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打破时间对称性的粒子 – 译学馆
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打破时间对称性的粒子

This Particle Breaks Time Symmetry

我们宇宙中大部分过程是具有时间可逆性的
Most processes in our universe are time reversible
即物理过程可沿同一路径向前或向后
in other words the physics works the same way forwards or backwards
这就是你为什么不能分辨出
Which is why you can’t tell
这个视频是正放还是倒放的原因
if I’m playing these videos normally or in reverse
熵是目前公认的唯一例外
People typically point to entropy as the only exception to this rule.
热力学第二定律指出
The second law of thermodynamics
系统的熵值或者是系统的混乱程度
states that the entropy of a system or the amount
总是沿着时间单向增加(警告声)
of disorder always increases with time *buzzer sound*
但是熵值的增加是突现特性
But increasing entropy is an emergent property,
是许许多多微粒运动造成的结果
the result of the motions of many many particles
那么问题来了
Which raises the question:
基本粒子自身能识别时间的方向吗? 或者
Can fundamental particles themselves tell the direction of time? Or,
换句话说
in other words,
是否存在某个粒子级别上的物理过程
is there a physical process on the scale of individual particles that looks different
时间向前或向后时是不一样的?令人惊奇的是 答案是存在的
forwards vs backwards? The answer surprisingly is yes.
粒子物理学中存在三种主要对称:电荷 宇称和时间
In particle physics there are three major symmetries that were always expected to hold: charge, parity, and time.
时间对称 如前所述
Time symmetry, as we’ve already discussed,
是指在时间向前或向后时的相互作用是沿同一路径的
means interactions work the same way forwards or backwards in time
电荷对称指电荷间的交互作用
Charge symmetry means interactions are unaffected
不受电荷交换影响 换句话说
if all the charges are swapped in other words
我们所说的正电荷并不特殊
there is nothing special about what we call positive charge.
自然不会给予优待 它仅仅是与负电荷相反
Nature treats it exactly equal and opposite to negative charge
宇称对称是指物理定律与左右方向无关
and parity symmetry means the laws of physics are indifferent to left- or right-handedness.
为了更好的理解
Now to understand what that means imagine a
假设我们的宇宙中存在一面巨大的镜子
giant mirror were held up to our universe
镜中的z轴方向发生翻转
and in the mirror the z direction Is
我的右手变成了我的左手
reversed and my right hand becomes my left hand
但是物理定律不会变化 我的意思是说
but the laws of physics shouldn’t care I mean
它们在镜中世界起同样的作用
they should work exactly the same way in the mirror world
就像现实世界一样
as they do in the regular one
与左右无关
with no preference
或者换句话说
for left- or right-handedness or to put it another way
你是无法通过试验来证明
there should be no experiment that you could do that would tell you
自己是否处于镜中世界的
whether or not you are in the mirror world
这些对称按照首字母
Each of these symmetries is known
缩写为CPT
by its initials- C, P, and T
20世纪50年代
in the 1950s it was thought
人们认为所有的基本粒子都遵循这些对称性
that all fundamental particles obey these symmetries
但是在1956年李振道和杨振宁的
But then in 1956 a paper written
一篇论文中指出
by Li and Yang pointed out that parity symmetry
宇称对称尚未在弱作用力情况下试验证明过
had actually never been tested in experiments involving the weak force
所以那一年的圣诞 哥伦比亚大学
So at Christmas break that year physics professor Chien Shang Wu
物理学教授吴健雄原来的
of Columbia University had planned
和她的同为物理学家的丈夫
to go on vacation
去度假的计划被打破了
with her husband who is also a physicist
取而代之的是 她被弱相互作用可能打破宇称对称这件事激起兴趣
but instead, so intrigued by the possibility that the weak force might violate parity,
而决定留下来
she decided to stay behind
做第一个测试该现象的人
And be the first person to test it.
为了实现它 她和一些低温领域的科学家
To do this she and a team of
将一批钴60原子核冷却至
Low-temperature scientists cooled a collection
仅比绝对零度高三千分之一度的温度
of cobalt-60 atoms to just three thousandths of a degree above absolute zero
然后施加强磁场
Then they applied a strong magnetic field to
将原子核排成列并且沿同一方向旋转
align all the nuclei with their spin pointing in the same direction
这样钴60就具有了放射性
Now cobalt 60 is radioactive and it decays
会通过衰变释放出β粒子
via the weak nuclear force releasing a beta particle
β粒子只是一个电子
Which is just an electron and what the
而且经过试验测得它的出射方向
experiment measured was the direction in which these electrons were emitted
和钴60原子核的自旋方向有关
Relative to the spin of the cobalt 60 nuclei.
想要知道在弱相互作用中的宇恒对称是怎样的的话
To see how this would work under parity symmetry
让我们先想象一下在镜中可能的出现的情况
Let’s consider the mirror image version of this experiment
镜中z轴翻转
in the mirror the direction of the z axis is flipped
