The kg is dead, long live the kg
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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. hey this episode of veritasium was
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to audible.com/veritasium or text veritasium to 500 500 now the book I’m
listening to at the moment is by my friend Hank green it’s called an
absolutely remarkable thing and it’s a novel about a girl who
stumbles upon a giant robot in New York City and the social media circus that
ensues and I think Hank has some really interesting insights in this world and I
find it really well written of course I’m kind of biased because I’m his
friend but if you were intrigued you can actually listen to this book right now
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100 thoughts on “The kg is dead, long live the kg

  1. I'm trying to imagine the 14 people in the world who will go like
    "OMG the volt value changed by one part in 10 million! MY LIFE IS RUINED!"

  2. does measuring mass with that balance in 4:56 , have a difference when measured in different places on earth and also differ in vacuum??

  3. a kilogram is the average weight of a standard medieval European double edged single handed arming sword made of spring steel

  4. Oh come on, it took me a while to learn si stuff. Now changing it will create soooo many problems. Alot of formels have to change (physics), tabels, books and etc etc. This creats more problem than it solves.

  5. Now they need to fix metric distances and eradicate Imperial units. A distance that is tied to the length of someone's foot? Really? Talk about an ever changing number.

  6. I cant wait to see “Big K” come through the doors in pawn stars. “ This piece of metal used to be universal standard for a kilogram”, “wow so this really was the standard value for a kilogram for the past few centuries?”, “yup!”…… “i’ll guve you $20 bucks for it”. “How about $22?”, “deal”.

    -two hours later…
    (chumlee)—“DaD i MeLtEd It InTo A nEw KaTaNa¿!¡”

  7. no longer tied to physical objects is all well etc etc….but there is something called taking things too far..too much of a good thing is a common phrase and i think it applies even to the obsession(of people other than me)about THE MOST PRECISE NUMBERED MEASUREMENT..but pause to consider how much precise do numbers have to be to serve as tools?nature is not absolute and constant..the only things constant about nature is causality and variability(which means uncertainty cannot be eliminated)but people are trying to eliminate it by insisting on finding MORE PRECISE NUMBERS….in the end numbers words images sounds and motor sensations are human tools to make observations of the universe to make sense of it..its good we are improving the tools but we must also accept no improvement(and not even infinite precision) can eliminate uncertainty..we should use the tools as best as possible not by making precision infinite but by using a fixed set of tools in infinitely diverse ways….that's how DNA and protein are capable of so much information despite using only 4 and about 20 building blocks respectively..is nature trying to make these or anything more precise?with infinite potential already existing what,s the need of infinite precision of numbers?

  8. Due to all sorts of things, the pull of the moon, magma flow, etc., the gravity acting on a mass like "the big K" varies, so its weight varies. Surprised this dude didn't mention it.

  9. How does Heisenberg feel about all this? Seems leaving a little wiggle room for the Plank Constant might prevent our universe from making a quantum leap to…somewhere else.

  10. Audible books are not anyone's to keep. Audible uses DRM, which forever chains the works to be un-owned. Please do not misrepresent the situation on Audible's behalf.

  11. Another solution. Weigh the mass once. Save it the definition a computer. Email the weight all over the world. Glad the story finished out though. I've seen you hold the roundest ball before.

    Avogadro's number is Planck's constant? I need a moment. How the heck have they done that?

    Now think of it this way. We don't do this, and someone makes an error because they don't have all the info? Nuclear physics is incredibly touchy. I still need to watch a video on the gravity wave.

    Lastly, these are great videos. I would've had no idea this was happening without this.

  12. Hi, can you make a video on how a "metre" is defined?
    After watching all these videos on kilogram, it makes me curious on how metre is defined.

  13. But ummm… What if we set in the planck's constant a tiny bit off, and then when we'll require really REALLY precise measurement, what if the whole thing collapses ? I mean who's to say what is and what isn't an acceptable error? The whole future development could fall because of this.

