The kilogram — anywhere in the world, for any purpose — is based on the exact weight of a golf-ball-sized chunk of platinum and iridium stored under three glass bell jars in a vault in an ornate building outside of Paris. Accessing the vault requires three people with three separate keys and the oversight of the Bureau Internationale des Poids et Mesures, the international organization that oversees the International System of Units.

Despite all of this security, in the 129 years since the International Prototype of the Kilogram was forged, polished and sanctioned as an artifact of measurement, it seems to have lost a tiny amount of material.

Mass is internationally defined by this prototype, also nicknamed “Le Grand K,” which means that if this original kilogram loses material, as it has done, the whole rest of the universe gets heavier.

On Friday, metrologists — people who study the science of measurements — and representatives from 57 nations will gather in a conference room in Versailles, France to redefine the kilogram. In other words: the way we weigh the world is about to change.

Nearly every corner of the globe, including the U.S. and its customary units, relies on the International System of Units, referred to as the SI, to make measurements.

The International System of units, or SI, will be based entirely on universal constants like the speed of light, the charge of an electron, and Planck's constant. Image courtesy of BIPM. Conversions using universal constants Even the U.S. customary units like feet, pounds and temperature in Fahrenheit, are defined down to the same SI unit standards. For example, since 1959, one foot has been officially defined as 0.3048 meters. Since a meter is the distance light travels in a vacuum in 1/299,792,458 seconds, a foot can be defined using the same universal constant: 1 foot is equal to the distance light travels through a vacuum in (0.3048 meters)/(299,792,458 meters/second). Light in a vacuum will travel one foot in exactly 0.000000001016 seconds, approximately one nanosecond.

Systems of measurement stretch back across more than 5,000 years of human history, based on common concepts like the length of an arm or the weight of a seed. Modern measurements strive for more universality.

All SI measurements are meant to be based in universal constants, like the speed of light or the oscillations of an atom of cesium-133. These units are expressed without artifacts or physical objects that define them. That is, except the kilogram.

“You couldn’t have GPS if you didn’t have atomic clocks and the speed of light,” said metrologist Barry Wood, who worked on the effort to make the prototype kilogram obsolete. In 1983 when the speed of light was officially defined as a constant, he said, no one would have anticipated the commonplace use of GPS by millions, if not billions of people. Scientists think that someday, the precision used in defining a kilogram might have a similarly fundamental importance.

What scientists did

Though four measurements are set to be officially redefined during the meeting in France, the kilogram is arguably the most meaningful. People use it in everyday and scientific activities — as people measure their morning coffee, determine shipping costs and calculate rocket fuel use.

It has also stymied metrologists for decades.

“The ability to compare two light objects with a balance is phenomenal, and has been phenomenal since probably the time of the Egyptians,” said Jon Pratt, a mechanical engineer who was until recently the Chief of Quantum Measurements at the U.S. National Institute of Standards and Technology. “It’s literally 3,000 year old technology.”

But determining mass without another object for comparison can be much more abstract. For more than a decade, metrologists have been working on two different experiments meant to relate mass to a universal concept called Planck’s constant, which links the energy of a photon to its frequency.

One experiment, dubbed the Avogadro Project, united laboratories across Europe, Australia, Asia and the U.S. in an effort to create a perfect silicon crystal. They hoped to calculate the exact number of atoms of silicon in a sphere equal in weight to Le Grand K — by using a single isotope, silicon-28, and hiring a master lensmaker to smooth the silicon sphere with near atomic precision. From there, they could create a precise definition of Avogadro’s constant, the number of carbon-12 atoms in 12 grams, and derive a more precise Planck’s constant.

Despite pouring millions of dollars into the most perfectly round objects ever created, researchers were not satisfied.

Another effort began with two incredibly precise scales known as Kibble or watt balances.

“If we were going to define mass in terms of the Planck constant, we were first going to have to define the Planck constant in terms of mass,” Pratt said. He worked on the NIST Kibble balance in the U.S., while Wood, a researcher at the National Research Council of Canada, worked on the Canadian counterpart.

Kibble balances have some similarity to the ancient balances that compared heaps of grain or lumps of gold. Instead of comparing objects, Kibble balances use magnets, a coil of wire and precise electrical monitoring equipment to tease out the relationship between electrical force and physical weight.

Laser interferometers in the Kibble balances measure minute movements, and other instruments constantly monitor fluctuations in local gravity to cancel out any effects from shifting tides or changing densities in the Earth’s crust.