Scientists baffled as global benchmark to define kilo changes

THE guardians of the world’s most important standards of weights and measures have turned to quantum physics to try to resolve a dilemma.

To the bafflement of scientists, a cylinder of metal sitting in a closely-guarded strongbox that is the global benchmark for the kilogramme is changing mass.

The enigma doesn’t affect people who wants to buy 500mg tablets of aspirin, half a kilo of carrots or a 50,000-tonne cruise ship.

But it poses a theoretical challenge to physicists, and complicates the work of labs, which need ultra-precise, always-standard measurement.

Since 1889, the kilo has been internationally defined in accordance with a piece of metal kept at the International Bureau of Weights and Measures (known by its French acronym of BIPM), in Paris.

The British-made cylinder, 90% platinum and 10% iridium, was deemed at its founding to be as inalienable as the stars in the sky.

It is kept under three glass cases in a safe in a protected building, the Pavillon de Breteuil.

In 1992 came a shock: the famous kilo was no longer what it should be.

Measurements made over a century showed that the prototype had changed by around 50 microgrammes — the equivalent of a grain of sand 0.4mm in diameter — compared to six other kilos also stored in Sevres.

“Actually, we’re not sure whether it lost mass or gained it,” said Alain Picard, director of the BIPM’s Mass Department.

The kilo is a bedrock of the International System of Units (SI), the world’s most widely-used system of measurement units for daily life, precision engineering, science and trade.

The SI has seven “base units” — the kilo, metre, second, ampere, kelvin, mole and candela — from which all units are derived.

The kilo is the last unit that is still defined by a material object.

Moving at a pace best described as ponderous, the masters of the SI have now decided to phase out the kilo cylinder.

If all goes well, it will be replaced by a fixed value based on the Planck Constant, named after Max Planck, who discovered quantum physics in 1899.

The Planck Constant corresponds to the smallest packet of energy, or quanta, that two particles can exchange.

On October 21, the General Conference on Weights and Measures agreed to use the constant to calculate the value of the kilo.

But adopting this “will not be before 2014”, after experiments to assess the accuracy of measurement techniques to ensure accuracy to 20 parts per billion.

If the Planck Constant is adopted, nothing in everyday life will change. The kilo will still be a kilo.

“However, the changes will have immediate impact in the excruciatingly accurate measurements carried out by highly specialised laboratories,” the conference said in a press release.