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如果要你测量宇宙的大小,该怎么做? – 译学馆
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如果要你测量宇宙的大小,该怎么做?

How Do You Measure the Size of the Universe? | Space Time | PBS Digital Studios

[音乐]
[MUSIC PLAYING]
宇宙是巨大的
The universe is huge.
大家都知道
We know this.
但是 它到底有多大以及我们怎样才可能测量
But how huge and how can we possibly measure
它的大小呢
the size of the universe?
这么说吧
Put it like this.
天文学普遍用光年来测量宇宙距离
It’s common to put astronomical distances in light years.
光年是光在一年内传播的距离
That’s the distance light travels in a year or about 9
大概是9万亿千米
trillion kilometers.
知道这个以后 这里有一些头脑风暴的例子
Given that, here’s some facts to blow your mind.
太阳系的直径是8光时
The diameter of the solar system is about 8 light hours
或者说9亿千米
or 9 billion kilometers.
我们所在的星系 银河系
The Milky Way, our galaxy, that’s
大概有十万光年的直径
about 100,000 light years across or close
差不多等于一百万的三次方千米
to 1 quintillion kilometers.
下一个离我们最近的大型星系
And the next closest major galaxy,
仙女座 距离我们2500万光年
Andromeda, that’s 2 and 1/2 million light years away, which
这时还用千米来衡量长度就不太明智了
is just silly to talk about in kilometers.
这些距离都太大了 是吧
All pretty big distances, right?
现在 让我们来谈谈整个宇宙
Now, let’s talk about the whole universe.
我们不能看到宇宙的所有部分
We can’t see all of it.
宇宙中有的东西实在太远了以至于从那发出的光无法
Some things are so far away that light from there
到达我们的星球
hasn’t reached us yet.
所以 我们重点关注可观测范围内的宇宙
So we’re going to focus on the observable universe.
这部分宇宙 原则上
That’s the part that we, in principle,
是我们可以利用引力波和光来观测的
can see with light or gravitational waves.
那么 可观测的部分是多大呢
So how big is that?
好的 这是一个球体 半径大概是
Well, it’s a sphere with a radius of about 46
460亿光年 或者简单的
billion light years, or, rounding down
来说 从这个球体的一端到另一端大约是900亿光年的距离
to keep it simple, about 90 billion light years end to end.
嗯 这距离太疯狂了
Now, that’s crazy big.
但是更加疯狂的是天文学家
But it’s even crazier that astronomers
认为他们知道具体的数字
think they know that number.
他们并不是用一把尺子来测量宇宙的长度
It’s not like they measured the universal with a ruler.
那么 他们是如何知道的呢
So how do they know?
大体来说 首先从宇宙的年龄入手
In a nutshell, you start with the age of the universe.
现在的预测是大概138亿年光年
The current estimate is about 13.8 billion years.
光需要大量的光年时间
That’s the maximum amount of time light
才到达我们的星球
has had to travel to us.
然后你就能算出
And then you work out how far away
那个光的发射点距离我们有多远
the emission point of that light is right now.
你就得出来了可观测宇宙的半径
And that’s the radius of the observable universe.
记住 这个距离并不仅仅是138亿光年
Remember, that distance is not 13.8 billion light years.
真实的距离其实要大的多
It’s much bigger.
想要知道为什么 知道到宇宙自身在不断的膨胀
To understand why, it’s critical to realize that space
当中很关键
itself is expanding.
一个很流行的类比
A popular analogy, that I wish I could take credit for,
宇宙就像是一个在炉子里烘烤的葡萄干面包
is a loaf of raisin bread rising in the oven.
每个葡萄干都代表了面团空间内的一个星系团
Each raisin represents a cluster of galaxies in the dough space.
那么 葡萄干是不会在面团中移动的
Now, the raisins don’t move through the dough.
但是随着面团的烘焙和膨胀
But as the bread bakes and rises,
所有的葡萄干都离彼此越来越远
all the raisins get further apart from each other
因为 面团自身在膨胀
because the dough itself expands.
但是每个葡萄干自身的体积不会发生变化
Now, each raisin stays the same size.
星系团不会膨胀
Galaxy clusters aren’t expanding and neither
单个星系 地球 人类 树木也不会膨胀
are individual galaxies, or the Earth, or people, or trees.
只是各大星系团之间相对的
It’s just the relatively empty space
剩余空间在膨胀
between those large clusters of galaxies.
