未登录,请登录后再发表信息
最新评论 (0)
播放视频

如何探测额外的维度

How to Detect Extra Dimensions | Space Time

PBS digital studios
PBS数码工作室
Thanks to CuriosityStream for supporting PBS Digital Studios.
感谢CuriosityStream对PBS数码工作室的支持
The hunt for extra dimensions sounds like science fiction.
搜寻额外维度听起来像是科幻小说
Fortunately, with the discovery of gravitational waves,
幸运的是 随着引力波的发现
we’re now living in a science fiction future.
我们进入了科幻的未来世界
So how many dimensions are there?
那么宇宙究竟有多少维度呢?
[MUSIC PLAYING]
[音乐播放中]
We may have mentioned once or twice
也许我们已经提过一两次
that the new era of gravitational wave astronomy
引力波天文学开启新时代
is going to open new windows to the universe
将为宇宙探测提供新的突破口
and unlock many mysteries.
并解开诸多谜团
When does all of that start to happen?
那么这一切将从何时开始呢?
Oh, it has.
好吧 其实已经开始了
We’re already making definitive statements
我们已经对之前无法验证的假说
about hypotheses that were previously untestable.
提出了明确的定论
Today, on “Space Time Journal Club,”
今天在《时空杂志俱乐部》栏目
I want to tell you about one in particular described in a new paper,
我想和大家分享一篇讲述引力波的新论文
“Limits on the Number of Spacetime Dimensions from GW170817,”
《GW170817中的时空维数限制》
by Pardoa, Fishbachb, Holzb, and Spergela.
作者是Pardoa Fishbachb Holzb和Spergela
The key to this breakthrough was the gravitational wave event
突破的关键是2017年8月观测到的引力波事件
observed in August of 2017, GW170817.
即GW170817事件
A pair of neutron stars spiralled together and merged.
两个中子星旋近直至完全并合
These superdense remnants of dead stars
这些死亡恒星的超高密度残留物
churned the fabric of space and time in their death spiral.
在它们的死亡旋转中搅乱了时空的结构
And the LIGO and Virgo gravitational wave observatories
而LIGO和Virgo两大引力探测器
detected the resulting ripples.
探测到其产生的引力波
Unlike merging black holes, which are invisible,
不同于两个黑洞的并合 其过程不可见
merging neutron stars explode spectacularly.
中子星的并合产生了巨大的爆炸
The resulting kilonova is first observed in gravitational waves
让我们首次在引力波中观测到千新星
and then as a gamma ray burst.
爆炸随后发生伽马射线暴
In GW170817, the flash of gamma radiation
在GW170817事件中 伽玛射线暴
arrived 1.7 seconds after the gravitational waves.
比引力波信号晚1.7秒到达探测器
This was followed by a glow across the electromagnetic spectrum
紧接着观测到电磁波信号
and ultimately, with the discovery of the distant galaxy in which the explosion happened.
最终发现了此次爆炸所处的位置 即遥远的星系
Among other things, this optical identification
除了其它方面 此次引力波事件
gave a completely independent measurement
还为测量引力波的传播距离
of the distance traveled by the gravitational waves.
提供了一套独立方法
This allowed us to make some really important conclusions
我们由此得出一些关于
about how gravity travels through space.
引力如何穿越空间的重要结论
In the case of today’s paper, it allows us to measure
依据这篇论文
how many dimensions that space actually has.
我们能够测出宇宙到底有多少维度
Yeah,that’s actually a serious question.
这确实是一个重要的问题
We think of space as three-dimensional.
我们认为空间是三维的
Add one dimension of time to give us 4D space-time,
加上时间维度构成四维时空
which we’ll also refer to as 3-plus-1-dimensional space-time
我们也称之为3+1时空维度
But adding the extra spatial dimensions
但要是加上额外的空间维度
beyond the usual three could actually explain a lot,
就能够解释很多问题
from the difference between gravity and the other forces of nature
例如重力与自然界其它力之间的区别
to the nature of dark energy.
以及暗能量的本质等等
But before we get all hyper-dimensional,
不过在我们讨论多维空间之前
let’s think a bit more about 3 plus 1D space-time
我们先仔细思考一下3+1时空维度
and how gravity, light, and matter behave there.
