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  • On November 16th, representatives from nearly 60 nations

  • will be meeting in Versailles, France

  • to vote to change the definition of a kilogram.

  • Not only that, they will also be changing the fundamental unit of temperature, the kelvin,

  • the unit for amount of substance, the mole,

  • and the unit for electric current, the ampere.

  • That is four of the seven base S.I. units in one day!

  • And after that all S.I. units will be based on fundamental constants of nature,

  • and not physical artifacts.

  • The kilogram is the last base SI unit to be defined by a physical object.

  • Since 1799, one kilogram has been defined as exactly the mass

  • of a single metal cylinder stored in Paris.

  • It was swapped out once in 1889.

  • But this International Prototype Kilogram (or Big K as it's affictionately known) has problems!

  • I mean, weighting it with in theory identical cylinders, scientists have found

  • that their masses are diverging.

  • So it doesn't even seem to maintain it's mass.

  • Plus, it's really hard to get access to Big K,

  • and that makes using this definition really difficult.

  • So how do you create a mass standard that will never change,

  • and also be available to everyone everywhere?

  • With the solution is you set Planck's constant to have a fixed, exact value.

  • Now I know that sounds a little strange, so bear with me for a moment.

  • I mean, Planck's constant is best known for relating the frequency of a photon,

  • particle of light, to it's energy.

  • But we also know that energy and mass are related through E = mc²,

  • so, hopefully, you can see how Planck's constant is involved in mass.

  • But problem as it stands today as I'm recording this video

  • is that Planck's constant has some uncertainty.

  • I mean we know the value of Planck's constant to a large number of decimal places

  • but those last couple of digits...

  • They're actually uncertain.

  • What is certain is the mass of that platinum-iridium cylinder stored in.

  • a climate-controlled vault in a basement in Paris it is exactly one kilogram

  • No uncertainty. So the solution is to flip this on its head set Planck's constant

  • to have an exact fixed value and then that cylinder in Paris will no longer be

  • exactly 1 kilogram I mean it'll be a kilogram but not exactly the thing that

  • is now exact is Planck's constant which determines how big a kilogram is.

  • But if you're gonna fix the value of Planck's constant well you better get that value

  • right, so that it's consistent with all of our current measurements and all of

  • the masses that exist in the world right now. and so for the last several years

  • And so for the last several years,

  • scientists around the world have used multiple different techniques to try to

  • measure Planck's constant as accurately as they possibly can.

  • One of the major methods was using a watt balance, where essentially, they

  • balance the weight of a kilogram with the force from an electromagnet if you

  • want more detail you should check out my video on that topic.

  • Scientists also created arguably the roundest object in the world made of one type of silicon

  • atoms these methods have been complementary because now they're able

  • to compare all of their different findings from physics and from this more

  • chemistry method of Avogadro's constant and determine what Planck's constant

  • really should be. So if the vote goes well the future definition of Planck's

  • constant will be that it is exactly this number.

  • Planck's constant is fixed.

  • That cylinder in Paris, no longer exactly equal to a kilogram.

  • But you can't redefine the kilogram in isolation, because other base S.I. units depend on it.

  • Take the mole for example. Currently, the mole is defined as the amount of

  • substance that contains the same number of particles as there are atoms in 12

  • grams of carbon-12 that's Avogadro 's constant and it depends

  • on what 12 grams is which depends on what a kilogram is so again Avogadro's

  • constant currently has some uncertainty but after the vote the plan is to fix

  • Avogadro's constant to be exactly this number in such a way that it is internally

  • consistent with the new definition of Planck's constant. There's a direct

  • relationship between Avogadro's constant and Planck's constant.

  • Likewise, ampere will no longer depend on the kilogram. Instead, it will be defined

  • based on this newly fixed value for the charge on an electron; and the Kelvin

  • will be based on the newly fixed Boltzmann constant, which relates the

  • temperature of a gas to the average kinetic energy of the molecules, and this

  • will be its exact value with no uncertainties.

  • Now will these new definitions change anything?

  • Well for most people, no. I mean, your food is still

  • going the way the same, as are you. And temperature is still gonna work the same way.

  • You know, everything basically stays the same,

  • and that is as it should be. The point of this definition change is not

  • to shake things up, but to keep things consistent and reliable forever.

  • All we're doing is removing the dependence on a physical object, which

  • theoretically, at least, makes it possible for anyone, anywhere to make incredibly

  • precise measurements.

  • Now, I should point out that a volt will actually change by

  • about one part in ten million, and resistance will change by a little bit

  • less than that. And that's because back in 1990, the electrical metrologists

  • decided to stop updating their value of effectively Planck's constant and just

  • keep the one they had in 1990 and there was a benefit to that.

  • They didn't have to update their definitions, or their instruments, but now that we've realized

  • that Planck's constant is actually slightly different than the 1990 value

  • because of better measurement techniques.

  • Well, now the electrical metrologists will

  • have to change, but that's a very tiny change for a very tiny number of people.

  • I think they'll be fine.

  • You know I've been trying to ask myself the question,

  • why am I so interested in this topic? I mean, I made like four videos on it and

  • the reason is, you know, to me the world and the universe is a big complicated place.

  • And when we're actually able to ascribe numbers to it, it's like we are

  • resting some sort of order out of the chaos that is our universe and that

  • is the beginning of our understanding of the way things work. You know

  • measurements are the foundation of science they allow us to make observations.

  • I think it's no surprise that, you know, Kepler was really able to

  • figure out what was going on with the planet that they were actually moving

  • in elliptical orbits. Once Tycho Brahe he had made the most accurate measurements

  • of their positions that people had ever made I mean I think that's no

  • coincidence and if you look at the discovery of the Higgs boson at CERN or

  • the detection of gravitational waves. These are, in my view, the pinnacle of

  • human achievement. I think there are orders of magnitude greater than the

  • achievements that then we make in literature, and art, and fashion; and I

  • don't say that to disparage those disciplines. I know that they're hard I

  • know they take a lot of human brain power and I'm not saying scientists are

  • smarter but the tools that scientists work with and the system in which they

  • work is what allows them to make such great leaps because science builds on

  • itself in almost, you know, an exponentially improving way and that to

  • me is why this is so important is because it allows us to take our

  • measurements to the next level. No longer are we bound to physical objects. I mean,

  • face it, up until now, we've essentially been doing a glorified version of Indiana Jones.

  • Now, we are taking that next leap to the abstraction that all of

  • our units are based on the way nature is and the way the universe is. We're no

  • longer tied to physical objects.

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  • veritasium to 500 500 now the book I'm listening to at the moment is by my

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On November 16th, representatives from nearly 60 nations

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kgは死んだ kg万歳 (The kg is dead, long live the kg)

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    林宜悉 に公開 2021 年 01 月 14 日
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