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伽马射线的死光

Extinction by Gamma-Ray Burst | Space Time

PBS数字工作室
PBS Digital Studios
本集由23andMe赞助播出
This episode is supported by 23andMe.
在宇宙中 有相当多的灾难可能会降临在
The cosmos has so many catastrophes in store
我们这颗脆弱而娇小的星球上
for our fragile little planet.
其中最可怕的灾难之一是
Among the scariest is that one day,
我们几乎注定会在某一天暴露在
we will almost certainly find ourselves
伽马射线暴的死亡威胁中
in the path of a gamma-ray bursts: death ray.
太空时间
SPACE TIME
世界末日或许没有近在眼前
The end of the world may not be nigh,
但它终将到来
but it will come.
事实上 世界末日有很多种可能
In fact, there are quite a few ends of the world on their way.
每一种都很耐人寻味
Each one were interesting
却也无法避免
and less avoidable than the last.
我们有很大机会阻止那些最紧迫的灾难
There’s a good chance we can prevent the most imminent,
比如小行星撞击
like asteroid strikes,
或者至少能应对其带来的的影响
or at least deal with their effects,
例如应对由伽玛射线暴和超新星所带来的灾害
like the damage caused by gamma-ray bursts and supernovae.
但后面提到的灾难就不是现有技术能够阻止的了
But the later ones will be beyond any conceivable technology to prevent,
比如
for example,
太阳热度的逐渐上升并最终走向寂灭
the gradual heating and eventual death of the sun.
充满希望的是 也许用不了几代
Hopefully, our super advanced
我们的高度先进的后代们
and probably not quite human descendants
就能通过移民到其他恒星系来躲避那些灾难
will be able to escape those by traveling to other star systems.
但假如银河系注定会与仙女座星系相撞会如何?
But what about the Milky Way’s inevitable collision with Andromeda,
或者最后的恒星也油尽灯枯了呢?
or the final burning out of the last stars,
亦或是连仅剩的黑洞也消失了
or the evaporation of the last black hole
最后一枚质子也衰变了呢?
and decay of the last proton?
那世界乃至宇宙的终结也就到了
The ends of the world and of the universe will come.
最好能未雨绸缪
Better to be forewarned.
我们会逐一讲述这些无法避免的宇宙浩劫
We’ll get to each of these inevitable cosmic catastrophes,
首先要讲的是一个随时都可能发生的灾难——
but let’s start with the one that could happen anytime–
能灼烧我们大气层的超新星爆发或伽马射线暴
a supernova or gamma-ray burst frying our atmosphere.
每隔一亿年左右
Every hundred million years or so,
地球上就会有大批生物灭绝
a good deal of Earth’s life gets wiped out.
在过去的五亿年中的
At five or six different point in time
五到六个时间点上
over the past half billion years,
化石记录显示 许多物种突然就消失了
a large fraction of species simply vanished from the fossil record.
有些大灭绝是因为大型小行星撞击
Some of these mass extinctions were due to giant asteroid impact,
包括距今最近的那次
including the most recent,
发生在6500万年前的恐龙大灭绝
which wiped out the dinosaurs 65 million years ago.
但这样的末日情景是最有可能阻止的
But this is the most preventable end of world scenario.
事实上 我们早就谈过这种情况以及如何使其转向
In fact, we’ve already talked about asteroid impacts and how to deflect them.
但无论如何 至少有一场大灭绝
However, at least one mass extinction may have been caused
是由人力无法阻止的东西造成的
by something we’ll never have the technology to stop.
四亿四千万年前的奥陶纪-志留纪灭绝事件
The Ordovician-Silurian extinction 440 million years ago,
很可能是由于地球遭到了
may have resulted from the earth being blasted
来自远处的恒星爆炸产生的
by the intense radiation jets
密集辐射流的轰击
from a distant exploding star.
可能就是一次伽马射线暴
It may have been caused by a gamma-ray burst.
在本集里 我们会给出相关证据
In this episode, we’re going to look at the evidence
来证明GRB是导致那次大灭绝的罪魁祸首
that the O-S extinction was caused by a GRB.
我们也会探索下一次的伽马射线暴会何时到来
And we’ll also figure out how long before the next gamma-ray burst hits.
但让我们先看一下提到的场景
But first let’s go over the proposed scenario.
很多人都知道 超新星是
As many of you know, a supernova is the explosion
