clipka wrote:


>> 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.

We can't get out of the medium to observe light waves as we can with 
water waves.

> [* 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.]

I have heard the apparent decrease in the velocity of light is explained 
by the interference of light
re-emitted by the material so as to give the appearance of a decrease in 
velocity. But the "true" velocity
of the light remains that in free space. (I did not altogether 
understand this and may have it wrong.)


>> 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?

That may be what I was getting at, but also the fact that the 
"relativistic" effects
such as time dilation or increase in mass that I would observe in an object
(say a space ship) moving with 99.99% the speed pf light relative to me 
are *not*
observed by its occupants and my observation is no more (or no less) 
valid than theirs.


A lot of the paradoxes of relativity seem to arise from treating the 
observations from two
or more frames of reference as if they were made from the same frame of 
reference.

David

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