Published online 28 January 2011 | Nature | doi:10.1038/news.2011.51

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Kilogram adjustment courts controversy

Redefinition of SI unit may require number fudge.

silicon sphereOne method of redefining the kilogram uses this silicon sphere, containing a well-defined number of atoms.ANDREW BROOKES, NATIONAL PHYSICAL LABORATORY/SCIENCE PHOTO LIBRARY

The scientists who define the world's weights and measures pride themselves on their fastidiousness — so a proposal this week to smooth diverging measurements of the kilogram, the SI unit of mass, is bound to cause waves.

During a conference at the Royal Society in London on 24–25 January, Richard Davis, the former head of the mass division at the International Bureau of Weights and Measures (BIPM) in Sèvres, France, suggested a workaround that would allow a long-planned redefinition of the kilogram to move forward. According to his plan, the results of two types of experiments that don't quite agree would be averaged, and the mean would be used to set the new standard.

The compromise seems to run contrary to the exacting standards of metrology, but without it, the kilogram's redefinition could be delayed for years or even decades. "In some sense, the redefinition of the kilogram is being held hostage," Davis told conference attendees.

Not everyone was pleased with the plan. "Deciding to just average these two results would be perfectly proper mathematics, but it would not be science," says Michael Hart, a physicist at the University of Manchester, UK. But the effort to redefine the kilogram has been dragging on for more than thirty years, and others believe that smoothing differences between the numbers — which are too small to be of much practical importance — is reasonable. "Ideally, of course, these two would come together, but if they don't, this is the way forward," says Terry Quinn, one of the conference organizers and a former director of the BIPM.

At stake is what Quinn describes as "the biggest change in metrology since the French Revolution", when the metric system was invented. Since 1889, shortly after SI units were adopted, the kilogram has been defined as the mass of a cylinder made of platinum and iridium that is locked in a vault at the BIPM. That definition may be easy to understand, but it carries risks: if the cylinder gets chipped, the world's scales have to be recalibrated. Moreover, the kilogram's mass relative to several identical copies seems to be decreasing ever so slightly. The shift is troubling because there is no way to tell whether the copies are getting heavier, or the original is getting lighter.

Fixed definition

The longstanding plan has been to replace the venerable cylinder with a kilogram defined in terms of a fundamental constant of nature. Fundamental constants are unchanging, and a definition based on them would make the kilogram as fixed as the laws of the Universe.

watt balanceThe other method uses a watt balance to define the kilo in terms of Planck's constant.Steiner, NIST

Researchers are using two different types of experiment to work towards the redefinition. In the first, a 'watt balance' — a sophisticated scale — weighs the kilogram using electric and magnetic fields. The mass measurement can then be used to define the kilogram in terms of Planck's constant, a number used in quantum mechanics.

The second method involves counting the atoms in a sphere of crystalline silicon. That result can be used to redefine the kilogram in terms of Avogadro's constant, which relates an element's atomic mass to its bulk weight.

In recent years, each method has taken measurements accurate to around 30 parts per billion (in relative uncertainty); within reach of the most accurate measurements of the platinum–iridium cylinder. But each experiment's best measurements diverge from each other by around 175 parts per billion, a quantity far larger than metrologists have been prepared to accept.

Davis says that, should the experiments prove impossible to reconcile, the results of these measurements and others should be averaged. The compromise values would be converted into precise numbers for the fundamental constants, which would be used to define the kilogram. A series of test masses made of different materials would then act, collectively, as a practical kilogram for everyday measurements.

Critical mass

The idea of setting the fundamental constants of nature by consensus may seem troubling, says Davis, but it is all part of the metrics game. "It's the nature of metrology that things don't agree perfectly." Most metrics are already set by averaging data, he says, and in the case of the kilogram, the difference is too slight to matter in even the most precise manufacturing processes. "The discrepancies are small compared to what you can live with," says Davis.

But whether physicists can live with them is another matter. Averaging incompatible results wouldn't pass muster in most labs, warns Hart.

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Some remain hopeful that Davis's compromise won't be needed. Ian Mills, a chemist at the University of Reading, UK, believes that the existing measurements can be reconciled by 2015, when the international General Conference on Weights and Measures will meet to discuss the redefinition in earnest. Several new watt balances will take measurements in that time, which could help to end the current conflict. "I think it is likely that the present disagreement will be resolved in the next couple of years," he says.

But Peter Becker of the Federal Institute of Physical and Technical Affairs (PTB) in Braunschweig, Germany, who has been conducting the silicon-sphere measurements, seems less certain. "Maybe we can do it," he says with a shrug. "I don't know."

Regardless, says Quinn, the time has come to make the switch. "The longer we wait, the larger might be the drift of the international prototype," he says. "The present situation is pretty flaky." 

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