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量子电动力学理论如何被证实? – 译学馆
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量子电动力学理论如何被证实?

Quantum Theory's Most Incredible Prediction | Space Time

Thanks to the Great Courses Plus for supporting PBS Digital Studios.
感谢The Great Courses Plus对PBS数码工作室的支持
Let’s talk about the best evidence
今天我们来谈谈
we have to the theories of quantum physics truly
现有的量子物理学理论
represent the underlying workings of reality.
体现在现实生活中的最佳证据。
Quantum field theory is notoriously complicated,
量子场论十分复杂,
built from mind-bendingly abstract mathematics.
建立在抽象数学的基础之上。
But could it be that the underlying rules that
但是它是
govern reality are really so far from human intuition,
掌控远离人们所设想的现实的潜在原因吗?
or a physicist just showing off?
还是说这只是物理学家在卖弄学识?
For better or worse, the physicists are definitely on the right track.
不管怎样,物理学家们是正确的,
We know this because the predictions of quantum field
这是因为量子场论的众多预言
theory stand up to experimental test time and time again.
已经一次次的得到了实验的证实。
Quantum field theory describes a universe
量子场论认为宇宙中
filled with different quantum fields in which particles
充斥着不同的量子场,场中粒子
are excitations, quantum vibrations
被激发,形成量子谐振。
We talk about QFT many times before
之前,我们也多次聊过量子场论,
starting with the very first quantum field theory
从最初的量子场论—
quantum electrodynamics
量子电动力学开始。
QED talks about the electromagnetic field whose
量子电动力学描述的是电磁场
excitations give us the photon.
可以激发产生光子。
The calculations of QED describe how this field interacts
量子电动力学公式体现了这个磁场是如何
with charged particles to give us the electromagnetic force,
与带电粒子相互作用形成电磁力的,
which binds electrons to atoms, atoms to molecules
从而使电子聚集形成原子,原子聚集形成分子。
and therefore, you know, allows you to exist.
这样,就构成了你我存在的基础。
QED is a much deeper and more complicated
量子电动力学更深刻和复杂地
description of electromagnetism than
描述了电磁场。
the simple opposite charges attract, like charges repel
相比之下,“同性相斥,异性相吸”的
of classical electrodynamics.
经典电动力学理论则简单许多。
But how do we do it’s right? Well,
但怎样来证实它呢?
because it makes some predictions that clash
因为它做出了一些预测
with the classical theory.
与经典理论完全相悖。
And those predictions are the most
而且这些预测大多已经
precisely tested and thoroughly verified in all of physics.
在物理学各领域中得到了精确的验证和证实。
Today, we’re going to talk about the theory and experiments behind one of these tests,
今天我们来谈谈其中一个理论及实验——
measuring the “g factor”,
“g因子”的测量。
or in simple English,
简单来说,
measuring the anomalous magnetic dipole moment of the electron.
就是测量电子的反常磁偶极矩。
OK, first up, what on earth did I just say?
那么,首先,我刚刚说的到底是什么?
What is the anomalous magnetic dipole moment?
电子的反常磁偶极矩是什么?
Well, it’s just like the regular magnetic dipole
它和正常的磁偶极矩类似,
moment but more anomalous.
只是更加反常。
OK, not helpful.
好吧,当我没说。
Let’s break down these magnetic dipole moment thing.
让我们把这个“磁偶极矩”分解一下。
Consider a bar magnet.
想象这是一块磁铁,
It has a dipole magnetic field,
它具有偶极磁场,
basically meaning it has a north and south pole.
就是说它有一个南极,一个北极。
Dipole — two poles.
偶极—两个极端。
If we put a bar magnet
如果我们将一块磁铁
in a second external magnetic field,
放在一个外加磁场中,
it’ll feel a torque, a force causing it to rotate
它会产生一个扭矩,一种导致它旋转的力,
to align with that field.
使的它与磁场的方向一致。
The tendency of a dipole magnet to rotate in an external magnetic field
这种偶极磁铁在外加磁场中旋转的倾向,
is its magnetic dipole moment.
就是它的磁偶极矩。
Anything with a dipole magnetic field
任何带有偶极磁场的物体,
has a magnetic dipole moment is basically
基本上都具有磁偶极矩。
a measure of how much it would interact
它衡量的是物体是如何与
with an external magnetic field if one existed.
