sci.physics.particle

On 16 Mar, 18:14, Tom Roberts wrote:
> The Speaking Clock wrote:
> > On 15 Mar, 08:57, Pentcho Valev wrote:
> >> [... his usual nonsense]
>
> > Would the speed of light be a non wiggly constant at the very edge of
> > the universe - bending around it?
>
> The speed of light is a constant, c, for any LOCAL measurement anywhere
> in the universe. I guess that includes "non wiggly". It also includes
> essentially all measurements of light speed here on earth. At least that
> is what GR predicts, and there are no observations that contradict it.
>
> =A0 =A0 =A0 =A0 [Ignore Valev when he brings up Pound-Rebka and similar
> =A0 =A0 =A0 =A0 =A0experiments -- they do not measure speed.]
>
> But if you want to see light "bending around the universe" then that
> would certainly require a non-local path, and there's no solid
> prediction of the result (given the uncertainties in the physical
> situation you have in mind).
>
> It is rather difficult to perform a non-local measurement of the speed
> of light, but it has been done by sending radar waves across the solar
> system to reflect back from mars and venus. By measuring the round-trip
> delay repeatedly over several years, and fitting to the planetary
> orbits, one can deduce the round-trip speed of light for these paths.
> That speed varies, depending on how close to the sun the path passes;
> look up "Shapiro time delay". The results are in excellent agreement
> with the predictions of GR.
>
> =A0 =A0 =A0 =A0 Some people claim this is due to the solar atmosphere.
> =A0 =A0 =A0 =A0 They are wrong: by doing this for multiple wavelengths the=

> =A0 =A0 =A0 =A0 effect of the solar atmosphere can be identified, and is
> =A0 =A0 =A0 =A0 found to be negligible. Of course it would be highly
> =A0 =A0 =A0 =A0 unlikely that such a different effect could mimic the GR
> =A0 =A0 =A0 =A0 dependence on path. Indeed, measurements using pulsars
> =A0 =A0 =A0 =A0 agree with the GR predictions for paths that never go
> =A0 =A0 =A0 =A0 inside earth's orbit, and we know what the solar
> =A0 =A0 =A0 =A0 atmosphere is out here.
>
> Tom Roberts

Thank you Tom for that excellent explanation. Good ole c is still a
dependable non wiggly constant! :)

Cheers,

TSC