大质量的恒星在演化末期发生剧烈爆炸后的产物
that follows the catastrophic collapse of a massive star at the end of its life.
太阳不会这样毁灭
Our sun won’t die that way,
但任何一颗质量相当于8个太阳的恒星就有可能
but any star more than around eight times the sun’s mass will.
爆炸会喷射出高能射线
The resulting explosion sprays high-energy light.
比如紫外线 x射线 伽马射线
So ultraviolet, x-rays, gamma rays
以及宇宙射线这样的近光速粒子
and near-light speed particles, so as cosmic rays,
射线会进入周边的星际空间
into the surrounding interstellar space.
任何距超新星几光年内的行星都会受到影响
Any planet within a few tens of light years of a supernova is in trouble.
如果恒星在爆发前高速旋转 那会更加危险
It’s even more dangerous if the star was rapidly rotating before it exploded.
这种情况下 强大的恒星磁场
In that case, the powerful magnetic fields
会将爆炸约束成狭窄的喷射流
can channel the explosion into narrow jets
从而极大地集中并增强了喷射
that massively focus and amplify the blast.
差不多每天 都会有形如此的来自遥远星系的喷射
Roughly once per day, the jet from such an explosion in a distant galaxy
抵达地球 并被斯威夫特卫星或费米卫星监测到
reaches the earth and is detected by the Swift or Fermi satellites.
观测到的来自爆炸恒星的伽马射线的微弱迹象
The observed faint flash of gamma-rays from exploding stars
能够持续数秒到数分钟
can last anywhere from a couple of seconds to a few minutes.
这种是属于长时间的伽马射线暴
These are long-duration gamma-ray bursts.
持续时间短于2秒的短时射线暴
Short-duration bursts that last less than two seconds
来自于中子星合并
are caused by merging neutron stars.
所以对GRB来说 多近才算是太近呢?
So how close is too close for a GRB?
发生在银河系中的射线暴的主要危险
Well, the main danger of the burst within the Milky Way
不在辐射本身
is not the direct radiation itself.
本质上来说 所有的伽马射线 x射线
Essentially all of the gamma-rays and x-rays
都会被大气层所阻挡
are going to be blocked by atmosphere.
部分紫外线辐射会到达地表
Some extra ultraviolet radiation will reach the ground,
但还到不了十分危险的地步
but not at seriously dangerous levels.
相反 危险存在于它对大气层的长期影响
Instead, the danger is in the long term effects on the atmosphere.
伽马射线会破坏大气中的氮气和氧气分子
Gamma-rays break apart nitrogen and oxygen molecules in the atmosphere,
后者又结合成多种氮的氧化物
which then recombine into various oxides of nitrogen.
这些新分子才是致命杀手
Those molecules are the real killers.
一氧化氮促进了臭氧分子的解裂
Nitric oxide catalyzes the destruction of ozone molecules,
而后者通过吸收太阳紫外线来保护我们
depleting the ozone layer that protects us from solar UV.
二氧化氮会吸收可见光
And nitrogen dioxide absorbs visible light,
减少地球从太阳接收到的能量
reducing the energy received from the sun.
这些危险的分子在大气层中能滞留数年时间
These dangerous molecules can remain in the atmosphere for a few years.
而这已经长到足够使紫外线的增加量
And that’s potentially long enough to cause a UV increase
对许多物种产生致命威胁
deadly to many species
并引发不可控的全球降温
and to initiate runaway global cooling.
它们还会导致硝酸雨
Also they result in nitric acid rain.
据估计 一次一万光年内发生的典型伽马射线暴
It’s estimated that a typical gamma-ray burst within ten thousand light years
所消耗的臭氧就足以
could deplete ozone enough
使海平面处的紫外线量上升30个百分点
to cause up to a 30% increase in ultraviolet at sea level.
这足以毁灭大多数对紫外线敏感的生物
This is enough to devastate the most sensitive organisms
包括海洋食物链底层的浮游植物
including phytoplankton, the basis of the marine food chain
以及地球上的主要制氧生物
and Earth’s main oxygen producer.
单这一点就足以导致大规模灭绝事件的发生
That alone is enough to cause a mass extinction event.
而短短几年内二氧化氮对光照的吸收引发的全球变冷
And this could be exacerbated by the global cooling
会使这场灾难进一步加重
triggered by a few years of NO2 absorption of sunlight.
那么 为什么有些科学家会认为
So why do some scientists think that