它所存在的外加磁场相互作用的。
Let’s talk about this dipole thing a bit more.
我们再来聊聊偶极体。
Magnetic fields are produced by moving electric charges.
磁场由运动的电荷产生。
A perfect dipole field is produced
1个完美的磁场是由
by charges moving in circles, for example,
电荷环形作用形成的。
a loop of wire with an electric current
比如一圈通电的电线
or the planet Earth with its dynamo core.
或是具有发电内核的地球。
But in the case of a bar magnet, the source
但是条形磁铁,
of its magnetic field is a bit weirder.
它的磁场源有一点奇怪。
It mostly comes from the summed dipole magnetic fields
它主要产生于叠加的偶极磁场,
of individual electrons in the outer shells of its atoms.
偶极磁场是由原子表面独立的电子形成的。
And those electron dipole fields are, indeed, very weird.
并且这些电偶极场非常奇怪。
As we’ll see, their nature is predicted by quantum theory,
如我们所知,量子力学预测了这种情况,
measure electromagnetic moments,and you verify your quantum picture of reality.
测量电磁力矩,证实你存在的现实是量子化的。
Electromagnetic fields seem intuitive
把电磁场想象为
if you think of them as tiny balls of rotating electric charge,
一个带电的旋转小球,这样看起来很贴切。
except electrons aren’t balls and they
但是电子不是球,
aren’t really rotating.
而且它们也不会旋转。
As far as we know, electrons are pointlike.
众所周知,电子是点状的。
They have no size.
它们没有大小。
And it doesn’t really make sense to think of
假设一个无穷小的点在旋转
an infinitesimal point as rotating.
是毫无意义的。
Nonetheless, electrons do have a sort of intrinsic angular momentum,
然而,电子确实有一种内在的角动量,
a fundamental quantum spin that is as intrinsic as mass and charge.
这是一种与质量和电荷量一样的电子的内在属性——量子自旋。
Despite not being the same as classical rotation,
尽管与常规的旋转不同,
this quantum spin does grant electrons
这种量子自旋的确赋予电子
a dipole magnetic field.
产生一个偶极磁场的能力。
So electrons have a magnetic dipole moment,
所以电子具有磁偶极矩,
meaning they feel magnetic fields
意味着它们能感受到磁场
and act as little bar magnets.
并且产生与小型条形磁铁一样的反应。
Electrons in atoms feel the magnetic fields
原子中的电子感受到了磁场
produced by their own orbits around the atom.
围绕于原子的自身轨道所形成的磁场。
This results in a subtle torque on these electrons,
这使得电子受到微小扭矩,
changing their energy states, and resulting
改变了其能态,
in the fine structure splitting of electron energy levels.
在电子能级水平形成精细结构的分裂。
The fine structure constant is named after this effect.
精细结构常数因此而命名。
And we talked about this fundamental constant
我们在之前的视频里
in an earlier episode.
讨论过这一基本常数。
Thinking of electrons as little bar magnets
把电子想象成小型条形磁铁
or as rotating balls of charge is a nice starting point.
或是旋转的球状电荷是一个很好的开始。
But in the end, it’s misleading.
但最后,这会产生误导。
It also gives you completely the wrong answer
你将会得到一个完全错误的答案,
if you try to calculate the electron’s magnetic moment.
如果你试图去计算电子的磁矩。
So that electron diagram you did in middle school, it’s time to kill that idea
所以我们应当放弃像中学一样画电子图的想法,
just like you kill your tamagotchi.
就像你该放下你的电子宠物了一样。
In fact, weirdly, if you measure the magnetic dipole moment of an electron
实际上,奇怪的是,如果你测量电子的偶极矩,
you get almost exactly twice the value you’d expect for a tiny classical sphere
你将得到一个常规小球偶极矩两倍的数值,
with the same charge and angular momentum as an electron.
这个小球与电子具有相同电荷和角动量。
This difference between the quantum versus
这种电子的量子磁矩
classical magnetic moments for the electron
与传统磁矩的不同,
is called the “g factor”.
叫做“g因子”。
It’s the number you need to multiply the classical value by
你需要用常规值乘以这一因子
to get the right answer.
来得到正确答案。
So apparently, g equals 2.