以及在此维度中引力 光和物质的活动
Imagine a pulse of light traveling from some distant source.
想象一束光从遥远的光源发出
We can think of light rays spreading up evenly
我们认为光线均匀的
over an expanding spherical shell.
从膨胀的球壳表面发出
If we see that pulse,
如果我们看见了那束光
it means our eye or our telescope
意味着我们的眼睛或者望远镜
intercepts some of those light rays.
接收到了那束光线
The brightness of the pulse is determined by how many rays we intercept.
光的强度取决于我们接收的光线数量
So as this shell expands, the light rays become more spread out.
因此 随着“外壳”膨胀 光线也扩散开来
Intensity drops proportional to the surface area of the shell,
光的强度与壳体表面积成反比
which is proportional to the square of its radius,
而表面积与壳体半径的平方成正比
the square of the distance to the source.
所以光线强度与光源距离的平方成反比
This is the famous inverse square law.
这就是著名的平方反比定律
But what if we, instead, lived in 2D space?
要是我们生活在二维空间呢?
Then the same pulse would spread out over an expanding circle,
同样的一束光会像一个圆形一样扩散
not a sphere.
而不是一个球形
It would diminish in intensity proportional
它的强度会按比例减小
to the circumference of that circle,
与圆的周长成反比
and also proportional to the radius —
所以和圆的半径成反比
so the distance to the source,
因此它与到光源的距离成反比
not the distance squared.
而不是与距离的平方成反比
The way pulses fade in brightness
光强度的下降与距离的关系
depends on the number of dimensions,
取决于维度的数量
typically proportional to 1 over the distance
光照强度通常与距离的维度减一次方
to the power of the number of dimensions minus 1.
成反比
So in 4-plus-1-dimensional space-time,
所以在4+1维时空中
brightness should drop off more quickly than in 3D space.
亮度应该比在三维时空中下降的更快
This relationship also applies to the forcefelt in a gravitational field.
这个关系式也适用于引力场产生的力
In our universe,
在我们的宇宙中
gravity appears to diminish according to the inverse square law,
引力的减弱往往遵循平方反比定律
as reflected in Newton’s law of universal gravitation.
如牛顿的万有引力定律
We do see slight deviations in very strong gravitational fields, like close to the sun.
但我们也确实在强引力场中发现了微小偏差 比如在靠近太阳的地方
But even there, Einstein’s general relativity
但就算是在那里 爱因斯坦的广义相对论
describes gravity perfectly with three spatial dimensions.
也完美地用三维理论描述了引力
In general, general relativity in 3 plus 1 space-time
总之 在宇宙的大尺度内
does a great job at describing gravity in the large-scale universe.
3+1维的广义相对论很好地描述了引力
But there are some things about this version of gravity that seem peculiar–
但是在这种描述引力的理论中 也有一些罕见现象
for example, its pathetic strength.
例如 它的力非常微小
While the electromagnetic strong and weak forces are all in the same ballpark in terms of strength,
虽然电磁强弱力在活动范围内强度都一样
gravity is vastly weaker,
但引力大体上要弱一些
10 to the power of 32 times weaker, than even the weak nuclear force.
比弱核力还要弱10至32倍
The only reason we see so much gravity is that
而引力容易被检测到的唯一解释是
its range is infinite–and unlike the nuclear forces,
与核力不同 引力的作用范围没有限制
and it doesn’t cancel out, like the electromagnetic force.
它也不会像电磁力那样互相抵消
This mismatch in strength might be because
这种力的强弱差距可能是因为
gravity is really fundamentally different to the other forces.
引力本质上不同于其它的力
But that idea makes some physicists sad.
但是这一说法令一些物理学家感到遗憾
Many would like to find a”theory of everything”
许多人想找到一个“万物理论”
which merges the forces of nature into the same über force.
即自然界中所有力都适用的法则
That means gravity has to look
这意味着在高能量状态下
just like the other forces at very high energies.
引力必须像其它作用力一样
It needs to be intrinsically strong,
它需要在本质上变强
but then become weakened