但是
But the direction
原子核旋向不变
of nuclear spin is not that’s because an object
因为顺时针旋转的物体在镜中也是顺时针
That’s rotating clockwise is still rotating clockwise in the mirror
所以这就意味着
So this means that the spins
本来的原子核与镜中原子核的旋向一致
of the regular and mirror nuclei are aligned the mirror experiment is actually the same
镜像试验结果也确实这样 现在当一个
Experiment as the original now when a cobalt
钴60原子核衰变并释放一个电子
60 nucleus decays and emits an electron that electron
这个电子可向任何方向 如向左或向右运动
Could go say to the left or to the right
若宇称对称成立
Now if parity symmetry is respected the electrons
电子向任何方向的运动都是等概率的
should be equally likely to go in any direction
这样本来正常情况下实验的结果与镜中实验的结果应该是相同的
That way both the normal and mirror image experiments would
然而如果电子向某一方向发射
give the same results however if the electrons were emitted in one direction
比如优先向z轴正向(发射)
preferentially say in the positive z direction
那么
well
在镜像中的
Then in the mirror experiment the
电子也会优先向
electrons would also have to fly off
z轴正向发射 而这
preferentially in the mirror positive Z direction which
和本来的z轴是相反的
is opposite to the original positive z direction
所以 现实中的电子逆旋向发射
So in the normal experiment the electrons would be emitted say
但是镜中的电子则会
opposite the direction of nuclear spin, but in the mirror version
顺旋向发射
They’d be emitted in the same direction as
这有什么意义
nuclear spin, so how does this make any
这能让你发现
sense it would allow you to determine?
自己是否身居镜中吗
Whether you’re in the mirror world or not
这就像《盗梦空间》中的陀螺
It’s like the spinning top from Inception
如果这个钴60发射的电子向
if the electrons from the cobalt 60 nuclei
这个方向移动 那你就知道你是在镜中
Go one way, you’re in mirror world and
而如果它们向另一个方向移动
if they go the other way
那你就知道你是在现实世界中了 这看起来真疯狂啊
You know you’re in normal world now crazy as this may seem.
这就是吴教授所观测到的:
This is exactly what professor Wu saw
电子会优先向一个方向发射
Electrons were emitted preferentially
而且偏好十分明显
in one direction and not just by a little bit
它们明显地向着原子核自旋的反方向发射
They were predominantly emitted opposite nuclear spin,
所以弱作用力不仅弱化了宇称守恒
so not only does the weak force violate parity
它还尽它所能地
It comes close to violating parity as much
打破了这一守恒
as is physically possible
这打破了理论物理学
This destroyed a basic assumption
(相信了)几十年的一个基本定理
of theoretical physics that had been around for decades
宇宙以着某种方法
Somehow the universe cares
区分着左右手边(镜内外)的世界 当吴教授宣布她的实验结果时
about left or right-handed nests when wu announced her results
整个物理界都震惊了 听闻这个实验
They shocked the physics world
著名物理学家 诺贝尔奖得主
After been told of the experiment famous physicist and Nobel laureate
沃尔夫冈·泡利说“一派胡言”
Wolfgang Pauli said”That’s total nonsense,”
并坚持认为实验结果有误 但是当实验再一次进行并获得同样结果时
and insisted the result was mistaken when the experiment was independently replicated
理论物理学家们不得不承认
Theoretical physicists had to accept that the universe we
宇宙与他们原先所想的不同
live in is not the one they had imagined
1957年 打破宇称守恒的这个发现
the Nobel Prize was actually awarded for
被授予了诺贝尔奖
The discovery of parity violation
并在同年予以出版
in 1957 the very same year these results were published
这需要物理学家们思想上有极大的改变
This required a profound shift in thinking for physicists,
但是在他们抛掉所有曾经所认知的东西重新开始寻找之前
but before throwing everything out and starting again from scratch
他们想到了一个折中
They formulated a workaround
宇称守恒定律会在弱作用力下被打破这事实是可以接受的
Maybe it was okay that the weak force broke parity
因为它并不是宇宙自身的
because it’s not a real
一个真定律 而只是
symmetry of the universe itself just part
一个更强的电荷宇称对称定律的一部分
of a larger symmetry charge parity or CP symmetry
这个观点是指 如果镜中不仅翻转了坐标轴
The idea was if the mirror flipped not only these axes
还将微粒的
But it also swapped the particles
电荷的极性翻转
for anti particles with their charges reversed
那么
well
这个对称定律将会被修复 而且这条定律的
Then the symmetry would be restored and the mathematics
计算公式将同样适用
behind our laws of physics would still work now
这给了物理学家们极大的安慰 直到
this gave physicists some comfort until in
1964年
1964 it was found
发现一些微粒也能打破电荷宇称的综合对称性
that some particles can also violate the combined charge parity symmetry and
然后另一个诺贝尔奖产生了 现在
Boom you got yourself another Nobel Prize now two rules
两个当初被物理学家认为是自然定律的定律被打破了 因此
which physicists once thought were fundamental laws of nature were broken so
他们退守最后一套理论:
They retreated behind their last set
