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如何理解黑洞的图像 – 译学馆
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如何理解黑洞的图像

How to Understand the Image of a Black Hole

On Wednesday April 10th 2019
2019年4月10日 星期三
you will probably see the first-ever image of a black hole.
你可能会看到有史以来第一张黑洞照片
That’s when the Event Horizon Telescope will be releasing their results.
那是事件视界望远镜发布成果的时间
And I haven’t seen them yet.
我还没有看过黑洞照片
But I think they’re going to look something like this.
但我想想应该是这样
And I can be relatively confident because well
我还是比较有信心的
It’s gonna look a bit like a fuzzy coffee mug stain.
看着就像咖啡杯上的污渍
But if you are disappointed by this image.
但如果你对这张图像失望了
I think that misses the gravity of the situation.
我想 是因为这张图像忽略了万有引力
From this image we should be able to tell
从图里我们应该能够判断
whether the general theory of relativity accurately predicts
相对论是否准确地预言了
what happens in the strong gravity regime.
在强引力区会发生的事
That is what happens around a black hole.
也就是黑洞周边发生的一切
What I want to do here is
此刻我想要做的是
understand what exactly we are seeing in this image.
理解从这张图像我们到底看到了什么
So here is my mock black hole of science.
这个是我的黑洞模型
And this sphere represents the event horizon.
这个球体代表’视界’
That is the location from which not even light fired radially away
在这个位置 即使是径向射出的光
from the black hole could be detected by an outside observer.
都无法被外部的观察者观测到
All of the world lines end up in the
所有’世界线’都终止在
center of the black hole in the singularity.
黑洞中心的的奇点
Once you’re inside here
一旦你进到里面
there is no coming back, not even for light.
就有去无回 连光也回不来
The radius of the event horizon is known as the Schwarzschild radius.
‘视界’的半径被称为史瓦西半径
Now if we were just to look at a black hole
现在如果我们单单观测黑洞
with nothing around it,
排除周围一切
we would not be able to make an image like this,
就不会得到这张照片
because it would just absorb all electromagnetic radiation that falls on it.
因为黑洞会吸收所有投向它的电磁辐射
But the black hole that they’re looking at
但是被观测的的黑洞
specifically the one in the center
特别是银河系中间的
of our Milky Way galaxy, Sagittarius A Star,
人马座A*
has matter around it in an accretion disk.
在它周围有一个物质组成的吸积盘
In this accretion disk,
在这个吸积盘里
there is dust and gas swirling around here very chaotically.
有无序旋转的尘埃和气体
It’s incredibly hot.
这里非常热
We’re talking to millions of degrees
只在百万尺度上讨论
And it’s going really fast a significant fraction
而它旋转的速度也非常快
of the speed of light
接近光速
and it’s this matter
这也是黑洞
that the black hole feeds off and
主要的物质来源
gets bigger and bigger over time.
让黑洞随时间越来越大
But you’ll notice that the accretion disk
但你会发现吸积盘
does not extend all the way into the event horizon.
并没有一路延伸到’视界’
Why is that?
为什么呢?
Well that’s because there is an inner most stable circular orbit
因为最内侧有最稳定的圆形轨道
and for matter around a non spinning black hole
同时对于不自转黑洞周边的物质而言
that orbit is at three Schwarzschild radiu
该轨道在3倍史瓦西半径处
now, in all likelihood
现在 只有一个可能
the blackhole at the center of our galaxy will be spinning
在银河系中心的黑洞是自转的
but for simplicity I’m just considering the non spinning case.
但为了简化问题 此处仅考虑不自转的情况
You can see my video on spinning black holes
如果你对此感兴趣 可以去看
if you want to find out more about that.
我的自转黑洞视频
So this is the innermost orbit
然后 这是最内侧轨道上的
for matter going around the black hole.
围绕黑洞的物质
If it goes inside this orbit it very quickly goes
如果它们更靠近黑洞的话
into the center of the black hole.
就会落进黑洞的中心
And we never hear from it again.
然后就音讯全无了
but there is something that can orbit closer
但还有一些物质
to the black hole and that
可以在更靠近黑洞的轨道上运行
is light because light has no mass it can
那就是光 因为光没有质量
actually orbit at 1.5 Schwarzschild radii.
它能在1.5倍史瓦西半径处公转
Now here i’m representing it with a ring
在这儿 我用一个圆环来代表它
but really this could be in any orientation
但实际上光子能朝任何方向运动
so it’s a sphere of photon orbits
所以这是一个光子轨道球
And if you were standing there
如果你站在那儿
