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弦理论中的弦是什么

What are the Strings in String Theory? | Space Time

你可能听过大众科学对弦理论的介绍
You may have heard the usual pop sci description of string theory.
一些微小 振动的弦
There are these tiny, vibraring strings
是宇宙中所有的力 粒子
That’s where all the forces particles,
包括万有引力出现的原因
including gravity in the entire universe come from
这就更奇怪了
This raises more questions than it answers
为什么是弦
Like why strings?
它们是由什么构成的
What are they made of?
这些额外的空间又是什么鬼
And what’s all this nonsense of extra dimensions?
物理中我们倾向于
In physics we like to reduce our description
将现实中的力学最简化
of the mechanics of reality down to the simplest possible form
我们假设的最基本的力学模型
We expect the most fundamental machinery to have
是有最少的活动部件或自由参数
the fewest possible moving parts or free parameters
所以粒子物理中标准模型其实是不标准的
This is why the standard model of particle physics is considered incomplete.
这个方程式推算的结果非常准确
Its equations predict many things with stunning accuracy
但是我们首先需要
But they first require us to
调试许多旋钮和仪表盘
tune many mathematical knobs and dials
需要用物理测量来固定19个自由参数
we need to use physical measurement to fix 19 free parameters
比如粒子的质量
like the masses of particles and
还有引力这种东西
Then there’s gravity which doesn’t fit
根本不该存在于标准模型中
into the standard model at all
所以肯定有一套更深层次的机制
so surely there exists a deeper set of cogs and wheels
一套理论来使所有现象
A theory that brings all observable phenomena
都能代入到同一个力学框架中
into the same mechanical framework
这会是一个通用理论
That would be a theory of everything
这也是弦理论的使命
and this is the great hope of string theory.
在接下来的剧集中
In the following episodes
我们将探索弦理论中这些纷繁复杂的细节
We’re going to explore the gory details of string theory,
但是今天是弦理论的第一讲
but today it’s string theory 101
为什么会提出这个疯狂的概念
Where did this crazy idea come from?
为什么会是微小振动的弦
I mean why tiny vibrating strings?
而不是其他微小振动的东西
Versus literally any other tiny vibrating anything?
到底什么是弦理论中的弦
What exactly are the strings of string theory first?
首先让我们看看弦理论的起源
Let’s do a quick primer on the origins of string theory.
这个概念产生于六十年代
The idea started in the 60s
人们想了解强子行为方式
with efforts to understand the behavior of hadrons
它是一群夸克 由胶子互相牵引拥有强大的核力
collections of quarks bound by the gluons of the strong nuclear force.
它包括质子 中子 和介子
That includes protons and neutrons as well as mesons,
这种夸克与反夸克的组合体
which are a combination of a quark and an antiquark
介子内这对夸克独特的交互方式
Peculiarities in the interactions between pairs of mesons as well
他们尖角运行轨迹和质量间奇怪的关系
as an odd relationship between their angular momenta and masses
暗示了介子内的夸克是由什么来连接
Suggested that the quarks in mesons are connected by,
你猜到了 是弦
you guessed it, strings
在这种状态下
In this case,
弦是一种向外延伸管状物 拥有强大核力
the strings are stretched out tubes of strong nuclear force
由胶子组成 像抖动的橡皮筋
Vibrating elastic bands made of gluons.
人们花了很多精力用量子论
A lot of work went into figuring out a quantum theory
来解释弦理论中这种强烈的交互方式
for the strong interaction based on the physics of strings
理论成功了一部分
The theory had some success but kind
但是还是卡壳 最终由量子色动力学解决
of got stuck and was ultimately replaced by quantum chromodynamics
这种强力的弦理论卡壳的原因之一是
One of the reasons this strong force version of string theory got stuck
它涵括了不知道的和不需要的
is that it predicted the existence of unexpected and unwanted
胶子运行模式
vibrational modes in the gluon field of these strings.
量子论中的振动模式是什么样的呢
What’s a vibrational mode in a quantum field?
是一个质点
It’s a particle.
其中一种模式是
And one of those modes appeared
个无重量自旋为2的质点
to be a massless spin-2 particle
但是假设无重量自旋为2的质点只有引力子
But the only hypothetical massless spin-2 particle is the graviton,
是引力场推测出的量子质点
the conjectured quantum particle of the gravitational field
如果引力场由量子微粒组成
If the gravitational field is made of quantum particles
可能是但还不确定
Which it might be we really don’t know.
但如果是
But if it is,
那么引力的量子应该
then the quanta of gravity should have an uncanny