显然,g=2。
Experiments point to this but so does the Dirac equation.
实验和狄拉克方程均得出这一结果。
This equation is the origin of quantum electrodynamics
这个等式就是量子电动力学的起源,
and the first to correctly capture the notion of quantum spin.
并且第一次准确描述了量子自旋的概念。
It describes electrons as weird for component objects
它将电子描述成一种奇怪物质,
with quantum spin magnitudes of half.
这种物质具有一半量子自旋振幅
That’s a whole bunch of crazy we talk about here.
这是我们讨论中最具争议的部分。
So measurements say the g factor is around 2.
测量表明,g因子约为2。
And Dirac says it’s exactly 2.
而狄拉克说g一定是2。
Case closed, right? Wrong,
结束了,对吧?错,
oh so very wrong. See,
大错特错了。
even though the Dirac equation tells us
即使狄拉克方程描述了
how a relativistic electron would interact with an electromagnetic field,
相对的电子在电磁场中是如何相互作用的。
it still treats this EM field classically.
但它将电磁场视为经典场,
It doesn’t consider the quantum nature of the field.
没有考虑场的量子性。
Only the fully developed quantum electrodynamics,
只有经过多方验证的量子电动力学,
the first true quantum field theory, does this.
可以称为第一个真正的量子场理论,因为它考虑到了场的量子性。
And QED tells us that the quantum electromagnetic field
量子电动力学告诉我们量子电磁场
is a messy, messy place.
是一个非常杂乱的场。
It seethes with a faint quantum buzz,
起先是微弱的量子嗡鸣,
infinite phantom oscillations that add
接着是无限虚振 ,
infinite complication to any electromagnetic interaction.
使得电磁相互作用更加复杂。
This messiness messes with the interaction
这种杂乱会与电子和磁场的相互作用
of the electron and the magnetic field to shift the G factor slightly.
混杂在一起,从而略微改变g因子。
So it’s not exactly 2, it’s 2.0011614 et cetera.
所以准确来说g不是2,而是2.0011614…。
That little bit extra is the anomaly.
确实有点不寻常。
And this is the anomalous magnetic dipole moment.
这就是反常的磁偶极矩。
It’s really incredible
真的令人难以置信。
that we can even begin to calculate the effect of the messy buzzing
我们甚至可以计算杂乱微弱的
electromagnetic field.
电磁场效应。
But in fact, we can calculate its effect extremely precisely
但事实上,我们能非常精确地计算
and test this through experiments,
并且通过实验证明这一效应,
showing the underlying truth of quantum theory.
揭示量子理论的内在本质。
So one way to think about this quantum buzz
一种解释量子嗡鸣的方法,
is with virtual photons.
是通过虚拟光子。
Quantum field theory describes the interactions
量子场论认为
between particles as the sum total
粒子间的相互作用是
of all possible interactions that can lead to the same result.
可能导致相同结果的所有相互作用的总和。
In the case of electromagnetism, those interactions
在电磁学的情况下 ,这些相互作用
are mediated by virtual photons,
以虚拟光子为媒介。
which just a mathematical way to describe quantum buzz.
这只是解释量子嗡鸣的一种数学方法。
Every interaction with virtual photons that can happen, does,
虚拟光子间的任何相互作用都有可能发生,
at least in a sense.
至少在某种意义上来说。
And the sum of the infinite possible interactions
无限可能的相互作用的总和
defines the strength of the one real interaction.
代表实际相互作用的强度。
And if that doesn’t make your head hurt, try thinking about it again.
如果这都没令你感到头痛,那就再想一遍。
So yeah, quantum field theory is a type of madness.
没错,量子场论是疯狂的。
And again, we’ve been down that rabbit hole.
我们再次被这一理论迷惑
In particular, we’ve been at Feynman diagrams,
特别是在费曼图中。
which are our best tool for dealing with the absurd complexity
费曼图是我们处理量子场特殊复杂性的
of quantum fields.
最佳工具。
They represent the possible interactions
它展现了量子场中
of the quantum field by way of virtual photons.
可能通过虚拟光子产生的相互作用。
And they tell you which interactions are the most important and which are insignificant.
它告诉你哪些相互作用是最重要的,哪些是不重要的。
So you know, you don’t have to calculate infinity of them.
