[volt-nuts] Physics !
jfor at quik.com
Fri Mar 11 00:24:01 UTC 2011
One Watt per square meter:
The scandal of the kilogram
By Matin Durrani
With Tunisia in political turmoil, parts of Australia under water and
dozens dead in a Moscow bomb blast, a meeting on SI units in the confines
of the Royal Society in London might seem absolutely right at the bottom
of anyones news agenda. Surely the conservative world of metrology, where
physicists spend years sharpening up their measurements of the seven
fundamental base units, is unlikely to cause much of a stir?
But the two-day meeting, which ended yesterday, did attract a dozen or so
journalists, that led to reports in the Wall Street Journal, the Guardian,
New Scientist and the BBC.
They were no doubt attracted in part by the presence of the worlds top
metrologists, but also by the meetings focus: to discuss whether to
revamp the SI system of units so that it is based purely on the
fundamental constants of physics.
The importance of the meeting was underlined by the fact that the
organizers had managed to snare the UKs minister for universities and
science David Willetts, who in his opening remarks gave a good impression
of at least seeming to understand what metrology is all about; he isnt
nicknamed two brains for nothing.
As Willetts pointed out (thanks no doubt to his speechwriters),
metrology and the measurement system are important on three counts.
First, its vital for us as consumers to be confident about what we buy
we dont want to be ripped off at the checkout with an underweight bag
of carrots or, more seriously, be given the wrong dose during
radiotherapy for cancer treatment.
Second, metrology is key for advanced technology accurate timekeeping
via atomic clocks has proved essential for GPS, for example. Third, and
this is what the meeting was about, the work is essential if we are to
define our measurement system entirely in terms of fundamental
Thats the name of the game in metrology these days finding a way
of defining mass without just resorting embarrassingly, as we do now, to
a lump of metal in the basement of the International Bureau of Weights
and Measures (BIPM) outside Paris and saying thats a kilogram. After
all, periodic inspections of the lump have shown its been changing its
mass slowly over time. As laser physicist Bill Phillips
from the National Institute of Standards and Technology (NIST) told
delegates during one question-and-answer session on Monday, Its a
scandal that weve got this kilogram hanging around thats changing its
In among the audience at the meeting was Physics World columnist Robert
Crease from Stony Brook University in New York, who in December wrote
about visiting the BIPM last autumn for what could be one of the last ever
annual inspections of the kilogram.
Crease was on hand to get the latest goings-on among the worlds
metrology community for a feature on the redefinition of the kilogram in
the March issue of Physics World magazine so keep an eye out for that.
But redefining the kilogram is not that easy. One option is to take a
large, nearly perfect silicon sphere, count how many atoms are in it
(which determines Avogadros constant) and then multiply that number by
the mass of each atom. If youre interested, a new paper in Physical
Review Letters provides the most accurate value for the Avogadro constant
to within 30 parts in a billion the result of a collaboration between
eight different national metrology institutes around the world.
The other is to use a Watt balance, which does not require big
collaborations, but is conceptually harder to understand. It involves
balancing the force through a coil with the mass of an object, and then
doing another bit of jiggery pokery involving the quantum-Hall effect
(to measure resistance) and the Josephson junction (to measure voltage).
The plan is for the worlds metrology community represented by the CIPM
to put forward a proposal at its meeting next October that the SI
system should be revamped. That proposal will go to the organization to
which the CIPM reports the General
Conference on Weights and Measures (CGPM) which is basically a bunch
of diplomats in a smoke-filled room (without the smoke). If they give it
the nod, well then its time to rewrite the physics textbooks.
In the current system, the kilogram, ampere, kelvin and the mole are
all linked to exact numerical values of the mass of the international
prototype kilogram in Paris, the permeability of the vacuum, the
triple-point temperature of water, and to the molar-mass of carbon-12
respectively. The plan is to change all that so that these four units
are linked to exact numerical values of the Planck constant, the charge
of the electron, the Boltzmann constant and to the Avogadro constant
Its likely that the CIPM proposal will
seek to redefine the kilogram in terms of Plancks constant when and if
the experiments the Watt balance and the Avogadro approach come into
reasonable agreement. Which they arent now. The metrologists clearly
dont want to play favourites regarding the technology, if only because
they dont want to get burned if one or the other doesnt live up to
As you can see, and as I soon discovered at the meeting, theres more
much more to SI units than meets the eye. And without wanting to
steal Creases thunder hes busily putting the finishing touches to
his Physics World feature on a plane back to the US as I write I think I
had better stop.
Just to say that on display in the foyer at the Royal Society are
copies of what used to be known as the standard yard and the standard
pound (see above), which made the venue a suitably appropriate place
for this weeks meeting. I cant help feeling, though, that despite the
flaws of artefacts like the standard pound, theres more of an emotional
connection with a real object like it than a seemingly esoteric
definition based on the Planck constant.
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