面团不仅仅在膨胀
And not only does the dough expand,
而且在不同的阶段 它膨胀的速率也不同
it can expand at different rates during different stages
在这个烘烤的阶段
of the baking process.
也许在几十亿年内膨胀很快 然后逐渐变慢
Maybe really fast for a billion years, then slower
在接下来的20亿年中
for the next 2 billion.
这两个因素 空间的膨胀
These two facts, the expansion of space
和膨胀速率变化无常的事实
and the fact that it can expand at a variable rate,
使得对可观测的宇宙
complicate how we measure the size
的测量变得更加困难
of the observable universe.
实际上 空间可以以任意速率膨胀
In fact, space itself can expand at any rate
只要它愿意 甚至会高于光速的速率膨胀
it wants to, even faster than the speed of light.
所以 在宇宙的一生中
So over the lifetime of the universe,
一束光的发射地点
the birthplace of a beam of light
可以被正在膨胀的面团空间带到
can be carried ridiculously far away by the expanding space
无限远处
dough.
为了知道确切的数值并且计算出
To know exactly how far though, and calculate
宇宙的尺寸 你需要
the size of the universe, you need
知道空间膨胀的速率有多快
to know how quickly space has been
在历史中每一个时刻
expanding at every moment in history, ever.
所以 我们怎样才能最大程度地了解这个宇宙
So how can we possibly know the expansion history
的膨胀历史
of the universe?
使用一种叫宇宙红移的工具
Using something called cosmological redshift,
红移就像宇宙膨胀留下的印记
which is like a fingerprint that the expansion of space
留在一束束光线上
leaves on beams of light.
让我解释一下
Let me take a moment to explain.
光的颜色是由波长决定的
Light has a color determined by its wavelength.
波长长的光更红 波长短的光更蓝
Longer wavelength light is redder, shorter bluer.
如果空间没有膨胀 那么
If space were not expanding, then light
从一个遥远的星系传过来的光
from a distant galaxy would be the same color
到达地球后的颜色和它初始的颜色将会是一样的
when it arrived on Earth as it was when it first set out, blue
离开时是蓝色 到达时也是蓝色
on departure, blue on arrival.
但是因为宇宙正在膨胀 光的波长
But because space is expanding, the wavelength of light
当它到达地球时会被拉长 使蓝光变为红光
gets stretched as it travels to us, making the blue light red;
这就是红向移动的意思
hence, the term redshift.
在更多极端的例子
In more extreme cases, the wavelength
波长可以被拉长到不可见光谱中
can be stretched out of the visible spectrum
到微波和无线电波段
altogether, into microwaves or radio waves.
那么 这又引出另外一个重要的问题
Now, here comes another important point,
需要注意的
so pay attention.
来自遥远星系的光
The light from more distant galaxies
会比从紧邻的星系传来的光更容易红移
is redshifted more than light from nearby ones.
你要知道 远处的光
You see, the light from more distant places
穿过的距离更长
has further to go.
所以它要在膨胀的宇宙中花更长的时间
So it spends more time in the expanding space,
在这个逐渐变大的面团中
in the rising dough.
因此 它的波长被拉的更长
And thus, it has its wavelength stretched more.
这样就说得通了
It makes sense.
但是红移和距离
But how are redshift and distance
怎么在数量程度上联系起来
related quantitatively?
这个问题非常重要
That is the million dollar question.
如果我们知道确切的数量关系
If we knew the answer in numerical detail,
我们就可以了解宇宙膨胀的历史
we could figure out the expansion history
从而算出宇宙的大小
and in turn the size of the universe.
那么你能做到这点吗
So can you do this?
你能发现出距离和红移的关系吗
Can you measure the distance-redshift relationship?
不会太困难
No sweat.
只需找一群遥远的星系
Just find a bunch of faraway galaxies,
比仙女座还要远的星系
much further than Andromeda.
测出他们之间的距离和红向移动
Measure their distances and their redshifts.
然后将距离和红移画在一张图上
Then put those distances and redshifts on a graph
找出最佳的曲线
and find the best-fit curve.
现在 你就得到了距离和红移的关系
Voila, you now know the distance-redshift relationship.
基于这个关系 宇宙每个时刻膨胀
And from that, how fast the universe was
的速度有多快
expanding at every moment ever.
一旦你了解了膨胀的历史
Once you have the expansion history,
你怎样断定宇宙的实际大小呢
how do you actually determine the size of the universe?
