Weight, what? The perfect kilogram is nearly in Planck's grasp

Only Russia can balls up the mass standard now

CSIRO's silicon sphere
CSIRO's silicon sphere

One of the standards world's toughest nuts, how to redefine the kilogram in terms of universal constants, is close to being resolved – unless a Russian experiment in 2017 throws a spanner in the works.

That's exciting for standards boffins, since it means anyone can have an accurate kilogram without having to trek to Paris for their reference – and it might happen in time for the kilo to take its place alongside the ampere, mole, and kelvin in a redefinition of units due in 2018.

The news comes courtesy of Nature, but first the background.

Since the 19th century, the world has had a single authoritative kilogram – a platinum-iridium cylinder kept near Paris by the International Bureau of Weights and Measures.

Over the years, the seven base measures – the metre, kilogram, second, Kelvin, ampere, mole, and candela – have been progressively replaced by fundamental constants instead of physical artefacts.

To turn the kilogram into a fundamental unit, boffins want to relate it to the quantum unit Planck's constant. This ties a fundamental value to the physical kilogram via E=mc2 but they need a precise measurement of Planck's constant, and that's been the stumbling block.

One technique is to count the atoms in a sphere of material (Australia's CSIRO has helped contribute to this with its own silicon sphere).

By comparing two silicon spheres that match the reference kilogram, the kilogram can be related to the mole and thence to Planck's constant.

The other is to use a watt balance, which Nature says works by “weighing a test mass calibrated according to the reference kilogram against an electromagnetic force”, and back to Planck.

Before the weights-and-measures boffins would accept either definition, the two have to be brought into agreement, and that's been the stumbling block.

“Although experiments that could define it in terms of fundamental constants were described in the 1970s, only in the past year have teams using two completely different methods achieved results that are both precise enough, and in sufficient agreement, to topple the physical definition”, Nature writes.

Researchers in NIST, the International Avogadro Coordination project, the UK's National Physical Laboratory, Canada's National Research Council and others have spent the last few years poring over results and building and re-building watt balances, and Nature reports the two result sets have finally converged enough to satisfy the Committee for Data on Science and Technology (CODATA).

The required accuracy? The various approaches now agree on Planck's constant within 12 parts per billion.

Unless something goes wrong, that's it: the kilogram will get its definition and the reference kilogram, “Le Grand K”, will slide into gentle retirement.

The only possible fly in the ointment, Nature says, is if some reference spheres being prepared in Russia ruin the previous results. ®

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