Let's look at the Doppler shift in light due to the velocity v of an observer. The wave has a phase phi = 2 pi (k x - f t), f/k = c, where k = 1/wavelength, f = frequency. If we use the Lorentz transformation, x = gamma (x' + v t'), t = gamma (t' + v x/c^2), gamma = 1/sqrt(1 - v^2/c^2), and do the algebra, then phi = 2 pi (k' x' - f' t'), k' = gamma (1 + v/c) k, f' = gamma (1 + v/c) f, f'/k' = f/k = c. So we see that there is a Doppler shift in both wavelength and frequency that is 1 + v/c to lowest order. But look at what the ether theory predicts. [The ether is a hypothetical medium filling the vacuum whose vibrations constitute light.] Now we use the Galilean transformation x = x' + v t, t = t'. We get instead k' = k, f' = (1 + v/c) f, c' = f'/k' = (1 + v/c) c. There is no wavelength Doppler shift. This is peculiar. A hundred years ago, experimentalists like Albert Michaelson worked very hard trying to detect the motion of the Earth through the ether, looking for effects of order (v/c)^2 = 10^-8. Why does the existence of the easily measured v/c wavelength Doppler shift not trivially demolish the ether theory of light? Gene ____________________________________________________________________________________ Be a better Heartthrob. Get better relationship answers from someone who knows. Yahoo! Answers - Check it out. http://answers.yahoo.com/dir/?link=list&sid=396545433