但在我们熟知的宇宙
in the low-energy, large-scale regime of the familiar universe.
即低能态 大尺度的宇宙中又变得弱化
One fun way to do that
一个有趣的方法是
is to throw in an extra spatial dimension.
额外增加一个空间维度
If you recall, intensity drops off more quickly the more dimensions you have.
我们之前说过维度越多 强度下降得越快
So you drain gravity into an extra dimension.
因此引力就流入了额外维
But you restrict all the other stuff in the universe–
但是宇宙中的其它事物——
matter, radiation, astronomers–
物质 辐射和天文学家等等
to only three spatial dimensions.
还限制在这个三维空间
All of that stuff will behave relatively normally,
当引力被削弱时
while gravity is weakened.
所有这些东西都会表现得相对正常
Let’s get a little bit more technical.
更专业地说
There are these theoretical objects called branes.
有一种叫做膜的假想型物质
We can think of them as geometrical structures of potentially any number of dimensions
我们可以把它们看作是能存在于任何维度的几何结构
on which the quantum field and their corresponding particles can live
量子场及其相关粒子都可以存在于此结构中
They’re used in string theory,
膜常用于解释弦理论
where they typically have a large number of dimensions.
弦理论认为空间有很多维度
11 is popular.
一般认为有11维度
But in string theory, all but three spatial dimensions of the brane are inaccessible.
但是在弦理论中 只有三维空间膜能被感知
They’re finite and coiled up on themselves, compactified,
其它额外维自我卷曲压缩
allowing us to cram them into three spatial dimensions.
使它们隐匿于三维空间而不能被我们感知
But you can also flip this idea around.
但你也可以反向思考
You can imagine a three-dimensional brane, a 3-brane,
想象一张三维空间膜 一张三维膜
embedded in a space-time with four spatial dimensions,
嵌入到四维空间的时空维度中
where the extra dimension of space is extended rather than compact.
这里第四维空间是展开的而不是压缩的
Most of the stuff in such a universe,
在这样的宇宙中
including all of the fundamental forces besides gravity,
大多数东西包括引力以外的所有基本力
would be restricted to the 3-brane.
都会局限于三维膜内
Tune your theory just right, and you get normal physics
根据维度相应调整你的理论
for matter and radiation in three spatial dimensions–
你就能得出物质和辐射在三维空间的普通物理规律
for example, the usual inverse square law for light.
例如 光的平方反比定律
On some spatial scales, you even get the inverse square law for gravity.
在某些空间尺度上 你甚至能得出引力的平方反比定律
But on other spatial scales, gravity can behave very differently.
但在其它空间尺度上 引力可能表现得大不一样
If gravity spreads out in four dimensions rather than three,
如果引力在四维展开而不是在三维展开
then it should become much weaker.
那么它应该变得更弱
This can be used to explain the general weakness
这个观点可以用来解释除了微观层面之外
of the gravitational force on all but the tiniest scales.
普遍情况下万有引力较弱的特性
It can also be used to explain another mysterious phenomenon, dark energy.
也可以用来解释另一神秘现象 暗能量
This is something we’ve gone into in great depth.
这是我们已经深入探究过的东西
But in short, the expansion of the universe seems to be accelerating.
总而言之 宇宙似乎在加速膨胀
This is usually thought of as coming from the action of the energy of the vacuum.
大家普遍认为这是真空能量的作用结果
But there’s another way to get this type of acceleration.
但也有另外一种可能会导致这种加速
In our hypothetical universe with four spatial dimensions,
在我们假想的四维空间宇宙中
gravity is already weak on the scale of the solar system and the galaxy.
太阳系和银河系的引力已经大大减弱
But it can become even weaker on larger scales,
但是在更大尺度上它们会变得更弱
depending on how you tweak the theory.
这取决于你如何理解这个假说
Gravity can obey an inverse square law on galactic scales,
在星系尺度上 引力遵循平方反比定律
where it’s sort of coupled to the three spatial dimensions of the 3-brane.
就像被耦合进三维空间的三维膜上
