bnp (02/07/83)
This query may be a repeat of an old question, but it has been bothering me for some time so... I have never had a good qualitative understanding of Einstien's' theories of relativity (both special and general), and a friend recommended a book entitled "Einstein's Universe" by Nigel Calder, as a good introduction to the subject. I got a copy from a local library and began reading, but quickly ran into trouble when the author began describing the differences in the Doppler shift mechanisms which apply to light vs. sound. I was hoping that someone on the net could provide a better explanation, or recommend a good book. I can see how in air, the magnitude of a Doppler shift will depend on whether the observer or the source of the sound is moving, but I cannot understand how the two cases can be treated identically for light. I have heard the arguments about the lack of an "either" through which the light moves, but that doesn't seem to explain the mechanism causing the Doppler shift when the observer is moving relative to the source of the light. Please respond by mail, and if other similiar queries appear, I will summarize. Thanks, Bruce Peters BTL - Naperville
larson (03/09/83)
#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