  14. But 2.2 is such a simple number. Now I'm fucked. You scientists can worry about moles and kelvin. Amps might be a problem if this isn't an effort to make it easier.

  15. Doesn't that mean that the definition of a meter changed slightly too? There is a relation between the two after all.

  16. Will this complicate my homework or make it easier. Becuz if this is gonna give me harder math I don’t want it

  17. In a 100 years, we'll laugh at ourselves for using unstable pleb constants derived from matter instead of the length of some hyperdimensional electron spacetime folding mechanism.

  18. I'd really love to see a video on ground-up definitions of all units from the basics. Mass of a proton, charge of an electron, etc, and see what the human-scale metric system would look like built that way. I'm sure there are people out there who have proposed such systems.

  19. STEAM>>STEM I honestly couldn't see science the same without those beautiful observations that I simply can't rationalize, rather intuit. Once I do that I can work my way back to the answers.

    My math teachers hated I had the answers but didn't know how to show the work. Now that I'm older I do understand the importance of checking your work.

    Some of the hardest problems I've found and answer and had to work back to the question.

    My whole life..

  20. Hey NIST, if you guys don't need K4 and K20 anymore, I'd be willing to take it off your hands.
    Ya know, for posterity ^.^

  21. And I studied the definition of kilogram in August 2018! When everything was still based on that block! I only got to know this today, 2 months after the transition was complete….. Wow…. The definition of something so fundamental I knew last summer has been changed, that is so exciting

  22. Why not start with the smallest unit of measurement and simply extrapolate from there… take the atomic weight of… hydrogen and go from there… or carbon…

  23. So yeah cool what those scientists are doing but couldn't that money and knowledge be put to better use helping the planet

  24. Before this video: Hah, I’m american, what’s the difference, lol.

    After the vid: Hah, I’m (somewhat smaller version of an american), ᴡʜᴀᴛ’s ᴛʜᴇ ᴅɪғғᴇʀᴇɴᴄᴇ, ʟᴏʟ.

  25. but isnt it still technically somewhat arbitrary, being based on a vote from scientists for what they think is the most accurate measurement? since we cant actually measure things exactly and can only get very close, arent we just picking the closest measurement and saying its "good enough"? it almost seems like we are moving from one "good enough" to another "good enough" and not actually pinning the real thing down in nature.

  26. When people go to Mars, will they create new units of measurement and the equivalences with those of Earth?

  27. I'm kinda dumb, but remember one artifact.

    In the old days, before we recorded a secong in time as the distance that a photon transitted a certsin space; we were more complex.

    When I PhD'd physics as an ungrad laymen at Barkeley in the late Ninetie's; the French International Standard for the 'second of time'; was differently defined.

    How? Some may ask.

    Well, back in the day we were antiquated. We defined a 'second' of time as the period of time that it took for 'one co-valent electron of Cesium134 to go through one hyper-fine dehradation of it's orbit'.

    Im'a call that superior science.

  28. It'd be so nice for future generations dealing with scientific/engineering calculations if SI had decided to shake things up by setting these constants to nicer values without so many baked in digits. For example you could imagine SI deciding to retain only enough digits that all the values will change by less than 1%; so for example, h would be just 6.6*10^(-34) Js and c would be just 3*10^8 m/s. Of course some of our constants are composites of constants that we treat as "more fundamental", so there's only so much you can do, but still.

  29. You know this doesn't really matter to me because I'm an American. We use the imperial system because we're fuckong idiots.

  30. Everything seems nice. But I have one problem. These measurements presented on a graphic at around 3:30 seems strikingly dissimilar. Their average value vary wildly between projects (about 1,44% discrepancy) and its distribution seems skewed. Moreover, in at least two projects raw values shows huge variability (over 1%). It seems way to much in physics. If so, then how the measure was calculated? By averaging averages? But why? With such a method you risk that including results from just one more project you would get totally different outcomes. If you choose only some results, than based on what? Is it only a matter of consensus? If we agreed on that prematurely, we will have to update it once we make next technological leap forward.

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