You’ve probably read by now that the base unit of mass — the kilogram — was redefined on Tuesday. Perhaps you read about how realizing this new measurement now has something to do with Planck’s constant and Kibble balances. Maybe you even heard that the new kilogram could lead to breakthroughs in micro and nanotechnology.
What you likely haven’t heard is just how epic the quest was to get here.
"As of today, the world will never be the same," began Wolfgang Ketterle, a professor at the Massachusetts Institute of Technology and a Nobel-prize-winning physicist, as he began a lecture on the new kilogram in front of an auditorium packed with bright young minds and eminent scientists on Monday. "The fundamental definition of the kilogram has changed. And that should affect all of us."
Indeed. Until this week, the kilogram was the final holdout from an earlier time — the last standard unit of measure that was still defined by a physical artifact. Now, like the meter and the second before it, its true value is no longer man-made, but naturally occurring.
"This dream of having them based on nature is the original dream," said Ken Alder, a professor of science history at Northwestern University.
This dream of standardizing the way we measure the world goes way back to France during the heady days of its famed revolution, Alder said.
"Within the country, units differed," he said. "They would differ from town to town, sometimes from parish to parish. Like, a pint of beer in Paris would be one-third smaller than a pint of beer in Saint-Denis."
And so was born the metric system — famous for its easy-to-understand divisibility by 10. It's perhaps less famous for its other, original goal: to set all base units of measure on fundamental, unchangeable aspects of nature. For instance, how did they decide how long a meter should be in the first place?
"They decide they're gonna base it on the size of the earth," said Alder. "They send out, in the 1790s, these two guys — one north and one south from Paris — to measure the quarter meridian. That's the distance from the north pole to the equator."
The meter was set as one ten-millionth of that distance. The second was established as a precise fraction of the average solar day. And so, too, it went for the base unit of mass: the kilogram.
"They took a cubic centimeter of water at a particular temperature and said that would be a gram," said Alder. "And a thousand, obviously, is a kilogram."
The idea was that these standards would be perfectly precise, always the same, and able to be derived at any time by anyone. It didn’t work. It turns out that each quarter-meridian of the globe is not quite the same distance. The solar day varies perhaps a little more than they thought. Water, no matter how purified, isn’t exactly the same mass every time you measure it.
And so, these “natural” values were approximated.
"Once they came up with the definition, they built these standards — these physical things — to be that amount," explained Alder.
Those original "uber" standards were housed in France, with precise replicas distributed around the world. But as science and technology advanced, this too began to prove problematic. At the smallest of scales, these exact copies, over time, changed ever so slightly. So did the originals. All at different rates.
And so, we turned back to nature. In the 1960s, we redefined the second in terms of the frequency of a Cesium atom. In 1980s, the physical meter stick was retired in favor of the distance that light travels in a vacuum over a precise fraction of a second. But the standard for mass remained a metal cylinder sitting under glass in Paris.
"This was limiting everything in science and technology related to mass," said MIT’s Wolfgang Ketterle.
Limiting, because of those known small changes over time meant measuring mass could only be precise to a certain point. No longer. The new kilogram is defined as the mass of a set number of elementary particles: A perfect, unchangeable, naturally occurring, infinitely accurate number.
"Every laboratory can create those particles and, in principle, count them," said Ketterle. "They have a kilogram which is directly linked to this definition. We are no longer limited by an artifact that has built-in imprecision."
This means unprecedented accuracy at amounts so small it’s beyond our perception. But lest you think that means it has nothing to do with you.
"Small quantities matter for our lives," said Ketterle. "Medicine, pharmaceuticals are small quantities. Our computers and our smartphones would not exist without nanotechnology."
A world without medicine or smartphones? By any measure, that's a pretty heavy thought.
