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!