When Einstein wrote the SR theory in 1905, he still thought the universe was “fixed” and that the highest speeds of astronomical bodies were the speeds of the planets and comets in their orbits, and those speeds were very slow, when compared to the speed of light.

So he postulated the speed of light to be “c” relative to “empty space”, thinking that “empty space” was pretty much “stationary” with all the stationary and “fixed” stars.

When he assumed this about light speed, and considered the motion of the planets and observers on the planets, then he had to alter the rates of the observers’ clocks in order to keep the speed of light always at “c”, relative to all observers.

You can do one or the other, admit that the speed of light changes as it travels through different areas of space, or claim that all the clock rates change so that light speed will always remain “c”, relative to all observers.

But, the entire situation is a little more complicated than that. Turns out that light speed changes in space, AND clock rates change due to different environmental factors.

First, all the electrodynamics experiments of the 19th Century were conducted at the surface of the earth. That means that some of the results might have been influenced by the earth’s local “fields”, such as the electric, magnetic, and gravitational fields, and, thus, some of Maxwell’s equations and theories might have reflected that fact. In other words, all of the 19th Century electrical and mechanical experiments produced “geocentric” results. This is why several nations are having many specific scientific experiments conducted on the International Space Station, so they can be conducted outside the direct “pulling” influence of the earth’s gravitational field.

Normally, we don’t think of the earth’s fields putting up any “resistance” to our motion through them. In fact, the urban legend is that anything that moves through “space” in orbit above the earth, will feel no “resistance” to its motion through any of the fields, but that’s not always true, as revealed in the NASA tether experiment.

The long wire of the tether feels a “drag effect” on it, caused by the wire moving through the earth’s magnetic field. This drag effect is what caused the NASA tether to fall behind the space shuttle after it snapped (you can find this information on several science websites).

It was Lorentz who first predicted that such a “force” existed when atoms moved through fields. In fact, this was the basis of his “speed limit of c” hypothesis, which Einstein later made famous.

Lorentz thought there was a “universe stationary ether”, and in his theories he treated the ether as a kind of “field”. He hypothesized that “c” was the “speed limit” for objects (“ponderablen Körpern”, “ponderable bodies”) moving through that “field”, and he also hypothesized that atoms would “shrink” in the direction of motion through that “field”. This was the origin of the “length contraction” concept, and the “speed limit of c”, since at the speed of “c”, he thought all bodies would shrivel up to “plane figures”.

A modern interpretation of the results of the Michelson Morley experiment suggests that the MM apparatus was NOT moving through any “universal ether”, but it was stationary inside the earth’s own “local ether”, which some people think might be the earth’s own local gravity field.

In fact, after developing his 1911 gravitational redshift theory, Einstein began to think of local gravity fields as a kind of light-speed-regulating “ether”. As he said in one of his 1918 papers:

“There, empty space in the previous sense has physical qualities, mathematically characterized by the components of the potential of gravitation that determine the metrical behavior of that portion of space as well as its gravitational field. This situation can very well be interpreted by speaking of an ether whose state varies from point to point.”

And also in one of his 1920 papers:

“Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense.”

One of the top urban legends in science today is the belief that Einstein never admitted to the possible existence of an “ether”. This legend persists because so many people who believe it have never read his papers in which he discussed the “fields” of space acting as a kind of light propagating “ether”.