1 00:00:02,040 --> 00:00:07,160 Speaker 1: Welcome to brain stuff from how Stuff Works. Hey, brain Stuff, 2 00:00:07,200 --> 00:00:10,280 Speaker 1: I'm Lauren Vogelbaum. And for nearly a hundred and fifty years, 3 00:00:10,320 --> 00:00:13,120 Speaker 1: the official weight of a kilogram was determined by a 4 00:00:13,160 --> 00:00:16,480 Speaker 1: shiny cylinder of platinum locked away in a French vault. 5 00:00:17,440 --> 00:00:19,880 Speaker 1: The kilogram, like the meter and the second, is one 6 00:00:19,920 --> 00:00:22,720 Speaker 1: of the seven fundamental units of measurement, also known as 7 00:00:22,760 --> 00:00:25,599 Speaker 1: the International System of Units or the metric system the 8 00:00:25,840 --> 00:00:29,240 Speaker 1: SI for short. These were first formalized in the eight 9 00:00:29,520 --> 00:00:32,919 Speaker 1: seventy five Treaty of the Meter. Back then, the best 10 00:00:33,000 --> 00:00:34,960 Speaker 1: way to agree on the weight of a kilogram was 11 00:00:35,040 --> 00:00:37,880 Speaker 1: to forge a single hunk of metal and dubit le 12 00:00:38,040 --> 00:00:41,720 Speaker 1: grand K, And for more than a century, all scientific 13 00:00:41,800 --> 00:00:45,000 Speaker 1: scales were calibrated back to that one physical reference point, 14 00:00:45,120 --> 00:00:49,200 Speaker 1: with copies stored in a dozen countries. But even solid 15 00:00:49,240 --> 00:00:52,240 Speaker 1: objects can change over time. When le grand K was 16 00:00:52,280 --> 00:00:54,240 Speaker 1: weighed in the nineteen eighties, it was a couple of 17 00:00:54,240 --> 00:00:58,440 Speaker 1: micrograms lighter, meaning that all highly accurate scientific scales, not 18 00:00:58,560 --> 00:01:01,640 Speaker 1: like the one in your bathroom, to be recalibrated, and 19 00:01:01,640 --> 00:01:05,280 Speaker 1: that's what nerds call a real pain in the mass. Luckily, 20 00:01:05,360 --> 00:01:09,039 Speaker 1: a team of metrologists was already on the case. Metrology 21 00:01:09,080 --> 00:01:11,920 Speaker 1: being the science of weights and measures. They were searching 22 00:01:11,920 --> 00:01:14,840 Speaker 1: for a universal constant that would generate a fixed value 23 00:01:14,840 --> 00:01:17,520 Speaker 1: for the kilogram that's true now and will still be 24 00:01:17,600 --> 00:01:20,800 Speaker 1: true a million years from now. They had already found 25 00:01:20,800 --> 00:01:23,600 Speaker 1: such a physics fix for the second, which was redefined 26 00:01:23,600 --> 00:01:26,240 Speaker 1: in nineteen sixty seven from its previous value of a 27 00:01:26,280 --> 00:01:29,440 Speaker 1: fraction of a day one six thousand, four hundredths of 28 00:01:29,440 --> 00:01:32,600 Speaker 1: a day to be precise, to something much more confusing, 29 00:01:32,800 --> 00:01:36,760 Speaker 1: but much more constant. It takes nine billion, one d 30 00:01:36,920 --> 00:01:39,520 Speaker 1: and ninety two million, six hundred and thirty one thousand, 31 00:01:39,600 --> 00:01:43,520 Speaker 1: seven hundred and seventy oscillations of a special microwave beam 32 00:01:43,720 --> 00:01:46,880 Speaker 1: to excite atoms of the isotope caes M one thirty 33 00:01:47,000 --> 00:01:50,520 Speaker 1: three to a higher energy level. Since that number will 34 00:01:50,520 --> 00:01:53,560 Speaker 1: never change unlike the exact length of a day, that's 35 00:01:53,680 --> 00:01:57,120 Speaker 1: your new second. Same for the meter. Instead of being 36 00:01:57,120 --> 00:01:59,480 Speaker 1: defined as the length of a single meter long metal 37 00:01:59,520 --> 00:02:02,000 Speaker 1: poll for orged back in eighteen eighty nine, it was 38 00:02:02,040 --> 00:02:04,880 Speaker 1: redefined in nineteen eighty three as the distance light travels 39 00:02:04,920 --> 00:02:07,120 Speaker 1: