer distance, so does the length of M&M’s apparatus, and so does any measuring device, and anything at all within that space that we are talking about.

    I don’t really think I can explain it very well, but if you are interested in physics this should be a fascinating study for you. Do some research on “Length Contraction”. Here is a quote from…


    “Length contraction was postulated by George Francis FitzGerald (1889) and Hendrik Antoon Lorentz (1892) to explain the negative outcome of the Michelson-Morley experiment and to rescue the hypothesis of the stationary aether (Lorentz–FitzGerald contraction hypothesis).”

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