In a previous message he commented on how the meter's length was "changing" as its definition did. I responded:
Carleton:
> Length stayed the same, only how it was calculated got more and more precise.
Friend:
Well, yes, but... one could have constructed a Platinum-Iridium
bar whose length corresponded to the average of the lengths of the feet
of 16 randomly selected individuals. Or the distance from some Monarch's
nose to the tip of his outstretced hand. My point was not the standard,
but the arbitrary way it was chosen. _NOT_ that I'm saying feet or
noses are _less_ arbitrary, just that they are no more. In each
case a "secondary standard" (e.g. those metal bars) is needed to
convey the representation beyond the Royal Court. In contrast,
a system based on things like absolute zero, wavelength of primary
hydrogen emission, etc, can be _described_ without reference to
purely local phenomena, and hence duplicated at a distance (think,
a _l_o_n_g_ distance, like to some extraterrestial civilization
or our "descendants" 10 million years from now). No need for anybody
to travel to the holy bar. Now, the newer standards tend to be of this
form, but they also tend to be pretty arbitrary multiples, because they
_really_ are descriptions of the form "All our measurements are based on
this holy bar, which _happens_ by chance to be 299792458 times as
long as the distance traveled by light in a Vacuum during a time
which is related to our planet's orbit but _happens_ by chance
to be (insert constant here, no time to look it up) times the duration
of one cycle of the primary component of the Cesium spectrum."
Any reasonably competetent alien equivalent of a high-school physics
student could figure out from that description how long a meter and
second are, but would be left wondering why we chose such awkward
numbers. They aren't powers of any small integer, let alone sane numbers
like 2, pi, or e. In fact, I wonder about the regression expressed
below, as "Ma Be" also did, and was not answered:
> Ma Be quoted me in USMA 19107:
>
> >>The advantage of these definitions of the second and the metre is that they
> >>do not depend on the preservation of physical prototypes. They can be
> >>replicated by any well equipped standardizing laboratory anywhere in the
> >>world.
>
> And added:
>
> >Perhaps so, but what was wrong with the previous one, that didn't need the
> >second for its definition? We're told that this was largely to increase
> >accuracy to 8 decimal places or something to that effect. Now, does this
> >mean we may not have another similar alternative? Just wondering...
...
> In 1957 the CIPM declared the metre to be equal to 1 650 763.73 wavelengths
> in vacuo of the radiation corresponding to the transition between the level
> 2p10 and 5d5 of the krypton 86 atom. This definition had an estimated
> precision of =B1 4 nm.
Friend:
Note that this does _not_ require the observer to know
how long a "second" is, while the below does. I suspect that was
the crux of Ma Be's question, which has been ignored in the answer.
Ma Be:
> In 1983 the CGPM decided that "The metre is the length of the path
> travelled by light in vacuum during a time interval of 1/299 792 458 of a
> second".
Friend:
Of course, folks like me have "informally defined" the meter
as "wavelength at 300 MHz" for years, for purposes of accuracy along
the lines that most folks use yardsticks, but then, I _am_ human,
and make no pretense not to be. If you regularly measure to better
than one part in 2^^-10 then you get one free sneer. Nonetheless, I
just had this impish thought that perhaps the change was motivated
by the fact that 1,650,763.73 "looks messy", exposing the arbitrariness
of the measure. Although 1/299792458 is a _fraction_, which all good
Meter-heads claim to have eliminated the need for. OTOH, it's not a
_continued_decimal_, as crop up so often when one tries to shoe-horn
real, physical phenomena into arbitrary systems of measure. :-)
Actually, I have a sneaking suspicion that the transition
above _did_ in fact alter the "defined length" by some minuscule
amount, but was undertaken because the "Metric for ideological reasons"
folks realized that the astronomers and physicists, who took the universe
as it was, had a much better handle on "metrics" (that is, the science
of measurement) than they did, and by pegging the political unit
to something with real science behind it, they could avoid looking
increasingly silly as our capabilities to measure increase. Sort of a
two-hundred-years-late "Well, that's what I meant to say all along" :-)
You do realize that the folkloric value of 98.6 F for mean
human body temperature is the result of a false gain in precision
from a double conversion from Fahrenheit (in which the original
data were collected) to Celsius (then called Centigrade) and then
_back_, right? In real life, the temperature of a human body varies
depending on what part of the body one measures, let alone the
difference in ambient temperature and among individuals. The
original 99F (good to 2 sig-figs, matching the error-bounds of
the data) was converted to 37 C for publication in Europe, then
erroneously back-converted to 98.6 for "English". MacNab would
have flunked them. If they had stuck with even approximately correct
error-bound, they would have made the Celsius number 37.2, which
would back-convert to 98.96 which even a football star would see
was 99, to the proper precision. One has to wonder whether the
folks who settled on 37 were clueless about the precision, or
deliberately chose the loss of precison to make it "look nicer".
I know what I believe.
xxxx
