# String theory - Brian Greene

### Summary

TLDR这个视频讲述了德国数学家卡卢扎在1919年提出的大胆假设，即我们的宇宙可能存在超过三个维度的空间。这一理论对20世纪的物理学产生了重大影响，并继续影响着前沿研究。视频还探讨了爱因斯坦如何通过广义相对论解释重力，并尝试寻找一个统一理论来描述自然界的所有力量。卡卢扎和克莱因的理论认为，额外的维度可能是微小且卷曲的，以至于我们无法观察到它们。弦理论的兴起为这一理论提供了新的支持，并且可能在未来几年内通过大型强子对撞机的实验得到验证。

### Takeaways

- 🌌 1919年，德国数学家卡卢扎提出了一个大胆的想法，即我们的宇宙可能存在超过三个维度的空间。
- 🔍 爱因斯坦在1907年提出了广义相对论，解释了引力是由空间本身的弯曲造成的。
- 🌐 卡卢扎的额外维度理论在1926年由奥斯卡·克莱因进一步发展，提出了微小的卷曲维度可能存在。
- 🎻 超弦理论是一种尝试统一所有物理定律的新方法，它认为宇宙中的基本构成是由振动的弦组成。
- 📈 弦理论的数学模型只有在存在十个空间维度和一个时间维度的宇宙中才有效。
- 🔬 额外的维度可能对于解释宇宙中约20个基本常数的特定值具有关键作用。
- 🌀 额外维度的复杂几何结构可能影响弦的振动模式，从而决定了宇宙中粒子的性质。
- 💥 在CERN的大型强子对撞机上进行的实验可能在接下来的几年内直接测试额外维度的存在。
- 🔋 如果在高能碰撞后测量到的能量少于碰撞前的能量，这可能是额外维度存在的证据。
- 🌟 这些理论的发展和实验验证是人类对宇宙理解的一次重大飞跃，可能会揭示宇宙更深层次的秘密。
- 🚀 从牛顿的绝对空间到爱因斯坦的弯曲空间，再到超弦理论的多维宇宙，我们对宇宙的认识正不断深化。

### Q & A

### Theodor Kaluza 提出了一个关于宇宙维度的什么大胆假设？

-Theodor Kaluza 提出了我们的宇宙可能不仅仅包含我们所知的三个维度，而是存在额外的空间维度，这些维度由于某种原因我们目前还看不到。

### 爱因斯坦是如何描述引力的？

-爱因斯坦描述引力是通过空间本身作为传输介质。他提出，如果没有物质存在，空间是平坦的。但是，当有物质存在时，比如太阳，它会导致空间结构发生弯曲，从而传递引力。

### 卡尔·施瓦西是如何尝试将电磁力与爱因斯坦的引力理论联系起来的？

-卡尔·施瓦西尝试用类似描述引力的方式，即通过空间的弯曲和扭曲来描述电磁力。他想象如果存在第四个空间维度，那么电磁力就可以在这个额外的维度中以弯曲和扭曲的形式存在。

### 奥斯卡·克莱因是如何解释额外维度的不可见性的？

-奥斯卡·克莱因提出维度可能有两种类型：大到容易看到的维度和小到卷曲起来的维度。卷曲起来的维度可能非常小，即使它们就在我们周围，我们也看不见它们。

### 超弦理论是如何复活了关于额外维度的概念的？

-超弦理论是一种尝试回答世界基本构成要素的问题的理论。它提出所有物质和自然力都可以归结为振动的弦。当研究超弦理论的数学时，发现它只在存在十个空间维度和一个时间维度的宇宙中才有效，这复活了卡尔扎和克莱因关于额外维度的概念。

