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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for watching
Awesome, you can see his Eng Phys grad pic downstairs in Stirling hall!
Wonder what bars he has lol