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重新定义千克 – 译学馆
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重新定义千克

How We're Redefining the kg

-我应该按什么 负二层吗?-负二层
What do I have to push, sub-basement?>>Woman: Sub-basement.
[蜂鸣安全报警器]
[Buzzing safety alarm]
我现在在华盛顿的国家标准与技术研究所
I’m at the National Institute of Standards & Technology in Washington D.C.
准备去负二层
and I’m going to the sub-basement.
这里变暗了
It’s getting dark down here.
我们要找出他们将如何重新定义千克
We’re going to find out how they’re going to redefine a kilogram.
千克有麻烦了
The kilogram is in trouble.
1799年 它被定义为
Since 1799, it’s been defined
在巴黎一个上锁的地下室里
as the mass of a metal cylinder,
一个金属圆柱体的质量
in a locked vault in a basement in Paris.
但在上个世纪
But over the last century,
仔细测量了这个国际标准千克原器
careful measurements of this international prototype kilogram
理论上和世界各国家的标准原器完全相同
and in-theory-identical national standardsfrom around the world,
表明这些质量数据上是有差距的
have shown that their masses are diverging.
这一差距已经扩大到约50微克 或者说亿分之5
The spread has grown to around 50 micrograms, or 50 parts per billion.
而质量标准的改变是不能接受的
And having a standard of mass that changes is unacceptable.
另外 千克是最后一个
Plus, the kilogram is the last
仍然由物理对象定义的基本国际单位
of the base SI units to still be defined by a physical object.
例如 米
The metre, for example,
在巴黎 米曾经被定义为一根铂管的长度
used to be defined as the length of a platinum bar in Paris,
但在1983中 它被重新定义为光
but in 1983 it was redefinedas the distance light travels
在每299 792 458分之一秒的传播距离
in 1/299,792,458 of a second.
这个定义意味着
This definition means that the speed
光的速度被设定为精准的299792458
of light is set to exactly 299,792,458
. 00000… 米每秒
point 00000… et cetera, metres per second.
怎么来的呢:首先你把现有的定义说成
Note how this works:first, you take the existing definition, say
那米杆的长度
the length of that metre bar,
仔细测量
and you measure as carefully
你可以知道它是如何与宇宙的一个物理常数相联系的
as you can how it relates to a physical constant of the universe:
光速
the speed of light.
然后用你设置这个常数的确切值
Then you set the exact value of that constant
来重新定义一米有多长
and use it to redefine how long a metre is.
我知道这看起来像是陷入死循环了 但重要的是
I know this might seem circular, but, importantly,
它把实质的点
it moves the point of truth
从物质实体转移到不会改变的宇宙常量上
off of the physical object, and onto the unchanging constant of the universe.
所以 自然地 我们的想法是用千克做同样的事情
So, naturally, the thought is to do the same thing with the kilogram.
但是该使用哪种常量 怎样使用呢
But using which constant, and how?
#重型机器噪音#
[Heavy mechanical noises]
我们尝试过许多不同的方法
Well, there are a number ofdifferent strategies that were attempted
但只有两种方法最为成功:
but the two that achievedthe greatest success were:
1 用硅球体确定阿伏伽德罗常数
1) using a silicon sphere to determine and setAvogadro’s number
2 用瓦特天平确定普朗克常数
and 2 ) to use a Watt balance to determine and set Planck’s constant.
-你好 我是德里克-好
-Hi, how ya’ doin’? I’m Derek-Pretty good.
很高兴见到你
Nice to meet you.
瓦特天平在哪?
Where is the Watt balance?
瓦特天平在这些关着的门后
The Watt balance isbehind these closed doors, and…
在那里?
It’s in there?
是的 现在的问题是
It’s correct, and right now the problem is that…
我们急需在5月底之前得到一个数值
We are in a crunch to get a number by the end of May.
什么数值?
What’s the number?
普朗克常数
The Planck’s constant.
这是我们用瓦特天平测量的
This is what we measure with the Watt balance.
在2011年 国际度量横大会决定
In 2011, the General Conference on Weights and Measures
根据普朗克常数 应该重新定义千克
decided that the kilogram should be redefined based on Planck’s constant,
但是这并不意味着阿伏加德罗的方法是无效的
but that doesn’t mean thatthe Avogadro approach was futile.
我的意思是 你可以用阿伏德罗常数
I mean, you can use Avogadro’s number to calculate