### 额外维度可能对理解宇宙的基本参数有什么帮助？

-许多科学家相信额外维度可能掌握着解释宇宙基本参数的钥匙。这些参数包括粒子的质量、重力的强度、电磁力的强度等，它们精确地决定了宇宙的结构和存在。

### 大型强子对撞机（LHC）如何帮助我们测试额外维度的存在？

-大型强子对撞机通过高能粒子碰撞实验，尝试将粒子从我们的维度推向额外维度。如果在碰撞后测量到的能量比碰撞前少，这将是额外维度存在的迹象。

### 为什么说额外维度的发现对我们理解宇宙至关重要？

-如果额外维度的发现得到证实，这将是对我们理解宇宙的一个重大突破。它不仅会验证超弦理论，还可能解释宇宙的基本参数为何具有当前的值，这些参数决定了宇宙的结构和我们的存在。

### 卡尔·施瓦西的额外维度理论最初遇到了哪些问题？

-卡尔·施瓦西的额外维度理论最初在细节上并不完全符合实际物理现象，例如，它无法正确解释电子的质量。这些问题导致这一理论在20世纪中叶被搁置。

### 超弦理论中额外维度的几何结构是什么样的？

-超弦理论中的额外维度具有非常丰富的、交织在一起的几何结构。这些维度以复杂的方式卷曲并相互缠绕，形成了一个微观层面上的复杂景观。

### 弦理论是如何统一物质粒子和力的粒子的？

-弦理论提出所有物质粒子和力的粒子都是由振动的弦构成的。不同的振动模式产生不同的粒子，这些粒子负责构成我们周围的世界，从而实现了物质和自然力的统一。

### 未来几年内，科学家们如何可能验证额外维度的存在？

-科学家们计划在未来几年内通过大型强子对撞机（LHC）进行高能粒子碰撞实验，通过观察碰撞后能量的变化来验证额外维度的存在。如果实验结果显示能量减少，这将是额外维度存在的第一个直接证据。

### Outlines

### 🌌 卡尔萨斯的多维宇宙理论

这段视频脚本介绍了德国数学家卡尔萨斯在1919年提出的大胆而奇特的理论，即我们的宇宙可能不仅仅包含我们熟知的三个空间维度。卡尔萨斯提出，除了左右、前后和上下之外，可能还存在我们尚未观察到的其他空间维度。尽管这个想法当时看似离奇，但它对20世纪的物理学产生了重大影响，并继续影响着当今的前沿研究。视频还提到了爱因斯坦在1907年对重力的探索，以及他如何发现重力是通过空间本身传递的。卡尔萨斯的想法启发了后续的实验，这些实验可能在未来几年内揭示其正确与否。

### 🔍 额外维度的探索与理论

视频的第二段深入探讨了卡尔萨斯理论的后续发展。卡尔萨斯试图用额外的空间维度来描述电磁力，他设想如果存在第四个空间维度，电磁力就可以用这个维度中的弯曲和扭曲来解释。当他将这个想法写成数学方程时，意外地发现了一个描述电磁力的方程，这让他非常兴奋。然而，这个理论在细节上存在问题，例如无法正确解释电子的质量。直到20世纪中叶，这个理论才被重新审视，并在超弦理论中得到了新的生命。超弦理论试图回答一个基本问题：构成我们周围一切的基本、不可分割的组成部分是什么？

### 🎻 弦理论与基本粒子的构成

第三段视频脚本详细阐述了弦理论的基本概念，即所有物质和自然界的力都可以归结为振动的弦。在这个理论中，深入到物质的微观层面，我们会发现传统粒子物理学的终点——夸克和轻子内部，存在着类似能量弦的结构。这些弦的不同振动模式对应着不同的基本粒子，从而构成了我们周围的世界。这个理论的关键在于，它提供了一个统一的框架，将物质粒子和力的粒子统一为同一实体的不同表现形式。

### 🔬 测试额外维度的可能性

视频的最后一部分讨论了如何可能直接测试额外维度的存在。首先，它解释了额外维度可能对宇宙的基本参数产生影响，这些参数包括粒子的质量、重力和电磁力的强度等。弦理论认为，这些参数的特定值可能与额外维度的几何形状有关。此外，视频还提到了在欧洲核子研究中心（CERN）的大型强子对撞机（LHC）上进行的实验，这些实验可能在未来几年内为我们提供额外维度存在的直接证据。如果实验观察到粒子碰撞后能量的减少，并且这种减少符合特定模式，那么这将是额外维度存在的有力证据。

