Ian Wilson wrote:

> On 10:10 AM 6/03/2001 +1100, John Haddy said:
> >By definition, 1 inch = 25.4mm exactly. This was standardised
> >some decades ago.
> Off topic a little...
>
> But the metre (and hence inch) are derived quantities (variables) so all
> our footprints are totally useless! Do you hear - *useless*!
>
> The speed of light, c, is constant.  Any improvements in the how fast light
> travels does not affect the number in metres per second.  The length of the
> metre is changed instead.  So there is no point in trying to design
> footprints as the next time a physicist improves the estimate of c we have
> to go back and update all out libraries.

This is actually not true.  The Metre is based on an exact number of
wavelengths
of the Neon emission line that is characteristic of the HeNe laser, somewhere
around 637 nM, I think.  So, the length of objects is not changed by speed
of light measurements.  And, the wavelength of the Neon atom will remain
quite stable when measured properly.

Jon


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