[volt-nuts] Physics !

J. Forster 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 anyone’s 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 world’s top
metrologists, but also by the meeting’s 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 UK’s 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 isn’t
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, it’s vital for us as consumers to be confident about what we buy –
we don’t 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

That’s 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 “that’s a kilogram”. After
all, periodic inspections of the lump have shown it’s 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, “It’s a
scandal that we’ve got this kilogram hanging around that’s 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 world’s
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 Avogadro’s constant) and then multiply that number by
the mass of each atom. If you’re 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 world’s 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 it’s 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

 It’s likely that the CIPM proposal will
 seek to redefine the kilogram in terms of Planck’s constant when and if
 the experiments – the Watt balance and the Avogadro approach – come into
  reasonable agreement. Which they aren’t now. The metrologists clearly
 don’t want to play favourites regarding the technology, if only because
 they don’t want to get burned if one or the other doesn’t live up to

 As you can see, and as I soon discovered at the meeting, there’s more
  – much more – to SI units than meets the eye. And without wanting to
 steal Crease’s thunder – he’s 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 week’s meeting. I can’t help feeling, though, that despite the
 flaws of artefacts like the standard pound, there’s 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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