和这种微粒极其的相似
resemblance to the type of particle produced
就是这些强子弦中查到的微粒
by this little investigation into hadronic strings
除了
except that there’s no way anything
那种弦中没办法有引力子
like the graviton should appear in that sort of string.
70年代初有一种新想法
This realization came in the early 70s
一个大胆的提议 不要管介子
A bold new proposal emerged forget mesons
如果这种理论的数学方程可以运用到
What if the math of this theory could be used
量子引力理论中去
in a theory of quantum gravity?
事实上如果所有的牵引粒子的力
in fact, what if all force carrying particles result
都是由微小弦振动产生
from oscillations in tiny strings?
那只需把弦细化
All we needed was to make the strings a bit smaller.
小20个数量级
Like 20 orders of magnitude smaller
把规模从质子降到普朗克量级
Shrinking from the size of a proton to the Planck scale
差别大约是
Roughly the scale of the difference
银河系和你家客厅那么大
between the Milky Way galaxy and your living room Oh,
我们还要把4维转变成22维
and we needed to add 22 dimensions to the familiar 4.
问题不大 这就是玻色弦理论
No biggie This was so-called bosonic string theory
如果可行它就是
If it worked it would have been a candidate
力学的通用理论
for a grand unified theory combining all known forces
但是为什么卡了
But why stop there?
如果抖动的弦可以解释玻色子的牵引力
If wiggly strings can explain force carrying bosons,
为什么不能解释组成物质的费密子
Why not also the fermions that comprise matter?
70到80年代 许多人提议
Through the 70s and 80s, several proposals
带入超对称性将费密子和玻色子
Introduced the idea of supersymmetry to bring the fermions and bosons
置于同一个理论框架中去
into the same theoretical framework
由此产生的超弦理论立志解释
The resulting super string theory sought to become an all-encompassing
整个世界潜在的运行机制
mechanism to explain the underlying workings of our entire reality.
一个关于所有的理论
A theory of everything.
另外托这个野心的福 也削减了一些维度
As an added bonus this ambition also shaved off a bunch of dimensions
在费密子加入后只需要十个维度
Only ten were needed once fermions were added.
1995年爱德华·威滕把多种超弦理论统一起来
Then in 1995 Ed Witten brought the many forms of super string theory together
构建出一个M理论
into the single framework of M theory
只在
All for the low price
十一维空间理论上多加了一个维度
of adding only one more spatial dimension for an eleven dimensional theory
历史课讲完了 我们来讲一下弦
Okay enough for the history lesson. Let’s talk strings.
这些抖动的弦可以解释整个宇宙
So wiggly strings could explain the whole universe
这句话很狂
That’s a hell of a claim.
要了解量子弦
To understand quantum strings,
首先我们得了解普通的弦
first we need to look at regular strings
比你想的还要酷
they’re cooler than you think.
关键在于弦可以运载波
The key is that strings can carry waves and
如果弦截断了
If the string has ends or is tied
或者打结了这个波会覆盖原来的轨迹
in a loop then a wave will end up overlapping with itself
这样你就得到一个驻波
In that case you get a standing wave
简单来说如果这些运动的波彼此重叠
Roughly speaking when these travelling waves overlap each
他们会堆叠或者相消
other they can either stack up or cancel out
增益或者损毁性影响
constructive or destructive interference
增益性影响只会在波长为
Constructive interference only happens if the wavelength of the wave fits a neat number
弦的整数倍出现
of times along the length of the string
如果波长刚好与波调谐
Then the phases of the overlapping wave match in the right way
那么这波长/频率的波就增强了
and that wavelength / frequency of the wave is enhanced
其他频率的就消亡了
All other frequencies tend to die out
结果就是弦只接受单一频率
The result is that for a given string only certain frequencies
也就是单一能量
Corresponding to certain energies are possible.
这些共振频率取决于弦长
These resonant frequencies depend on the length of the string Also,
和松紧度
it’s tension which defines wave
松紧度决定波速 频率 和波长
velocity and so relates frequency to wave length
就像
For example, this leads to
吉他单弦上的特定频率
the specific frequencies of vibration on a guitar string
但是这种现象
But this sort of behavior,
只有特定的离散能量模式才有
where only specific discrete energy modes are allowed
听起来很量子学
sounds very quantum like.
这并不是由弦理论家首先发现的
String theorists weren’t the first to notice this
尼尔斯·波尔提出第一个电子运行轨迹量子模型
