For nearly 150 years, the official weight of a kilogram was determined by a shiny cylinder of platinum locked away in a French vault.
The kilogram, like the meter and the second, is one of the seven fundamental units of measurement (also known as the International System of Units or the metric system, the "SI" for short). These were first formalized in the 1875 Treaty of the Metre. Back then, the best way to agree on the weight of a kilogram was to forge a single hunk of metal and call it "Le Grand K." And for more than a century, all scientific scales were calibrated back to that one physical reference point (with copies stored in a dozen countries).
But even solid objects can change over time. When Le Grand K was weighed in the 1980s, it was a couple of micrograms lighter, meaning that all highly accurate scientific scales (not the one in your bathroom) had to be recalibrated. That's what nerds call a real pain in the mass.
Luckily, teams of metrologists were already on the case (metrology is the science of weights and measures), searching for a universal constant that would generate a fixed value for the kilogram that's true now and a million years from now.
They had already found such a physics fix for the second, which was redefined in 1967 from 1/86,400th of a day to something much more confusing, but constant. It takes 9,192,631,770 oscillations of a special microwave beam to excite atoms of the isotope cesium-133 to a higher energy level. Since that number will never change (unlike the exact length of a day), that's your new second!
Same for the meter. Instead of being defined as the length of a single, meter-long metal pole forged back in 1889, it was redefined in 1983 as the distance light travels in a vacuum in 1/299,792,458th of a second.
It wasn't until 2017 that scientists working at the U.S. National Institute of Standards and Technology (NIST) and similar bodies worldwide finally agreed on a universal constant for the kilogram. The achievement required solving one of the thorniest physics problems of the last century, coming up with a numerical value for Planck's constant.
Without getting too technical (you can do that here), physicist Max Planck proved in 1900 that matter releases energy in discrete chunks called "quanta." His equation for measuring those packets of energy included a constant called h, hitherto known as Planck's constant. Thanks to Einstein, we know that energy and mass are mathematically related, so physicists figured out that Planck's constant (which is a fixed unit of energy) could yield the world's most accurate measurement of mass.
Calculating the exact value of Planck's constant took decades and some serious technological innovation (specifically a nifty device called a Kibble Balance), but now even distracted kindergarteners know that Planck's constant is 6.626070150 × 10-34 kg⋅m2/s. I mean, duh.
In mid-November, at the annual meeting of the International Bureau of Weights and Measures (BIPM) in Versailles, France, representatives from more than 60 countries voted to approve a new and everlasting definition of the kilogram as calculated by the Planck constant. No more hunk of metal — the kilogram's mass is now tied to Planck's constant. New definitions were also announced for SI units the ampere (electrical current), the kelvin (temperature) and the mole (number of molecules or atoms in an element). These new definitions took effect on May 20, 2019.
The original platinum kilogram prototype will remain in that underground French vault, while countless generations of scientists make life-changing discoveries using the kilogram 2.0.