Reference: http://www.relativitybook.com/resources/Einstein_space.html
NOTE: Einstein’s statements are in black italics. My understanding follows in bold color italics.
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CeThe aether-theory brought with it the question: How does the aether behave from the mechanical point of view with respect to ponderable bodies? Does it take part in the motions of the bodies, or do its parts remain at rest relatively to each other? Many ingenious experiments were undertaken to decide this question. The following important facts should be mentioned in this connection: the “aberration” of the fixed stars in consequence of the annual motion of the earth, and the “Doppler effect”, i.e. the influence of the relative motion of the fixed stars on the frequency of the light reaching us from them, for known frequencies of emission. The results of all these facts and experiments, except for one, the Michelson-Morley experiment, were explained by H. A. Lorentz on the assumption that the aether does not take part in the motions of ponderable bodies, and that the parts of the aether have no relative motions at all with respect to each other. Thus the aether appeared, as it were, as the embodiment of a space absolutely at rest. But the investigation of Lorentz accomplished still more. It explained all the electromagnetic and optical processes within ponderable bodies known at that time, on the assumption that the influence of ponderable matter on the electric field – and conversely – is due solely to the fact that the constituent particles of matter carry electrical charges, which share the motion of the particles. Concerning the experiment of Michelson and Morley, H. A. Lorentz showed that the result obtained at least does not contradict the theory of an aether at rest.
Aether was assumed to have material properties but it didn’t seem to interact with matter. In all scientific experiments aether appeared, as it were, as the embodiment of a space absolutely at rest.
In spite of all these beautiful successes the state of the theory was not yet wholly satisfactory, and for the following reasons. Classical mechanics, of which it could not be doubted that it holds with a close degree of approximation, teaches the equivalence of all inertial systems or inertial “spaces” for the formulation of natural laws, i.e. the invariance of natural laws with respect to the transition from one inertial system to another. Electromagnetic and optical experiments taught the same thing with considerable accuracy. But the foundation of electromagnetic theory taught that a particular inertial system must be given preference, namely that of the luminiferous aether at rest. This view of the theoretical foundation was much too unsatisfactory. Was there no modification that, like classical mechanics, would uphold the equivalence of inertial systems (special principle of relativity)?
Thus at the foundation of electromagnetic theory we have luminiferous aether at rest. According to Classical mechanics, there is invariance of natural laws with respect to the transition from one inertial system to another. It was unsatisfactory to regard an aether to be at rest in two different inertial systems while not also interacting with matter.
The answer to this question is the special theory of relativity. This takes over from the theory of Maxwell-Lorentz the assumption of the constancy of the velocity of light in empty space. In order to bring this into harmony with the equivalence of inertial systems (special principle of relativity), the idea of the absolute character of simultaneity must be given up; in addition, the Lorentz transformations for the time and the space co-ordinates follow for the transition from one inertial system to another. The whole content of the special theory of relativity is included in the postulate: The laws of Nature are invariant with respect to the Lorentz transformations. The important thing of this requirement lies in the fact that it limits the possible natural laws in a definite manner.
Einstein thus rejected the idea of a mechanical ether. He assumed space to have the properties of light which were constant from one inertial system to another. This constancy was represented by the “speed” of light as measured in any inertial system. But this again is unsatisfactory because it assumes that light has zero inertia and it does not interact with matter.
Light has a finite velocity. That means the acceleration of light is balanced by some internal resistance. This resistance is represented by the properties known as permittivity and permeability. In fact, the speed of light is determined by permittivity and permeability measured in space. Permittivity and permeability represent resistance and they seem to be related to inertia.
As described earlier, the background SPACE acts as the reference point of zero dimension, zero inertia, zero change and zero frequency. In this background exists the electromagnetic and inertial fields that are also continuous with each other. Thus the dimension and inertia appears to be the property of these fields and not of the background SPACE.
The key error has been to attribute the property of dimension to space.
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Earlier notes by Vinaire:
Electromagnetic field was at first thought to describe the states of some mysterious substance called aether. Experiments then showed aether to embody “a space absolutely at rest”. But, this idea contradicted the principle that natural laws work the same way in all inertial systems (special principle of relativity). The principle, however, was supported by Lorentz transformations for the time and the space co-ordinates.
This special principle of relativity thus required that speed of light must be constant in “empty space” and the idea of the absolute character of simultaneity must be given up.
This means that there is no “now” in an absolute sense.
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