But it starts to obey the inverse cubed law on much larger scales.
但是在更大尺度上 引力开始遵循立方反比定律
In fact, the 3-brane itself, which defines the three-dimensional structure
事实上 定义了三维结构的三维膜本身
on which our observable universe exists,
就可以扩展出第四维空间
can actually expand into the extra fourth spatial dimension.
而我们可观测的宇宙就存在于这个三维膜中
To us, that would look like an accelerating expansion of the universe.
对我们来说 看上去就像宇宙在加速膨胀
It would look like dark energy.
看起来就像是暗能量在起作用
So how do you even test a wild idea like this?
那么像这样疯狂的想法要怎么验证呢?
Well, here’s where we finally get back to our gravitational waves.
这里我们就要回到引力波的问题上了
If the gravitational field can extend into this hypothetical extra spatial dimension,
如果引力场可以扩展到这个假想的额外维中
then gravitational waves should lose energy
那么引力波在空间传播时
to that extra dimension as they travel through space.
一部分能量就会丢失而进入额外维
Here’s where I have to complicate things a tiny bit more.
这里我不得不稍微将事情复杂化一下
But I promise, we’re nearly there.
不过我保证 我们快接近答案了
Wild light and the force of gravity appear to obey the inverse square law.
光和引力似乎都符合平方反比定律
In regular 3D space,
在常规三维空间
gravitational waves drop in intensity
引力波强度下降与距离成正比
proportional to just distance, not distance squared.
而不是距离的平方成正比
But it’s the same general trend.
但总体趋势是一样的
If space has four or more dimensions,
如果空间有四个或更多维度
then gravitational waves should drop off in intensity
那么引力波的强度就会下降得更快
faster than you’d expect in three dimensions.
比在三维空间中快得多
So that gives us a simple test.
这就给我们提供了一个简单的验证方法
Just observe a gravitational wave,
只需要观测引力波
and figure out how much its intensity dropped off over the distance traveled.
弄清楚它随着传播距离下降的强度
Does that match what you expect in a universe with three spatial dimensions?
判断这和你预计的在三维宇宙中下降的强度是否相符
If the dropping intensity was too much,
如果引力波强度下降得太多
then you have evidence for extra dimensions–
那就证明了额外维的存在
basic stuff, right?
很简单 是吧?
All you need is a billion-dollar network of gravitational wave detectors
你只需要有一个价值十亿美元的引力波检测器网络
and a way to independently measure the distance the wave traveled.
和一种能独立测量引力波传播距离的方法
Fortunately, we have both.
幸运的是 这两样我们都有
We have LIGO and Virgo.
我们有LIGO和Virgo检测器
And now we also have GW170817.
现在我们又有了GW170817引力波事件
The electromagnetic signal from these merging exploding neutron stars
这些来自中子星并合爆炸产生的电磁信号
allowed us to measure its distance completely independently to the gravitational wave signal,
让我们能够独立测量引力波的传播距离
something that’s impossible with black hole mergers.
这在黑洞并合中是无法测量的
One other important factor here —
还有另外一个重要因素——
in order to determine how much intensity was lost by the gravitational wave,
为了测量引力波在传播过程中的损失程度
we need to know how intense it was when it started its journey.
我们得知道它开始传播时本身的强度
A super convenient property of gravitational waves
引力波有一个超级便利的特性
is that you can figure this out by looking at other properties of the merger event
就是你可以通过观察 并合过程的其它特性来得知波的强度
— namely, the masses of the merging objects and the frequency of the wave
——顾名思义 就是并合物的质量和引力波的频率
combined with our independent distance measurement.
加上我们独立测量传播距离的方法
OK. So what’s our conclusion?
那么 我们得出什么结论呢?
How many extra dimensions did we discover?
发现了几个额外维呢?
Uh, zero, precisely zero.
0 真的一个也没有
The gravitational wave lost the right amount of intensity
引力波丢失的强度
for a 3-plus-1-dimensional space-time.
刚好符合3+1维时空中丢失的强度
