Speed of light? [was Re: GPL vs BSD Licence]

[Mike Jeays]

On Tue, 2004-11-02 at 14:12, Marc Ramirez wrote:
> On Tuesday 02 November 2004 01:21 pm, Mark Murray wrote:
> > Marc Ramirez writes:
> > > > IANAPhysicist but, isn't the speed of light in a vacuum constant? Well,
> > > > it may be being actively debated by cosmologists attempting to explain
> > > > the origins of the universe; but, VSL aside... the speed of light is
> > > > 2.998something x10^8 m/s in vacuum.
> > > >
> > > > Sorry, but this is chat, and I figured I ask.
> > >
> > > Yes, it is a fixed speed in a vacuum; it gets redshifted in a
> > > graviational field.
> 
> Again, I will preface by saying I am a professional doofus and amateur 
> windbag.
> 
> > Not quite. The Speed Of Light in a Vacuum Constant is constant in all
> > inertial frames of reference, and is exactly 299792458 m/s. In other media
> > (air, water etc, it may be slower than this. It is never faster.
> 
> True.  More precisely, the slowing of light within materials can be derived 
> from first principles by assuming a fixed SoL, c, and factoring in the time 
> involved in absorption/re-emission of photons by the molecules.
> 
> > If the originator of the light is moving away from you, atomic spectra in
> > the light are shifted towards the red end of the spectrum ("redshift"). If
> > the originator of the light is moving towards you, atomic spectra are
> > blueshifted. This is called the Doppler shift.
> 
> All true.
> 
> > Gravity may bend light beams.
> 
> More precisely, gravity is what we call the curvature of spacetime.  Light 
> always heads in the straighest possible line, but in a curved spacetime...  
> Also, if light is emitted in an area of lower potential and is absorbed in an 
> area of higher potential (e.g., from Sun to Earth) it will be redshifted. 
> Other way around, it will be blueshifted. If you are using the frequency of 
> light as a clock (and you basically have no other choice), you will notice 
> this effect as "time moving more slowly around massive bodies."
> 
> Marc.

Doesn't this equate to defining the speed of light as constant, and then
adjusting the metre and second appropriately?  I thought the constancy
of the speed of light was a basic assumption taken by Einstein; the
slowing of time and reduction of lengths of moving objects, or those in
a gravitational field are a consequence.