New kilogram could have mass appeal, say scientists

Scientists are working to liberate the standard unit of mass from its physical prototype.

John Kehe
Over the years, the International Prototype Kilogram and its replicas have diverged ever so slightly in weight. The difference amounts to only 50 micrograms, about the weight of a fingerprint, but even that slight discrepancy can hamper precision measurements.

You won’t feel it happen, but the kilogram, used to measure the mass of electrons, galaxies, and everything in between, is about to be transformed.

The General Conference on Weights and Measures is set to meet in Versailles, France, in November to vote to redefine the kilogram in terms of a fundamental physical constant, Planck’s constant, making it the final metric unit to be uncoupled from a material artifact.

“This in some unusual way represents a very fundamental pivot point in humanity,” says Jon Pratt, a mechanical engineer at the National Institute of Standards and Technology who is working on the revision.

Since 1889, the kilogram has been defined by the International Prototype of the Kilogram, a platinum-iridium cylinder held in a vault at the International Bureau of Weights and Measures near Paris that weighs exactly – and we do mean exactly – 1 kg. 

Sealed inside a series of three bell jars, Le Grande K, as it is known, has been handled, on average, only once every 40 years, when it is carefully washed, dried, and then weighed against dozens of official replicas from national measurement standards laboratories. Every precision scale in the world – even those that measure in pounds – can, in principle, trace its calibration back to this particular metal slug. 

But there’s one problem: Physical objects, even meticulously preserved ones, have a nasty habit of changing over time. Nobody knows if it’s because Le Grande K has been losing matter or if the replicas have been gaining it, but over time the values have been diverging over the years, to the tune of about 50 micrograms, about the weight of a fingerprint. 

“It has been remarkably stable and remarkably well handled over 130 years and has served us very, very well,” says Dr. Pratt of Le Grande K. 

Nevertheless, deriving a unit like a microgram – a billionth of a kilogram – from a 1 kg mass can be cumbersome, particularly in a world of increasingly sensitive scientific measurements. Ronald Fox, a professor emeritus of physics at the Georgia Institute of Technology and an early advocate of redefining the kilogram, mentions the Laser Interferometer Gravitational-Wave Observatory, which led to the architects receiving a Nobel Prize last year.

“If you think of the recent LIGO experiment,” says Dr. Fox, “the unit of mass is very important because you’re looking at a very, very delicate effect.”

Scientists are therefore eager to liberate the kilogram from its physical prototype. Instead, it will be linked to Planck’s constant, about 6.62607015 × 10-34 m2 kg/s. 

The trick is to calculate Planck’s constant as accurately and precisely as possible, by measuring a kilogram’s mass in terms of current and voltage.

That’s what scientists around the world have been doing in recent years, using gigantic devices known as watt balances, which measure the magnetic field generated to counteract the weight of a kilogram, thus defining its mass electrically. Once all the measurements are in, Planck’s constant will be permanently fixed. At that point the watt balances will cease to be a device for measuring Planck’s constant using kilograms and instead become a scale that measures mass using Planck’s constant. And all the metric units that depend on the kilogram – newtons, pascals, joules, watts, amperes, volts, teslas, and even lumens – will then rest on Planck’s immutable number. 

Scientists have done this sort of thing before. Until 1960, the meter was defined by a platinum-iridium rod that was sealed in the same vault as Le Grande K. But, in recognition of the relativity of space-time, it was subsequently redefined in terms of the speed of light in a vacuum, which in 1983 was redefined as exactly 299,792,458 meters per second.

“The kilogram was left out in the cold. It was the only fundamental unit that was left [that] was determined by an artifact,” says Fox.

On Dec. 31, 2018, the General Conference on Weights and Measures will stop accepting values for Planck’s constant, and the new units are set to be fixed on May 20, 2019. In addition to redefining the kilogram, the conference will also redefine the ampere, the kelvin, and the mole.

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