If things go as planned, by the end of this week, the world will have a new definition of the kilogram.
The change will not require any adjustments to bathroom scales, or alter the heft of a bag of potatoes. But the milestone will serve to make our universal unit of mass a lot more universal.
In short, instead of basing it on a lump of precious metal locked in a vault in France, scientists have decided to recast the kilogram as something truly immutable, tied by a mathematical umbilical cord to the fundamental constants of nature that have endured since the Big Bang banged.
For those who play in the rarefied world of high-precision metrology − the science of measurement − it just doesn’t get any better.
“I’ve always said this is the best time to be the chief metrologist for your country,” said Alan Steele, who plays that role for Canada as director-general of the National Research Council’s Metrology Research Centre in Ottawa.
Dr. Steele is set to cast Canada’s vote to adopt the new kilogram definition on Friday.
The kilogram was originally devised as part of the metric system, a byproduct of the French revolution that sought to break with traditional measures such as the pound − an arbitrary quantity that harks back to the Roman libra − in favour of more scientifically derived units.
Initially, the kilogram was defined as the mass of a litre of water at the freezing point. But by 1889, the countries who were then part of the General Conference on Weights and Measures agreed that the value of the unit mass had to be pegged to something that could be measured far more precisely.
They settled on a reference weight machined out of a platinum-iridium alloy that has served as the world’s prototype kilogram ever since.
Canada was not among the original signatories, having joined the conference in 1907. But this time around, Canada has played a key role in supplanting the prototype with a new definition that no longer depends on a physical artifact.
“It’s something we’re really proud of,” Dr. Steele said. “Metrology is about credibility and demonstrating you’re as good as you say you are.”
What Canada turns out to be good at is measuring an exceedingly small number, known as Planck’s constant, that serves as the fundamental increment of action in the universe. In theory, the constant emerges whenever you divide the energy in a particle of light by its frequency. In practice, it’s not so easy to measure. Right now, Canada holds the record for setting the value of the tiny constant with the least amount of uncertainty − about 9.1 parts per billion.
The device that made it possible is the Kibble balance, a supercharged version of the standard laboratory scale. But instead of comparing the masses of two objects, the Kibble balance very precisely sets the mass of one object against the magnetic force generated by an electric current flowing through a coil of wire. The ingenious design bridges the mechanical realm with the electromagnetic, and thereby allows the kilogram to be bound firmly to Planck’s constant for all time.
This has long been a goal of scientists who serve as the arbiters of measurement. The metre and the second have been defined by physical constants, such as the speed of light, for decades. But because of the difficulty of measuring Planck’s constant, the kilogram has been a holdout until now.
“It was only in the last two years or so that it became clear that we could vote for the redefinition,” said Michael Stock, director of physical metrology for the International Bureau of Weights and Measures in Sèvres, France. The effort, he said, is not driven by the needs of today but by the possibilities that more precise units of measurement open up for scientists in years to come.
Canada first got into the kilogram game in 2009 when it took over a Kibble balance from Britain and, in Dr. Steele’s words, “went to town and made almost every aspect of the experiment better and better.”
The work ultimately allowed Canada’s blue-chip measurement to be included with a handful of other labs who have combined to set the value of Planck’s constant and, by extension, the kilogram.
Carlos Sanchez, who was part of the team that conducted the work at the NRC, said that the challenge was not just about being precise but about relentlessly beating down more than a dozen sources of uncertainty in the equipment for several years to get the cleanest possible result.
“It takes patience,” Dr. Sanchez said. “You have to have a plan, otherwise you can spend your life doing experiments that lead nowhere.”
Happily, the NRC’s experiments have led to the Palace of Versailles, where representatives from 60 countries have gathered to officially approve the new definition, not only for the kilogram but also for units of electric current (the ampere), temperature (the kelvin) and particulate quantity (the mole). A unanimous vote is expected, after which the global edifice of measurement, perhaps humanity’s greatest tribute to objective reality, will stand on firmer ground − all with nothing apparently having changed.
Which is precisely the point, Dr. Steele said. At the end of it all, "you want the kilogram to still weigh a kilogram.”