### Mindmap

### Keywords

### 💡Theodor Kaluza

### 💡额外维度

### 💡爱因斯坦

### 💡广义相对论

### 💡电磁力

### 💡超弦理论

### 💡维度的几何结构

### 💡粒子物理学

### 💡大型强子对撞机

### 💡宇宙的精细调谐

### Highlights

1919年，一位鲜为人知的德国数学家Theodor Kaluza提出了一个大胆而奇特的想法，即我们的宇宙可能实际上拥有超过三个维度的空间。

除了我们熟知的左右、前后、上下三个空间维度外，Kaluza提出可能存在我们尚未看到的空间维度。

尽管这个想法当时看起来古怪，但它对20世纪的物理学产生了重大影响，并继续影响着许多前沿研究。

爱因斯坦在1907年发现了特殊相对论后，决定开始一个新项目，试图完全理解广泛而深远的重力力量。

牛顿在17世纪晚期提出了重力理论，很好地描述了行星、月球等的运动，但爱因斯坦意识到牛顿遗漏了一些关于重力如何实际工作的内容。

爱因斯坦发现，传递重力的介质是空间本身。当空间中存在物质，如太阳时，它会导致空间结构发生弯曲，从而传递重力。

1919年通过天文观测验证了爱因斯坦关于重力的理论，这使爱因斯坦在世界范围内获得了声誉。

Kaluza试图用额外的空间维度来描述当时已知的另一种力量——电磁力。

Kaluza的理论与爱因斯坦的相对论相结合，意外地得出了一个描述电磁力的新方程。

尽管Kaluza的发现令人兴奋，但实际应用中存在问题，例如无法正确计算电子的质量。

1926年，Oskar Klein提出了维度可能以大的容易看到和微小卷曲的形式存在，微小到我们看不见。

超弦理论是一种尝试回答世界基本构成要素是什么的理论，它提出所有物质和力量都来源于振动的能量弦。

超弦理论只有在一个具有十个空间维度和一维时间的宇宙中才有效。

超弦理论提出，我们宇宙的微观结构是由大量微小的、振动的能量弦构成的。

超弦理论可能解释了为什么宇宙中的某些基本参数具有它们特定的值。

在CERN的瑞士日内瓦，正在建造的大型强子对撞机可能在未来几年内测试额外维度的存在。

通过粒子碰撞实验，科学家们希望能够探测到额外维度的存在，这将是物理学史上的重大突破。

从牛顿的绝对空间到爱因斯坦的弯曲空间，再到超弦理论的多维宇宙，我们对宇宙的理解正在不断深化和扩展。

### Transcripts

in the year 1919 a virtually unknown

German mathematician named Theodor

Kaluza suggested a very bold and in some

ways very bizarre idea he proposed that

our universe might actually have more

than the three dimensions that we are

all aware of that is in addition to

left-right back-forth and up-down gluts

to propose that there might be

additional dimensions of space that for

some reason we don't yet see now when

someone makes a bold and bizarre idea

sometimes that all it is both bizarre

but has nothing to do with the world

around us this particular idea however

although we don't yet know whether it's

right or wrong and at the end I'll

discuss experiments which in the next

few years may tell us whether it's right

or wrong this idea has had a major

impact on physics in the last century

and continues to inform a lot of

cutting-edge research so I'd like to

tell you something about the story of

these extra dimensions so where do we go

to begin need a bit of backstory go to

1907 this is a year when Einstein is

masking the glow of having discovered

the special theory of relativity and

decides to take on a new project to try

to understand fully the grand pervasive

force of gravity and in that moment

there were many people around who

thought that that project had already

been resolved Newton had given the world

the theory of gravity late 1600s that

works well describes the motion of

planets the motion of the moon and so

forth the motion of apocryphal apples

falling from trees hitting people in the

head all of that could be described

using Newton's work but Einstein

realized that Newton had left something

out of the story because even Newton had

written that although he understood how

to calculate the effect of gravity he

had been unable to figure out how it

really works how is it that the Sun 93

million miles away somehow defect

the motion of the earth how does the Sun

reach out across empty inert space and

exert influence and that as a task to a

Jain Stein set himself to figure out how

gravity works and let me show you what

it is that he found so Einstein found

that the medium that transmits gravity

is space itself the idea goes like this

imagine space is a substrate of all

there is Einstein said space is nice and

flat if there's no matter present but if

there is matter in the environment such

as the Sun it causes the fabric of space

to warp to curve and that communicates

the force of gravity even the earth

warps space around it now look at the

moon the moon is kept in orbit according

to these ideas because it rolls along a

valley in the curved environment that

the Sun and the moon and the earth can

all create by virtue of their presence

we go to a full frame view of this the

earth itself is kept in orbit because it

rolls along a valley in the environment

that's curved because of the sun's

presence that is this new idea about how

gravity actually works now this idea was

tested in 1919 through astronomical

observations it really works it

describes the data and this gained

Einstein prominence around the world and

that is what got Colusa thinking he like

Einstein was in search of what we call a

unified theory that's one theory that

might be able to describe all of

nature's forces from one set of ideas

one set of principles one master

equation if you will so Coolidge's said

to himself

Einstein has been able to describe

gravity in terms of warps and curves in

space in fact space and time to be more

precise maybe I can play the same game

with the other known force which was at

that time known as the electromagnetic

force we know of others today but at

that time that was the only other one

people were thinking about you know the