in a vacuum in a particular fraction of a second 40 00:02:07,400 --> 00:02:11,079 Speaker 1: one two million, seven hundred and nine two thousand, four 41 00:02:11,160 --> 00:02:14,280 Speaker 1: hundred and fifty eight of a second. It wasn't until 42 00:02:14,320 --> 00:02:17,320 Speaker 1: twenty seventeen that scientists working at the u S National 43 00:02:17,360 --> 00:02:21,320 Speaker 1: Institute of Standards and Technology and similar bodies worldwide finally 44 00:02:21,360 --> 00:02:24,959 Speaker 1: agreed on a universal constant for the kilogram. The achievement 45 00:02:25,040 --> 00:02:27,840 Speaker 1: required solving one of the Thorny's physics problems of the 46 00:02:27,919 --> 00:02:30,920 Speaker 1: last century, coming up with a numerical value for planks 47 00:02:30,960 --> 00:02:35,720 Speaker 1: constant without getting too technical. A physicist, Max Planck proved 48 00:02:35,720 --> 00:02:39,560 Speaker 1: in nineteen hundred that matter releases energy in discrete chunks 49 00:02:39,600 --> 00:02:43,560 Speaker 1: called quanta. His equation for measuring these packets of energy 50 00:02:43,800 --> 00:02:48,160 Speaker 1: included a constant called h hitherto known as planks constant. 51 00:02:48,720 --> 00:02:51,160 Speaker 1: Thanks to Einstein, we know that energy and mass are 52 00:02:51,200 --> 00:02:55,680 Speaker 1: mathematically related, that whole equals mc squared thing, so physicists 53 00:02:55,720 --> 00:02:58,919 Speaker 1: figured out the planks constant, being a fixed unit of energy, 54 00:02:59,080 --> 00:03:03,720 Speaker 1: could yield the world most accurate measurement of mass. Calculating 55 00:03:03,720 --> 00:03:06,640 Speaker 1: the exact value of plunks constant took decades and some 56 00:03:06,760 --> 00:03:11,520 Speaker 1: serious technological innovation, specifically a nifty device called a Kibble balance, 57 00:03:11,960 --> 00:03:14,000 Speaker 1: but they did that work, and we now know that 58 00:03:14,040 --> 00:03:17,440 Speaker 1: plunks constant is six point six to six zero seven 59 00:03:17,480 --> 00:03:20,200 Speaker 1: one zero times ten to the power of negative thirty 60 00:03:20,200 --> 00:03:25,640 Speaker 1: four jewels per second I mean duh. In mid November, 61 00:03:26,560 --> 00:03:29,040 Speaker 1: at the annual meeting of the International Bureau of Weights 62 00:03:29,040 --> 00:03:32,560 Speaker 1: and Measures in Versailles, France, representatives from more than sixty 63 00:03:32,560 --> 00:03:35,840 Speaker 1: countries voted to approve a new and everlasting definition of 64 00:03:35,840 --> 00:03:39,560 Speaker 1: the kilogram, as calculated by the plank constant. No more 65 00:03:39,640 --> 00:03:42,200 Speaker 1: hunk of metal. The kilograms mass is now tied to 66 00:03:42,320 --> 00:03:45,920 Speaker 1: plunks constant. New definitions were also announced for SI units, 67 00:03:46,000 --> 00:03:49,440 Speaker 1: the ampire electrical current, the kelvin for temperature, and the 68 00:03:49,520 --> 00:03:52,360 Speaker 1: mole the number of molecules or atoms in an element. 69 00:03:52,960 --> 00:03:57,720 Speaker 1: These new definitions will take effect on nineteen The original 70 00:03:57,760 --> 00:04:01,200 Speaker 1: platinum kilogram prototype will remain in that underground French vault, 71 00:04:01,360 --> 00:04:04,400 Speaker 1: while countless generations of scientists will make world change in 72 00:04:04,440 --> 00:04:12,640 Speaker 1: discoveries using the kilogram to point oh. Today's episode was 73 00:04:12,680 --> 00:04:15,160 Speaker 1: written by Dave Ruse and produced by Tyler Clang. For 74 00:04:15,200 --> 00:04:17,320 Speaker 1: more on this and lots of other weighty topics, visit 75 00:04:17,360 --> 00:04:30,720 Speaker 1: our home planet, how Stuff Works dot com.