Niels Bohr came up with the first quantum model for electron orbits
假定他们是氢原子的驻波环
by thinking of them as ring like standing waves around the hydrogen atom,
但是量子弦的野心比“无聊的”电子环大
but quantum strings are much more ambitious than boring electron orbits
如果扭动量正确
The hope is that tweaked just right,
这些离散的振动模式可以用来描述已知微粒
those discrete vibrational modes can be made to match the properties of known particles
质点的重量来源于弦长和它的松紧度
Particle mass just comes from the length of the string and it’s tension
松紧度也只是每个单元长度的能量
Tension is after all just energy per unit length.
弦长决定重量
string length defines mass
也决定会采用哪种振动模式
But also defines which complex vibrational modes are possible
这些模式最终确定质点的属性
possible and those modes in turn define particle properties
比如带电量和自旋
like electric charge and spin
这就是弦理论的目标
So this is the great promise of string theory.
假设一个参数 也就是弦的松紧度
By defining a single parameter the string tension
或同等的弦长的量
Or equivalently string length scale all
所有可能的粒子将会自动标定位置
of the possible particles should be automatically defined
一个参数
Compare that one parameter to the
和十九个自由参数的标准模型比
19 free parameters of the standard model
感觉更接近于基本原理了
It sure sounds closer to a fundamental theory.
好的 概括一下 这些普兰克数量级的
Okay recap, we have these Planck scale
一维结构
One-dimensional structures that can be
可以是圈或线 他们的振动模式可确定粒子属性
in loops or extended they have vibrational modes that define particle properties
顺便提一下 这些振动的驻波
By the way, those vibrations the standing waves
不是一些内部似波的东西
You’re not some abstract internal wave the strings are
这些弦是实体 这些波在现实中摇摆
real physical strands and the waves are wiggles in actual space
但是是什么做的实体线
But physical strands of what?
常规答案包括纯能量团
common answers include pure mass energy
最基本的存在
fundamental irreducible existence
现实中不规则的拓扑或更常见的答案
Topological irregularities in the fabric of reality or the most common answer.
这问题没意义
It’s a meaningless question.
他们是基础所以不是由什么组成
They are fundamental so not made of anything or
或换言之是一种“闭嘴去计算鎓元素”
in other words a material known as shut-up-and-calculate-onium
大多数弦理论学家
Most string theorists are more interested
对弦能做什么更感兴趣
in what strings do not what they’re made of So,
所以他们能做什么呢
what do they do?
振动当然可以储存能量
Well vibrate obviously they can hold energy
可以拉伸 融合 断开
They can stretch they can also merge and split apart
这些属性很重要
These last properties are important because it gives
因为这种机制使由弦理论组成的粒子
a mechanism for the particles of string theory
可以交互和跟其他粒子融合
to interact and to decay into other particles
这张弦靠近连接分开的图
This picture of strings coming together joining and
是弦理论的巨大优势
splitting apart is a huge strength of the theory
它解决了许多量子化引力的影响
It solves one of the main problems with quantizing gravity
你应该记得之前那集
Maybe you remember from our episode
量子引力 如果你想知道
on quantum gravity if you try to describe
更小量级的引力交互
gravitational interactions on the smaller scales,
那么小层次中所需的交互能量可产生黑洞
the energies required to interact on that scale produce black holes
更别提
There’s no way to even think about the shape
普兰克级别引力场的形状了
of the gravitational field on the Planck scale
不会产生无用的矛盾
That doesn’t produce a hopeless conflict
弦理论可以解决这个问题
string theory fixes this
因为引力子是个圈不是有方向的质点
because the graviton is a loop not a point particle.
它均匀的在弦表面活动
It’s interactions are smeared around that string
不受普兰克长度之下
handily avoiding the explosion of mathematical infinities
无限远的点影响
You get below the Planck length
这些听起来不错
All this stuff sounds great
顺便说一下
and by the way
对于一维外其他几何模型没用
doesn’t work for any other geometrical structure other than a 1d string
所以振动的吉他弦 可行 鼓面 不行
So vibrating guitar strings, yes. Drum skins, no. Unfortunately,
但是没这么简单
it’s not going to be this easy Yeah,
弦本身是一维的
the strings themselves are 1d but
但是让这些粒子拥有属性
to even start to produce the properties of known particles
他们需要振动的空间
They need to vibrate in more
多于三维