去计算普朗克的常数 反之亦然
Planck’s constant and vice-versa.
最终 这两种方法都将被用来同时重新定义
So, ultimately, both approachesare going to be used to redefine
普朗克常数和阿伏伽德罗常数
Planck’s constant andAvogadro’s number simultaneously.
在硅球体方面有一个好消息
One good thing about having silicon spheres,
就是 你只想
is that you only want to redefine
在不同常数之间达成一致时重新定义 对吗?
if you have agreement between different numbers, right?
硅球法
And the silicon sphere method is a method
在我看来是来自化学
in my mind that comes out of chemistry.
你测量的阿伏伽德罗常数
You measure Avogadro’s constant,
是一个源自于化学的常数
which is a constant that comes out of chemistry.
而我们测量普朗克常数的这种方法源自于物理学
This method comes out of physics,we measure Planck’s constant.
如果这两种方法结果一致 那真是个很重要标志 对吧
So if they both agree, it’s a pretty strong sign, right?
因为你懂得化学和物理学结果一致
Because you know chemistry and physics agree.
现在 因为我已经
Now, since I’ve already
在之前的视频中讨论过阿伏伽德罗的方法
discussed the Avogadro approach in a previous video,
在这里我想把重点放在瓦特天平上
here I want to focus on the Watt balance.
实际上 它现在被叫做基布尔天平
It’s actually now called a Kibble balance
为了纪念他的发明者 布莱恩•基布尔
in honor of its inventor, Bryan Kibble,
事实上他在2016 年去世了
who actually passed away in 2016.
我们知道 传统的天平是通过将
You know, traditional balances work by equating the gravitational forces
托盘两边物体的重力等效
on objects in two pans.
基布尔天平看似相似
The Kibble balance looks kind of similar,
但是所有的平衡状态都发生在左边
but all of the balancing happens on the left-hand side,
在磁场中
where a mass pan is attached to a coil
质量盘与线圈相连
of wire in a magnetic field.
在右边是一个发动机
On the right-hand side is a motor.
整个装置在真空中密封和操作
The whole apparatus is sealedand operated in vacuum.
天平以两种方式运行
The balance operates in two modes:
即称重模式和速度模式
Weighing mode and velocity mode,
这两者都需要确定普朗克常数
and both are requiredto determine Planck’s constant.
在称重模式中
In weighing mode,
一千克质量标准原器放在质量盘上
a kilogram mass standard is placed on the mass pan
然后电流通过
and then current is passed through the coil
磁场中的线圈
in the magnetic field
并且调整到
and adjusted until the weight
千克的重量与
of the kilogram is equal and opposite
线圈上的电磁力相同或相反
to the electromagnetic force on the coil.
这个方程是质量乘以当地的重力加速度
The equation for this isMass times the local gravitational acceleration
等于磁场
is equal to the Magnetic field,
乘以线圈中导线的长度再乘以流过线圈的电流
times the length of wire in the coil, times the current flowing through it
在这个方程中 难以精确测量的变量是
In this equation the variables that are difficult to
磁场强度和
measure exactly are the magnetic field strength,
线圈中导线的长度
and the length of wire in the coil
但幸运的是 基布尔天平允许我们
But luckily the Kibble balance allows
使用速度模式来解决这个问题
us to get around this problem using velocity mode
在速度模式下 千克原器从质量盘上被抬起
In velocity mode the kilogram mass is lifted off the mass pan
而此时 天平另一端的发动机用来
and now the motor on the other side of the balance is used to
在磁场中以恒定速度来回移动线圈
move the coil back and forth at constant velocity through the magnetic field.
这种运动在线圈中
This motion induces a voltage
产生的电压等于磁场
in the coil which is equal to the magnetic field,
乘以线圈中导线的长度 再乘以其速度
times the length of wire in the coil, times its velocity.
现在我们有两个方程
Now we have two equations which we can
我们可以求出B和L的乘积
solve for B times L and so we
这样我们就可以把它们设为相等
can set them equal to each other and
来消除这些变量
eliminate these variables
而不必知道它们的值是多少
without having to know precisely what their values are
如果我们重新排列一下
and if we rearrange a little bit you
你可以得到:电压×电流=质量×重力×速度
get voltage times current equals mass times gravity times velocity.
在左手边
On the left hand side,
是电力 右手边是机械动力
there is electrical power and on the right hand side, mechanical power,
