David H. Burns wrote:
> clipka wrote:
>> That's how this whole "no absolute frame of reference" thing started.
> I think it started long before that even as far back as Newton (?)
>>
>> By the way, Einstein did /not/ show that light was travelling at 
>> constant speed - that was Maxwell - 
> 
> 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?
> 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?

As far as I understand it it's a bit misleading (although technically 
correct I suppose) to claim that Maxwell "showed" that the speed of 
light is a constant.  A constant speed of light does fall out of 
Maxwell's equations, but I believe that the original interpretation was 
that they were only correct when considered by an observer at rest with 
respect to the aether (this is where clipka's next sentence about 
different reference frames comes in).

Historically speaking I think that the reasoning actually happened the 
other way around.  From Maxwell's equations he determined that 
electromagnetic waves propagate at a particular speed, which happened to 
match the speed of light, and thereby inferred that light was actually a 
form of electromagnetic radiation.


>> All Einstein did was do just that: Take the "c appears to be constant 
>> in all frames of reference" as a given for argument's sake, and see 
>> what weird predictions he'd wind up with.
> 
> I think two of his basic assumptions were (in my words):
> 
> 1) The speed of light is constant in all frames of reference.
> 
> 2) Other than that, the observations made from within any frame of 
> reference are valid
> only within it.

I think a better way of phrasing these might be as follows:

1) All the laws to physics are invariant to the velocity of the observer

2) This includes Maxwell's equations (and thus light must have a 
constant speed independent of the velocity of the observer.)


Side note: I am not a physicist, so take everything with a grain of salt.

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