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B Michelson Morley, Lorentz and Einstein, evolution of principles

  1. Jan 16, 2017 #1
    Hi forum,
    I followed through Feynamn's derivation to show the different times taken for light by the parallel and perpendicular paths of the Michelson Morley apparatus. He showed that it took longer for light to go to the far mirror and back if it were parallel to the direction the whole apparatus was going at compared to the perpendicular path. He then said that Lorentz suggested if a correction was made to the parallel length of the apparatus it could account for the null result of interference shift. It then said that Einstein thought well this was fine but you still need to introduce a time dilation because the path time for someone moving with the apparatus is different to someone observing it from a stationary reference frame.

    At what point along this story did Einstein come up with his postulates because it wasn't originally to do with the M and M experiment but something about Maxwells equations ( don't understand this part yet) but also wasn't Lorentz criticised for just adding a length contraction to make the theory fit. At what point did it become accepted? And at what point along all of this does Einstein come up with his famous postulates?
    Would really appreciate any sort of brief running order of events if possible.

    Kind regards Glenn.
  2. jcsd
  3. Jan 16, 2017 #2


    Staff: Mentor

    If the light clock is moving, yes. I think Fitzgerald also suggested this.

    I don't know if Einstein was the one who first brought this up (I don't think he was), but he certainly recognized it, yes.

    Here is my (possibly incorrect) understanding of the basic sequence of Einstein's reasoning:

    Before Einstein considered the implications of the M-M experiment, he already knew that Maxwell's equations were not consistent with Newtonian mechanics. He knew this by imagining moving along with a light wave: according to Newtonian mechanics, if you did this, the light wave would be a spatial standing wave--i.e., a wave that varies only in space, not in time. But Einstein knew that there are no solutions of Maxwell's equations that have this property: all of the wave solutions vary in both space and time.

    So either Maxwell's equations are no longer valid in a moving frame, or Newtonian mechanics has to be modified to deal with light. Einstein's intuition told him that the second alternative was the one to pursue; that led him to postulate that the correct transformations should leave the speed of light invariant in all inertial frames. And that led him to the Lorentz transformations. Once he had those, he realized that they could explain the null result of the M-M experiment as well, since they implied exactly the length contraction and time dilation that had already been suggested by others.
  4. Jan 16, 2017 #3
    Thanks Peter
  5. Jan 17, 2017 #4
    Lorentz Theory had a notion of local time. Chapter 3, page 13 of dissertation.


    A Comparison between Lorentz's Ether Theory and Special Relativity in the Light of the Experiments of Trouton and Noble. Michel Janssen. Dissertation. University of Pittsburgh, 1995. Posted on the website of the Max Planck Institute for the History of Science.

    Chapter 3 in particular.

    Chapter 3, 3.5.5 The reciprocity of the Lorentz transformation. p. 93
    Last edited: Jan 17, 2017
  6. Jan 17, 2017 #5


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    It's difficult to reconstruct what physicists like Einstein were really thinking to come to the discovery of groundbreaking new theories like special relativity. Later in 1948, Einstein wrote that the MM experiment was not an important thought in his finding of SR but rather the missing Galileo symmetry of Maxwell's equations, and that's what he indeed states in the famous paper on "Electrodynamics of Moving Bodies" of 1905 with an example of induction seen from different frames of reference, where in one the magnet is moving close to a wire loop at reat and in the other one where the magnet is at rest and the loop moving. In both cases an EMF is induced, but at Einstein's time the explanation was different in both cases although "the phenomena" in fact only depended on the relative motion between magnet and wire loop.

    For sure Einstein was aware of Lorentz's works on the issue, and the great achievement is Einstein's conclusion that it is mechanics which has to be adapted to the relativistic transformation between inertial reference frames, which he called the Lorentz transformations (referring particularly to the here relevant boosts only) already in his first famous paper of 1905. In fact all the mathematics was well known in his time, and also Poincare has come to the mathematics, but the ingenious unique step by Einstein was to reinterpret this mathematics in the physically most sound and simple way, and that's the interpretation of SR we still consider valid today.
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