WEBVTT - BrainStuff Classics: How Do We Define the Kilogram?

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<v Speaker 1>Welcome to brain Stuff, a production of iHeartRadio. Hey brain Stuff,

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<v Speaker 1>I'm lorn vogel bomb, and this is a classic episode

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<v Speaker 1>of the podcast. In this one, we dig into the

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<v Speaker 1>weighty history of the kilogram. It turns out this standard

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<v Speaker 1>measure has been redefined over time. Hey brain stuff, I'm

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<v Speaker 1>lorn vogel bomb. And for nearly one hundred and fifty years,

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<v Speaker 1>the official weight of a kilogram was determined by a

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<v Speaker 1>shiny cylinder of platinum locked away in a French vault.

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<v Speaker 1>The kilogram, like the meter and the second, is one

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<v Speaker 1>of the seven fundamental units of measurement, also known as

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<v Speaker 1>the International System of Units or the metric system the

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<v Speaker 1>SI for short. These were first formalized in the eighteen

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<v Speaker 1>seventy five Treaty of the Meter. Back then, the best

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<v Speaker 1>way to agree on the weight of a kilogram was

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<v Speaker 1>to forge a single hunk of metal and dubit legrand k,

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<v Speaker 1>and for more than a century all scientific scales were

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<v Speaker 1>calibrated back to that one physical reference point, with copies

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<v Speaker 1>stored in a dozen countries. But even solid objects can

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<v Speaker 1>change over time. When La grand k was weighed in

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<v Speaker 1>the nineteen eighties, it was a couple of micrograms lighter,

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<v Speaker 1>meaning that all highly accurate scientific scales, not like the

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<v Speaker 1>one in your bathroom, had to be recalibrated, and that's

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<v Speaker 1>what nerds call a real pain in the mass. Luckily,

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<v Speaker 1>a team of metrologists was already on the case. Metrology

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<v Speaker 1>being the science of weights and measures. They were searching

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<v Speaker 1>for a universal constant that would generate a fixed value

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<v Speaker 1>for the kilogram that's true now and will still be

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<v Speaker 1>true a million years from now. They had already found

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<v Speaker 1>such a physics fix for the second, which was redefined

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<v Speaker 1>in nineteen sixty seven from its previous value of a

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<v Speaker 1>fraction of a day one eighty six thousand, four hundredth

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<v Speaker 1>of a day to be precise, to something much more confusing,

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<v Speaker 1>but much more constant. It takes nine billion, one hundred

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<v Speaker 1>and ninety two million, six hundred and thirty one thousand,

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<v Speaker 1>seven hundred and seventy oscillations of a special microwave beam

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<v Speaker 1>to excite atoms of the isotope caesium one thirty three

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<v Speaker 1>to a higher energy level. Since that number will never

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<v Speaker 1>change unlike the exact length of a day, that's your

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<v Speaker 1>new second. Same for the meter, instead of being defined

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<v Speaker 1>as the length of a single meter long metal pole

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<v Speaker 1>forged back in eighteen eighty nine. It was redefined in

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<v Speaker 1>nineteen eighty three as the distance light travels in a

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<v Speaker 1>vacuum in a particular fraction of a second one two

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<v Speaker 1>hundred and ninety nine million, seven hundred and ninety two thousand,

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<v Speaker 1>four hundred and fifty eighth of a second. It wasn't

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<v Speaker 1>until twenty seventeen that scientists working at the US National

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<v Speaker 1>Institute of Standards and Technology and similar bodies worldwide finally

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<v Speaker 1>agreed on a universal constant for the kilogram. The achievement

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<v Speaker 1>required solving one of the thorniest physics problems of the

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<v Speaker 1>last century, coming up with a numerical value for planks

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<v Speaker 1>constant without getting too technical. A physicist, Max Planck proved

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<v Speaker 1>in nineteen hundred that matter releases energy in discrete chunks

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<v Speaker 1>called quanta. His equation for measuring these packets of energy

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<v Speaker 1>is included a constant called H hitherto known as planx constant.

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<v Speaker 1>Thanks to Einstein, we know that energy and mass are

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<v Speaker 1>mathematically related, that whole e equals mc squared thing, so

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<v Speaker 1>physicists figured out the planks constant, being a fixed unit

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<v Speaker 1>of energy, could yield the world's most accurate measurement of mass.

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<v Speaker 1>Calculating the exact value of planks constant took decades and

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<v Speaker 1>some serious technological innovation, specifically a nifty device called a

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<v Speaker 1>kibble balance. But they did that work, and we now

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<v Speaker 1>know that planks constant is six point six two six

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<v Speaker 1>zero seven one five zero times ten to the power

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<v Speaker 1>of negative thirty four jeweles per second I mean duh.

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<v Speaker 1>In mid November twenty eighteen, at the annual meeting of

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<v Speaker 1>the International Bureau of Weights and Measures in Versailles, France,

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<v Speaker 1>representatives from more than sixty countries voted to approve a

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<v Speaker 1>new and everlasting definition of the kilogram as calculated by

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<v Speaker 1>the plank constant. No more hunk of metal. The kilogram's

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<v Speaker 1>mass is now tied to planks constant. Definitions were also

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<v Speaker 1>announced for SI units, the ampeer electrical current, the kelvin

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<v Speaker 1>for temperature, and the mole the number of molecules or

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<v Speaker 1>atoms in an element. These new definitions will take effect

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<v Speaker 1>on May twentieth, twenty nineteen. The original platinum kilogram prototype

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<v Speaker 1>will remain in that underground French vault, while countless generations

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<v Speaker 1>of scientists will make world changing discoveries using the Kilogram

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<v Speaker 1>two point zero. Today's episode is based on the article

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<v Speaker 1>the Kilogram is Dead Meet the Kilogram two point zero

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<v Speaker 1>on how stuffworks dot Com, written by Dave Ruse. Brain

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<v Speaker 1>Stuff is production of iHeartRadio in partnership with HowStuffWorks dot

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<v Speaker 1>Com and is produced by Tyler Klang. Four more podcasts

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<v Speaker 1>from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever

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