The kilogram is forever changed. Here's why that matters.

 This cylinder is an exact replica of the International Prototype of the Kilogram, or IPK. Stored at the National Institute of Standards and Technology in Gaithersburg, Maryland, this particular copy is the basis for all weight calibrations in the United States.  PHOTOGRAPH BY ROBERT RATHE

This cylinder is an exact replica of the International Prototype of the Kilogram, or IPK. Stored at the National Institute of Standards and Technology in Gaithersburg, Maryland, this particular copy is the basis for all weight calibrations in the United States.

PHOTOGRAPH BY ROBERT RATHE

Sealed under a trio of nested glass bell jars, a gleaming metal cylinder sits in a temperature-controlled vault in the bowels of the International Bureau of Weights and Measures in Sèvres, France. Dubbed Le Grande K, or Big K, this lonely hunk of platinum and iridium has defined mass around the globe for more than a century—from bathroom scales to medical lab balances.

But that is all about to change.

Today, representatives from more than 60 countries voted during the 26th meeting of the General Conference on Weights and Measures in Versailles, France to redefine the kilogram. Rather than basing the unit on this physical object, henceforth, the measure will be based on a fundamental factor in physics known as Planck's constant. This infinitesimally small number, which starts with 33 zeros after its decimal point, describes the behavior of elementary packets of light known as photons, in everything from the flicker of a candle flame to the twinkle of stars overhead.

“That fundamental constant is woven into the fabric of the universe,” says Stephan Schlamminger, leader of the National Institute of Standards and Technology team who, along with an international cohort of scientists, worked to refine Planck's constant for the kilogram redefinition. Most importantly, this value will remain the same for all time, no matter the location.

Read the full story on National Geographic’s website