Speaker 1: Welcome to brain stuff from how Stuff Works. Hey, brain Stuff,
Speaker 1: I'm Lauren Vogelbaum. And for nearly a hundred and fifty years,
Speaker 1: the official weight of a kilogram was determined by a
Speaker 1: shiny cylinder of platinum locked away in a French vault.
Speaker 1: The kilogram, like the meter and the second, is one
Speaker 1: of the seven fundamental units of measurement, also known as
Speaker 1: the International System of Units or the metric system the
Speaker 1: SI for short. These were first formalized in the eight
Speaker 1: seventy five Treaty of the Meter. Back then, the best
Speaker 1: way to agree on the weight of a kilogram was
Speaker 1: to forge a single hunk of metal and dubit le
Speaker 1: grand K, And for more than a century, all scientific
Speaker 1: scales were calibrated back to that one physical reference point,
Speaker 1: with copies stored in a dozen countries. But even solid
Speaker 1: objects can change over time. When le grand K was
Speaker 1: weighed in the nineteen eighties, it was a couple of
Speaker 1: micrograms lighter, meaning that all highly accurate scientific scales, not
Speaker 1: like the one in your bathroom, to be recalibrated, and
Speaker 1: that's what nerds call a real pain in the mass. Luckily,
Speaker 1: a team of metrologists was already on the case. Metrology
Speaker 1: being the science of weights and measures. They were searching
Speaker 1: for a universal constant that would generate a fixed value
Speaker 1: for the kilogram that's true now and will still be
Speaker 1: true a million years from now. They had already found
Speaker 1: such a physics fix for the second, which was redefined
Speaker 1: in nineteen sixty seven from its previous value of a
Speaker 1: fraction of a day one six thousand, four hundredths of
Speaker 1: a day to be precise, to something much more confusing,
Speaker 1: but much more constant. It takes nine billion, one d
Speaker 1: and ninety two million, six hundred and thirty one thousand,
Speaker 1: seven hundred and seventy oscillations of a special microwave beam
Speaker 1: to excite atoms of the isotope caes M one thirty
Speaker 1: three to a higher energy level. Since that number will
Speaker 1: never change unlike the exact length of a day, that's
Speaker 1: your new second. Same for the meter. Instead of being
Speaker 1: defined as the length of a single meter long metal
Speaker 1: poll for orged back in eighteen eighty nine, it was
Speaker 1: redefined in nineteen eighty three as the distance light travels
Speaker 1: in a vacuum in a particular fraction of a second
Speaker 1: one two million, seven hundred and nine two thousand, four
Speaker 1: hundred and fifty eight of a second. It wasn't until
Speaker 1: twenty seventeen that scientists working at the u S National
Speaker 1: Institute of Standards and Technology and similar bodies worldwide finally
Speaker 1: agreed on a universal constant for the kilogram. The achievement
Speaker 1: required solving one of the Thorny's physics problems of the
Speaker 1: last century, coming up with a numerical value for planks
Speaker 1: constant without getting too technical. A physicist, Max Planck proved
Speaker 1: in nineteen hundred that matter releases energy in discrete chunks
Speaker 1: called quanta. His equation for measuring these packets of energy
Speaker 1: included a constant called h hitherto known as planks constant.
Speaker 1: Thanks to Einstein, we know that energy and mass are
Speaker 1: mathematically related, that whole equals mc squared thing, so physicists
Speaker 1: figured out the planks constant, being a fixed unit of energy,
Speaker 1: could yield the world most accurate measurement of mass. Calculating
Speaker 1: the exact value of plunks constant took decades and some
Speaker 1: serious technological innovation, specifically a nifty device called a Kibble balance,
Speaker 1: but they did that work, and we now know that
Speaker 1: plunks constant is six point six to six zero seven
Speaker 1: one zero times ten to the power of negative thirty
Speaker 1: four jewels per second I mean duh. In mid November,
Speaker 1: at the annual meeting of the International Bureau of Weights
Speaker 1: and Measures in Versailles, France, representatives from more than sixty
Speaker 1: countries voted to approve a new and everlasting definition of
Speaker 1: the kilogram, as calculated by the plank constant. No more
Speaker 1: hunk of metal. The kilograms mass is now tied to
Speaker 1: plunks constant. New definitions were also announced for SI units,
Speaker 1: the ampire electrical current, the kelvin for temperature, and the
Speaker 1: mole the number of molecules or atoms in an element.
Speaker 1: These new definitions will take effect on nineteen The original
Speaker 1: platinum kilogram prototype will remain in that underground French vault,
Speaker 1: while countless generations of scientists will make world change in
Speaker 1: discoveries using the kilogram to point oh. Today's episode was
Speaker 1: written by Dave Ruse and produced by Tyler Clang. For
Speaker 1: more on this and lots of other weighty topics, visit
Speaker 1: our home planet, how Stuff Works dot com.