sci.physics.particle

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.

[Ignore Valev when he brings up Pound-Rebka and similar
experiments -- 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.

Some people claim this is due to the solar atmosphere.
They are wrong: by doing this for multiple wavelengths the
effect of the solar atmosphere can be identified, and is
found to be negligible. Of course it would be highly
unlikely that such a different effect could mimic the GR
dependence on path. Indeed, measurements using pulsars
agree with the GR predictions for paths that never go
inside earth's orbit, and we know what the solar
atmosphere is out here.


Tom Roberts