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.

Last year, 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 behaviour 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.

**A massive change**

The kilogram is one of seven base units in the International System of Units, which defines all other measurements. (The other six base units are the metre, the second, the mole, the ampere, the Kelvin, and the candela.) It's easy to overlook the importance of units, but these seven underpin everything in our universe. They ensure stability in manufacturing, commerce, scientific innovation, and more.

The metric system, which later became the International System of Units, was conceived in the late 1700s as a way to make measurements “something for all times, for all people,” Schlamminger says. The hope was to simplify day-to-day life in a world where venturing into a different town meant the possibility of needing to learn a different system of measures.

Many of these early metric units were based on things in nature, explains Richard Davis, emeritus research physicist at the International Bureau of Weights and Measures, the organisation that regulates all things measurement-related. But in the end, these proved impractical to use. For example, the metre was defined as 1/10,000,000 the distance from the North Pole to the equator, passing through Paris. The kilogram was the mass of a litre of distilled water at its freezing point.

“They just didn't have the technology or the science to succeed,” Davis says. So, in June of 1799, two platinum standards—a metre rod and a kilogram cylinder—were forged, marking the creation of the decimal metric system. To increase their stability, the prototypes were reforged in 1889 out of a platinum-iridium alloy and stowed under lock and key.

Yet this reliance on physical objects also had its issues. “A material object will not be for all time,” Schlamminger says. Coffee cups break; clothes tear; pipes rust. What's more, locked in a vault, these objects certainly aren’t “for all people.”

In the intervening century, these physical objects have one by one been replaced with fundamental constants. The kilogram was the final holdout.

**Years of weight loss**

Save for its inaccessibility, Big K got the job done. Scientists forged a series of copies for researchers around the world to use. Only three times in its nearly 130 years did researchers release Big K from its vault to compare the precious cylinder with its doppelgängers.

But with each of these comparisons, scientists became increasingly concerned: Big K seemed to be losing weight.

Compared to its copies, the tiny cylinder appeared to be getting progressively lighter. That, or its copies were getting progressively heavier. It's impossible say which, since Big K, by definition, is exactly one kilogram. Even if someone took a file and shaved off a corner, Big K would still weigh one kilogram, and kilograms around the world would have to adjust.

In total, Big K’s mass differs from its copies by about 50 micrograms—almost the mass of a grain of salt. And though this might not seem like a lot, it's a tremendous issue for exacting fields like medicine. To top it off, this loss doesn't just affect mass, it affects any other units, like the Newton, which are defined in relation to mass.

**How is this happening?**

To resolve this weight loss, the General Conference on Weights and Measures unanimously passed a resolution in 2011 to redefine the kilogram and three additional units—the ampere, the kelvin and the mole—based on “invariants of nature.” Since then, scientists around the world have raced to find a solution.

Two different possibilities for the kilogram emerged, both of which are tied to Planck's constant. The first is based on something known as a Kibble balance. It's a little like the classic beam balance, which is, in essence, a bar with a hanging pan on either side. To measure the weight of something, place a known mass on one side and the object of interest on the other. Thanks to the gravitational force, you can tell how much that object weighs in relation to the known mass.

For a Kibble balance, however, one of these pans is essentially replaced with a coil in a magnetic field. And instead of using a gravitational force to balance the mass, it uses an electromagnetic force. By comparing a mass with aspects of this electromagnetic force, scientists can make exacting measurements of Planck's constant.

The other solution is based on crafting another gleaming object: a perfect sphere of crystalline silicon-28. This idea is based on a constant known as Avogadro's number, which defines the number of atoms in a mole to be roughly 602,214,000,000,000,000,000,000. By counting the number of atoms in a silicon sphere that is exactly 1 kilogram, scientists can figure out Avogadro's number with extreme accuracy. That can then be converted to Planck's constant. (Learn more about the use of Avogadro's number in the kilogram redefinition.)

The final value of Planck's constant is unimaginably small: 0.000000000000000000000000000000000662607015 metre-squared-kilograms per second.

**The wait is over**

With the two methods, scientists can now measure a kilogram with an uncertainty of one part in 100,000,000—a difference that is about a quarter of the weight of an eyelash, Schlamminger says. “That's the thing in science—there's no such thing as perfection," he says. "There's always random effects, and there's always a little bit of scatter. And you have to decide: Is it good enough?” Today's unanimous vote suggests this is indeed good enough.

The change will go into effect on May 20, 2019. "On that day, you won't see any change in our in our daily lives," Davis says. But in one way or another, every single scale on the planet is connected to the international kilogram standard. While measuring flour in your kitchen will remain the same, the new standard makes a world of difference for things like manufacturing car components, developing new drugs, and crafting scientific instrumentation.

Today's vote was not only remarkable for the incredible precision with these measurements can now be made, but for the international cooperation at the foundation of this work. After representatives unanimously approved the new definition, Sébastien Candel, president of the French Academy of Sciences concluded: "I hope that such will also be possible for many other issues for the world."