Tom Davidson wrote: > > David Wilkinson wrote: > > (snip) > > > > Two compelling arguements for constant c in vacuo: > > 1) It has been repeatedly measured to be constant for all inertial > > observers independant of the velocity of source and receiver and its > > frequency. It is an experimental finding. > > > (snip) > > > Actually the overwhelming reason is that only a constant c in a vacuum is consistent with basic electromagnetic theory as found out by James Clerk Maxwell. In a vacuum the electromagnetic forces are the same as the displacement forces, ie forces actually observed. When matter is present, then the basic EM forces are perturbed by the interaction of matter leading to displace- ment forces that are different from "applied" forces. This results in an index of refraction for EM waves and the relationship v = c/n > Citations, please? I would be very interested in the mechanics of these > measurements, as I have been trying for years to design a light > speedometer that lacks lenses, mirrors and (the tough one!) G-fields > that would affect the speed on light within the instrument. > > Tom Davidson > (former government scientist) David Wilkerson in a followup post mentions Michaelson- Moreley, which is the most famous. Nevertheless, if you go to the basic Maxwell equations and solve the for wave equations you will find that in a vacuum c^2*epsilon0*mu0 = 1 thus c depends only on the space parameters which are fundamental constants. Thus c *must* be a constant, otherwise electromagnetism could not exist as it is observed. You can find a speed of light in the presence of matter independent of resorting to refraction (sort of). To do it, you start with the basic Maxwell equa- tions and apply the condition that displacement forces are not the same as the "applied" forces. Then the resulting space-time differential equation gets real messy. However, if you just look at the structure of the equations (2 of them) you can easily realize that the perturbing term reflects dispersion. The conclusion is that if the speed of light is not constant, then any wave consisting of multiple frequencies will not arrive intact if *any* component is allowed to exceed c. So information can not be sent faster than c. The really surprising thing is that in this method the result is just due to basic electromagnetism, not relativity!