Folks,
Awhile back I called attention to a new page at the BIPM's website on
"the New SI". Much discussion ensued. I had the opportunity to feed
comments from the members of this USMA mail list back to the CCU and I
did so, as I reported here at that time.
We tried unsuccessfully to get the phrase changed from "the New SI" to
"the New SI Definitions". However, we were successful in having many
other issues explicitly addressed. Those issues and the responses to
them are posted now as FAQs on that website. Please see
http://www.bipm.org/en/si/new_si/faqs.html
for what I think to be a forthright and elegant discussion of the issues
we raised here.
A key point that must be comprehended is put forth clearly in those
FAQs. That is, there is a distinction between the value of a constant
and the numerical value of a constant. For example, the speed of light
in vacuum is constant.* The numerical value for the speed of light
depends on the units in which that value is expressed. Consider that we
can state the value of the speed of light as
299 792 458 m/s
or as
186 282 397 mi/s (using mi to stand for the international mile)
and the numerical values obviously are different. The reason for that is
that the unit m/s differs in magnitude from the unit mi/s. The speed of
light is unaffected by that. Since the value is constant, changing the
size of the unit changes the size of the numerical value. Or, the other
way around, changing the size of the numerical value changes the size of
the unit.
* For the sake of this discussion I am ignoring the effects of
relativity here. We deal with units of the SI in a proper frame of
reference.
Jim Frysinger
Chair, IEEE SCC14
Vice Chair, IEEE/ASTM Joint Committee for Maintaining SI 10
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