所以你不需要计算他们的无穷性。
A basic interaction of an electron with an EM field
一个电子在电磁场中的基本相互作用,
is illustrated by this partial Feynman diagram.
体现在这个局部费曼图中。
An electron encounters a real photon
电子与真实光子相遇
that could represent an external magnetic field.
可以形成一个外在磁场,
And it is deflected in some way.
并且会发生某些偏离。
But the same encounter could look like this.
电子和真实光子相遇,也可以这样解释:
The electron first emits a virtual photon,
电子首先发射一个虚拟光子,
then gets deflected, then re-absorbs the virtual photon.
然后发生偏离,之后再回收虚拟光子。
Same particles in and out, so it leads
相同的粒子进出,所以它得到
to the same overall result. But now
完全相同的结果。
the electron undergoes an additional interaction
但现在电子在嗡鸣量子场中
with the buzzing quantum field.
发生了额外的相互作用。
We need to include this sort of secondary interaction
我们需要把这种继发的相互作用考虑在内
when we calculate, say, the overall strength
在计算磁场中
of an electron’s interaction with the magnetic field
电子相互作用的总强度时。
when we calculate the electrons magnetic dipole
在我们计算电磁偶极矩
moment and it’s G factor.
和其g因子时,也应这样考虑。
If we consider only the first interaction I showed along
如果我们仅仅考虑之前所说的第一种相互作用
with similar primary ones, you calculate
你会计算得出一个
a G factor of exactly 2.
完全等于2的g因子。
But if you include this secondary interaction,
但是如果考虑这种继发的相互作用,
you get g equals 2.0011614.
将会得到 g=2.0011614的结果。
This correction was first calculated
这一更正最初是由
by American physicist Julian Schwinger in 1949.
美国物理学家朱利安·施温格于1949年做出的。
It was an amazing result for the time,
这在当时是一个令人震惊的结果。
but a lot of time has passed since then and physicists were not
但随着时间流逝,物理学家们不再
content to simply stop at this first correction. See,
满足于最初的这个的更正。
there really are infinite ways
确实有很多方式
the electron can interact with the EM field,
使电子在量子电磁场中相互作用,
with crazy networks of virtual particles
通过虚拟微粒,
and virtual matter, anti-matter loops between the real ingoing
虚拟物质和反物质进出微粒
and outgoing particles.
形成的网络。
The more complicated the interaction,
相互作用越复杂,
the less it contributes to the overall effect.
其对整体效应的贡献就越小。
But contribute they do.
但并不是没有贡献。
Over time, physicists have included more and more corrections
随着时间的推移,物理学家对预测的g因子
refining the prediction of the g factor
进行了越来越多的修正。
to increasing precision.
从而提高g因子的精确度。
For each new degree of precision,
每获得一个更高的精确度,
the number of Feynman diagrams needed explodes.
就需要运用更多的费曼图。
Schwinger did his 1949 calculation by hand.
施温格在1949年通过手算得出结果。
Since 2008, all calculations are done
而自2008年以后,所有的计算
on large, supercomputing clusters. However,
都通过大型超级计算机进行。
the ultimate arbiter of any physical theory
然而,对于任何物理理论的最终验证
is experiment.
都是实验。
To actually measure the g factor with the same high precision
在如此高精度下计算出g因子,
as these calculations requires some cunning.
需要一些技巧。
One way to do it is to watch the way electrons process
一种方法是通过粒子回旋加速器,
in the constant magnetic field of a cyclotron,
一种粒子的加速装置,来观察电子
a type of particle accelerator.
恒定磁场中的运动过程。
Electron spin axes are always slightly misaligned
电子的自旋轴在外加磁场总会有些偏离。
with an external magnetic field, due to quantum uncertainty in the spin direction.
因为量子自旋方向的不确定性。
As a result, they feel a torque from that field
结果是,它们在场中产生了扭矩
and persist like a top.
并像陀螺一样转动。
This is called Larmor precession.
这一过程叫做“拉莫尔旋进”。
And the rate of this precession tells us the electron g factor.
旋进的速率就是电子的g因子。
And the results are staggering.