of course you could never go there.
当然你不可能真站那儿
But if you could, you could look forward
假设你可以的话 你朝前看
and actually see the back of your head.
看到的会是你的后脑勺
Because the photons could go around and complete that orbit.
因为光子会绕着轨道走完整圈
Now the photon sphere is an unstable orbit,
光子球是不稳定轨道
meaning eventually either the photons have to spiral in to the singularity,
这意味着光最终要么被卷进奇点
or spiral out and head off to infinity.
要么螺旋而出投向无尽宇宙
Now the question I want to answer is
现在我要回答的问题是
what does this black quote-on-quote shadow in the image correspond to
这张图片中的“黑影”
in this picture of what’s actually going on around the black hole.
对应于此模型中的什么?
Is it the event horizon?
是’视界’吗
Are we simply looking at this?
我们只能看到它吗?
Or is it the photon sphere?
或者是光子?
Or the inner most stable circular orbit?
还是最内稳定圆形轨道?
Well things are complicated
好吧事情有点复杂
and the reason is this black hole warps space-time around it
因为黑洞周围扭曲了的时空
which changes the path of light ray.
改变了光的路径
So they don’t just go in straight lines
因此 不同于我们的常识
like we normally imagine that they do.
光线并不只沿直线传播
I mean they are going in straight lines.
光确实是沿直线传播
But space-time is curved.
但是时空被扭曲了
So yeah they go in curves.
所以就走曲线了
So the best way to think of this
可能最好办法
is maybe to imagine parallel light rays
可能是想象一束平行光
coming in from the observer
从观察者处射出
and striking this geometry here.
打在这里的模型上
Of course if the parallel light rays cross the event horizon,
当然 如果这束平行光穿过’视界’
we’ll never see them again.
就看不到光了
So they’re gone. That will definitely be a dark region.
它们消失了 只留下一片黑暗
But if a light ray comes in just above the event horizon,
但如果一束光从’视界’上方经过
it too will get bend
它会被弯曲
and end up crossing the event horizon.
并最终穿过视界
It ends up in the black hole.
结束于黑洞之中
Even a light ray coming in the same distance away as the photon sphere
甚至 从据此等距远处而来的光线或是光子
will end up getting warped into the black hole
最终也会向黑洞弯曲
and curving across the event horizon.
穿过’视界’进入黑洞
So in order for you to get a parallel ray
所以如果你想要一束
which does not end up in the black hole
不会落于黑洞的平行光
you actually have to go out 2.6 radii away
那就需要远离黑洞2.6个史瓦西半径
if a light ray comes in 2.6 Schwarzschild radius away,
如果一束光从 2.6倍史瓦西半径处射出
it will just graze the photon sphere at its closest approach
它将紧贴着掠过光子层
and then it will go off to infinity
之后重返宇宙
and so the resulting shadow that we get looks like this.
所以我们看到的最终的黑影是这样的
It is 2.6 times bigger than the event horizon.
它比’视界’大2.6倍
We say what are we really looking at here?
所以我们在这看到的到底是什么呢
What is this shadow?
这个黑影是什么?
Well in the center of it is the event horizon.
它的中心是’视界’
It maps pretty cleanly onto, onto the center of this shadow.
图像中的’视界’的界限十分清晰
But if you think about it, light rays going above or below
但你想想看 上上下下的光线
also end up crossing the event horizon just on the backside.
最终也会以从背面穿过’视界’
So in fact what we get is
事实上我们看到的
the whole back side of the event horizon mapped
是整个视界的背面的图像
onto a ring on this shadow.
在这个阴影上被绘制成环状
So Looking from our one point in space at the black hole.
因此 从我们在空间中的视角观测黑洞
We actually get to see the entirety of the black hole’s event horizon.
我们会实在的看到黑洞整个的’视界’
I mean maybe it’s silly to talk about seeing it
谈论’看到’可能会有点傻
because it’s completely black.
因为它完全是黑的
But that really is where the points would map to on this shadow.
但其实 其中某一点 会弥漫在阴影上
It gets weirder than that.
实际的景象会更加古怪
Because the light can come in and go around the back
因为光进来会在背面环绕
and they get absorbed in the front.
然后他们被吸到前面
You get another image of the entire horizon next to that.
你会得到另一个完整的’视界’图像
And another annular ring and then another one.
就是另一个圆环 然后又是一个
after that and another one after that and you get
然后再一个
basically infinite images of the event horizon
当你靠近阴影的边缘时
as you approach the edge of this shadow.
会得到无穷多的视界图像
So what is the first light that we can see?
所以我们看到的第一束光是什么?
