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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.
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And he continued to assume the same while formulating GR as well. You implied in the Doppler shift thread that GR was a sort of apology for SR's postulate of c being a universal constant. Edwin Hubble didn't show the universe to be expanding until the late 1920's, more than 10 years after General Relativity.
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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.
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No. No no no no no. SR doesn't say anything about space. Einstein still considered space to be nothing but emptiness in 1905, i.e. the "fabric of space-time" hadn't even been thought of. So, what's the speed of anything relative to nothing? Einstein's postulate is, very simply, that the speed of light is a constant for all observers in an inertial (i.e. unaccelerating) frame.
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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.
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And Einstein chose to advance both physics and philosophy by choosing the latter. Subsequent experiments have since validated his choice.
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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.
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Yes, the speed of light depends on the medium through which it passes. A body's clock speed depends on both the speed and acceleration (or an equivalent external gravitational field) of said body.
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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).
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That's some urban legend. Usually, they involve people dying, disappearing, or something else spooky, not fully predictable phenomena. In fact, I performed a rather simple experiment in the lab this year dealing with electromagnetic induction and drag.
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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.
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Stellar aberration is widely considered to be the downfall of the "local aether" or "aetheral dragging" hypotheses. Basically, if there was a stationary aether surrounding the Earth which passed with it through a universal aether, starlight would be refracted in such a way as to cancel out stellar aberration.
And if you want to think of the gravitational field as some sort of medium for the passage of light waves, then you'd have to expect the speed of light to depend on gravitational field strength. The stronger the field, the more quickly one would expect light to propagate. As such, one would expect any light coming from a deep gravitational well to be blue shifted. I don't think that agrees with observations...
Err, ignore that bit. I really need to learn to not do any thinking before my morning tea.
From
superstringtheory.com's forum:
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Originally Posted by DickT
Recapitulating, we may say that according to the eneral theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists ether.
Because of General Covariance, space(time) had different properties at one point than another; it was not a fixed backgound but a participant in the physics. So Einstein felt justified in calling it an ether. Today we call this property "background independence", and it may well survive relativity if the loop quantum gravity researchers succeed as they desire in incorporating it into their quantum version of spacetime.
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So, as far as ol' Al goes, you seem to be getting hung up on a word which was used as an analogy to demonstrate his concept of space-time.