force responsible for electricity and

magnetic attraction and so forth so

Colusa says maybe I can play the same

game and describe electromagnetic force

in terms of warps and curves

that raised a question warps and curves

in what Einstein had already used up

space and time warps and curves to

describe gravity didn't seem to be

anything else to warp a curve so Colusa

said well maybe there are more

dimensions of space

he said if I want to describe one more

force maybe I need one more dimension so

he imagined that the world had four

dimensions of space not three and

imagine that electromagnetism was warps

and curves in that fourth dimension now

here's the thing when he wrote down the

equations describing warps and curves in

the universe with four space dimensions

not three he found the old equations

that einstein had already derived in

three dimensions those were for gravity

but he found one more equation because

of the one more dimension and when he

looked at that equation it was none

other than the equation that scientists

had long known to describe the

electromagnetic force amazing it just

popped out he was so excited by this

realization that he ran around his house

screaming victory that he had found the

unified theory now clearly Colusa was a

man who took theory very seriously he in

fact there's a story that when he wanted

to learn how to swim he read a book a

treatise on swimming then toven to the

ocean this is a man who would risk his

life on theory now but for those of us

who are a little bit more practically

minded to questions immediately arise

from his observation number one if there

are more dimensions of space where are

they we don't seem to see them and

number two does this theory really work

in detail when you try to apply it to

the world around us now the first

question was answered in 1926 by a

fellow named Oskar Klein

he suggested that dimensions might come

in two varieties there might be big easy

to see dimensions but there might also

be tiny curled up dimensions curled up

so small even though they're all around

us that we don't see them let me show

you that one visually so imagine you're

looking at something like a cable

supporting a traffic lights in Manhattan

in Central Park that's it's kind of

irrelevant but the cable looks

one-dimensional from a distant viewpoint

but you and I all know that it does have

some thickness it's very hard to see it

though from far away but if we zoom in

and take their perspective to say a

little ant walking around little ants

are so small that they can access all

the dimensions the long dimension but

also this clockwise counterclockwise

direction and I hope you appreciate this

it took so long to get these ants to do

this but this illustrates the fact that

dimensions can be of two sorts big and

small and the idea is that maybe the big

dimensions around us are the ones that

we can easily see but there might be

additional dimensions curled up sort of

like the circular part of that cable so

small that they have so far remained

invisible let me show you what that

would look like so if we take a look say

at space itself I can only show of

course two dimensions on a screen some

of you guys will fix that one day but

anything that's not flat in the screen

is a new dimension goes smaller smaller

smaller and way down in the microscopic

depths of space itself this is the idea

you could have additional curled up

dimensions here is a little shape of a

circle so small that we don't see them

but if you were a little

ultra-microscopic ant walking around you

can walk in the big dimensions that we

all know about that's like the grid part

but you could also access the tiny

curled up dimension that's so small that

we can't see with the naked eye or even

with any of our most refined equipment

but deeply tucked into the fabric of

space itself the idea is there could be

more dimensions as we see there now

that's an explanation about how the

universe could have more dimensions than

the ones that we see but what about the

second question that I asked does the

theory actually work when you try to

apply it to the real world well it turns

out that Einstein includes and many

others worked on trying to refine this

framework and apply it to the physics of

the universe as was understood at the

time and in detail it didn't work in

detail for instance they couldn't get

the mass of the electron to work out

correctly in this theory so many people

worked on it but by the 40s certainly by

the 50s

this strange but very compelling idea of

how to unify the laws of physics had

gone away until something wonderful

happened in our age in our era a new

approach to unify the laws of physics is

being pursued by physicists such as