It is those light rays that come in at just such an angle
是那些刚好以某一角度到达
that they graze the photon sphere and
能掠过光球层
then end up at our telescopes.
然后到达我们的望远镜中的光线
And they produce a shadow
然后他们会形成一个
which is 2.6 times the size of the event horizon.
2.6倍’视界’大的阴影
So this is roughly what
如果我们恰好以垂直吸积盘
we’d see if we happen to be
的方向看过去
looking perpendicular to the accretion disk
上述就是粗略的我们会看到的东西
But more likely we will be looking at
但人们更有可能
some sort of random angle to the accretion disk.
以一种随机的角度观测吸积盘
We may be even looking edge-on.
甚至可能会从边缘看过去
And in that case do we see this shadow of the black hole?
这样 我们还能观测到黑洞阴影吗?
You might think that we wouldn’t.
你可能认为不能
But the truth is:
但事实是:
because of the way the black hole warps space-time and bends light rays,
由于黑洞扭曲了时空和光线
We actually see the back of the accretion disk.
我们确实可以观测到吸积盘的背面
The way it works is light rays coming off the accretion disk bend over the top
原理是逃离吸积盘的光 会从其顶部绕过
and end up coming to our telescope.
最终到达我们的望远镜
So what we end up seeing
所以最终我们观测到的
is something that looks like that.
看起来就像是这样的
Similarly light from the bottom of the accretion disk
同样 从吸积盘底部逃离的光
comes underneath gets bent underneath the black hole
从下方弯曲上来
and comes towards us like that.
然后就这样朝向我们运动
And this is where we get an image that looks something
这时我们得到的图像
like the interstellar black hole.
就像是《星际穿越》中的黑洞
It gets even crazier than this
实际可能更疯狂
because light comes off the top of the accretion disk here
因为从吸积盘上方过来的光
can go around the back of the black hole,
会绕着黑洞的背面运动
graze the photon sphere and come at the bottom right here,
掠过光球层 恰好到达底部
producing a very thin ring underneath the shadow.
在阴影下面生成一个很薄的环
Similarly light from underneath the accretion disk in the front
同样 来自底部的光到前面
can go underneath and around the back
会向下运动 绕过后面
and come out over the top
从顶部逃离出来
which is why we see this ring of light here.
所以我们会在这里看到光环
This is what we could see
如果我们距离黑洞很近
if we were very close to the black hole,
就会看到这样的图像
something that looks truly spectacular.
非常壮观
One other really important effect to consider is that
另一个很重要的因素应当考虑在内
the matter in this accretion disk is going very fast,
物质在吸积盘上高速运动
close to the speed of light
接近于光速
and so if it’s coming towards us
所以如果他向我们射过来
it’s gonna look much brighter than than if it’s going away.
会比它离开时更亮
That’s called relativistic beaming or Doppler beaming.
这就是相对论成束或多普勒成束
And so one side of this accretion disk
所以这个吸积盘的一侧
is going to look much brighter than the other.
看起来会比另一侧亮得多
That’s why we’re gonna see a bright spot in our image.
所以在图像中我们会看到一个很亮的点
So hopefully this gives you an idea
希望以上讲解能让你大概了解
of what we’re really looking at
在看黑洞图像时
when we look at an image of a black hole.
我们到底在看些什么
If you have any questions about any of this
如果你对此有任何问题
please leave them in the comments below
请在下方评论区留言
and I will likely be making a video for the launch
后期首张黑洞照片真正发布时
of the first ever image of a black hole.
我可能还会制作一期视频
So I’ll try to answer them then.
届时将尽量回答这些问题
Until then I hope you get as much enjoyment
在那之前 希望你们能够像我一样
out of this as I have.
乐在其中
Because this has truly been my obsession for like for last week.
过去的一周我都痴迷于此
I guess what will be exciting
我猜最令人兴奋的是
is to watch it over time how it changes, right?
观看事物随时间的变化 对吗?
There’s a lot of hope that there are blobs moving around.
很可能会有四处运动的斑点
If you see a blob going round the front
如果你看见一个点在前面
and then it goes around the back
他又会到后面
then you see it in the back image etc.
然后你会在背面图像里看到它
And that’s gonna be kind of cool.
这就比较酷了

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视频概述

世界上第一张黑洞的图像被科学家“拍摄”到了,黑洞的图像为什么会是这个样子的?我们能从这张黑洞的图像中看到什么呢?在视频中,你将会找到问题的答案

听录译者

收集自网络

翻译译者

Vnihc

审核员

审核员1024

视频来源

https://www.youtube.com/watch?v=zUyH3XhpLTo

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