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Difficulty with imagine ( MM experiment)

  1. Sep 15, 2008 #1
    I have difficulty with imagine why i must see the shift in the interference fringes when we rotate the apparatus 90 degrees !!!
    what exactly happen to make me assuming see this shifting?( in michelson-morley experiment)

    because when i finish from rotating, the same rays will be interference?! and i will see the same pattern!!

    and during the rotating the time difference between the two rays it will be constant because any decreasing time in one ray facing increasing time of the other ray ( of course this is when we take ether in account)like what was expect when did this experiment..

    thanks in advance
  2. jcsd
  3. Sep 15, 2008 #2


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    Staff: Mentor

    What you are probably missing is that the transit time of the ray that moves perpendicular to the ether travels a longer path and thus is delayed.
  4. Sep 15, 2008 #3
    Huh? :confused:

    Note however that while the orbital path of the Earth does not influence the MM measurements, the rotation around the Earth's axis does influence it.
  5. Sep 15, 2008 #4


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    On the timescale of the experiment neither is really important, although the experiment is repeated multiple times at different times of year so that even if the apparatus happened to be at rest relative to the ether on one trial, it wouldn't be on all of them. But the idea is just that as long as the apparatus isn't at rest relative to the ether on a given trial, then if the arms are adjusted so there's purely constructive interference when the apparatus is oriented one way, then if the stone table the apparatus was sitting on was then rotated, they would expect to see some destructive interference as the orientation was changed. There's a good writeup of the how the experiment works on this page, it's a little different than what I had imagined based on more schematic descriptions I'd seen elsewhere (for example, there was no attempt to make sure that the lengths of the arms were identical, just that the positions of the mirrors were such that there was no destructive interference in the original orientation before the table was rotated).
  6. Sep 16, 2008 #5
    Hi again :0

    first of all (russ, jennifer, jessem)thanks for the effort ..

    yes , i know that (by the way I think you reversed the hypothesis.. perpendicular ray take less time than horizontal ray)

    and what I mean here was the properties of the rays when they are interference is the same ( i.e time different it will be the same because we just exchange the position of the rays but still one of them perpendicular and the other horizontal of the ether so we see the same pattern and we don't note the rotating )

    and here I mean the components of each ray according to the influence of the ether..

    sorry I don't get your point here and how it is relating to my question ?

    I looked to the page and it will be clear my confusing if I am sure about this point ( he Suppose that initially, the apparatus was arranged so that both paths were oriented 45 degrees away from the motion of the Earth through the ethe, then oriented by 90 degrees and see the different in the pattern)>>>>> i have no problem with this :)

    but in four textbook i read it about ( MM experiment) they don't say that the initial position of apparatus was 45 degrees? they make you think that initial position was at zero degree then oriented by 90 degrees!! >> and here was my difficulty in imagine

    >>> by the way the reason behind this rotating is because in technical view point we cann't exactly make two lenghts L equal , so to make this small difference in length become unimportant , we rotate the apparatus>>> cause if we sure about the equality of the lingths we just need to check if the rays in phase or not !!

    the last thing I want to mention is the apparatus must be vacuumed from the air because if it isn't... why dosn't make the same influence like the ether??!!

    thanks ..and sorry for long respond :)
  7. Sep 16, 2008 #6


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    Staff: Mentor

    Sorry, misread that the question was specifically about the rotation of the apparatus.
  8. Sep 16, 2008 #7
    Just a theoretical aside.
  9. Sep 17, 2008 #8
    Your analysis is correct, it is your visualization of how the interference pattern is formed that is incorrect.

    Take two incoming beams A & B at right angles that will produce a interference pattern on a film plate marked with a center line. As you said the phase alignment of that pattern will not fit that center mark correctly since if the A path is slow the pattern will pulled to the A side of the film plate.
    Can you measure that shift – NO it is small and you have no confirmed Reference Measurement to compare.
    Solution assume A is the slow path; rotate 90° and B will be the slow path pulling the pattern toward B away from the center line. You still don’t have a center line standard to measure against but it must be out of phase with A side slow measurement since the pattern phase shifted in the other direction on the film. Remember the film turns with the experiment.

    Any difference between the two paths must show up as a phase shift between the two film plate exposures as the experiment move though the ether – if not some new physics may be needed to resolve such a paradox i.e. SR.
  10. Sep 18, 2008 #9
    russ >>> no problem :wink:

    MeJennifer >>> thanks :smile:

    RandallB >>> big thanks for you :rofl: you find the wrong point in my imagination :cool:

    I think the answer is we ignore the influence of the air because we are assumed that the air moving with the earth so it is stationary relative to the earth <<<< and I think there must be a little different between initial and second pattern(because air is a fluid) but it is not the scope of the study.

    finally,thank you all for the cooperation :smile:

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