是GRB导致了奥陶纪-志留纪灭绝事件的发生呢?
the Ordovician-Silurian extinction event resulted from a GRB?
有几条证据比较符合事实
Well, a couple of pieces of evidence fit nicely.
从化石记载来看 一种强相关性似乎体现在了
Looking at the fossil record, there seems to be a strong correlation
某个物种灭绝的可能性
between the likelihood of a given species going extinct
和该物种所受到的强紫外线之间
and the exposure that species would have had to ultraviolet light.
奥陶纪晚期生活在海洋表面附近
Species in the late Ordovician that live near the ocean surface
或浅水域的物种更有可能灭绝
or in shallow water were more likely to go extinct,
或者比生活在深水域的物种更早灭绝
or went extinct earlier than those living in deeper water.
同样的模式在其他大规模灭绝中并未表现出来
The same pattern isn’t clear for the other mass extinctions.
对这种反常的灭绝模式的一种解释是
One explanation for this unusual extinction pattern is that
更深水域的生物在伽马射线暴引发的
deeper dwelling organisms had more protection
紫外线增长面前得到了更多的保护
against the increased UV following a gamma-ray burst.
我需要说明的是 奥陶纪-志留纪灭绝事件
Now I should mention that the O-S extinction is
必然与冰川时期的开始有所关联
definitely associated with the beginning of an ice age.
科学家们认为 那个时代多数物种的灭绝
Scientists agree that many of the extinctions of that era
都源自气候的改变
resulted from the changing climate.
奥陶纪是一个非常温暖的时期
The Ordovician was a very warm period,
而冰川作用的相对突然的发生
and the relatively sudden onset of glaciation
不用一些触发事件来解释的话很难说通
is hard to explain without some triggering events.
这类事件可能是GRB导致的二氧化氮对光照吸收的增加
That event may have been the increase in sunlight absorbing NO2 after a GRB.
而且 灭绝事件似乎在冰川时期真的来临前就开始了
Also, extinctions appear to have started before that ice age really got underway.
这符合GRB假设
That fits the hypothesis of a GRB.
暴露在更多的紫外线下导致灭绝的开始
Extinction started due to the sudden UV exposure,
而气候变化则使其继续发生
and continued due to climate change.
无论该次大灭绝事件是否是由伽马射线暴引起
Whether or not this particular extinction event was due to a gamma-ray burst,
我们能确定的是 地球会周期性地遭受宇宙射线的冲击
we’re pretty confident that the earth does get blasted periodically.
基于GRB在其他星系的发生频率
Based on the rates of GRBs we see in other galaxies
以及银河系中恒星的数量
and the population of stars in the Milky Way,
大概每十亿年 就会有1到3次发生在1万光年以内的GRB
it’s estimated that every billion years Earth finds itself in the path
从地球附近经过
of between 1 and 3 GRBs within 10,000 light years.
不幸的是我们没法得知
Unfortunately there’s no way to tell
GRB是否会降临到我们身上
whether a GRB will point it our way until it happens.
距离我们最近的还在酝酿中的GRB
The nearest potential GRB in the brewing
在8000光年之外 也就是在危险区域内
is 8,000 light years away, so within the danger zone.
这是一颗沃尔夫-拉叶星 编号WR 104
This is a Wolf-Rayet star, WR 104,
是一颗处于生命最后阶段的大质量恒星
the massive star in the last phase of its life,
它在最近将外壳喷射了出去 形成了风车形状的星云
currently blasting off its outer shells into a pinwheel-like nebula.
暴露出来的恒星内部要比太阳热数倍
The exposed inner star shines several times hotter
而亮度更是相当于成千上万个太阳
and hundreds of thousands of times brighter than the sun.
这颗恒星是双星体系的一部分
This star is part of a binary system
而正是这个双星轨道产生了螺旋状的星云
and it’s this binary orbit that produces the spiraling nebula.
这个螺旋似乎面对着我们
The fact that the spiral appears to be face-on
这也意味着 整个星系的公转轴是正指向地球的
suggests that the axis of the entire system is pointed directly at the earth.
在这颗恒星产生伽马射线暴的情况下
The rotational axis of the star will define the direction of the jet
恒星的自转轴将确定喷射的方向
in the event that this Wolf-Rayet star does produce a gamma-ray burst.
如果真是这样 双星系统的运行轨迹表明
