1 00:00:01,840 --> 00:00:07,640 Speaker 1: Welcome to brain Stuff, a production of iHeartRadio. Hey brain Stuff, 2 00:00:07,680 --> 00:00:10,600 Speaker 1: I'm lorn vogel bomb, and this is a classic episode 3 00:00:10,640 --> 00:00:13,640 Speaker 1: of the podcast. In this one, we dig into the 4 00:00:13,680 --> 00:00:18,120 Speaker 1: weighty history of the kilogram. It turns out this standard 5 00:00:18,120 --> 00:00:23,480 Speaker 1: measure has been redefined over time. Hey brain stuff, I'm 6 00:00:23,520 --> 00:00:26,400 Speaker 1: lorn vogel bomb. And for nearly one hundred and fifty years, 7 00:00:26,440 --> 00:00:29,240 Speaker 1: the official weight of a kilogram was determined by a 8 00:00:29,280 --> 00:00:32,560 Speaker 1: shiny cylinder of platinum locked away in a French vault. 9 00:00:33,560 --> 00:00:36,040 Speaker 1: The kilogram, like the meter and the second, is one 10 00:00:36,040 --> 00:00:38,839 Speaker 1: of the seven fundamental units of measurement, also known as 11 00:00:38,880 --> 00:00:41,720 Speaker 1: the International System of Units or the metric system the 12 00:00:41,960 --> 00:00:45,600 Speaker 1: SI for short. These were first formalized in the eighteen 13 00:00:45,640 --> 00:00:49,120 Speaker 1: seventy five Treaty of the Meter. Back then, the best 14 00:00:49,120 --> 00:00:51,120 Speaker 1: way to agree on the weight of a kilogram was 15 00:00:51,159 --> 00:00:54,720 Speaker 1: to forge a single hunk of metal and dubit legrand k, 16 00:00:55,680 --> 00:00:58,440 Speaker 1: and for more than a century all scientific scales were 17 00:00:58,440 --> 00:01:02,040 Speaker 1: calibrated back to that one physical reference point, with copies 18 00:01:02,040 --> 00:01:06,000 Speaker 1: stored in a dozen countries. But even solid objects can 19 00:01:06,120 --> 00:01:08,680 Speaker 1: change over time. When La grand k was weighed in 20 00:01:08,720 --> 00:01:11,280 Speaker 1: the nineteen eighties, it was a couple of micrograms lighter, 21 00:01:11,600 --> 00:01:14,880 Speaker 1: meaning that all highly accurate scientific scales, not like the 22 00:01:14,880 --> 00:01:17,959 Speaker 1: one in your bathroom, had to be recalibrated, and that's 23 00:01:18,000 --> 00:01:21,440 Speaker 1: what nerds call a real pain in the mass. Luckily, 24 00:01:21,480 --> 00:01:25,160 Speaker 1: a team of metrologists was already on the case. Metrology 25 00:01:25,200 --> 00:01:28,040 Speaker 1: being the science of weights and measures. They were searching 26 00:01:28,080 --> 00:01:30,960 Speaker 1: for a universal constant that would generate a fixed value 27 00:01:30,959 --> 00:01:33,679 Speaker 1: for the kilogram that's true now and will still be 28 00:01:33,720 --> 00:01:36,920 Speaker 1: true a million years from now. They had already found 29 00:01:36,920 --> 00:01:39,720 Speaker 1: such a physics fix for the second, which was redefined 30 00:01:39,760 --> 00:01:42,319 Speaker 1: in nineteen sixty seven from its previous value of a 31 00:01:42,400 --> 00:01:45,480 Speaker 1: fraction of a day one eighty six thousand, four hundredth 32 00:01:45,520 --> 00:01:48,720 Speaker 1: of a day to be precise, to something much more confusing, 33 00:01:48,920 --> 00:01:53,000 Speaker 1: but much more constant. It takes nine billion, one hundred 34 00:01:53,040 --> 00:01:55,640 Speaker 1: and ninety two million, six hundred and thirty one thousand, 35 00:01:55,720 --> 00:01:59,640 Speaker 1: seven hundred and seventy oscillations of a special microwave beam 36 00:01:59,760 --> 00:02:03,280 Speaker 1: to excite atoms of the isotope caesium one thirty three 37 00:02:03,520 --> 00:02:06,840 Speaker 1: to a higher energy level. Since that number will never 38 00:02:07,000 --> 00:02:10,000 Speaker 1: change unlike the exact length of a day, that's your 39 00:02:10,000 --> 00:02:13,560 Speaker 1: new second. Same for the meter, instead of being defined 40 00:02:13,600 --> 00:02:15,880 Speaker 1: as the length of a single meter long metal pole 41 00:02:16,000 --> 00:02:18,720 Speaker 1: forged back in eighteen eighty nine. It was redefined in 42 00:02:18,760 --> 00:02:21,160 Speaker 1: nineteen eighty three as the distance light travels in a 43 00:02:21,200 --> 00:02:23,880 Speaker 1: vacuum in a