My question is very simple. As far as I know, once upon a time there was the Lorentz Ether Theory. This theory admitted existence of so-called Ether as a certain preferred frame, which was undetectable because of physical contraction of moving material bodies in direction of their motion. In very simple words – this theory explained null result of Michelson – Morley experiment (and null result of all other Ether drift experiments) by physical contraction of length of interferometer in direction of its motion. Albert Einstein admitted: “Concerning the experiment of Michelson and Morley, H. A. Lorentz showed that the result obtained at least does not contradict the theory of an ether at rest.” http://www.relativitybook.com/resources/Einstein_space.html In Lorentz theory due to interaction with Ether moving measuring rod with proper length L0 was gamma times shorter, than a measuring rod with the same proper length which is “at rest” in the Ether. Thus, a rod “at rest” was gamma times longer than moving rod. Though this theory could also explain null – results of Ether Drift experiments, Ether theory was finally rejected and SR was accepted. In Lorentz theory velocity of light in the Ether was c, hence velocity of light in different directions was different for moving and observer and one at rest. Hence, frame of reference “at rest” had a specific feature – velocity of light in all direction in this frame was c (hmm… just like in the rest frame in SR). Article in Wikipedia claims: A defining feature of special relativity is the replacement of the Galilean transformations of Newtonian mechanics with the Lorentz transformations. But, to my knowledge, Lorentz theory employed the same transformations. As I understood, relativity of simultaneity is inseparable feature of the SR which is a consequence of equivalence of inertial reference frames. Exactly relativity of simultaneity causes reciprocity of observations, like length contraction by every observer. In Lorentz theory, relatively moving frames are not equivalent. Thus, it is exactly Special Relativity predicts some astonishing effect like hotly - debated on pages of this wonderful forum “Embankment and Train paradox” a.k.a. “barn and ladder paradox” a.k.a. “train and tunnel paradox”. This effect cannot take place in Lorentz Theory due to lack of symmetry. This is significant difference in description of properties of space – time. Also in Special Relativity Maxwell equations take invariant and independent of chosen reference frame form. This is what makes Special Relativity preferable, am I right?