Welcome to brain stuff from how Stuff Works. Hey, brain Stuff, I'm Lauren Vogelbaum. And for nearly a hundred and fifty 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 eight
seventy five Treaty of the Meter. Back then, the best way to agree on the weight of a kilogram was to forge a single hunk of metal and dubit 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 nineteen eighties, it was a couple of micrograms lighter, meaning that all highly accurate scientific scales, not like the one in your bathroom, to be recalibrated, and that's what nerds call a real pain in the mass. Luckily, a team of metrologists was already on the case. Metrology
being the science of weights and measures. They were searching for a universal constant that would generate a fixed value for the kilogram that's true now and will still be true a million years from now. They had already found such a physics fix for the second, which was redefined in nineteen sixty seven from its previous value of a fraction of a day one six thousand, four hundredths of a day to be precise, to something much more confusing,
but much more constant. It takes nine billion, one d and ninety two million, six hundred and thirty one thousand, seven hundred and seventy oscillations of a special microwave beam to excite atoms of the isotope caes M one thirty three 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 poll for orged back in eighteen eighty nine, it was redefined in nineteen eighty three as the distance light travels in a vacuum in a particular fraction of a second one two million, seven hundred and nine two thousand, four
hundred and fifty eight of a second. It wasn't until twenty seventeen that scientists working at the u S National Institute of Standards and Technology and similar bodies worldwide finally agreed on a universal constant for the kilogram. The achievement required solving one of the Thorny's physics problems of the last century, coming up with a numerical value for planks
constant without getting too technical. A physicist, Max Planck proved in nineteen hundred that matter releases energy in discrete chunks called quanta. His equation for measuring these packets of energy included a constant called h hitherto known as planks constant. Thanks to Einstein, we know that energy and mass are mathematically related, that whole equals mc squared thing, so physicists figured out the planks constant, being a fixed unit of energy,
could yield the world most accurate measurement of mass. Calculating the exact value of plunks constant took decades and some serious technological innovation, specifically a nifty device called a Kibble balance, but they did that work, and we now know that plunks constant is six point six to six zero seven one zero times ten to the power of negative thirty
four jewels per second I mean duh. In mid November, at the annual meeting of the International Bureau of Weights and Measures in Versailles, France, representatives from more than sixty countries voted to approve a new and everlasting definition of the kilogram, as calculated by the plank constant. No more hunk of metal. The kilograms mass is now tied to
plunks constant. New definitions were also announced for SI units, the ampire electrical current, the kelvin for temperature, and the mole the number of molecules or atoms in an element. These new definitions will take effect on nineteen The original platinum kilogram prototype will remain in that underground French vault, while countless generations of scientists will make world change in discoveries using the kilogram to point oh. Today's episode was
written by Dave Ruse and produced by Tyler Clang. For more on this and lots of other weighty topics, visit our home planet, how Stuff Works dot com.