Sandia weighs in on new definition for kilogram, Change should have little impact on nuclear weapons complex
ALBUQUERQUE, N.M. —The kilogram is losing weight and many international scientists, including some at Sandia National Laboratories, agree that it’s time to redefine it.


“The idea is to replace the single master kilogram with something based on physical constants, rather than an artifact that could be damaged accidentally,” says mechanical engineer Hy Tran, a project leader at the Primary Standards Laboratory (PSL) at Sandia.

Sandia is a National Nuclear Security Administration laboratory.

The purpose of redefining the kilogram is based on risk reduction, says Tran.




“If and when the redefinition takes place, it will be done in such a fashion as to have minimal or no practical impact with other measured quantities,” Tran says. “In other words, if it is redefined so as to ensure better than 10 parts per billion agreement — rather than 20 parts per billion agreement — then we will see no major changes immediately.”




Sandia physicist Harold Parks agrees that the redefinition of the kilogram is inevitable and says there are a couple of issues that need to be resolved before it’s redefined.


The watt balance is based on an idea that compares electrical and mechanical power with a high degree of accuracy, he says.






Is Paris worth a mass?, A kilogram, it seems, is no longer a kilogram
IN THE 19th century, the heyday of European colonialism, those two great imperial rivals, Britain and France, agreed to carve up not merely the world, but the universe. The capacity to describe that universe depends on the ability of scientists to relate their discoveries to the fundamental quantities of length, time and mass. The British gained control of time, which is why the Earth’s prime meridian—from which the planet’s rotation and thus the lengths of the day, the hour, the minute and the second were then calibrated—runs through Greenwich, a suburb of London. The French annexed length and mass.

They kept them, in the form of two lumps of metal, in sealed jars in the Bureau International des Poids et Mesures in Sèvres, a suburb of Paris. The metre was the distance between two scratches on a bar composed of a special alloy of platinum and iridium. The kilogram was the weight of a lump of a similar alloy. The foot and the pound, to British chagrin, were legally agreed to be mere fractions of these fundamental units.

Science moves on. Time is now defined by readily available clocks that use caesium atoms as their pendulums, while distance is specified in terms of the speed of light in metres per second (the caesium atoms having already provided the value of a second), and these days that speed can be measured with great accuracy using easily purchased equipment. Mass, however, remains stubbornly stuck in Paris, under three concentric glass lids (pictured) strangely reminiscent of cheese covers, which are intended to stop it either absorbing or shedding matter and thus changing in value.

Unfortunately, the cheese covers have not worked. Over the years the standard kilogram has put on weight, or possibly lost it. Nobody quite knows which (see article). But they do know that it is not the same kilo that went under the lids in 1889. Which is wonderful news for writers of newspaper headlines, who have been handed a post-Christmas gift about seasonal weight-gain not being as bad as the bathroom scales might suggest, but is awkward for physicists, who are left unsure what anything in the universe actually does weigh.

Science would thus love to be free of this awkward lump of metal, but attempts to define mass objectively—with reference to, say, the mass of a proton—have always foundered on the question: “So how do you measure that?” For all the fancy equipment that scientists now have for monitoring the behaviour of caesium atoms and the value of the speed of light, no one has come up with a more accurate way of measuring mass than taking the Parisian ingot out of its sarcophagus from time to time, and putting it on a set of scales.

Some physicists do believe they have an answer. It involves Planck’s constant (the fundamental principle of quantum physics), the speed of light and an apparatus called a watt balance. If a meeting next year approves this idea, the Parisian mass will at last become redundant. But the watt-balance approach itself has critics, for such balances are costly.

Je pèse donc je suis

Being able to weigh things without reference to a bit of Bonapartist bullion would mean that all means of measurement were based on universal truths rather than mere artefacts. That would bring cheers from mensuration laboratories around the globe. But the measuring itself will still be done by human minds, with human ideas of what sorts of thing to measure, and of how to make those measurements make sense—what axes, in other words, to use to define the universe of the measured. And here, at least, there is a sop for French amour propre: though mass will no longer be Parisian, at least the co-ordinates will remain Cartesian.

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