I would love to get my head around this; I did a quick search for this, but only came across one thread with fringe shift in the title (in the first 5 pages of the search results). The OP will probably sound like me "thinking out loud", because it largely will be. I'm not entirely sure of what the right question to ask is, but I'm hoping that a point of departure might materialise. I suppose, in short, I'm trying to understand the conclusions drawn from the experiment, and the justification for them. The speed of light, in the MMX, is not measured in the sense of distance/time; is it the fringe shifts that are used to determine if there is a change in speed along either of the arms? Some of the questions I have: - what is a fringe shift? - what causes a fringe shift? - does frequency play a part? - can anything about wavelength be inferred?