这就是为什么这个功率平衡单位
and that’s why this was called the Watt,
被称为瓦特
the unit of power, balance
但是 你如何
But how do you go
从这个单位变换到将光子频率与其能量联系起来 普朗克常数
from this to Planck’s constant,the number that relates a photon’s frequency to its energy?
事实上有一种方法
Well it turns out there’s actually a way
可以利用约瑟夫森结的宏观量子效应精确测量电压
of measuring voltage accurately using a macroscopic quantum effect that involves Josephson junctions
约瑟夫逊结是由
so a Josephson junction consists
两个超导体组成 被一片绝缘体隔开
of two superconductors separated by a thin piece of insulator
现在 如果你在结上施加微波辐射
Now if you apply a microwave radiation to that junction,
你会在器件两端产生一个电压
you create a voltage across the device
我们可以精确地知道它的值是 2e除HF
and its value is precisely known to be hf over 2 e.
其中h是普朗克常数 f是辐射的频率
Where h is Planck’s constant, f is the frequency of the radiation,
e是电子上的电荷
and e is the charge on an electron
现在 通过调整频率
Now by tuning that frequency and stacking as many
把约瑟夫森结按你想要的顺序尽可能多地叠加起来
of these Josephson junctions as you want in series
你就可以非常精确地产生任何你想要的电压
you can create virtually any voltage you like very very precisely.
在基布尔平衡中使用的方式是
The way this is used in the Kibble balance
当线圈移动通过磁场时
is a stack of hundreds of thousands of Josephson junctions
将数十万个约瑟夫森结
are put into the circuit with the coil
堆叠在电路中
as it is moved through the field
这样就可以精确地让这些
and so you exactly balance the voltage which is induced
约瑟夫森结在线圈中感应的电压平衡
in the coil using those Josephson junctions
因此 你就可以非常精确地测量电压
So you can measure that voltage very very accurately.
但是我们如何测量电流呢?
But how do we measure current?
原来 这种电压测量方法是如此方便
Well it turns out this voltage measuring method is so good
以至于不用设法直接测量电流
that instead of trying to measure current directly
我们同样可以在R上测出V
we instead measure V on R which is the same thing
所以 电流通过电阻
So this current is passed through a resistor,
我们再用约瑟夫森结测量电压
and we measure that voltage again using Josephson junctions
然后为了测量电阻 我们使用另一种
And then to measure resistance we use another
宏观量子效应——量子霍尔效应
macroscopic quantum effect called the quantum hall effect.
这已经超越了本视频的范围 但是
Which is Beyond the scope of this video but,
只要电阻测量是一个整数部分 除以p
suffice is to say that the resistance measurement will be an integer fraction, one over p
乘以普朗克常数 再除以电子平方上的电荷
times Planck’s constant divided by the charge on the Electron squared
所以 如果我们把所有这些分解到我们的
So if we sub all of this into our
方程中 并求解h 我们得到
equation and solve for h, we have that
普朗克常数等于
Planck’s constant is equal to four
4/pn² 这些都是我们知道的常数
over p n squared, those are all constant numbers that we know,
乘以当地重力加速度 再乘以速度
times the local acceleration due to gravity times velocity
除以频率的平方 乘以质量——即1千克
divided by frequency squared times the mass which is one kilogram.
所以 就质量为1千克而言
So here we have a very precise equation
我们就有一个非常精确的普朗克常数方程
for Planck’s constant in terms of the mass of one kilogram
现在要得到一个很好的答案,
Now to get an answer that’s good to say,
1亿分之一
ten parts per billion
你需要精确的知道所有的这些值
You need to know all of these values very accurately
因此 为了测量V
So to measure V for example the velocity
例如线圈在磁场中移动时的速度
of the coil as it moves through the Magnetic field,
我们使用激光干涉仪
we use a laser interferometer
当到线圈的距离发生改变时
as the distance to the coil
干涉条纹通过一个检测器
changes, the interference Fringes pass over a detector
本质上 通过计算
And essentially by counting how many
一定时间内通过的条纹数量
fringes go past in a certain period of time
你可以非常准确地计算出线圈的速度
you can determine the speed of the coil very accurately
为了计算g 在建造平衡室之前
To measure g, a device called a gravimeter was used