myself many others around the world it's

called super string theory as you were

indicating and the wonderful thing is

that super string theory has nothing to

do at first sight with this idea of

extra dimensions but when we study super

string theory we find that it resurrects

the idea in a sparkling new form so let

me just tell you how that goes super

string theory what is it

well it's a theory that tries to answer

the question what are the basic

fundamental indivisible uncuttable

constituents making up everything in the

world around us the idea is like this so

imagine we look at a familiar object

just a candle and a holder and imagine

that we want to figure out what it is

made of so we go on a journey deep

inside the object and examine the

constituents so deep inside we all know

you go sufficiently far down you have

atoms we also all know that atoms are

not the end of the story

they have little electrons that swarm

around a central nucleus with neutrons

and protons even the neutrons and

protons have smaller particles inside of

them known as quarks that is where

conventional ideas stop here is the new

idea of string theory deep inside any of

these particles there is something else

there's something else is this dancing

filament of energy it looks like a

vibrating string that's where the idea

of string theory comes from and just

like the vibrating strings that you just

saw on a cello can vibrate in different

patterns these can also vibrate in

different patterns they don't produce

different musical notes rather they

produce the different particles making

up the world around us so these ideas

are correct this is what the

ultra-microscopic landscape of the

universe looks like it's built up of a

huge number of these little tiny

filaments of vibrating energy vibrating

in different frequencies the different

frequencies produce the different

particles the different particles are

responsible for all the richness

in the world around us and there you see

unification because matter particles

electrons and quarks radiation particles

photons

gravitons are all built up from one

entity so matter and the forces of

nature all are put together under the

rubric of vibrating strings and that's

what we mean by a unified theory and

here is the catch when you study the

mathematics of string there you find

that it doesn't work in a universe that

just has three dimensions of space

it doesn't work in a universe with four

dimensions of space nor 5 nor 6

finally you can study the equations and

show that it works only in a universe

that has ten dimensions of space and one

dimension of time leads us right back to

this idea of Kaluza and Klein that our

world when appropriately described has

more dimensions than the ones that we

see now you might think about that and

say well ok you know if you have extra

dimensions and they're really tightly

curled up yeah perhaps perhaps we won't

see them if they're small enough but you

know if there's a little tiny

civilization of green people walking

around down there you make them small

enough and we won't see them either that

is true one of the other predictions of

string theory no that's not one of the

other predictions of string theory but

it raises the question are we just

trying to hide away these extra

dimensions or do they tell us something

about the world and the remaining time

I'd like to tell you two features of

them first is many of us believe that

these extra dimensions hold the answer

to what perhaps is the deepest question

in theoretical physics theoretical

science and that question is this when

we look around the world as scientists

have done for the last hundred years

there appear to be about 20 numbers that

really describe our universe these are

numbers like the mass of the particles

like electrons and quarks the strength

of gravity the strength of the

electromagnetic force a list of about 20

numbers that have been measured with

incredible precision but nobody has an

explanation for why the numbers have the

particular values that they do now

the string theory often answer not yet

but we believe the answer for why those

numbers have devised they do may rely on

the form of the extra dimensions and the

wonderful thing is if those numbers had

any other values than the known ones the

universe as we know it wouldn't exist

this is a deep question why those

numbers so finely tuned to allow stars

to shine and planets to form when we

recognize that if you fiddle with those

numbers if I had 20 dials up here and

I'll let you come up and fiddle with

those numbers almost any fiddling makes

the universe disappear so can we explain

those twenty numbers and string theory

suggests that those twenty numbers have

