The kilogram is about to change forever

  Last updated February 7, 2019 at 10:34 am


A group of scientists are about to vote to change the definition of a kilogram – and an Australian will be playing a key role.

This week, scientists and delegates representing 60 countries, including Australia, are meeting in Versailles, France, to discuss the future of the way we measure our world. If all goes to plan, they will vote to change the very definition of the kilogram.

While it seems strange to decide to change the kilogram – something we consider to be a set weight – the reason has its origins over 140 years ago.

In 1875, the Metre Convention established the International Bureau of Weights and Measures to coordinate the definition and distribution of standard units of measurement (called SI units) for trade and scientific research.

Countries involved in Friday’s vote, in red. Credit: Bureau International des Poids et Mesures

Seven base units define our world, including the second, the metre, and the kilogram. Originally, the second was defined as 1/86,400th of a day (24 hours times 60 minutes times 60 seconds), and the metre was defined as 1/10,000,000th of the distance between the north pole and the equator.

But the Earth does not spin consistently on its axis and its tectonic plates do not keep still, so new definitions were developed as the old ones became too imprecise and too unstable.

In 1967, the second was redefined using atomic clocks that tick 9,192,631,770 times a second. In 1983 the metre was redefined as the distance a beam of light travels in a certain fraction of a second. Over the years, the SI base units have been redefined in terms of unchanging physical properties of the universe.

The kilogram is the exception

The exception, however, was the kilogram. It remains the only SI base unit defined by a physical object – a platinum-iridium cylinder called Le Grand K. This precious standard is kept in a vault in the basement of the International Bureau of Weights and Measures outside Paris.

“’Le Grand K’ has been part of a shared international language of measurement, underpinning international trade and supporting international collaborations” says Bruce Warrington, CEO of the National Measurement Institute and Australia’s Chief Metrologist.

Copies of Le Grand K, otherwise known as the international prototype kilogram (IPK), were distributed to each member nation of the Metre Convention – Australia’s National Measurement Institute has copies 44 and 87. Every measurement made, from your kitchen scales to chemicals in a research laboratory, can be traced back through a series of calibrations to the IPK.

The US copy of the kilogram, stored in Bethesda, Maryland.

While it seems logical to have a standard that weights are calibrated from, the system has two major problems.

Firstly, it relies on replica kilograms to be distributed around the world by the International Bureau of Weights and Measures. Every other unit can be measured independently using a set of instructions. A country can build its own atomic clock and measure a second, and build a laser and measure a metre. But the kilogram needs to be traced back to that single lump of metal in France.

Two, the kilogram is unstable. Three times in the last 80 years, kilogram replicas have been returned to France and weighed against the IPK, and all of them have changed. The average change is 50 millionths of a gram, about the weight of a fingerprint. It doesn’t sound like much, but to modern science and industry it is a massive problem. At the cutting edge, this level of uncertainty is unacceptable.

“Our current kilogram is the best science could make in the nineteenth century,” explains Warrington. “It has served us well for over a hundred years, but it’s time to move on.”

And so, on Friday, Warrington will join his colleagues at the General Conference on Weights and Measures to redefine the kilogram in terms of Planck’s constant, a fundamental physical constant of the universe.

Time to redefine

A precise measurement of this universal constant will allow laboratories around the world to carefully measure a kilogram without having to refer to the IPK.

“It’s exciting to be a part of this historic change in the international system of measurement,” says Warrington, who will be casting Australia’s vote at Friday’s vote. “It’s a key moment in history, moving on from the last of the physical artefact standards to a new definition that will keep pace with technology into the future.”

“The average Australian won’t notice a difference overnight.”

You are not going to need to get new bathroom scales, nor is it going to affect the price of bananas at the local supermarket, but the medical and high-tech manufacturing industries will benefit from the new precision. Pharmaceutical companies increasingly need to measure ingredients in millionths, or even billionths, of grams.

The NIST-4 Kibble balance, which began full operation in early 2015 at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, measured Planck’s constant to within 13 parts per billion in 2017, which was accurate enough to assist with the redefinition of the kilogram planned for 2019.

While the kilogram will be the star of the show, three of the other base units will also be getting a makeover. The kelvin unit of temperature, which is currently defined using physical properties of pure water, will be redefined in terms of the Boltzmann constant which relates the energy of particles in a gas compared to the temperature of the gas. The mole, which defines the number of particles in a substance, will no longer be defined based on carbon atoms, but will be fixed as 6.02214076×1023 particles. And the ampere, the unit of electric current, will be defined in terms of the elementary charge of an electron.

If the vote goes the way most measurement scientists are hoping, and the definitions of these four base units are changed, the new definitions will come into effect in May 2019 on World Metrology Day.

What does it mean? Day to day, not much says Warrington. But it means far more than just the result of your bathroom scales.

“It sets Australia up for the future so that our ability to measure will improve as our technology improves,” he says.

“Measurement plays a key part in underpinning our economic prosperity and our quality of life, and it’s important that measurement enables new industries and new technologies into the future.”

The international prototype kilogram, which has served us for more than 100 years, will remain in use a little while longer to calibrate the new measurement methods.

But once its duty is done it will be relegated to science history, and the kilogram will forever be the same.

About the Author

David Gozzard
David Gozzard is a Postdoctoral Fellow in the Department of Quantum Science at the Australian National University where he works on developing laser systems for communications and sensing. David also teaches physics and is a keen science communicator. At the 2017 WA Premier's Science Awards he was named Student Scientist of the Year. Twitter: @DRG_physics

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