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#### 为什么“千克”的质量变了？

The kilogram has changed forever. Here’s why.

I grew up in the United States,

which means I used pounds and ounces

to measure everything that I used growing up

except maybe soda bottles.

Right, and I grew up in the U.K.

where I use the metric system for pretty much everything

and pounds are what I pay for things in the store with.

So while we don’t really

agree on a unit of measurement yet, there is one thing

that everyone should be familiar with,

and that’s the kilogram.

Right, and not just because

it’s a superior unit of measurement

or because it’s used pretty much everywhere in the world,

but because it recently changed.

[rhythmic drumming]
[有节奏的鼓声]
How much does a kilogram weigh?

This is not a trick question.

When the kilogram was first introduced

as a unit of mass in the 18th century,

it was decreed to be the absolute weight

of a volume of pure water equal to the cubeof the 100th part of the meter. (Correction: the 10th part)

Basically, one cubic decimeter, or one liter.

Anyway, the French scientist who came up

with this definition soon discovered it was a little sloppy.

The weight of the water changes depending on things like purity and atmospheric pressure.

So they’ve replaced that definition with something like this.

For more than a hundred years,

a metal artifact similar to this has been the definition

of the kilogram.

The real one is made of platinum iridium

and it’s called the International Prototype Kilogram.

Since 1889, it’s been kept under lock and key

in a vault on the outskirts of Paris.

The Prototype Kilogram has been a big deal.

If we didn’t have reliable units of weight,

How would scientists repeat experiments and share their results?

How could you trust a recipe or even your bathroom scales?

That little chunk of metal is the foundation of the modern world.

But recently, scientists voted to get rid of it

and they replaced it with something far, far stranger.

[rhythmic drumming]
[有节奏的鼓声]
Last month, I visited the headquarters of

the International Bureau of Weights and Measures in France,

the body that acts as the guardian of the metric system

and I watched as delegates from 60 countries voted

to retire the Prototype Kilogram.

It was amazing to see.

Each country standing up to vote one by one

and then the whole hall erupts into applause and congratulations.

[applause]
[掌声]
For some of the scientists there,

it was the culmination of a lifetime of work,

slimming down the world’s system of measurement

so that all the definitions now fit onto a single card.

[contemplative music]
[冥想音乐]
But before we get into that,

let’s think about how weights work in general.

We know that the Prototype Kilogram

and its successor define standards of weights around the world,

but if you grab some scales

and weigh out a kilogram of flour,

how do those scales know how much a kilogram weighs?

Well, we called some scale companies to find out.

-[电话语音]您好 请问您需要什么？
– Can you tell me how you go

how do you make sure that they’re accurate?

– [Cory] Here’s what we learned.
-[科里]我们了解到
Basically, a scales company

like Escali buys calibration weights to place on their scales

and dial them in.

A weight like this one was measured against an even more

accurate kilogram, which itself might be measured against an even

more accurate kilogram,

all the way up the ladder of accuracy,

until, in the U.S. anyways, it reaches

the National Institute of Standards and Technology.

They have a couple of very special artifacts:

two of the 40 original weights that were measured directly
1889年直接根据国际原型公斤测量的
against the International Prototype Kilogram in 1889.
40个原始砝码中的两个
And they numbered them from one to 40

and they screwed one number up and they

left out eight and numbered it 41 by mistake.

[laughs]
[笑声]
Small little detail.

[Cory] That’s Stefan Schlamminger, a physicist at NST.
[科里]Stefan Schlamminger NST的物理学家
He says there was a global raffle

for countries to get the 40 original weights.

And the United States got two: number four and number 20.

[Cory] Today there are more than just 40.
[科里]现在不止40个了
New ones are measured against a final,

ultra-accurate stand-in for the IPK,

called the Working Standard.

The Working Standard is basically used to

calibrate now all masses from all nations.

That’s how things used to work,

but with the redefinition last month,

instead of tracing weights back to the Prototype Kilogram,

there’s a new top boss.

The kilogram is no longer a lump of metal,

it’s a constant of nature.

[godly chorus sings]
[神圣的合唱]
The kilogram isn’t the first unit to go through this change.

In fact, it’s the last.

The meter, for example, was once defined as one

ten-millionth of the distance from the North Pole to the Equator,

a length that was enshrined in a physical artifact.

Now, it’s defined as the distance that light travels

in a tiny fraction of a second, about a 300 millionth.

The kilogram, on the other hand, is now tied

to the smallest action that can be made by a photon,

which is essentially the smallest possible physical action there is.

It’s called Planck’s Constant

and in order to tie mass to it,

scientists had to create one of these,

a Kibble Balance.

It works like an old-fashioned balance beam,

but instead of weighing one mass against another,

it weighs one mass against an electromagnetic force.

This force can be measured with extreme precision using Planck’s Constant

and this will be the new definition of the kilogram.

But look, there’s one question we’ve not talked about so far.

Why?

Why are we going to all this trouble?

Well, when I spoke to the scientists at the big vote last month,

they gave me two reasons.

First, accuracy.

The problem with using a physical standard to

define the kilogram is that physical objects change.

None of them last forever.

And over the past few decades, the Prototype Kilogram itself has lost mass.

or the weight of a human eyelash, but still,

thanks to that huge chain

of calibrations we traced,

technically every kilogram in the world ends up shifting

by one eyelash weight.

Not ideal.

But more importantly, they said it was about ideology.

The Prototype Kilogram is based in France

because it was created during the French Revolution.

This was a time when revolutionaries were tearing down

and reconstructing so many of society’s norms.

And that included weights and measurements.

They wanted to make units that were consistent,

that were fair to the working man and woman.

And that was shared between countries.

They wanted to unite the world.

Their motto was”Pour tous les temps,

“pour tous les peuples, for all time, for all people.”

But if we define mass using

a single Prototype Kilogram locked away

in a basement in France,

then it can’t be for all men and for all women.

And if it changes when we measure it,

it can’t be for all time.

So the scientists say by tying the kilogram

to a constant of nature, we are,

in a way, freeing it from its physical constraints.

We’re achieving the dream of the creators of the metric system,

a journey that’s been centuries in the making.

Planck’s Constant is something,

it’s woven into the fabric of the universe,

so everybody can access it.

And I think that opens a whole world.

[claps]
[鼓掌]
[Man Offscreen] Um.
[屏外声]嗯
Ah.

[claps]
[鼓掌]
[laughs]
[笑声]
[claps and laughs]
[鼓掌和笑声]
Sorry I’ll stop laughing when I do that eventually.

YvonneYY