Pascal Bourguignon  <pjb@informatimago.com> wrote:
+---------------
| And don't be misled, very little physical constants we know with more
| than 4 digits, and  most of the time they are _defined_, like c which
| is _defined_ to be exactly 299792458 m/s.  And note how it's an
| integer number, not a real one ;-).
+---------------

Well, actually, as of 1983 the meter is *defined* to be:

    <http://en.wikipedia.org/wiki/Meter>
    ...the length of the path travelled by light in an absolute vacuum
    during a time interval of exactly 1/299,792,458 of a second.

So the meter is now a derived unit defined in terms of the speed
of light and the second, rather than a basic unit of its own.[1]

And as of 1967, the second is *defined* to be as follows:

    <http://en.wikipedia.org/wiki/Second>
    [The second] is defined as 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 at
    zero kelvins.
    ...
    The ground state is defined at zero magnetic field. ...


-Rob

[1] Between 1960 and 1983 the meter was defined to be "1650763.73
    wavelengths in vacuum of the radiation corresponding to the
    transition between levels 2p10 and 5d5 of the krypton-86 atom"
    (the orange-red emission line) in a vacuum.

-----
Rob Warnock			<rpw3@rpw3.org>
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