使用了一种叫比重计装置来测量由于重力
to map out the local acceleration due to gravity
而产生的加速度
in the balance room before it was built in there
重力仪实际上是把一个角反射器
The gravimeter actually drops a corner
放入真空管中 然后
reflector down a vacuum tube and measures its acceleration
再通过干涉法测量它的加速度 计算出经过的条纹
again through interferometry, counting the fringes as they pass
这是基布尔平衡室中
This is a 3D printed map of the acceleration
重力加速度的3D打印地图
due to gravity in the Kibble balance room
这个肿块来源于平衡中的强大而
The bump is due to the mass of the powerful
沉重的永磁体的质量
and very heavy permanent magnet that’s in the balance
由重力引起的加速度必须连续测量
The acceleration due to gravity must continually be measured
因为在这个精度水平上
because it can be affected at this level of precision
太阳和月亮的位置
By the positions of the sun and moon
甚至建筑物下面的地下水位都会影响加速度
and even the water table underneath the building
2018年 千克
In 2018 the kilogram will no
将不再由巴黎的物体来定义
longer be defined by an object in Paris
而是根据
Instead it will be defined based
目前正在最后确定的
on the fixed value of Planck’s constant
普朗克常数的固定值来定义
which is being finalized right now as a result
这是从基布尔天平和硅球进行所有这些测量的结果
of all these measurements from the Kibble balances and silicon spheres
所以现在我们要做的是
So right now what we do is,
把质量放进去 然后把H拿出来
we put the mass in, and we get h out
在2018年重新定义后
and in 2018, after redefinition,
H就固定了 你可以用它来求出质量单位
h will be fixed and you use that to realize the unit of mass
-简单-是的 就那么简单
-Easy-Yeah, just that– just that easy.
-就那么简单-简单
-Just that simple.-Simple
嘿 Veritisium的这一集得到了
Hey, this episode of Veritasium was supported
Patreon上像你这样的观众的支持
in part by viewers like you on Patreon
以及你可能知道的Audible的支持
and by Audible, who, as you probably know
因为他们是该频道的长期支持者
because they’re longtime supporters of the channel
是包括有声读物
are leading providers of spoken audio information
原创节目 新闻喜剧等在内的主要音频信息的提供者
including audiobooks original programming news comedy and more
对于这个频道的观众 他们提供30天的免费试用
And for viewers of this channel they offer a free 30-day trial.
登录audible.com/veritasium就可以获取
Just go to audible.com/veritasium
你们知道 最近我一直在环游世界
You know, recently I’ve been traveling
去了以色列 伦敦及墨西哥城
around the world to Israel, London, Mexico City
明天我就要出发去新奥尔良了
And tomorrow I’m off to New Orleans
因为我在为Netflix拍摄一些东西
because I’m shooting stuff for Netflix and
我会在飞机上听
What I’ll be listening to
史蒂芬·平克的《我们本性中善良的天使》
on the plane is Steven Pinker’s The Better Angels of Our Nature
这是一本很棒的书 它从科学和统计学的角度
This is an awesome book that takes a scientific and statistical
探讨了什么时候是人类最好的生存时间
approach to the question of when is the best time to be alive
而他的答案是:现在
as a human and his answer, is now.
不论是否如它所言
Whether it seems like it or not,
暴力及很多人类须解决的糟糕的事
violence and all the terrible things that humans have had to deal with
在过去的几个世纪一直在减少
has been on the decline for centuries and
如果你想要
if you want the
相关的统计数据
statistical proof for
你应该看看这本书 它太棒了
that you should check out this book it is fantastic
如果你想要 你可以上
And if you want you can download it
audible. com/veritasium免费下载
for free by going to audible.com/veritasium
你也可以选择任何其他的书
Or you can pick any other book
在你的一个月的试用期内
of your choosing for a one-month free trial
非常感谢audible网站的支持
So I want to thank audible for supporting me,
也感谢你们的观看
and I want to thank you for watching.

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视频概述

在本视频中,我们来到了位于华盛顿的国家研究所,看看现代科学家们是如何重新定义千克的。

听录译者

收集自网络

翻译译者

天虎

审核员

审核员 LD

视频来源

https://www.youtube.com/watch?v=Oo0jm1PPRuo

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