CPT的组合对称性(T是指时间)
of theoretical defenses the combined symmetry of CP T where T is time Sure,
他们说弱作用力虽然会打破宇称和电荷宇称对称性
they said the weak force violates parity and charge Parity,
但是他们坚信时间和电荷宇称结合的对称性是不会被打破的
but certainly not charge parity and time
直到今天 物理学家们都非常确定
together and to this day physicists are still pretty
CPT对称性是整个宇宙
sure that CP T is a real
都适用的 迄今为止也没有
Symmetry of the universe so far no experiment
实验能够打破CPT对称性的 实际上 如果CPT的
has found a violation of CPT in fact if CP T is
对称性真的被打破了 我们将改写上个世纪大部分的发现
Violated we would have to rewrite a lot of the last century’s work
因为它就意味着狭义相对论和量子场论
because it would mean that special relativity and quantum
都是错误的 好的
Field theory are both wrong ok
那么让我们说说CPT
So let’s say that CP T
它是正确的 想想它的含义
Is a true symmetry think about the implications
如果我们知道CP对称性是可以被打破的
of this if we know that CP can be violated?
而CPT对称性则不行 那么时间对称性也应该是可以被打破的
But CP t cannot, then time symmetry must also be broken
否则这三者(电荷 宇称 时间)结合的对称性是没有办法
otherwise there would be no way for the combined three-way symmetry to be maintained
在其中的两种都被打破了的情况下维持的
while two of the sub symmetries are broken and
实际上科学家们也确实通过实验
Physicists have actually conducted Experiments that
证实了有微粒能够打破时间对称性
confirm that certain particles directly break time symmetry
例如有一对夸克因受强作用力
For example when a pair of quarks are held together
而被束缚在一起 它们有两种可能的
by the strong force there are sometimes two different possible
状态 而它们可以通过弱作用力
Arrangements and they can switch back and forth
在两种状态间来回转换
between these two arrangements via the weak force
但从其中一个状态转换到另一个状态所花费的时间比转换回来所耗时间更长
But switching in one direction takes longer than switching back
所以如果你能给
So if you could make a recording
它们转换的情景进行录像
of this event it would look different
那么你会发现正放和倒放录像带
If you played the recording forwards than if
所看到的结果是不一样的 这就是真正的
you played it backwards and that’s exactly
打破时间对称性
what it means to break time symmetry
在某些情况下 基本粒子可以区分
In certain cases then fundamental particles can tell the difference
时间的向前与向后
between going forwards and backwards in time
热力学第二定律并不是唯一一个
The second law of thermodynamics is not the only physical process
在时间上会有偏好方向的物理过程
that prefers one direction in time
所以这是我们只能感知到时间往一个方向
Now is this the origin of our perception that time
流逝的原因 还是
only goes one way, or is it the reason
宇宙具有时间流向的原因
for the universe’s arrow of time?
事实是我们仍然搞不清楚
The truth is that we still have no
为什么时间只能向一个方向前进
idea why time only goes in one direction
有趣的是物理学家们曾经都怀疑
What’s interesting is that physicists once thought
宇称 电荷和时间的对称性真的是能被打破的吗?
that parity charge and time were these symmetries that were unbreakable?
但随着时间的流逝 这几个对称性都被实验打破了
But over time each of these symmetries was demonstrably violated
所以最终的CPT对称性将是不可打破的还是
So is the ultimate symmetry CPT also unbreakable or will
会随着量子场论和狭义相对论研究的深入而被打破?
it fall taking quantum field theory and special relativity with it?
这只是我们探索宇宙过程中
These are just some of the big basic unresolved mysteries left
遗留的未解之谜的一部分
in our quest to understand the universe
也许某天会有另一位物理学家
Perhaps one day another physicist will
放弃假期而去寻找这个问题的答案
give up their vacation to figure out the answer
本期视频多亏了我朋友
This episode of veritasium was animated and co-written
豪尔赫·卡姆
by my friend jorge
进行润色和修改 他还是
Cham who is also the creator
《博士漫画》的作者 现在他写了一本新书
of PhD comics and now he’s written a book.

Yeah.
这本书的名字叫“我们一无所知”
It’s called we have no idea
它是
It’s a book
我和物理学家丹尼尔·韦茨共同创作的
I wrote with physicist Daniel Weitz in’ and
主要讲述了我们宇宙的一些不了解的地方
it’s about all the things we don’t
里面有大如
know about the universe although the big things like
暗物质和暗能量的问题
dark matter and dark energy
也有小如
But also all the little things
基本粒子是什么的问题 它里面有许多插画 嗯
like what are all those fundamental particles for and it has cartoons in it Yep,
还有一些恶搞的双关语
and lots of bad puns.
这听起来是你的拿手好戏啊
So sounds like it’s up your alley.
你们真应该去看看它
You should check it out
我会把链接以及豪尔赫的其他项目
I’ll put a link to it in the description,
打在介绍里面
and all of jorge’s other projects.
谢谢观看
Thanks

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打破时间对称性的粒子

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视频来源

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