Some further points on the rotation of arms in the Michelson-Morley.
Frank Russo - December 25, 2012.
Crucial to the understanding of the rotation, is the fact that it takes place over a sort of 'fulcrum'... hence it's only points that have a synchronous starting point in both position and timing, that can have their arms rotated 'into each other'.
It is quite clear then that the two photons that start together from the origin, with one getting to the perpendicular mirror whilst the other gets to 1.752722466m away from the orbital mirror, can have their respective arms rotated into each other (so to speak). However, it is another matter altogether in trying to rotate the arms from the position of when the orbital photon reaches the orbital mirror, for the simple reason that their (mirrors) end-point simultaneity do not have the same positional starting point, despite having the same timing start!
The simultaneous perpendicular mirror photon has had its starting point some 0.335799009 m later than their previous common origin that we started from. However, the crux of the matter is that this point is dependant on the orbital velocity, and this means that the corresponding starting point in timing, for the other simultaneous end-point is 1.921923284 m forward of the mentioned previous common origin (without the beam-splitter there because this is still the original beam - just further along - whereas the other is part of a new beam!)... Obviously because the end-point has moved further forward by 1.921923284 m.
This fact that the end-point stretches forward with the orbital speed but is travelled at the velocity of light, whilst the rotational axis moves forward with the orbital velocity of the earth only, has caused massive confusion over the years... in actual fact it has spawned many very fatuous ideas of time-travel and of aberrant simultaneities!
One should of course study my rough diagram that illustrates this matter by clicking on the following link
(However, you should open it in a new window otherwise it will be a bit enlarged.)
In conclusion then, it's only synchronous starting points in both position and timing, that can be rotated...
Happy Birthday Sir Newton!