Date: Thu, 29 May 2003 00:09:20 -0400 (EDT)
From: "Keith F. Lynch" <kfl at KeithLynch.net>
To: WSFA members <WSFAlist at KeithLynch.net>
Subject: [WSFA] Re: reference kilogram slowly evaporating
Reply-To: WSFA members <WSFAlist at keithlynch.net>

ronkean at juno.com wrote:
> The kilogram is getting lighter, scientists say, sowing potential
> confusion over a range of scientific endeavor.

Interesting concept.  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 is 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.)

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 Avagodro'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).

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.

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.

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.

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.

You missed another good Balticon.
--
Keith F. Lynch - kfl at keithlynch.net - http://keithlynch.net/
I always welcome replies to my e-mail, postings, and web pages, but
unsolicited bulk e-mail (spam) is not acceptable.  Please do not send me
HTML, "rich text," or attachments, as all such email is discarded unread.