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.

Scientists are hoping to redefine the kilogram by basing it on standards of universal constants rather than on an artifact standard.

“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.

Of the seven units of measurement in the International System, or SI, the kilogram is the only base still defined by a physical object. In addition, copies of the kilogram have changed over time by either gaining or losing weight as compared to the standard kilogram.

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

“In the long term, the redefinition — especially if performed correctly — is beneficial because of risk reduction and because it may enable better measurements in the future,” he says.

By replacing the master kilogram — Le Grand K —with a unit based on physical constants, researchers at multiple laboratories and at national measurement institutes could establish traceability, he says.

Tran says the kilogram will remain the kilogram; it’s only the way it will be defined that will change. He says the earliest the kilogram would be redefined is 2011.

“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.”

Based on the current formal definition of the kilogram (the mass of the 1 kilogram prototype) and experimental dissemination to standards labs, the uncertainty (95 percent confidence) in PSL’s kilogram is about 40 parts per billion, compared to the IPK.

One part per billion is about the ratio of the area of a square 3/32 inch on a side, with respect to the area of a regulation NFL football field (including the endzones, or 120 yards by 53-1/3 yards), Tran says.

The target originally proposed by the Bureau International des Poids et Mesures (International Bureau of Weights & Measures) was to get one of the alternative kilogram definitions, such as the experimental measurement of force on the watt balance (or counting atoms on the silicon sphere), and deriving the kilogram, matched to experimental measurements of the prototype kilogram to within 20 parts per billion.

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 method of defining the kilogram makes the most sense for those of us in electrical metrology and so far it is the most accurate,” he says. “But other proposals, such as those based on counting the number of atoms in a silicon crystal, are being considered.”

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

Conflicts between the results of the watt balance and the atom counting experiments will also need to be resolved, Parks says.

“The NIST (National Institute of Standards and Technology) watt balance experiment has achieved the accuracy needed to redefine the kilogram, but the experiment will need to be confirmed by other groups in order for the results to be fully accepted,” he says.

Tran says redefining the kilogram will have little impact on the Primary Standards Lab or the broader nuclear weapons complex. The lab develops and maintains primary standards traceable to national standards and calibrates and certifies customer reference standards.

“It should not affect PSL or the complex if the international metrology community ensures that they fully consider the uncertainties, the necessary experimental apparatus to realize the kilogram, and implementation issues prior to agreeing to the redefinition,” Tran says.

In preparation for the change, PSL staff members are staying up to date in research in metrology and standards practices. Staff also participate in standards activities in order to ensure that any transition would be smooth.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the U.S. Department of Energy’s National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.

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.