Born-again units

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.


Metal shock

Nearly all measurements we make are ultimately based on the SI, with a chain of laws and rules leading back to just seven base units. The way that these units are defined doesn’t matter so much for weighing vegetables, say, but many scientific experiments require precise measurement, especially in areas like fundamental physics.

The first sign that the SI was flawed was noticed in 1949 in a check on a lump of metal kept inside a vault at the International Bureau of Weights and Measures (BIPM) in Paris. By definition, it is the only object in existence with a mass of exactly 1 kilogram – one of the seven SI base units – so metrologists were unsettled to discover that this mass had changed.

Not liking to rush into anything, however, no one checked the standard kilogram again until 1989. The problem had not gone away.

The metre and the second are two base units that don’t have anything like this to worry about. They are defined with reference to the speed of light – a link to a fundamental constant that makes them robust. Much of the rest of the SI, however, isn’t in great shape.

Planck and Avogadro

A drifting kilogram means that the mole, the unit that chemists use for measuring the amount of a substance, is in trouble too. The kelvin is currently defined according to the property of water in a certain state – a fact that makes precise measurements at very high or low temperatures impossible. Meanwhile, the ampere’s definition is so impractical that electrical researchers have had to turn to a definition outside the SI system based on quantum processes instead.

See more: Click here to see the present and proposed SI unit definitions

In 2006, after decades of little action, Mills and fellow metrologists Terry Quinn, Peter Mohr, Edwin Williams and Barry Taylor argued in the journal Metrologia that the four problematic units should be tied to fundamental constants of the universe instead. The seventh, the candela, could wait.

For the kilogram, they suggested using the Planck constant, which relates the energy of electromagnetic radiation to its frequency. The Planck constant can also be used to define the Avogadro constant, the number of atoms in 12 grams of carbon-12, which in turn can be used to obtain the mole.

A fixed value is also possible for another constant, the elementary charge carried by one proton or electron, which can be used to define the ampere. For the kelvin, you can use the Boltzmann constant, which relates thermal and mechanical energy.

Persuasive power

Mills and colleagues had their work cut out to actually make this happen, though. The process of changing the SI units requires a vote by representatives of the member states of the CGPM, which meets only once every four years and is notoriously slow and bureaucratic.

What’s more, the reformers had to persuade conservative elements within the CGPM of the necessity of the changes. The conservatives resisted such a switch, claiming that the extra precision was unnecessary and that there was a risk the changes could be wrong.

Mills and colleagues, however, believe that metrology should always be one step ahead: ever-increasing precision is what the field is all about. “We’re impeding progress if we refuse,” says Mills.

The metre used to be defined by a scratch on a metal bar and the second was linked to the rotation of the Earth, he points out. At the time people were happy enough, but without exquisite accuracy in measuring time and distance, for example, we couldn’t use satellites for GPS.

Tense lobbying

On 17 October, Mills stood up in front of the CGPM audience to make the case. It was only a 10-minute presentation, but among the most important of his career. He and colleagues had drafted a proposal that would set the body on a path to change the SI.

Five tense days of behind-the-scenes discussions and lobbying later, Mills and his colleagues learned that the CGPM had unanimously backed the proposal.

Although the decision will not be binding without another vote in four years’ time, this approval makes the switch much more likely. “This is a unanimous public statement,” says Mills.

At his age, Mills accepts that he himself may not get to see the new system put in place. “It’ll happen,” he told New Scientist earlier this month. “It may be after I’m gone. I’m 81 years old. But it’ll happen.”