clipka wrote:

>> We can't get out of the medium to observe light waves as we can with 
>> water waves.
> 
> Well, then compare it with the perspective of a person actively 
> swimming: Despite obviously being inside the medium, from his point of 
> view water waves going in the same direction as he is appear to be 
> slower than those going in the opposite direction.

Does he? Or does he just observe a frequency change, i.e. see the crests and
  troughs closer together or further apart, as with the doppler effect 
in sound and light?
> 
> You don't even have that with light, no matter how fast you "swim".
> 
>> 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.)
> 
> Given that refraction normally occurs due to different /phase/ 
> velocities of light in two materials, but at the same time there have 
> been experiments reducing the /signal/ velocity of light almost to a 
> standstill... no, I guess that's oversimplified.

I wasn't convinced by the explanation and couldn't find a physicist to 
comment
at the time.
> 
>> 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.
> 
> Which basically boils down again to saying that your /frame of 
> reference/ (which includes sort of a local definition of length) is no 
> more (or no less) valid than theirs.

Exactly, according to the relativity principle observations from within 
any frame
of reference are valid for it alone.

A curious result of this is that any mass measurements contain two 
components,
the inertial mass (the mass that would be measured if the object were at 
rest with
respect to the measurer) and the relativistic mass due to the relative 
velocity of the
object with respect to the observer.

David

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