Name: 世界一丸い物体! (World's Roundest Object!)
Uploaded: 2021-01-14T05:04:10.000Z
Duration: 11 min 44 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

様態の副詞

Only if you promise to be really, really careful.

程度の副詞

I promise I will be so incredibly careful. I will be incredibly careful with it. I promise.

So, it's slippery, be careful. Alright, are we ready?

I'm about to touch a 1kg sphere of silicon-28 atoms. There are about 2.15x10^25 of them.

It feels absolutely incredible. Wow, that is amazing.

Besides its creators, I am one of only a handful of people ever to hold this sphere.

The raw material used to make it was worth 1 million Euros but now that it has been so

It's priceless. ... This you are looking at now is the roundest object in the world.

How can you say for sure it's the roundest object? I mean the Earth is pretty round,

If this were the Earth then the highest mountain to the lowest valley would be... about 14m

That is shocking. That is shockingly round.

But why would you invest one million Euros and thousands of man-hours perfecting a pure,

Well the answer is grave. Or rather 'grave' as it would have been pronounced in the original

You see the grave was the original name for the base unit of mass in the metric system,

which became the Systeme International d'unites or SI units. In 1793, a commision which included

notable scientist and aristocrat Antoine Lavoisier, defined the base unit of mass as the weight

of a cubic decimeter of water at the melting temperature of ice -- essentially just a litre

of ice water. The name grave came from the Latin gravitas, meaning weight.

But it wasn't to last. It sounded too similar to the aristocratic title 'graf' -- which

is the equivalent of an earl or a count. And with the French revolution in full swing with

the rallying cry of equality for all, you couldn't exactly have one unit nobler than

the others. At this Lavoisier lost his head, literally, not because he helped devise one

of the greatest systems of measurement of all time, but because he was collecting taxes

The new republican government believed a grave would be too big for the things they wanted

to measure anyway and and so they settled on the gramme, which was just a thousandth

But soon they realized that a gram was too small and so they returned to the grave, but

since they couldn't call it that, they invented the kilogram -- a thousand grams. And that

is why out of the seven base SI units, the kilogram is the only one to have a prefix

In 1799 the kilogram definition was refined to be the mass of a litre of water at 4 degrees

Celcius -- the temperature at which it is densest. But water itself is obviously not

the most sensible thing to use as a mass standard. So a pure platinum cylinder was created to

have the same mass as the water definition and it was declared Kilogram of the Archives.

Now it's important to note at this point the kilogram is no longer tied to the mass of

a volume of water -- the kilogram of the archives is by definition THE kilogram.

90 years later, in 1889 the kilogram was upgraded to a platinum-iridium alloy cylinder. Now

it was much harder than the original but was otherwise basically identical. And to this

day, it remains the definition of the kilogram. It is officially called the International

Prototype Kilogram, though it's affectionately known as Le Grand K -- or Big K. Oh, and it's

It is the only thing in the entire universe with a mass of exactly one kilogram because

it IS the kilogram. It is also the only SI unit that is still defined by a physical object.

It sits under three bell jars, next to six sister kilograms, in a climate-controlled

vault locked by three independently controlled keys, in the basement of the International

Bureau of Weights and Measures on the outskirts of Paris.

Now if you were able to break into the vault and tamper with Big K, you would be changing

the definition of the kilogram, a definition on which many of our measurements rely, and

so you would throw the world into chaos! Well no, not actually-- but how would anyone ever

Well when it was first created, 40 identical replicas were also made. Well they weren't

quite identical - they had a mass which was slightly different to Big K but those offsets

were recorded. Now these replicas were sent out to countries around the world to serve

In 1948 the kilograms were reunited for a weigh-in. And this is when the problems started.

Because even though all the cylinders were made of the same alloy and stored under virtually

the same conditions, their masses had diverged over time. The mass of Big K wasn't even the

same as the six sister cylinders stored with it. And to make matters worse when they were

brought together again forty years later, their masses had further diverged, up to about

50 micrograms - that's about the weight of a fingerprint. But fingerprints were not the

culprits since the kilograms were carefully washed before their weigh-ins.

So some physical process must have actually changed the mass of the cylinders, but how

that exactly works remains a matter of speculation. One this is for certain, the mass of a platinum-iridium

cylinder is not stable over time. And this is a big problem. You can't have a unit which

changes its value. And the fallout isn't limited to measurements

of mass since of the seven base SI units, four of them depend on the mass of the kilogram,

not to mention all the derived units like Newtons, Joules, Volts and Watts.

At this point those of you in countries that have not adopted the metric system--yes I'm

speaking to you Liberia, Burma, and the US--you may be feeling rather smug that your unit

of mass, the avoirdupois pound, is no longer defined by a physical object. No, instead

So clearly something needs to be done to eliminate the kilogram's dependence on a physical object

and this is where the silicon sphere comes in, but how exactly does that help?

Here you have a physical object and it's beautiful but you know it's still a physical object.

You're trying to get away from that. We're trying to get away from the physical

object but what we're doing with this particular object is counting how many atoms are in there.

You can't actually count how many are in there can you?

You can't count how many are in there but you can calculate how many are in there because

this material is silicon, there's no voids or dislocations.

So this is like a perfect crystal of silicon. That's right.

Not only is it pure silicon, it contains only one isotope of silicon, silicon-28, and that

explains why the original material was so expensive.

And why a sphere? Well, a sphere is a pretty simple object.

If you know the diameter of the sphere you can characterise the entire dimension of the

object. Well that explains why the sphere has to be

the roundest object ever created, but how do you actually make something that round?

We actually start with an oversized sphere. So it was about two millimetres larger in

diameter and then we just grind it progressively finer and finer using abrasive. It's actually

massaging atoms. You're down at that level of trying to control the shape of an object

down at the atomic level. But making the sphere is only half the battle,

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世界一丸い物体! (World's Roundest Object!)

physical

constant

definition

object