clipka wrote:
> David H. Burns schrieb:
>> Yes and I used to think that it was Einstein who showed the velocity 
>> of light to be constant
>> in all frames of reference. But it wasn't he started with that. I was 
>> not able to find where
>> this idea actually came from. Any ideas?
> 
> Been around a while. People looking for the "aether" though which light 
> waves  were assumed to propagate, by measuring subtle changes in the 
> difference of c depending on direction (a) parallel to the earth's 
> movement on its orbit and (b) orthogonal to it, found that these subtle 
> changes were apparently... well, /very/ subtle indeed. Actually too 
> subtle to be measured, if there were any at all. At any rate, "aether" 
> theory would have predicted a lot more.

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.
> 
> 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?

>nor did he show that this was the 
> case for every frame of reference - that was the experimenters (who 
> didn't really show it for /every/ frame of reference of course, but for 
> enough FoR to provide reason enough to seriously toy around with this 
> idea).

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.
> 
> 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.
> 
> It was only the experimenters who showed that Einstein was right in his 
> predictions, and therefore it is prudent to assume that he was also 
> right in his initial presumptions.

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

David

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