speed of light

God day all you folks in NEC-LIST Land

I have just been browsing through the August 1995 Physics Today Buyer's
Guide. Of interest is the fact that the speed of light is given as
299 792 458 m/s, "exact." That's right EXACT. All the other constants
except permeability and permittivity of vacuum (also exact), have relative
uncertainties attached to them. This to me implies that c is DEFINED to
have this value. So, what about the meter and second I hear someone say.
What indeed.

Does anyone out there know when and how come c, Mu(0) and Epsilon(0) were
defined to have the values given??

Mu(0) = 4*Pi*10**(-7)
Epsilon = 1/[Mu(0)*c**2]

Apologies to the folks who did not grow up using the user abusive notation
of Fortran - I have no idea what it is in the latest and greatest computer
language.

Regretfully I have not been keeping tabs on the world of physics and
metrology as closely as I should. If anyone has a convenient reference, I
would certainly like to hear about it.

Have a great day, Duncan Baker.
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Received on Wed Aug 30 1995 - 21:34:00 EDT