On Sat, 17 Jul 1999, Michael S. Lorrey 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).
||: Look if you like, but you will have to leap. :||