There was no observable leakage of gravity into extra spatial dimensions,
没有观察到引力丢失到额外维的情况
pretty much ruling this out as an explanation for dark energy.
基本上可以排除是暗能量的作用
There still might be compactified extra dimensions.
还是有可能存在压缩的额外维
So string theorists are OK for now.
所以目前弦理论暂时安全
By the way, comparison of the electromagnetic and gravitational wave arrival times
同时 通过比较电磁波和引力波的到达时间
also allowed us to verify that gravity really does travel pretty much exactly at the speed of light.
我们得以验证 引力确实是以光速传播的
This ruled out or constrained various alternative theories to general relativity.
这排除或限制了各种替代广义相对论的其它理论
This sort of null result might sound like the less interesting outcome.
这种无价值的结果听上去可能没那么有趣
I mean, how cool would it have been to discover extra dimensions?
我是说 如果发现了额外维该多么酷啊?
But don’t be disappointed.
但是不要失望
It’s completely mind-blowing
我们能够去验证这种疯狂的想法
that we can even test these crazy ideas.
本身已经足够疯狂
Ruling them out narrows the vast scope of possible theoretical models for our universe,
排除它们也排除了一些可能的宇宙理论模型
bringing us closer and closer to the truth.
让我们越来越接近真理
And apparently, that truth doesn’t include
显然 这个真理排除了
a 3-brane embedded in an extended 4-plus-1-dimensional space-time
三维膜嵌入扩展的4+1维时空的这种理论
Thank you to CuriosityStream for supporting PBS Digital Studios.
感谢CuriosityStream对PBS数码工作室的支持
CuriosityStream is a subscription streaming service
CuriosityStream是一家流媒体订阅服务公司
that offers documentaries and nonfiction titles from a variety of filmmakers,
提供各种制片人拍摄的各种纪录片和非小说电影
including CuriosityStream originals,
包括CuriosityStream原创精品
such as their upcoming three-part series “Age of Big Cats,”
如即将和大家见面的《大猫时代》三部曲
which looks at the unique evolution of big cats,
讲述了大型猫科动物独特的进化史
their innovative hunting techniques,
它们是如何创新捕猎技术
and how they spread across the globe.
以及它们如何繁衍遍布及全世界的
You can learn more at curiositystream.com/spacetime.
访问网站:curiositystream.com/spacetime 还有更多精彩内容
And use the code SPACETIME during the sign-up process.
注册时请使用密码SPACETIME
Last week, we talked about the hardest problem in physics,
上周 我们讨论了物理学中最难的问题
exploring the conflicts between general relativity and quantum theory
即在量子引力理论发展过程中
towards the development of a theory of quantum gravity.
探讨广义相对论与量子理论的冲突
You guys had the best questions.
你们也提出了各种优秀的问题
Devin Faux asks whether gravity is maybe
用户Devin Faux问到:
the exception to the rule that the forces arise from quantizing the fields.
“在力都是场量子化产生的这一规则内 引力是否是个例外?”
Well, totally, it might be.
这是完全有可能的
Theories of quantum gravity go in both directions.
量子引力理论是双向的
So-called theories of everything try to quantize gravity
所谓的万能理论试图通过将引力
by placing it within the same framework as the other forces
与其它力放在同一框架内来量化引力
to show that it arises from the same underlying mechanism.
以表明它也来自于同一理论框架
These are grand unified theories.
这就是“大一统理论”
And string theories are an example.
弦理论就是一个例子
Other theories treat gravity very differently to the other forces.
其它理论则认为引力远不同于其它力
But they still end up with a space-time fabric
但它们最终还是形成了一种时空结构
that is fragmented on its smaller scales.
这种结构在小尺度上是碎片化的
An example is loop quantum gravity.
圈量子引力理论就是一个例子
One thing that it’s hard to do
比较困难的是
is to keep space-time continuous on the smaller scales.
要在小尺度上保证时空的连续性
If space-time is indefinitely divisible,
如果时空并非绝对分离
then you get hopeless conflicts with quantum theory.
