On Jul 26, 2009, at 2:42 PM, Olin Lathrop wrote:

>
> Today the imperial units are defined in terms of metric ones, which  
> are
> defined against evolving standards as we discover ways to measure  
> something
> more accurately.  For example, the inch is 25.4mm exactly, by  
> definition.
> That makes the foot exactly 304.8mm, whatever a mm really is.  It  
> used to be
> 1/1000 the distance between two scratches on a metal bar in Paris.   
> I think
> it's now a certain number of wavelengths of the red emissions of  
> Krypton 86,
> but that may have changed again and I may be a bit off about it in  
> the first
> place.

Yes, and the kilogram physical iridium standard has lost something  
like 30 micrograms of its mass.  So it might seem that as time and  
millennia evolve, standards themselves may change.

While I think time and distance can be measured without a physical  
reference (as per your example), I wonder how weight measurements  
might be referenced?  I don't think the scientists have figured out a  
way to use physical constants for a weight reference.

Joe
  
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