I am interested in the Sagnac effect which showed that light travels a greater distance in an apparatus that is moving with it. Does this mean length contraction is not a physically measurable quantity? On a related issue is there any evidence supporting physical shortening on rotating discs? What is the current status of this debate?

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    $\begingroup$ There is no debate at this point. The idea of length contraction as a dynamic effect was entertained by Larmor and Poincaré in the waning days of the ether theory, to which Sagnac was partial, and so he misinterpreted his own experiment. The modern view is that contraction is a purely kinematic effect, which essentially measures the extent to which the classical notion of "length" is inadequate (space distance is not relativistically invariant). $\endgroup$
    – Conifold
    Commented Jan 6, 2017 at 3:29
  • $\begingroup$ One can, of course, construct an argument that magnetic induction follows from length contraction though this is just a long-winded way of saying that the electromagnetic field is a skew-symmetric second-rank tensor field and the Lorentz contraction maps it into a group through the usual similarity transform. Or something like that. $\endgroup$ Commented Jan 6, 2017 at 7:00
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    $\begingroup$ Possible duplicate of: Proof of Lorentz contraction? $\endgroup$ Commented Jan 6, 2017 at 11:55


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