Agreement to tie kilogram and friends to fundamentals

• 25 October 2011 by Richard Fisher

After decades of worry, toil and argument, metrologists have officially begun the process of tying the definitions of four basic units to nature's fundamental constants.

The General Conference on Weights and Measures (CGPM) in Paris, France, has unanimously agreed on a proposal that would lead to reform of the mole, kilogram, kelvin and ampere, according to the international system of units (SI).

That puts us on the cusp of a historic change in the way science sizes up the world. If the next CGPM, in four years' time, confirms the plan, it will amount to the biggest change to the SI units for a century.

Proponents of the switch are thrilled. "Not a single vote against! It was unbelievable," says Ian Mills of the University of Reading, UK.

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