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The kilogram — anywhere in the world, for any purpose — is based on the exact weight of a golf-ball-sized chunk of platinum and iridium stored under three glass bell jars in a vault in an ornate building outside of Paris. Accessing the vault requires three people with three separate keys and the oversight of the Bureau Internationale des Poids et Mesures, the international organization that oversees the International System of Units.
Despite all of this security, in the 129 years since the International Prototype of the Kilogram was forged, polished and sanctioned as an artifact of measurement, it seems to have lost a tiny amount of material.
Mass is internationally defined by this prototype, also nicknamed “Le Grand K,” which means that if this original kilogram loses material, as it has done, the whole rest of the universe gets heavier.
The International Prototype of the Kilogram, also known as Le Grand K, was the internationally sanctioned artifact that defined the amount of weight equivalent to one kilogram. It is kept under three glass domes in a vault for protection. Image courtesy of BIPM
On Friday, metrologists — people who study the science of measurements — and representatives from 57 nations will gather in a conference room in Versailles, France to redefine the kilogram. In other words: the way we weigh the world is about to change.
Nearly every corner of the globe, including the U.S. and its customary units, relies on the International System of Units, referred to as the SI, to make measurements.
Systems of measurement stretch back across more than 5,000 years of human history, based on common concepts like the length of an arm or the weight of a seed. Modern measurements strive for more universality.
All SI measurements are meant to be based in universal constants, like the speed of light or the oscillations of an atom of cesium-133. These units are expressed without artifacts or physical objects that define them. That is, except the kilogram.
“You couldn’t have GPS if you didn’t have atomic clocks and the speed of light,” said metrologist Barry Wood, who worked on the effort to make the prototype kilogram obsolete. In 1983 when the speed of light was officially defined as a constant, he said, no one would have anticipated the commonplace use of GPS by millions, if not billions of people. Scientists think that someday, the precision used in defining a kilogram might have a similarly fundamental importance.
Though four measurements are set to be officially redefined during the meeting in France, the kilogram is arguably the most meaningful. People use it in everyday and scientific activities — as people measure their morning coffee, determine shipping costs and calculate rocket fuel use.
It has also stymied metrologists for decades.
“The ability to compare two light objects with a balance is phenomenal, and has been phenomenal since probably the time of the Egyptians,” said Jon Pratt, a mechanical engineer who was until recently the Chief of Quantum Measurements at the U.S. National Institute of Standards and Technology. “It’s literally 3,000 year old technology.”
But determining mass without another object for comparison can be much more abstract. For more than a decade, metrologists have been working on two different experiments meant to relate mass to a universal concept called Planck’s constant, which links the energy of a photon to its frequency.
One experiment, dubbed the Avogadro Project, united laboratories across Europe, Australia, Asia and the U.S. in an effort to create a perfect silicon crystal. They hoped to calculate the exact number of atoms of silicon in a sphere equal in weight to Le Grand K — by using a single isotope, silicon-28, and hiring a master lensmaker to smooth the silicon sphere with near atomic precision. From there, they could create a precise definition of Avogadro’s constant, the number of carbon-12 atoms in 12 grams, and derive a more precise Planck’s constant.
A silicon sphere similar to the ones created for the Avogadro Project. The spheres used in the project have been called “the roundest objects in the world.” Image courtesy of NIST
Despite pouring millions of dollars into the most perfectly round objects ever created, researchers were not satisfied.
Another effort began with two incredibly precise scales known as Kibble or watt balances.
“If we were going to define mass in terms of the Planck constant, we were first going to have to define the Planck constant in terms of mass,” Pratt said. He worked on the NIST Kibble balance in the U.S., while Wood, a researcher at the National Research Council of Canada, worked on the Canadian counterpart.
Kibble balances have some similarity to the ancient balances that compared heaps of grain or lumps of gold. Instead of comparing objects, Kibble balances use magnets, a coil of wire and precise electrical monitoring equipment to tease out the relationship between electrical force and physical weight.
Laser interferometers in the Kibble balances measure minute movements, and other instruments constantly monitor fluctuations in local gravity to cancel out any effects from shifting tides or changing densities in the Earth’s crust.
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By weighing an exact copy of the Le Grand K with immense precision, the Kibble balance is meant to define Planck’s constant to within eight decimal places. Then, Planck’s constant can be used to define weight in the absence of any physical artifact, anywhere in the universe.
“We couldn’t do it before because we couldn’t do it to the accuracy that could satisfy almost everybody,” Wood said. There wasn’t one leap in technology that allowed the redefinition of the kilogram now versus any other time in history, both researchers agreed. According to Wood, “It’s a series of little things that magically just make it a little better and a little better.”
The Avogadro Project’s numbers also agree with the Kibble balances’ more precise measurements, Wood said, which provides confirmation that their work is bringing them close to the universal constant they seek.
After the kilogram’s definition is changed officially — on May 20, 2019, also known as World Metrology Day — most people will never notice the difference, Wood said. It won’t change baking ingredients on a kitchen scale, or even have an effect on the tons of goods shipped globally every day.
For astronomers calculating the movements of stars and galaxies or for pharmacologists trying to define doses of medications down to the molecule, the new standard of measurement could change the way they work.
But for many metrologists, that day-to-day work isn’t necessarily what inspired this change. The metric system was intended to be a rational, universal set of units “for all people, for all time,” an idea attributed to the French mathematician, philosopher and revolutionary known as the Marquis de Condorcet.
According to Pratt, it was quantum physicist Max Planck himself who predicted that a system of measurements based on natural constants would someday be necessary for extraterrestrial communication. No alien civilization would agree with us on the standardized weight of Le Grand K, but we all should be able to measure the same energy coming off of a photon anywhere in the universe.
The International Prototype of the Kilogram is kept at the headquarters of the BIPM in the Pavilion de Breteuil, outside of Paris. Image courtesy of BIPM.
“Whether we know them or not, the natural laws governing the universe are necessary and constant,” Pratt said. “From the beginning of the universe till now, we haven’t seen any evidence of these constants varying. They pretty much cover the known universe and all time.”
After the upcoming vote, those constants will define the global system of measurements. For all people, and for all time.
“The SI unit system will finally be a truly universal system, free of any human artifacts,” said aerospace engineer Max Fagin, whose tweets about metrology recently went viral. “And that to me is a profoundly beautiful thing.”
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