Re: speed of light

[Leigh Palmer <palmer@SFU.CA>]

> > NOBODY has measured the speed of light since 1983 (sometime in
> > October, if I remember correctly)
>
> I both agree and disagree with this!  (How's that for waffling.)
>
> In the official SI system, the speed of light is indeed a defined quantity
> which cannot change.  The meter is experimentally determine from c and the
> definition of the second.
>
> However, in the practical SI system, where the tick marks on a strip of
> metal is my working definition of a meter, I can certainly measure the
> speed of light as so many metersticks per second.

But you cannot measure it sufficiently accurately with that
standard to detect a change. There was much effort put into
detecting secular change in the speed of light (and other
physical constants) back in the fifties and sixties. The
end result was that the length standard, a meter bar, was
too imprecise for the task; its precision (or the precision
with which it could be compared to the wavelength of light)
was the limit to the accuracy of the apparently unvarying
speed of light. Already enough time had passed in the modern
scientific era that any secular drift in c should have been
detectable. It may happen in the future that this topic will
be revisited, but it's certainly not something an eager
young assistant professor seeking tenure should undertake!

Leigh