Pascal Bourguignon <firstname.lastname@example.org> 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:
...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.
And as of 1967, the second is *defined* to be as follows:
[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
The ground state is defined at zero magnetic field. ...
 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 <email@example.com>
627 26th Avenue <URL:http://rpw3.org/>
San Mateo, CA 94403 (650)572-2607