What OS are you? fun

M. Warner Losh imp at bsdimp.com
Mon Nov 22 16:30:32 PST 2004


In message: <41A1684E.1020302 at itga.com.au>
            Gregory Bond <gnb at itga.com.au> writes:
: Rob wrote:
: 
: >>> You'd better cite your source and / or reasoning, as ~3*10^8m/s =is= 
: >>> the
: >>> accepted constant speed of light in vacuum.
: >>
: It's deeper than that.  The "second" and the "meter" are both defined in 
: terms of wavelengths of light, which (as a consequence) fixes the speed 
: of light _by definition_, at _exactly_ *299 792 458 m s^-1.

The second is not defined in terms of the speed of light.  It is
defined in terms of the number of hyperfine transitions of cesium:

http://www.bipm.fr/en/si/si_brochure/chapter2/2-1/second.html
	The second is the duration of 9 192 631 770 periods of the
	radiation corresponding to the transition between the two
	hyperfine levels of the ground state of the caesium 133 atom.
	This definition refers to a caesium atom at rest at a
	temperature of 0 K.

The meter used to be defined in terms the wavelength of Krypton-86
radiation, but that was changed in 1983.  It is nowdefined in terms of
how far light travels in a given time interval.  See
http://physics.nist.gov/cuu/Units/meter.html for a good historical
perspective.

So the definition of the meter is dependent on the second, but the
second is independent.

However, the definition implicitly assumes that today's fundamental
constants of the universe are indeed constant.  There's been some
evidence that suggests, but is so far inconclusive, that some or all
of the fundamental constants of the universe may vary on the order of
a few parts in 10^15 over the last few billion years or so.  The
definition of the meter was changed before this evidence was known.

And this is indeed, very off topic.

Warner



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