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 golf-ball-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.