那么对量子理论的争论就毫无意义
Something about its structure has to change on those scales.
在此尺度上其结构也会发生改变
But it may not be the same sort of changes you get when you quantize, say, the electromagnetic field.
但这种改变可能与你想的不同 比如量子化电磁场时的改变
Hecatonicosachoron doesn’t understand why renormalization works, conceptually speaking,
用户Hecatonicosachoron说自己 无法理解从概念上讲为何重正化会起作用
and when it is that it doesn’t work.
何时重正化又会失效
OK. So when you use perturbation theory to calculate an interaction in field theories,
好了 当你用微扰理论来计算场理论中的相互作用时
feedback effects give infinite loops of interactions.
你会得到无限循环的相互作用
More crudely, when you try to calculate something with a long series of approximate corrections,
简言之 当你想用一大串近似修正值来计算某个东西
those corrections can create infinities.
这些修正最终会得出无穷大的结果
Renormalization resets the scale to something finite
而通过计算该系统的一个或多个真实物理特性
by measuring one or more actual physical properties of the system.
重正化将结果重设成有限值
This only works if the number of measurements you need to make is finite.
条件是你需要计算的数据个数是有限的
For the simplest attempts at quantum gravity,
而在量子引力中
you need infinite measurements.
即使是最基本的尝试都需要无限多的观测值
So it’s non-renormalizable.
所以它是不可重正化的
Iago Silva and Rubbergnome jumped in to give better answers
Iago Silva和Rubbergnome两位用户针对
and then got into an extended and quite high-level argument
如何正确理解重正化这一概念提出了更好的方法
about the right approach to understanding renormalization.
并就这一问题进行了深层次高水平的争论
Eventually, they took it offline to continue.
最后一直争论到线下
They”got a room,” so to speak.
他们“自行解决”着这个问题
We can only assume that they are figuring it out to this day.
我们假装他们今天已经争论出结果了
Guys, when you’re done, please let us know how it went.
伙计们 你们讨论完记得告诉我们结果啊
Some quickfire answers– John Gibbs–
还有一些直接点的回答——John Gibbs说
yes, if general relativity and quantum mechanics are both right,
是的 如果广义相对论和量子力学都是对的
then we should have Planck-length virtual black holes popping into and out of existence.
那么我们身边应该随时会有 普朗克尺度的黑洞产生和消失
That would be bad.
那可不是什么好事
And we’d notice–hence, the conflict.
而且我们也会注意到 因此自相矛盾了
用户adamdecoder1说
用户adamdecoder1说
the difference between deleting quantum information
删除量子信息和将其从宇宙中移除
and just removing it from the universe, e.g., by dropping it into a black hole
例如将量子信息丢进黑洞 讨论这二者的区别
is an interesting philosophical point.
这倒是个有哲学价值的问题
Probably, they’re very different.
可能它们完全不是一回事
But do some reading on complementarity for more info.
建议你还是去多读一些补充资料吧
Michael Jordan asks: but why male models?
用户Michael Jordan问:为什么是男模特
Are you serious? I just told you that a moment ago.
你开玩笑吧?我刚不是告诉你了吗
Feynstein 100 is sad about not being featured on comments as often as before.
用户Feynstein100说 自己的评论不像以前那样经常上墙 有点不开心
Oh, boo, hoo, hoo.
哦 喔 嚯
You want back, Feynstein? Show me what you got.
Feynstein 你还想上墙吗?拿出点真本事
Show me that old brilliance and wit.
让我见识到你以往的聪明才智吧
And I see Vacuum Diagrams, Gareath Dean or Robert complaining– they just keep at it.
我还看到用户Vacuum Diagrams 用户Gareath Dean和Robert在抱怨 他们一直都抱怨
Whining won’t get you anywhere, except this time.
抱怨是没用的 除了这一次
This time, it really worked the tree.
这次就让你们上墙了 干的漂亮
Brilliance, wit, and/or whining–
聪明 智慧或抱怨
that’s what it takes to make it on”Space Time.”
这是你们上《时空节目》的好办法

发表评论

译制信息
视频概述

用量子理论来解释宇宙中隐藏的维度

听录译者

收集自网络

翻译译者

ZTT

审核员

审核员 EM

视频来源

https://www.youtube.com/watch?v=3HYw6vPR9qU

相关推荐