Jonathon McClure asked in USMA 22526:
>Why is it that the meter has been tied to the speed of light (in a vacuum)
>when we are finding out how variable that "constant" can be? What was so
>wrong with Krypton Lasers?
>
>
>Johnathan McClure
Yes, the speed of light of various colors varies in various media,
but the Michelson-Morley experiment failed to find any variation in
the speed of light in different directions. Einstein based his
theory of relativity on this result. The fact that variable stars do
not show a variation of color as they wax and wane shows that all
colors travel at the same speed in empty space.
The second is the most accurately determined unit that there is; it
is determined with a standard uncertainty of 2 parts in 10^15. This
is much more accurate than a prototype metal metre.
The 1997 Comit� International des Poids et mesures (CIPM) resolved:
"considering
"that science and technology continue to demand improved
accuracy in the realization of the metre:
"that since 1992 work in national laboratories, in the BIPM
and elsewhere has identified new radiations and methods for their
realization which lead to lower uncertainties;
"that such work has substantially reduced the uncertainty in
the determined value of the frequency and wavelength in vacuum of one
of the previously recommended radiations;
"that a revision of the list of recommended radiations is
desirable for many applications, which include not only the direct
realization of the metre by means of optical interferometry for
practical length measurement, but also spectroscopy, atomic and
molecular physics and the determination of fundamental physical
constants."
The CIPM listed wavelength and frequency for 15 different atoms. The
clearest result with found with calcium 40, with a relative standard
uncertainty of + or - 6 x 10^-13, that is + or - 0.6 picometres.per
metre. The former standard based on the wavelength of krypton 86 had
an uncertainty of + or - 160 nanometres.,which is 260 000 times
greater than that for the new calcium 40 standard.
I think that the CGPM in view of the uncertainties and the
interrelation of all measurements, decided to accept the second as
the best determined unit, and then to *define* the speed of light in
vacuum as 299 792 458 metres per second. Then from the measurement
of the frequency and wavelength of light from stabilized lasers, the
metre may be determined. The metre is no longer defined by scratches
on a metal bar in Paris, but can be regenerated by a properly
equipped laboratory anywhere.
