David H. Burns schrieb:
> There were (are ?) several different interpretations of the results of 
> these
> experiments: (1)that the "light bearing aether" didn't exist, (2) it 
> moved with the earth,
> (3) it exists but is theoretically undetectable, and perhaps more. 
> Didn't the "Lorentz-Fitxgerald"
> contraction also come out of this as an attempt to preserve the aether? 
> But that shows up
> in Einstein's relativity too.

Indeed. Maybe starting with the concept of "frame dragging" first (the 
"aether" moving along with the earth) might have resulted in the same 
equations. That's just pure speculation though.


> So the idea that the speed of light is a constant comes from Maxwell. Does
> simply "fall out" of his equations or is it an assumption or what?

It indeed "falls out" of his equations that electromagnetic waves in a 
vacuum must travel exactly with the same speed as had already been 
measured for light (which actually was the first hint that light /is/ an 
electromagnetic wave).


> Come to think of it, isn't the speed of any wave in an unchanging 
> unmoving medium
> constant and dependent on the properties of the medium?

It is [*]. But every medium (other than space) also provides a 
"standard" frame of reference: For instance, water waves on a stream 
will move at the same speed in all directions relative to an observer 
drifting on that very stream (the "standard" frame of reference), but 
relative to an observer on the bank they will move faster downstream 
than upstream.

This is not so for light: No matter whether you're a stationary (hah!) 
outside observer or zipping along at near-lightspeed, you'll always see 
the light go at some 300,000 km/s relative to you.

[* Well, it is not /strictly/ true, unless the direction and 
polarization of the wave is also a given, as the speed of a wave may be 
anisotropic regarding these parameters; for instance, there are 
materials out there in which the speed of light depends on polarization, 
causing what is known as "birefringence"; but taking these constraints 
into account as well, you're right.]


> The idea that the velocity of light is a fundamental constant is 
> something dictated by
> theory or maybe better a fundamental assumption of Einstein's theory.

The latter, exactly. Or, to be even more precise, it's the idea that the 
assertions "the velocity of light in vacuum is a fundamental constant" 
and "there is no preferred frame of reference with respect to the 
propagation of light in vacuum" are not mutually exclusive.


> I think two of his basic assumptions were (in my words):
> 
> 1) The speed of light is constant in all frames of reference.

That was /the/ basic assumption of his theory of special relativity, yes.

> 2) Other than that, the observations made from within any frame of 
> reference are valid
> only within it.

I'm not sure what exactly you mean by that. Maybe the "you can't tell 
whether your own frame of reference is being accelerated or at rest" 
assumption of general relativity?

> Of course the "light bearing aether" survives in a way as the 
> "electrical and magnet fields of space".

Yup.

Although the magnetic field, as we have just seen, appears to be only an 
illusion caused by relativistic effects in the electric field.

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