particular fraction of a second one two 44 00:02:24,000 --> 00:02:26,720 Speaker 1: hundred and ninety nine million, seven hundred and ninety two thousand, 45 00:02:27,000 --> 00:02:30,080 Speaker 1: four hundred and fifty eighth of a second. It wasn't 46 00:02:30,120 --> 00:02:33,440 Speaker 1: until twenty seventeen that scientists working at the US National 47 00:02:33,480 --> 00:02:37,440 Speaker 1: Institute of Standards and Technology and similar bodies worldwide finally 48 00:02:37,480 --> 00:02:41,079 Speaker 1: agreed on a universal constant for the kilogram. The achievement 49 00:02:41,200 --> 00:02:43,959 Speaker 1: required solving one of the thorniest physics problems of the 50 00:02:44,040 --> 00:02:47,040 Speaker 1: last century, coming up with a numerical value for planks 51 00:02:47,080 --> 00:02:51,840 Speaker 1: constant without getting too technical. A physicist, Max Planck proved 52 00:02:51,880 --> 00:02:55,680 Speaker 1: in nineteen hundred that matter releases energy in discrete chunks 53 00:02:55,720 --> 00:02:59,720 Speaker 1: called quanta. His equation for measuring these packets of energy 54 00:02:59,880 --> 00:03:04,239 Speaker 1: is included a constant called H hitherto known as planx constant. 55 00:03:04,840 --> 00:03:07,280 Speaker 1: Thanks to Einstein, we know that energy and mass are 56 00:03:07,320 --> 00:03:10,960 Speaker 1: mathematically related, that whole e equals mc squared thing, so 57 00:03:11,160 --> 00:03:14,520 Speaker 1: physicists figured out the planks constant, being a fixed unit 58 00:03:14,600 --> 00:03:18,080 Speaker 1: of energy, could yield the world's most accurate measurement of mass. 59 00:03:19,240 --> 00:03:22,600 Speaker 1: Calculating the exact value of planks constant took decades and 60 00:03:22,680 --> 00:03:26,760 Speaker 1: some serious technological innovation, specifically a nifty device called a 61 00:03:26,840 --> 00:03:29,799 Speaker 1: kibble balance. But they did that work, and we now 62 00:03:29,880 --> 00:03:32,680 Speaker 1: know that planks constant is six point six two six 63 00:03:32,919 --> 00:03:35,520 Speaker 1: zero seven one five zero times ten to the power 64 00:03:35,520 --> 00:03:39,800 Speaker 1: of negative thirty four jeweles per second I mean duh. 65 00:03:41,000 --> 00:03:43,720 Speaker 1: In mid November twenty eighteen, at the annual meeting of 66 00:03:43,760 --> 00:03:46,760 Speaker 1: the International Bureau of Weights and Measures in Versailles, France, 67 00:03:47,080 --> 00:03:50,000 Speaker 1: representatives from more than sixty countries voted to approve a 68 00:03:50,120 --> 00:03:53,760 Speaker 1: new and everlasting definition of the kilogram as calculated by 69 00:03:53,760 --> 00:03:57,400 Speaker 1: the plank constant. No more hunk of metal. The kilogram's 70 00:03:57,440 --> 00:04:00,800 Speaker 1: mass is now tied to planks constant. Definitions were also 71 00:04:00,840 --> 00:04:04,440 Speaker 1: announced for SI units, the ampeer electrical current, the kelvin 72 00:04:04,480 --> 00:04:07,440 Speaker 1: for temperature, and the mole the number of molecules or 73 00:04:07,440 --> 00:04:10,600 Speaker 1: atoms in an element. These new definitions will take effect 74 00:04:10,640 --> 00:04:15,280 Speaker 1: on May twentieth, twenty nineteen. The original platinum kilogram prototype 75 00:04:15,320 --> 00:04:18,799 Speaker 1: will remain in that underground French vault, while countless generations 76 00:04:18,800 --> 00:04:22,080 Speaker 1: of scientists will make world changing discoveries using the Kilogram 77 00:04:22,120 --> 00:04:30,679 Speaker 1: two point zero. Today's episode is based on the article 78 00:04:30,839 --> 00:04:33,480 Speaker 1: the Kilogram is Dead Meet the Kilogram two point zero 79 00:04:33,600 --> 00:04:36,440 Speaker 1: on how stuffworks dot Com, written by Dave Ruse. Brain 80 00:04:36,480 --> 00:04:39,400 Speaker 1: Stuff is production of iHeartRadio in partnership with HowStuffWorks dot 81 00:04:39,400 --> 00:04:42,400 Speaker 1: Com and is produced by Tyler Klang. Four more podcasts 82 00:04:42,440 --> 00:04:46,000 Speaker 1: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever 83 00:04:46,000 --> 00:04:47,800 Speaker 1: you listen to your favorite shows.