than just the three dimensions of space
实际上这个理论只作用于
In fact, the theory only works
九维空间 加一个时间维度
in precisely nine spatial dimensions plus one for time
再加一维给M理论
Plus one for M theory,
等等再说这个 总之若非有这些数量的维度
which we’ll come back to in short without exactly this number of dimensions
就不会得到引力子和其他无重量的粒子
You don’t get gravitons or any other massless particle.
接下来的剧集中会解释为什么
We’ll look into why in future episodes
但是很尴尬
But this is awkward to say the least.
这个理论作用于
It’s a theory that works in a universe
显然不是我们这个就只有三维空间的宇宙
That is clearly not our own with its measly three dimensions of space.
但是弦理论家才不管这个
But this sort of thing doesn’t deter string theorists
有办法加上另外的空间
There’s a way to add extra spatial
也能和我们三维空间接轨
dimensions that is still consistent with our perceived 3d universe
形象的说
to get our heads around this
想象一下我们生活在二维平面宇宙
Imagine we lived in a 2d flatland universe.
我只有大X与Y轴
We only perceive the giant x and y directions
但是如果平面不是平面 如果有一个Z轴的话
But what if flatland isn’t truly flat? What if the z-direction?
有一丢丢的宽度
Has a tiny tiny width
这是个吃豆人维度
This is a pac-man dimension.
游动在这么小的维度
Travel the tiny width
一出门就回家了
of this dimension and you’ll find yourself back where you started
非常小的东西像量子弦可以穿梭
Very tiny objects like quantum strings could explore
在那一维中更重要的是可以振动
that extra dimension and importantly oscillate in it
我们这些大笨木头人才发现不了这一维
But we giant lumbering Flatlanders would have no clue it existed
总结一下三大维空间
Okay now scale this up three large dimensions
还有六个小吃豆人维度
of space and six tiny pac-man dimensions
只有弦能存在 好了弦理论可以用了
That only strings experience. Voila string theory is saved
现代M理论提出一个
Modern m-theory proposes an additional large
额外的空间维度 我们三维空间
spatial dimension our universe of 3d space and
和一维时间像一个平面
1d time is like flatland
驻扎在五维物体上 它叫五膜宇宙
on this 5d object called a 5-brane
M理论统一了各种弦理论
m-theory unites different string theories because it demonstrates some
因为它哲学性的解释各种理论对空间想象有趣的对偶性
philosophically fascinating dualities between different ways of thinking about the dimensions Ultimately,
也最终引到终极对偶性
it also leads to the ultimate duality.
也就是全息主义
That is the holographic principle Patience,
耐心点蚱蜢们 等等会谈
grasshoppers. We will get there.
弦的轨迹取决于
The exact behavior of strings depend
他们运动维度的紧密程度
on the shape of their compact dimensions
实际上弦理论中的一个自由参数
In fact the single free parameter
就是多出维度的外形
in string theory becomes the configuration of the extra dimensions
找到正确的位置
Find the right location
在弦平面上然后你就可以完美的描述这个宇宙
in this string landscape and you perfectly describe the universe
唯一的问题是
the only issue is
大概有十个到
that there are an estimated 10 to the power of
五百个可能的选择
500 possible choices and almost no
没办法知道哪个是我们要的
way to figure out which one is ours
现在弦理论看起来是条死胡同
Right now string theory appears to be at an impasse.
它没有确定的轨迹
It has produced no confirmed predictions
一些人说没有可测的轨迹
Some would say it’s made no testable predictions
把弦的领域跟
Tuning that string landscape to match
我们的世界对接很困难而且几乎不可能
our universe is daunting and perhaps impossible
除了死胡同这一点它的前景
Yet despite this impasse its promise and its
和优雅让许多人确定
elegance has convinced many that it must be right
这是对的或者方向是对的
or at least the right path
往后的剧集中我们会深刻探讨
In coming episodes will look deeper into both the successes,
它的成功与失败 还有弦理论奇怪的地方
the failures, and the profound weirdnesses
之后你就能决定你自己能不能接受
of string theory then you can decide for yourself whether you accept the
基本弦的时空属性
fundamental stringy nature of space-time
感谢23andme对PBS工作室和本节目的支持
Thanks to 23andme for supporting PBS Digital Studios and space-time
23andme是个人基因公司
23andme is a personal genetic company created to
帮助大家了解自己DNA秘密
help people understand what their DNA says about them
十月是族谱月
the month of October is Family History Month,
正是一个了解
which is a great time to explore and learn
家人和祖先
more about your own family and ancestry a
