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Scientists say goodbye to physical definition of the kilogram

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A scientist holding a copy of the International Prototype Kilogram, which has now been replaced.
Photo: NIST

In between coffee breaks and presentations on quantum physics, scientists today voted to redefine the kilogram, the unit that underpins the world’s system of weights. Instead of basing the kilogram’s value on a physical artifact, as has been the case for more than 100 years, it will now be defined using a constant of nature.

Its weight, handily, will not change.

The vote has been centuries in the making, with scientists describing it as the biggest revolution in measurement since the French Revolution. Now, they say, the metric system will fulfill the guiding ideology of metrology established in that period: to create units of measurement “Pour les temps, pour les peuples” — for all time and for all people.

The use of physical artifacts has been a stumbling block in achieving this goal. For 129 years, the kilogram has been defined as the weight of the International Prototype Kilogram (IPK), a lump of platinum-iridium that’s stored in a vault in Paris. Slight changes to the IPK’s weight (caused by contamination, some speculate) have been a blemish on the metric system’s reputation and a threat to scientific experiments that depend on precise measurement.

Read more: How and why the kilogram had to die

With today’s vote, those problems are in the past. The kilogram is now defined using a calculation based on Planck’s constant, or h, which can be thought of as the smallest amount of energy possible. Along with the kilogram, three other units were also redefined: the ampere, Kelvin, and mole, all of which are now officially linked to constants of nature.

Bill Philips, a Nobel laureate whose work underpins the new definitions, told delegates that with the change, “you can now keep the international system of units in your wallet.” (Though you’d need some serious lab equipment to turn those definitions into actual units.)

A card showing the new definitions of the units of the metric system, which is handy if you have a high-tech lab nearby.

The vote was held by the International Bureau of Weights and Measures (BIPM), the agency that maintains the metric system, near its headquarters in Versailles, France. For scientists, attending the occasion was celebratory but also tinged with a little sadness.

“I do feel emotional about burying the kilogram,” Stephan Schlamminger, a physicist at the US National Institute of Standards and Technology (NIST), told The Verge. “I feel nostalgic and sad about it in a way, even though I know this new system is going to be better.”

Klaus von Klitzing, another Nobel laureate whose work played a key role in the redefinition, echoed these sentiments. “Today we celebrate a funeral and a marriage simultaneously,” he told delegates at the Palais des Congrès. But the benefits of the new system, he said, vastly outweighed any convenience in retaining the old. Stasis is not an option in metrology, as new types of measurement expand what we can observe and learn about the world.

“If you are doing high precision measurements, you discover new territories,” von Klitzing told delegates, before laughing and adding, “Therefore, there is a close connection between Nobel prize winners and metrology.”

The vote was as much a diplomatic event as a scientific one, which is fitting given that the Treaty of the Meter, which created the BIPM in 1875, is one of the oldest international agreements. Now, metric units are the official units for every nation on Earth, apart from Liberia, Myanmar, and the United States. (And even America is metric at heart: it has its own units, but those are defined using the metric system.)

Jon Pratt, a physicist from NIST, shows off his metrology-themed tattoo. It shows the original seal of the BIPM (with a statue holding a prototype meter and kilogram), with Planck’s constant written underneath.

Delegates from some 60 nations attended the vote, each responsible for the standardization of units in their own country. A question and answer sheet was given to those who attested to some of these practical matters, with questions like: “Will I get my standard of mass calibrated under the Revised SI in the same way as I do now?” (The answer: yes, with reference kilograms created using a device called a Kibble balance.)

Willie May, a former director of NIST, said that reaching this point had been grueling, requiring years of international debate. “Any time you have to have a whole lot of folks agree on anything, it’s going to be a long, tedious, and delicate process,” said May. But the payoff was more than worth it, he said. “It’s something that had to be done.”

Without a shared system of consistent units, trade is difficult, and scientific research is nearly impossible. The redefinition sustains this exchange of goods and information into the future, and it will support the development of new technology that requires precise measurement, like building microchips for quantum computers or medicine that’s tailored to an individual’s genome. Even at this conference, delegates were discussing new frontiers, including better measurements for biological responses to drugs and pharmaceuticals.

When it came time for the vote, delegates from each country stood in the auditorium to give their response. In the end, it was unanimous.

Delegates applaud after the final vote to accept the new definitions.

Martin Milton, director of the BIPM, said he was pleased with the result. “The feeling is that plans have worked,” Milton told The Verge. “We’re the international organization for measurement, and our job is to coordinate international effort. For me, success is if our member states think it’s a success.”

For metrologists, the redefinition also offered a previously unknown experience: a spot in the limelight. “This is something metrologists aren’t used to,” said May. “We work in the very backwaters of geekdom, and now we have the entire world coming to see what we have accomplished... I honestly don’t have words to describe what today feels like.”

But that’s the nature of metrology. It is a discipline that enables so much, but, if the job is done correctly, it goes almost completely unnoticed. As with the redefinition of the kilogram, the aim has not been to change anything, but to simply place it on stronger foundations — ones that will hopefully support civilization for a very long time to come.

Photography by James Vincent / The Verge