New definition for kilogram kicks in Monday
In a subterranean vault in a suburb of Paris lies a small, rarely seen metal cylinder known as Le Grand K.
For 130 years, this golfball-sized hunk of 90% platinum and 10% iridium has served as the international prototype kilogram. That means it was the single physical object by which all other kilograms across the planet were measured.
If microscopic contaminants in the air caused Le Grand K to grow a bit heavier, the kilogram itself grew a bit heavier. If a rigorous cleaning or small scratch caused it to become ever so slightly lighter, the kilogram itself became lighter as well. Indeed, it is estimated that over the course of its lifetime, Le Grand K has lost 50 micrograms of mass.
But the long reign of Le Grand K is about to come to an end.
Starting Monday, the kilogram will be redefined not by another object, but by a fundamental property of nature known as Planck’s constant. Like the speed of light, the value of Planck’s constant cannot fluctuate – it is built with exquisite precision into the very fabric of the universe.
“Unlike a physical object, a fundamental constant doesn’t change,” said Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. “Now a kilogram will have the same mass whether you are on Earth, on Mars or in the Andromeda galaxy.”
Researchers who have devoted their lives to the science of measurement say the new definition of the kilogram – and similar changes to the mole (which measures quantities of very small particles), the ampere (which measures electrical charge) and the kelvin (which measures temperature) – represents a profound turning point for humanity.
“The ability to measure with increasing accuracy is part of the advancement of our species,” said Walter Copan, director of NIST.
Most of us regular folks will hardly notice the switch. A 4-pound chicken (1.81437 kilograms) at the grocery store or a pound of coffee beans (0.453592 kg) at Starbucks will remain exactly the same.
“We don’t want to shock the system,” Schlamminger said.
The decision to redefine four base units of the International System of Units was made in November at the 26th General Conference on Weights and Measures in Versailles, France. Delegates from 60 member states assembled in a large auditorium for the historic vote. It was unanimous. A standing ovation and champagne toast followed.
“The meeting itself was an electric experience,” said Copan, who represented the U.S. “It was a long journey to get to this point.”
The origins of the metric system date back to the French Revolution in the late 1700s. At the time, an estimated 250,000 different units of measurement were being used in France, making commerce and trade a challenge. The new system was designed to be rational and universal, with units based on properties of nature rather than royal decree or the whims of local dukes and magistrates.
“The idea was that these measurements would be eternal and the same for everybody, everywhere,” said Ken Alder, a science historian at Northwestern University in Evanston, Ill.
The foundational unit of the system was the meter, which was supposed to be one ten-millionth the distance from the North Pole to the equator along the Paris meridian.
At the same time, the kilogram was defined as the mass of 10 cubic centimeters of water at 4 degrees Celsius.
These units were adopted by the French Republic in 1795. Countries in Europe and South America adopted the metric system throughout the 19th century. In 1875, delegates from the U.S. and 16 other countries signed the Treaty of the Meter in Paris. It established a universal system of units based on the meter, the kilogram and the second that would streamline trade among nations.
Although the meter and the kilogram were based on the size of Earth, they were officially defined by metal artifacts, including Le Grand K, that were cast in London in 1889 and kept in a vault in the basement of the newly created International Bureau of Weights and Measures in Sevres, France. Member nations received one of 40 precise replicas.
The Treaty of the Meter also established the General Conference on Weights and Measures (CGPM), an international group tasked with studying and voting on proposed changes to units of measurement for all member states.