Re: speed of light (fwd)

> 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.

The following definitions may have changed based on the date of the reference
used.
 
1. Meter: is a defined quantity. Based on the CRC book of 1962-63 (sorry
    for the dated material) 1 meter = 1,650,763.73 wavelengths of the orange-
    red line of Krypton-86.

2. Second: is a defined quantity. Ephemeris second = 1/31,556,925.9747 of
    the tropical year for 1900 January 0d 12h ephemeris time.

3. Speed of light. Is a measured quantity, not exact. Therefore, it has
    definite error. c = (2.997928 +/- 0.000004) * 10**(10) cm/sec (1955).
    I'm sure the error is smaller as of today, so maybe what the Buyer's
    Guide meant was that it is exact to the number of significant digits
    that they listed, BUT it is NOT a defined quantity.

    I use the "Handbook of Chemistry and Physics" by The Chemical Rubber
    Company to clarify issues of this sort. Though the one I have at home
    is more recent (approx. 1983) than the one I'm using right now.

4. I'm guessing that the free space permeability is defined, though I
    couldn't find it in this book. If indeed it is defined, then the
    permittivity [Epsilon(0)] must be derived from the two quantities
    c and Mu(0) (permeability).

Unfortunately, I don't have an answer for Mu(0). Rats! Oh well, this
is fun.

Thanks,
Rupert

>
> 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.

-- 
*********************************************************************
Rupert L. Seals
Internet: sealsr_at_netaxs.com
Web Page: http://www.netaxs.com/~sealsr
*********************************************************************