If it does, the orientation of the system suggests
我们将恰好处于它的射程内
we could be right in its firing line.
不过现在倒没必要收拾行李逃离太阳系
Well, no need to pack up and leave the solar system just yet.
首先 WR104还有将近五十万年的寿命
Firstly, WR 104 could have up to half a million years of life in it,
尽管很难判断这样的一颗恒星距离爆炸还有多久
although it’s hard to tell exactly how close a star like this is to exploding.
而且凯克天文望远镜的进一步观测发现
Also further observations with the Keck telescopes indicate
这个双星系的公转轴并没有直接指向地球
that the system’s orbital axis isn’t pointed directly at the earth.
但这并不能保证我们的安全
That doesn’t necessarily mean we’re safe.
恒星的自转轴决定了喷射的方向
It’s a star’s rotational axis that defines the direction of the jet.
但是双星系的公转轴
But the orbital axis of a binary system
和其中恒星的自转轴通常是相关的
and the rotational axis of its stars are often correlated.
所以在这种情况下 我们可能躲过了致命子弹
So we may have dodged the bullet in this case.
伽马射线暴相比于普通的超新星十分少见
Gamma-ray bursts are much less common than regular supernovae,
事实上 普通的超新星能造成与之同样的伤害
and in fact, regular supernovae can do just as much damage as a GRB.
然而 一颗超新星要达到相同的效果
However, for a supernova to produce the same effects,
距离就得很近 需要在20到30光年内
it needs to be much closer, within 20 to 30 light years.
这个范围内绝没有能在短时间内爆炸的恒星
There are definitely no stars in that range that could explode any time soon.
但是太阳却并不稳定
However the sun isn’t stationary.
它绕银河系运行 与星系内的邻居们来来往往
It orbits the Milky Way and its galactic neighbors come and go.
或许在几个周期为2.5亿年的运行过程中
Maybe in a few 250-million-year orbits,
我们就会碰上一颗定时的星球炸弹
a stellar time bomb will wander into our vicinity.
然而 实际上只有GRB才是最有可能
However, it’s really the GRBs that are most likely
率先并多次袭击我们的
to hit us first and hit us more often.
我们应该会在接下来的五亿年中遇到一次
We should certainly expect one in the next half-to-one-billion years,
即使它不是来自WR104星
even if it’s not WR 104.
而那一旦发生 我们根本来不及看到
And when that happens, well, we won’t see it coming
而且在太阳系中也无处可以避难
and anyway there’s nowhere in the solar system to hide.
但是运气好的话 届时我们的技术能先进到
But with any luck, we’ll have advanced to the stage
实施针对整个大气层的地球级工程
where geoengineering of the entire atmosphere is possible.
或许我们能重塑臭氧层
Perhaps we’ll be able to rebuild the ozone layer
并从大气中清理掉有害分子
and clean the bad molecules from the sky.
或许我们能够在一系列
Maybe we can hold out a little longer
从外太空接踵而至的
against the series of calamities flung at us
灾难中坚持更久一些
one after the other from outer space time.
感谢23andMe对PBS数字工作室的大力支持
Thanks to 23andMe for supporting PBS Digital Studios.
23andMe这一名字来源于
23andMe comes from the fact that
人类DNA由23对染色体组成
the human DNA is organized into 23 pairs of chromosomes.
23andMe是一家个人基因检测公司
23andMe is a personal genetic analysis company,
它帮助人们了解自己的DNA中蕴藏的个人信息
created to help people understand what their DNA says about them.
23andMe能帮你建立与家人的联系
23andMe can help connect you with your family
并且告诉你你们之间的共同点
and remind you of what you have in common,
而且在每年的这个时间特别重要
which can be particularly important this time of year.
你们可能执有不同观点
You may not share a lot of the same opinions,
但却一定执有相同的DNA
but you definitely share a lot of the same DNA.
23andMe从现在到12月26号有一个特别的假日优惠
23andMe has a special holiday offer now through December 26th.
你可以访问23andMe.com查看具体情况
You can go to 23andMe.com to check out their holiday offer
并为家人购置相关工具
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你可以通过访问23andMe.com/spacetime来支持本节目
