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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