Roger
Michelson 1881: "The result of the hypothesis of a stationary ether is thus shown to be incorrect...This conclusion directly contradicts the explanation of the phenomenon of aberration which has been hitherto generally accepted, and which presupposes that the earth moves through the ether, the latter remaining at rest"
Michelson & Morley 1886: "The result of this work is therefore that the result announced by Fizeau is essentially correct; and that the luminiferous ether is entirely unaffected by the motion of the matter which it permeates"
Now, in 1892 (not 1895), Lorentz writes: "I have sought a long time to explain this experiment [M&M] without success, and eventually I found only one way to reconcile the result with Fresnel's theory. It consists of the assumption, that the line joining two points of a solid body doesn't conserve its length, when it is once in motion parallel to the direction of motion of Earth, and afterwards it is brought normal to it...Such a change in length of the arms in Michelson's first experiment, and in the size of the stone plate in the second, is really not inconceivable as it seems to me...Anyway, it seems undeniable that changes of the molecular forces and consequently of the body's size of order of 1 - p2/2V2 are possible. Michelson's experiment thus loses its verification power for the question at which it was aimed. If one assumes the theory of Fresnel, then its meaning rather lies in the fact, that we can learn something about the change of dimensions".
By 1895 Lorentz is writing: "Thus we see, that the phase difference expected by the theory could also arise, when (during the rotation of the apparatus) sometimes one, sometimes the other arm would have the greater length. From that if follows, that they can be compensated by opposite variations of the dimensions...If we assume, that the arm lying in the direction of Earth's motion, is shorter by p2/2V2... than the other one, and simultaneously the translation would have an influence which follows from Fresnel's theory, then the result of Michelson's experiment is fully explained... Consequently we have to imagine, that the motion of a rigid body, e.g. a brass rod or of the stone plate used in later experiments, would have an influence on the dimensions throughout the aether, which, depending on the orientation of the body with respect to the direction of motion, is different... As regards the experimental confirmation, it is to be noticed at first, that the relevant elongations and contractions are extremely small... it would cause a contraction in the direction of motion in the ratio of 1 to √(1-p2/V2). In reality the molecules of a body are not at rest, but there exists a stationary motion in every "equilibrium state".
With the important proviso: "When I developed the present theory, I hoped at first to be able to explain this difference, but soon I found myself disappointed in my expectation" & "Everything taken together, the question is forced upon us, whether it might be possible to adapt the theory to observations, without ceasing to explain the other phenomena discussed in this work. I haven't succeeded in this, and I must therefore leave the whole question open, in the hope that others might overcome the difficulties that still exist".
The original argument is about the constitution and effect of ether. Correct assumptions about light, ie its independence and constancy, were already in place because this was the basis upon which the result of the M & M experiment was considered. Leading to the conclusion that it proved that matter had a certain 'elasticity'. The calibration of light speed (ie its speed wrt) will of course vary depending on the reference point. Lorentz dissasociates ether from having any effect, eventually. At first (as above 1892) he hypothesises the actual physical alteration of matter as a function of ether. But that becomes problematical (see above). By 1899 he is trying to resolve the problem (of differential movemnt and an associated effect on dimension) with "such small differences" having the "same local time". Under pressure from Poincare to 'propose a coherent theory and not keep adding to the current one', and bearing in mind his principle "that many electromagnetic actions are entirely independent of the motion of the system", by 1904 he puts one forward with "The only restriction as regards the velocity will be that it be smaller than that of light".
Essentially, Lorentz suggests that the alteration in dimension in the line of motion is a function of the flattening of electrons: "I shall now suppose that the electrons, which I take to be spheres of radius R in the state of rest, have their dimensions changed by the effect of a translation, the dimensions in the direction of motion becoming kl times and those in perpendicular direction l times smaller... Our assumption amounts to saying that in an electrostatic system, moving with a velocity, all electrons are flattened ellipsoids with their smaller axes in the direction of motion".
As Einstein later said (Ether & Relativity 1922): "He [Lorentz] achieved this, the most important advance in the theory of electricity since Maxwell, by taking from ether its mechanical, and from matter its electromagnetic qualities. As in empty space, so too in the interior of material bodies, the ether, and not matter viewed atomistically, was exclusively the seat of electromagnetic fields. According to Lorentz the elementary particles of matter alone are capable of carrying out movements; their electromagnetic activity is entirely confined to the carrying of electric charges.
Light was deemed, correctly, to always start with the same velocity, which will not alter unless impeded in some way. This is obvious, because light is just another entity. Forget that it enables organisms to 'see'. It results from an atomic interaction (ie always has the same start speed) and, like anything else, will continue to travel at that speed unless acted upon in some way. Furthermore, observation cannot affect reality.
The bottom line here is that, according to the theory, the variable is a quality in matter which results in an alteration in one particular dimension when a differential force is incurred, ie one which also causes it to speed up/slow down. So in the context of comparison, one must be aware of this, because an 'it' which is undergoing changing momentum does not have the same dimension as one which is not. It has nothing to do with light, observation, timing and any other spurious explanation which has been alluded to. Of course, whether this dimensional alteration does actually occur, and if so, why and what is its magnitude, are different questions.
Paul