结果是令人难以置信的,
The measured g factor agrees with the calculated value
g因子的测量值和计算值
to 10 decimal places. Now,
在小数点后十位内保持一致。
I need to add a little subtlety here
现在,我们需要在场中添加一些微扰,
to get from the QED calculations to a value for G,
通过QED来计算g的值。
you also need to know the fine structure constant
你需要知道精细结构常数,
that I mentioned earlier.
我之前提到的。
This is the fundamental constant governing the strength
这是一个基本常数
of the electromagnetic interaction of charged particles.
决定带电粒子间电磁相互作用强度。
This requires an independent experimental measurement.
g因子需要一个独立实验来测量。
So it’s really the relationship between
所以,我们需要验证的是
the electron magnetic moment and the fine structure
电子磁矩与精细结构常数
constant that we’re verifying.
之间的关系。
But that prediction is the most accurately verified prediction
但是,这个预测得到了最为准确的验证,
in the history of physics.
在物理学史上。
At it’s heart, physics is the study of the natural world.
究其本质,物理学是研究自然世界的。
We make observations of reality and then try
我们观察到了自然现象,然后开始尝试
to find theoretical frameworks that explain those observations.
去找到能够解释这些现象的理论框架。
If those theories are good,
如果理论是正确的,
they’re able to predict things beyond the observations on which the heory was built.
就能够在理论基础上预测出观察不到的现象。
The better these predictions,
所做的预测越准确,
the more universal and presumably the more correct the theory.
理论就越普遍且更有可能是正确的。
The theory of quantum electrodynamics
量子电动力学理论
has been pushed to the experimental limit
已经经受了实验的考验,
and come out unscathed.
并且毫发无损。
That means that it and the quantum mechanical principles on which it is founded
也就是说,量子电动力学理论和作为其形成基础的量子力学原理
are good representations of reality.
很好地再现了现实。
We have to conclude that we are getting closer and closer
可以说,我们越来越接近
to the truth in our search for theories
我们所寻找的
to explain the underlying mechanics of space time.
解释时空基本原理的理论真相了。
Thanks to the Great Courses Plus for supporting PBS Digital Studios.
感谢The Great Courses Plus对PBS数码工作室的支持。
The Great Courses Plus is a digital learning service
The Great Courses Plus是一个数字学习服务,
that allows you to learn about a range of topics from educators,
允许你学习教育者发布的多种主题,
including Ivy League professors and other experts
教育者包括常青藤教授和其他
from around the world.
来自世界各地的专家。
You can go to the greatcoursesplus.com/spacetime
你可以登录greatcoursesplus.com/spacetime网站
to get access to a library of different video lectures
获得图书馆权限,观看视频课程。
about science, math, history, literature,
关于科学,数学,历史,文学
or even how to cook, play chess or become a photographer.
或是如何做饭,下象棋,成为摄影师。
New subjects, lectures, and professors are added every month.
每月都会有新增的主题,课程和教授
You could get a free trial by clicking on the link below
通过点击下面的链接,你将得到一些免费的训练
or going to thegreatcoursesp lus.com/spacetime.
或者登录thegreatcoursesplus.com/spacetime网站。
And now onto your comments about getting close to the sun
现在根据近期发射的派克太阳探测器
with the very recently launched Parker Solar Probe.
谈谈你关于太阳的看法
Emma Faman asks about how satellites and service
Emma Faman提出疑问关于卫星和电子服务设备
electronics could be protected given
是如何被保护的,
advanced knowledge of a Carrington-like geomagnetic storm.
通过卡林顿式地磁风暴的前沿知识。
So advanced warning definitely helps a lot.
提前预警很重要。
Power grids can be shut down to prevent major damage.
可以通过关闭电网来预防主要损失。
That damage occurs when powerful currents are induced
当电缆中引入强电流时
in long range power cables.
会出现损伤。
When those currents hit transformers at power stations,
当这些电流击中发电站的变压器时,
those transformers can be fried.
变压器就会起火。
Just disconnecting the transformers is enough to save them.
仅仅将变压器分离开就足以拯救它
There are various ways to automatically dissipate
即使没有预警,也有多种方法
excess current even without forewarning,
将变压器自动分离开,
but that would take proper investment in infrastructure
而这将花费数百亿美元来
to update our antiquated power grid to the order
投资建设基础设备,
of tens of billions of dollars.
来升级过时的电网。
Satellites are tougher.
卫星十分稳固。
Their electronics can withstand smaller currents.