Show your support for this show by checking out 23andMe.com/spacetime.
上周我们讲到了第一次观测到的来自太阳系外的访客
Last week we talked about our first detection of a visitor from outside our solar system.
那是一块巨大的岩石 我们称其为Oumuamua
It’s a giant chunk of rock that we’ve named Oumuamua.
名叫supreme84x的网友问 如果它撞击了地球会发生什么
supreme84x asks what would happen if this thing hit us.
根据它的亮度以及亮度的变化
Well, based on its brightness and variations in that brightness,
可以估计出其大概有180米长 30米宽
the body is estimated to be 180 meters long and maybe 30 meters wide.
假设反照率或反射率为0.1
Now assuming an albedo or reflectivity of 0.1,
其体积大概为13万立方米
that gives it a volume of around 130,000 cubic meters.
典型的小行星密度为2000千克/立方米
A typical asteroidal density is 2,000 kilograms per cubic meter,
那么由此得出的岩石质量大约为2.5亿千克
so that gives it a mass of around a quarter of billion kilograms.
它的速度比太阳系内的大多数天体都要快
It was moving much faster than most solar system objects
离地球最近时能达到每秒50千米左右
at around 50 kilometers per second at its closest approach to Earth.
所以它的动能(质量乘以速度的平方再除以二)
So its kinetic energy, 1/2 × m × v^2
大约为3 × 10^17焦耳
was around 3 by 10 to the power of 17 joules
或是70兆吨的TNT当量
or 70 megatons of TNT.
那比一枚“沙皇炸弹”的能量都要大
That’s greater than the yield of Tsar Bomba,
后者可是有史以来最大的氢弹
the greatest hydrogen bomb ever detonated.
假设Oumuamua完全是岩石 并且正面对着大气砸下来
Assuming Oumuamua is entirely rocky and hit the atmosphere pretty much head-on,
它可能会在崩解之前就到达地面 并释放能量
it might actually reach the ground before disintegrating to deliver that energy.
届时它会成为一个“城市毁灭者”
It would become what we call “a city killer”.
它对全球的影响则会受限
The global effects however would be limited.
名叫PixelatedDonkey的网友问 有没有可能
PixelatedDonkey asks whether there’s a chance
当银河系与仙女座星系相撞时
that some stars like solar systems will be ejected from the galaxy,
像太阳系这样的恒星会被逐出星系
when the Milky Way collides with Andromeda?
其实呢 有这样的可能
Well, in fact, yes.
最近的研究表明 大概有3%的几率
Recent studies suggest there’s a 3% chance
太阳会在仙女座的第一次飞越中跳过星系
that the sun will jump galaxies on Andromeda’s first fly-by.
更小的几率下会表现不佳 错过这次跳跃
And there’s some smaller chance that it’ll miss that jump, coming up short,
最终落入星际空间
and end up in intergalactic space.
有些恒星会被仙女座星系和银河系相撞时产生的
And some stars will be slingshotted out of the galaxy by the two supermassive black holes
两个超大质量黑洞弹出星系之外
of Andromeda and the Milky Way as they fall together.
那么哪些被放逐的恒星系中的居民会看到什么呢?
And what will the residents of one of those exiled systems see?
一个处处充斥着黑暗的夜之苍穹
Well, a very dark night sky, dark everywhere,
除了巨大无比的椭圆形状的星系——银河-仙女座
except for the gigantic elliptical galaxy–Milkdromeda,
那是最终的融合之结果
the final result of the merger.
不过不用担心
But don’t worry.
我们有数十亿年的时间
We have a few billion years
来为这个新星系起一个更好的名字
to come up with a better name for that galaxy.
网友Timothy Judge在Portegies Zwart及其合作者的一篇文章中
Timothy Judge points out a significant potential error
指出了一处重要的潜在错误
in the paper by Portegies Zwart and collaborators.
克林贡语中的“mob nagh”应被更确切地翻译为
translates from the Klingon more accurately to
“这石头感到孤独”而非“孤独的石头”
“The stone is lonely.”, not “Lonely rock”.
希望审核员能在文章发表之前发现这个错误
Let’s hope the referee caught that one before publication.
愚蠢的错误

发表评论

译制信息
视频概述

飞向宇宙,浩瀚无垠,但宇宙中充满了未知与危险。今天我们来谈谈伽马射线暴对地球的影响。

听录译者

jm

翻译译者

伽卡

审核员

审核员1024

视频来源

https://www.youtube.com/watch?v=8HvwNOSnveE

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