
Originally Posted by
Scotsman Hosie
It has been demonstrated that the speed of light is a constant. Which, despite differences − demonstrated through various media, as well as the effects of gravity − means that speeds are not additive, with respect to the speed of light. A moving body will either compress or stretch wavelengths − in relation to an observer. Hence, the attendant red-shift, of a sources frequency is noted. The speed at which the wave is propagated, however, remains the same. It's how astronomers are able to tell whether stars are moving towards or away from us. And provides established evidence, for an expanding universe.
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