From: Joseph C Fineman (jcf@world.std.com)
Date: Sun Jul 18 1999 - 07:23:29 MDT
On Sat, 17 Jul 1999, Michael S. Lorrey wrote:
> Ron Kean wrote:
> > In the 1980s, the meter was redefined in terms of the speed of
> > light, whereas earlier it had been defined as a certain number of
> > wavelengths of a certain spectral line of Krypton 86. The meter
> > is now defined as the distance light travels in 1/299,792,458
> > second. That means we no longer have to wonder what is the exact
> > speed of light. It is 299,792,458 meters per second, exactly, by
> > definition.
>
> Wasn't it originally based on the distance from the north pole to
> the equator along the surface of the earth?
Along the meridian thru Paris! Yes, but that was a long time ago.
For most of its history, IIRC, the meter was defined as the distance
between two scratches on a certain rod in that city.
It is true that now, in a sense, the speed of light is a matter of
arithmetic definition like the number of feet in a mile (or the number
of francs in a Euro). However, that is true only of the light from
the abovementioned spectral line. Whether _all_ electromagnetic
radiation travels at the same speed is still an empirical question.
In particular, if the photon turned out to have a nonzero rest mass
(which would be shocking, but is not inconceivable), then the constant
c in Einstein's equation would not be the actual speed of any light,
but only the limit as the frequency approached infinity (wavelength
approached zero).
--- Joe Fineman jcf@world.std.com
||: Look if you like, but you will have to leap. :||
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