记住一点 就像我很久之前说的
Remember, as I said a long, long time ago,
首先 我要知道宇宙的年龄
we first need to get the universe’s age.
所以 让我们回到葡萄干面包模型
So let’s go back to the raisin bread.
如果我们将宇宙膨胀的历史反过来看
If we run the movie of the rising dough backwards,
以我们了解到的膨胀速率作为缩小速率
at the rate given to us by the expansion history,
最终葡萄干会彼此重叠在一起
eventually nearby raisins will sit on top of each other.
这就是大爆炸
That is the Big Bang.
要回到这一点会用多久
And how long it takes to get back to this point
会用像宇宙现在年纪那么长的时间
is the current age of the universe.
目前我们最准确的估计 根据膨胀历史
Our best current estimate, using that expansion history,
长达13800000000年
is 13.8 billion years, give or take.
第二步 是烦人的数学问题
Step two is, well, annoying math.
但是我们可以把它形象化地表示出来 像下面这样
But we can represent it visually as follows.
想象在大爆炸发生后的几秒中
Imagine that seconds after the Big Bang happens,
每个葡萄干发出一束光
every raisin emits a beam of light
这束光可以无阻碍的穿行
that can then travel without hitting any obstacles.
让这个过程正向继续发展下去 以宇宙膨胀历史
As we run the movie forward, at the rate given
告诉我们的速率 这些想象中的光束
to us by the expansion history, those imaginary beams
将会穿过正在膨胀的空间
would travel through the expanding space
在不同的时间到达我们
and reach us at different times.
其中一束光
One of those beams of raisin light
会穿行13800000000年
would be just switching as the clock hits 13.8 billion years.
然后我们看见了这束光现在在哪里
We then see where that raisin is now.
答案是 大约460亿光年之外
Answer, about 46 billion light years away.
这么远
Boom.
因此这就是我们怎么知道可见宇宙
So that’s how we know that the observable universe is
直径大约为900亿光年的方法
about 90 billion light years in diameter.
但是不可见的部分怎么办
But what about the unobservable universe?
是否有星系距离更远 它的光
Aren’t there galaxies even further away, whose light
还未到达地球
hasn’t reached us yet?
是的 有
Well, yeah, there are.
但是 这是另外一集的内容了
But I’m afraid that’s a subject for another episode.
如果你有兴趣 可以在
If you’re inclined, go ahead and get the discussion
下面的评论区加入讨论
about that started in the comments below.
我会报道所有有趣的内容
I’ll report on any interesting threads
这些内容来自于在下一集””宇宙时间””
from that conversation on the next episode of ‘Space Time.’
上周 我们问到相信有外星人存在
Last week, we asked whether it’s irrational to believe
是否合理
in aliens?
以下是你们的观点
Here’s what you guys had to say.
PantsuMann和其他人指出
PantsuMann and others point out that it
它的几率和偶然间在太空中
would really low odds to accidentally
碰到”千年鹰号”一样低
bump into a ‘Millennium Falcon’ in space.
我认为你们把我的例子理解的太随便了
I think you guys took my glib example a little too literally.
费米悖论的重点是
The real point of Fermi paradox is
银河范围内的殖民
that colonization on a galactic scale
有可能会非常迅速的进行 以至于太空中生命真正很常见
could happen so quickly that if life is really that common,
至少到目前为止一种文明应该已经形成了
at least one civilization should have already done it by now
它们拥有普适性以至于我们已经看到了一些证据
and they should be so widespread that we’d see some evidence.
Nik G评论说 一种真正先进的文明
Nik G commented that a really advanced civilization might not
可能并不会向外太空扩张 而是在内部壮大
try to expand outward into space, but inward into, let’s
比如说 先进的计算机电路 这是一个很棒的想法
say, advanced computer circuitry, pretty good thought.
VegetaAFH提问那基本指令呢
VegetaAFH asks what about the prime directive?
好的 那詹姆斯·T·柯克呢
Well, what about James T. Kirk?
他不太在乎基本指令
He didn’t care about the prime directive much.
所以为什么外星人会这样呢
So why should all the aliens?
Rob LaRosa给了一个非常深刻的评论
And Rob LaRosa left us with a deeply insightful comment
从”卡尔文和霍布斯”的角度
from ‘Calvin and Hobbes.’
非常棒
Good work.

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