Speaker 1: Welcome to brain Stuff, a production of iHeartRadio. Hey brain Stuff,
Speaker 1: I'm lorn vogel bomb, and this is a classic episode
Speaker 1: of the podcast. In this one, we dig into the
Speaker 1: weighty history of the kilogram. It turns out this standard
Speaker 1: measure has been redefined over time. Hey brain stuff, I'm
Speaker 1: lorn vogel bomb. And for nearly one 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 eighteen
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 legrand k,
Speaker 1: and for more than a century all scientific scales were
Speaker 1: calibrated back to that one physical reference point, with copies
Speaker 1: stored in a dozen countries. But even solid objects can
Speaker 1: change over time. When La grand k was weighed in
Speaker 1: the nineteen eighties, it was a couple of micrograms lighter,
Speaker 1: meaning that all highly accurate scientific scales, not like the
Speaker 1: one in your bathroom, had to be recalibrated, and that's
Speaker 1: 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 eighty six thousand, four hundredth
Speaker 1: of a day to be precise, to something much more confusing,
Speaker 1: but much more constant. It takes nine billion, one hundred
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 caesium one thirty three
Speaker 1: to a higher energy level. Since that number will never
Speaker 1: change unlike the exact length of a day, that's your
Speaker 1: new second. Same for the meter, instead of being defined
Speaker 1: as the length of a single meter long metal pole
Speaker 1: forged back in eighteen eighty nine. It was redefined in
Speaker 1: nineteen eighty three as the distance light travels in a
Speaker 1: vacuum in a particular fraction of a second one two
Speaker 1: hundred and ninety nine million, seven hundred and ninety two thousand,
Speaker 1: four hundred and fifty eighth of a second. It wasn't
Speaker 1: until twenty seventeen that scientists working at the US 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 thorniest 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: is included a constant called H hitherto known as planx constant.
Speaker 1: Thanks to Einstein, we know that energy and mass are
Speaker 1: mathematically related, that whole e equals mc squared thing, so
Speaker 1: physicists figured out the planks constant, being a fixed unit
Speaker 1: of energy, could yield the world's most accurate measurement of mass.
Speaker 1: Calculating the exact value of planks constant took decades and
Speaker 1: some serious technological innovation, specifically a nifty device called a
Speaker 1: kibble balance. But they did that work, and we now
Speaker 1: know that planks constant is six point six two six
Speaker 1: zero seven one five zero times ten to the power
Speaker 1: of negative thirty four jeweles per second I mean duh.
Speaker 1: In mid November twenty eighteen, at the annual meeting of
Speaker 1: the International Bureau of Weights and Measures in Versailles, France,
Speaker 1: representatives from more than sixty countries voted to approve a
Speaker 1: new and everlasting definition of the kilogram as calculated by
Speaker 1: the plank constant. No more hunk of metal. The kilogram's
Speaker 1: mass is now tied to planks constant. Definitions were also
Speaker 1: announced for SI units, the ampeer electrical current, the kelvin
Speaker 1: for temperature, and the mole the number of molecules or
Speaker 1: atoms in an element. These new definitions will take effect
Speaker 1: on May twentieth, twenty nineteen. The original platinum kilogram prototype
Speaker 1: will remain in that underground French vault, while countless generations
Speaker 1: of scientists will make world changing discoveries using the Kilogram
Speaker 1: two point zero. Today's episode is based on the article
Speaker 1: the Kilogram is Dead Meet the Kilogram two point zero
Speaker 1: on how stuffworks dot Com, written by Dave Ruse. Brain
Speaker 1: Stuff is production of iHeartRadio in partnership with HowStuffWorks dot
Speaker 1: Com and is produced by Tyler Klang. Four more podcasts
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