From: utzoo!decvax!harpo!seismo!hao!hplabs!sri-unix!larson Newsgroups: net.space Title: Re: Einstein's Universe?! - (nf) Article-I.D.: sri-unix.5343 Posted: Wed Mar 9 07:26:30 1983 Received: Fri Mar 11 21:02:59 1983 #R:ihldt:-131300:sri-unix:13400003:000:1196 sri-unix!larson Mar 7 00:25:00 1983 From "Physics" by Resnick & Halliday, section 20-7: There are differences, however, in the Doppler effect formula for light and for sound. In sound is is not just the relative motion of source and observer that determines the frequency change. In fact, as we have seen, even when the relative motion is the same, we otain different quantative results, depending on whether the source of the observer is moving. This difference occurs because Vo and Vs are measured relative to the medium in which the sound wav is propagated and because this medium determines the wave speed. Light, however, does not require a material medium for its transmission, and the speed of light relative to the source or the observer is always the same value c, regardless of the motion of those bodies relative to each other. This is a basic postulate of the special theory of relativity. Hence, for light only the relative motion of the source and observer can lead to physical changes, there being no material medium to use as a reference frame. I cannot think of any way to make it more clear without including the formulas given in the section, and they are just more than I wanted to type in. Alan