to do with the extra dimensions let me

show you how so when we talk about the

extra dimensions in string theory it's

not one extra dimension as in the older

ideas of Kaluza and Klein this is what

string theory says about the extra

dimensions they have a very rich

intertwined geometry this is an example

of something known as a claw be a shape

name isn't all that important but as you

can see the extra dimensions fold in on

themselves and intertwine in a very

interesting shape interesting structure

and the idea is that if this is what the

extra dimensions look like then the

microscopic landscape of our universe

all around us would look like this on

the tiniest of scales when you swing

your hand you'd be moving around these

extra dimensions over and over again but

they're so small that we wouldn't know

it so what is the physical implication

though relevant to those twenty numbers

consider this if you look at the

instrument of French horn notice that

the vibrations of the airstreams are

affected by the shape of the instrument

now in string theory all the numbers are

reflections of the way strings can

vibrate so just as those air streams are

affected by the twists and turns in the

instrument strings themselves will be

affected by the vibrational patterns and

the geometry within which they are

moving so let me bring some strings into

the story and if you watch these little

fellas vibrating around they'll be here

in a second right there notice that the

way they vibrate is affected by the

geometry of the extra dimensions so if

we knew exactly what the extra

dimensions look like we don't yet but if

we

we should be able to calculate the

allowed notes the allowed vibrational

patterns and if we could calculate the

allowed vibrational patterns we should

be able to calculate those 20 numbers

and if the answer that we get from our

calculations agrees with the values of

those numbers that have been determined

through detailed and precise

experimentation this in many ways would

be the first fundamental explanation for

why the structure of the universe is the

way it is now the second issue that I

want to finish up with is how might we

test for these extra dimensions more

directly is this just an interesting

mathematical structure that might be

able to explain some previously

unexplained features of the world or can

we actually test for these extra

dimensions and we think this is I think

very exciting that in the next five

years or so we may be able to test for

the existence of these extra dimensions

here's how it goes in CERN Geneva

Switzerland a machine is being built

called a Large Hadron Collider it's a

machine that will send particles around

a tunnel opposite directions near the

speed of light every so often those

particles will be aimed at each other so

there's a head-on collision the hope is

that if the collision has enough energy

it may act some of the debris from the

collision from our dimensions forcing it

to enter into the other dimensions how

would we know it well we'll measure the

amount of energy after the collision

compare it to the amount of energy

before and if there's less energy after

the collision than before this will be

evidence that the energy has drifted

away and if it drifts away in the right

pattern that we can calculate this will

be evidence that the extra dimensions

are there let me show you that idea

visually so imagine we have a certain

kind of particles called a graviton

that's the kind of debris we expect to

be ejected out if the extra dimensions

are real but here's how the experiment

will go

you take these particles you slam them

together you slam them together and if

we are right some of the energy of that

collision will go into debris that flies

off

into these extra dimensions so this is

the kind of experiment that we will be

looking at in the next five seven to ten

years or so and if this experiment bears

fruit if we see that kind of particle

ejected by noticing that there's less

energy in our dimensions then when we

began this will show that the extra

dimensions are real and to me this is a

really remarkable story and a remarkable

opportunity going back to Newton with

absolute space didn't provide anything

but an arena a stage in which the events

of the universe take place

Einstein comes along and says well space

and time can warp and curve that's what

gravity is and now string theory comes

along and says yes gravity quantum

mechanics electromagnetism all together

in one package but only if the universe

has more dimensions than the ones that

we see and this is an experiment that

may test for them in our lifetime

amazing possibility thank you very much

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