还有与其他人联系的好时候
discovery that can lead to new connections with others
到这个网站了解更多
You could learn more by going to 23andme.com/spacetime
上周我们谈到
Last week we talked
我们宇宙的基本计算界限
about the fundamental computational limits of our universe and
顺便说一下要什么才能在黑洞表面
Incidentally what it would take to compute a universe simulation
运行一个宇宙模拟器
on the event horizon of a black hole
看一下你们的评论
Let’s see what you had to say
RomanR说
RomanR asks
计算黑洞边界会经历
Whether computation at an event horizon would experience
巨大的时间差
massive time dilation relative to an outside observer
我们怎么知道计算结果
So how do we see the results of the computation? Yeah,
是个问题
that’s an issue Really?
真的不可能读取到结果
You can’t read off the results
黑洞表面的计算几乎永远不可能
of an event horizon computation until practically ever
我提到一个视频
I mentioned in the video
你可以知道霍金辐射的结果
that you’d read off the result in Hawking radiation
那时最后一颗星
Which would take until long after the last star
都泯灭了才能读取出
in the universe has died to even give you
一小部分
a small fraction of that read out
霍金辐射这种
The slow read
缓慢的读取就跟时间延长一样
out by Hawking radiation is equivalent to the time dilation issue
你知道吗这些黑洞计算机烂透了
You know what these black hole computers suck.
不要弄了
Let’s not build one
你们提出
A few of you pointed out
一个黑洞计算机存不下其他黑洞的信息
that a black hole computer couldn’t store the information about other black holes and
对的
You’re right this is one
这是我们计算中的一个假设
of the assumptions we made in the calculation
我们特大黑洞计算机只够
Our supermassive black hole computer is only large
装辐射跟物质的内容
enough to contain all information in radiation and matter
但是宇宙中大部分内容
But most of the information
是在黑洞中或更确切的说
in the universe is in black holes or more accurately
大部分熵和隐藏的信息在黑洞里
Most entropy or hidden information is in black holes
所以我们黑洞计算机存不下
So our black hole computer can’t contain the information hidden
黑洞里所有信息
in all black holes.
他都记不下黑洞里
It can’t even contain the information from black holes
比他自己体积大的信息
larger than itself
尤瓦尔·尼希米查了一个老视频
Yuval Nehemia went back to an old video
是我跟尼尔·得格拉斯·泰森说
that quotes me saying to Neil deGrasse Tyson
想要模拟宇宙你需要宇宙那么大的计算机
To simulate the universe you need a computer the size of the universe.
直接反驳了
In direct contradiction
我最近说的贝肯斯坦界
with everything I’ve said recently about the bekenstein bound
很难相信
As hard as it is to believe
我说错了
that I have ever said anything wrong before
我好担心尤瓦尔发现了
I’m afraid Yuval has caught me out.
你可以建一个宇宙模拟器比宇宙小
You can build a universe simulator smaller than the universe
好了
That’s it.
你建的宇宙模拟器在这个宇宙有界限
The universe simulator that you’d build inside this universe has limits
不能模拟的很完美
It couldn’t simulate a universe so perfectly
宇宙模拟器内也可以有个一样好用的宇宙模拟器
that the simulated universe could also contain an equally good universe simulator
也不能无线嵌套模拟器
There’s no infinite set of nested simulators
我说过我们的黑洞计算机
Like I said our black hole
只模拟粒子不是黑洞
computer is only simulating particles not black holes
简单的逻辑
It’s like the simple logical
就像我的世界里的排列大门游戏
gate arrays that people build inside Minecraft
模拟器永远都不会比原版本软件好用
Emulators are never as efficient as the original hardware
山姆·吉尔说这视频史上最无聊的视频
Sam Gil tells us this was the most boringest video I’ve ever seen.
我必须说承蒙夸奖
I have to say I’m kind of flattered
你看过油管吗
Have you been on YouTube?
即使PBS工作室都有很多优秀的竞争者
Even PBS Digital Studios has spectacular contenders
还有那个约翰逊什么都不做
What about that one where Johansson does nothing
就舔了十七分钟波板糖的视频
but lick a lollipop for over 17 minutes?
还有瓦娜莎希尔直接标的“最无聊的视频”
Or Vanessa Hill’s literal”The Most Boring Video Ever”?
那个其实还蛮好看
That one’s actually quite interesting

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视频概述

物理学家喜欢无限细分,找出最小的物质。寻求一个理论解决所有问题。这里是弦理论的科普第一讲。

听录译者

收集自网络

翻译译者

刘倩Rachel

审核员

审核员1024

视频来源

https://www.youtube.com/watch?v=k6TWO-ESC6A

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