To: WSFAlist at keithlynch.net
Date: Thu, 29 May 2003 10:38:34 -0400
Subject: [WSFA] Re: kilogram evaporating, wine production
From: ronkean at juno.com
Reply-To: WSFA members <WSFAlist at keithlynch.net>

On Thu, 29 May 2003 00:09:20 -0400 (EDT) "Keith F. Lynch"
<kfl at KeithLynch.net> writes:
...If the kilogram is *defined* as the mass of
> that> platinum-iridium cylinder, then how can that cylinder ever be
said
> to> have changed in mass, even if a prankster were to knock a big chunk
> out of it?
>
> Obviously there's some more natural concept of mass which the
> official> definition unsuccessfully attempts to capture.
>
> The metric system's original approach was to define a particular
> volume (one cubic centimeter) of a particular substance (pure
> water)> at a particular temperature (that at which it is maximally
dense,
> about 4 C or 39 F) as having a specific mass (one gram).  They
> could> go back to something like that.  (If they choose water again,
> they'd> better specify the isotope ratio, as "pure water" is otherwise
> ambiguous, with a range of densities.)
>

The kilogram was intended to be the mass of 1000 cc of water at 4 deg C,
and the full length article mentioned the difficulty of making precise
measurements of the mass of water (and you have mentioned the isotope
ratio, a difficulty which was not known in 1889), so the metal cylinder
was made in 1889 as a more convenient standard for the most precise
calibrations, while the wide availability of distilled water,
thermometers and measuring calipers means that water remains perhaps the
most accessible standard for casual use.  If I had to calibrate a
weighing scale at home, and if I did not have or trust commercial
weights, I could use water, and it would be close enough.

It would be taking too literally the statement that 'the kg is defined as
the mass of the one original reference cylinder', to mean that the
defined kilogram mass would automatically change if a chunk were knocked
off the standard.  It might be better to say that 'the kg is defined as
the mass of the original reference cylinder, as originally made'.  Dozens
of copies of the original standard were made, and if the original primary
standard were to be lost or damaged, they would presumably just make a
new primary standard based on the remaining copies.

> Another approach to a disaster-proof mass standard is to define it
> as> a specific number of atoms of a specific substance.  If that
> substance> is carbon 12, this has the effect of defining Avogadro's
number as
> a> particular exact number, in much the same way as the redefinition
> of> the meter twenty years ago had the effect of defining the speed of
> light as a particular exact number (299,792,458 meters per second,
> exactly).
>

In the full length article, it was mentioned that they are considering
defining the kg as a particular number of atoms of silicon, or possibly
gold.  Silicon is attractive because it is easier to count or estimate
the number of atoms in a monocrystal.

> Another approach would be to define mass in terms of energy instead
> of> (as is currently done) vice versa.  But then how to define the unit

> of> energy?  Define Planck's constant as a particular exact number, and
> the energy unit will come from the second, just as the meter does.
> Alternatively, define Boltzmann's constant as a particular exact
> number, and the energy unit will come from the temperature unit,
> which> comes from the triple point temperature of pure water.  Several
> other> ways of defining an energy unit are possible.
>

I think the practical problem with those methods is that they require
making physical measurements of energy which are more difficult, for a
given degree of precision, than comparing the mass of a given number of
atoms (or the mass of a selected block of metal), with a mass to be
measured.

> One thing we don't want to do is define the gravitational constant
> as> some particular exact number.  Since that constant is only known to
> four place precision, setting up our system of units such that it's
> an> exact number would make everything else measurable only to four
> place> precision.
>

Yes, the gravitational constant would be a particularly bad choice as the
basis for a mass standard, because of the difficulty of measuring G
precisely.  It has been suggested that G might not even be the same at
very short distances (a few millimeters or less) as it is at astronomical
distances.

> This is a fascinating field, as it really illuminates the fuzzy
> border> between what is true because we've measured it and what is true
by
> definition.  Mass isn't just one concept, it's several very similar
> but subtly different concepts.  Similarly with distance, time, and
> electric charge.
>

According to a reference chart I found, the SI base units are the meter,
kilogram, second, ampere, kelvin, mole, and candela.  But those are
'base' units because they are conventionally used to derive the other
units, and not necessarily because they need to be at the core of a
universal system of units.  For example, the candela is defined for
convenience in lighting applications and in accord with how the human eye
responds to light, so it is not really a fundamental physical unit.  The
candela can be defined as a particular power (watts) of light of a
particular wavelength, or its equivalent brightness to the human eye.
The mole depends on the ratio of the mass of a particular type of atom to
the chosen standard mass, so it's not really fundamental either.  Kelvin
can be defined in terms of energy per degree of mechanical freedom, so
it's not truly fundamental; it's just a unit of convenience.  The ampere
can be defined in terms of the force between two parallel wires a certain
distance apart carrying current, so that's not fundamental, considering
that force is derived from kilograms and seconds.  We are left with the
meter, kilogram, and second.  But the meter has now been defined in terms
of the second and the speed of light, which leaves just the kilogram and
the second.  Does that mean that a complete system of physical units can
be based on arbitrary definitions of the kilogram and the second, by
application of known physical laws?  Would there be any way to define the
second in terms of the kilogram, or vice versa, without introducing an
additional arbitrarily defined unit?

> There's some weak evidence that the fine structure constant, a
> dimensionless number which equals the square of the charge of the
> electron divided by the product of the speed of light and Planck's
> constant, may have been slightly different in the distant past.
> If so, it's not clear whether it's meaningful to attribute that
> difference to a change in the speed of light, a change in Planck's
> constant, or a change in the charge of an electron, or whether any
> such choice would be completely arbitrary, no better or worse than
> any other.  Our system of units pretty much forces the speed of
> light to have been constant.  We can also nail down either Planck's
> constant or the charge of the electron, by defining it as a certain
> number.  But not all three at once.
>

If we woke up one morning, and everything was slightly smaller than the
day before, by one inch per foot, presumably we would be able to detect
and measure the change by noticing that the speed of light had increased
by some 9.09%.  But if the speed of light was slightly different 5
billion years ago than what it is today, I don't how we would know that,
or what significance it might have to us.

> You missed another good Balticon.
> --

The Maryland Libertarians took a winery tour Sunday afternoon, at the
Basignani Winery north of Baltimore, and I went on that tour.  I asked
the winemaster how many bottles of wine each grape vine produces per year
(on the average), and he said that many people ask that question, but
that he does not know the answer because he has never computed it.  Upon
further questioning, I found that an acre of that vineyard can produce 2
1/2 to 3 tons of grapes per year, that the vines are planted with varied
spacing but that the average land area per vine may be taken as about 64
square feet, and that it takes 12 to 15 pounds of grapes to make a gallon
of wine, depending on the type of wine.  From all that, I calculated that
the average grape vine at that vineyard produces grapes for about three
bottles of wine per year, assuming the bottles are 750 ml in size .

Ron Kean

.

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