它们的电子设备可以承受较小的电流。
But a Carrington-size event, it’s not clear.
但并不确定能否承受住卡林顿规模的事件。
Presumably better Faraday cages and integrated surge
也许采用更好的法拉第笼和电涌保护器
protections are the answer.
才是正确做法。
Again, it requires investment in long term stability.
而且,这需要长期稳定的投资。
All of these measures produce benefits outside
这些措施都会在下一季度报告
of the next quarterly report or election cycle.
或选举周期之外产生效益。
And that’s the real impediment.
这真的无法预测。
Andrea Smith asks whether the Suns million Kelvin corona
Andrea Smith设想,太阳日冕温度高达数百万开尔文
temperature is caused by magnetic reconnection.
是不是由磁力重联引起的。
The answer is, possibly, even probably, at least in part.
答案是有可能,至少有部分可能性是这样的。
So the mystery here is why the sun’s corona,
但问题是,为什么太阳的日冕,
the extremely diffuse layer of material surrounding the sun,
围绕在太阳表面的极度扩散层,
can be so hot compared to the 5800 Kelvin solar surface.
它的温度能比5800开尔文的太阳表面高这么多。
It’s pretty firmly established that energy
可以十分确定的是,
must be pumped into the corona by magnetic fields.
能量必须通过磁场进入日冕。
Just radiating it from the solar surface
仅仅从太阳表面辐射的能量
would lead to temperatures below 5800 Kelvin.
可以使温度低于5800开尔文。
Magnetic fields can do the job in two ways.
磁场通过两种方式来完成这一过程。
One is this magnetic reconnection thing.
一种是通过磁重联:
When magnetic loops extending from the surface
从太阳表面扩展的磁力环
break and reconnect into different forms,
断裂并以不同形式重联,
they can dump huge amounts of energy into the plasma of the corona.
将会释放大量能量到日冕的等离子体中。
Another possible mechanism is through turbulence
另一种可能的机制是通过
in waves generated by the rapid motion of magnetic fields.
磁场快速运动产生的波动扰动。
Francios Lacombe drops some knowledge
Francios Lacombe写下了一些关于
on the 1859 Carrington event.
1859年太阳风暴事件的知识。
In his words, the Carrington event was actually a pair
他认为太阳风暴事件实际上是两次
of coronal mass ejections,
日冕物质喷射。
a lesser one that reached the earth on August 29, 1859,
其中较小的一次喷射在1859年8月29日到达地球,
and caused widespread auroral activity,
造成了毁灭性打击。
and the big one that occurred
较大的一次发生在
on the sun on September 1, 1859,
1859年9月1日,
and reached the earth 17.6 hours later.
经过17.6小时后到达地球。
It’s been speculated that the first coronal mass ejection cleared the way
可以推测出第一次日冕物质喷射扫除了障碍,
to allow the second one to travel so quickly.
使得第二次喷射的物质飞速传输。
CMEs usually take several days to cross
日冕物质喷射通常需要数天时间来
the distance between Earth and the sun,
越过地球和太阳之间的距离,
and making its effect even more powerful
使这种影响
than if it had happened alone.
比单独发生的喷射更强烈。
Well, thanks, Francois.
感谢Francois科普。
I didn’t know that.
在这之前,我并不了解这些知识。
Master Therion expressed his enjoyment
Therion表示他很喜欢
at being mentioned in the comments.
在评论中被提到内容。
Sebastian Elytron cautioned him
Sebastian Elytron告诫他
to enjoy this one,
享受这一次吧。
the second time it feels nowhere near as good, speaking from experience.
依据经验,第二次感觉可没这么好。
Well, master Therion, this is your second.
Therion,这是你的第二次了。
How much worse was this?
有很糟糕吗?
And what about you, Sebastian, was number three worse still?
Sebastian,你怎么看,第三次被提到会更糟吗?
Let us know in the comments and we’ll see
让我们在评论中看结果吧。
how boring these shout outs can get.
看评论好无聊。

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视频概述

量子电动力学与经典电动力学差异颇大,关于量子电动力学的实验研究中通过测量朗德因子(简称g因子)实验证实量子电动力学

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收集自网络

翻译译者

马达

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视频来源

https://www.youtube.com/watch?v=7UwigY4SjKY

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