The greatest long-term threats facing humanity
末世猜想：可能导致人类灭亡的几大威胁

Can we actually say anything about the far future? If we can’t predict when it will rain next month, forecasting billions of years hence might seem impossible.

遥远的未来会是什么样子，我们真能说得准吗？连下个月什么时候下雨都不知道，预想几十亿年以后的事情似乎只能是天方夜谭。

However, not everything is as chaotic as the weather: even predictions very far ahead are sometimes possible, especially in astrophysics and cosmology. We can be confident that there will be a total solar eclipse in the UK on 23 September 2090 because the Moon, Sun and Earth move in stable, predictable orbits with very minor disturbances, and the laws of gravity are now well-tested. Similarly, we can use known astrophysics to predict what will likely happen across the Universe as it expands.

不过，并非所有事情都像天气那样混沌难测，预测遥远的未来也是有可能的，特别是在天体物理学和宇宙学方面。我们能够肯定，2090年9月23日这一天英国一定会出现日全食，因为月亮、太阳以及地球都是沿着可预知的轨道在稳定运行，受到的干扰微乎其微，而且万有引力定律也已经得到全面证实。同样地，我们也可以利用已知的天体物理学知识预测出在宇宙膨胀的过程中可能会发生哪些事情。

This approach can be described as “physical eschatology” – a term coined by the astronomer Martin Rees for using astrophysics to model where the Universe is going. Rees took a cue from theology, in which “eschatology” is the study of ultimate things such as the end of the world. And the classic paper on the topic is Freeman Dyson’s 1979 paper on life in open universes, which outlined likely or possible existential catastrophes that could threaten life far into the future, from the death of the Sun to the detachment of stars from galaxies.

这种预测方法被称之为“物理末世论”，由天文学家马丁‧里斯（Martin Rees）提出，利用天体物理学建构的模型，预测宇宙的发展。末世论在基督教神学领域研究的是世界末日等终极问题，里斯正是借鉴于此。相关的经典之作是戴森（Freeman Dyson）在1979年发表的一篇文章，讨论开放宇宙中的生命前景。论文认为太阳死亡，以及恒星逃离银河系等可能存在的灾难都会对遥远未来的地球生命造成威胁。

So, what are the biggest challenges humanity will face if we survive into the far future? We cannot say how (or if) they will be overcome (I will make some guesses) but we can be confident these threats to our existence are coming.

那么，如果人类能够延续到如此遥远的未来，会面临什么样的最严峻的挑战？我们并不知道该如何克服这些挑战，或能否克服这些挑战，此处我只能做一些猜测。但可以肯定的是，这些事关人类文明存亡的威胁正在向我们迎面而来。

Problem 1: Survive better than other mammals

问题1：人类能否超越哺乳动物宿命

The typical lifespan of a mammalian species is about a million years or so. From nuclear war to bioengineered pandemics, humanity clearly has other risks it needs to reduce urgently: right now the natural extinction rate is far smaller than risk we pose to ourselves.

一个典型哺乳动物物种的存在时间大约为100万年。对人类而言，除了从核战争到生物工程可能引发的传染病，还有其他可能毁灭人类的危险迫切需要面对和解决。今天，人类自作孽造成的灭绝风险已远远高于发生自然灭绝事件的概率。

Were we to fix our current existential risk and sustainability problems we would still have to deal with some other challenges to stay around.

要想解决现有的人类生存风险以及可持续问题，我们还得应对许多其他挑战。

For starters, in a few tens of thousands of years we will have to cope with the end of the current interglacial period: we are living during a brief interruption of a long ice age. Our ancestors have survived ice ages, so it is likely not a big deal – except that they were nomadic hunter-gatherers rather than a global civilisation.

人类在未来几万年内，首先面临的是本次间冰期（两个冰河期之间的时期）的结束。目前人类正处于一个漫长冰川时代中的一个短暂温暖的间冰期之中，间冰期结束，地球将会变得非常寒冷。不过我们的祖先活过了冰川时代，再过寒冷生活可能问题不大，只不过我们的祖先是游牧人，靠狩猎采集维生，还没有一个全球性的文明。

We may also face dramatic climate variations between different geological eras. In the past the Earth has been not just colder, but also warmer. During the Eocene, temperatures were 10C warmer, with palms and alligators in the Arctic and equatorial regions too hot for unprotected humans to survive in. Even further in the past there has been “snowball Earth” episodes where almost all of the Earth was covered with ice.

人类还可能经历不同地质时期之间的气候大变迁。在远古时代，地球曾经非常寒冷，但也曾经非常暖和。在5600万年前到3400万年前的始新世地质年代，地球的气温要比现在高10摄氏度，北极也有棕榈树和鳄鱼，而赤道地区则太过炎热，人类如生活在这样的赤道，不采取保护措施根本无法生存。再往前还有过“冰雪地球”时期，当时整个地球基本都被冰雪所覆盖。

Then there is the risk of supervolcanism, meteor impacts, gamma ray bursts, or emergent ecological disruptions, which we know have led to natural mass extinctions about once every 100 million years.

此外人类还可能遭遇超级大火山喷发、小行星撞地球、来自宇宙的伽马射线暴，或紧急生态大灾难等能毁灭人类文明的种种危险。我们人类已知，上述大灾难曾导致地球约每1亿年就会出现一次物种大灭绝事件。

Ultimately, Homo Sapiens may not endure as a species because we could evolve into something else. We are constantly mutating and subject to natural selection (even today with good healthcare, road accidents are slowly selecting away people likely to kill themselves in traffic while young), and modern biotechnology allows us to modify our genes deliberately. Not to mention technologies that allow us to merge with the artificial. Over millions of years it is unlikely that we will stay the same – unless we make a deliberate decision to preserve our genetics and can make that decision stick over geological timescale.

最终可能不再有智人这个物种存在，人类可能会进化成一个新的物种。人类一直在演化变异中，并受到物竞天择的自然筛选，即或如今医疗条件良好，但交通事故也会让人英年而逝，犹如是一种自然淘汰。现代生物科技也让我们能够改良自身基因，甚至还有能将人与人工合为一体的科技。相信再过几百万年，未来的人类将会与我们是面目全非，除非我们有意决定维护我们的现存基因，不让其发生任何的改变，并在未来漫长的地质年代中一直坚守到底。

If “we” are around in a billion years from now, we would have arbitrarily sustainable civilisations able to handle disasters on a planetary scale, plan ahead for geologic time, and likely be as different from us as we are from the trilobites. The irony is that to survive longer than our fellow mammalian species, we have to become something very different from what we are.

如果“人类”（姑且仍称之为人类）10亿年后还存在，将一定会拥有可传之久远的文明，能够应对行星级别的大灾难，能够规划以地质年代为单位的人类前景，那时的人类跟现在的人类相比，差异之大就跟我们当今人类和二亿五千万前已灭绝的三叶虫的区别差不多。人类要想比其他哺乳类动物活得更长久，就得大变身成为其他物种，说来也是讽刺。

Problem 2: Survive the end of the biosphere lifespan

问题2：能否存活到地球生物圈终结之后

In about a billion years (give or take a few hundred million years) the increased brightness of the Sun will doom the Earth’s biosphere.

再过10亿年（或者前后差个几亿年），太阳光亮度的增强会给地球上的生命带来灭顶之灾。

The problem is that the heat of the Sun leads to increased weathering of rock, which in turn leads to chemical reactions that removes significant amounts of carbon dioxide from the air as part of the carbon cycle, eventually starving plant-life. In addition, the planet eventually overheats as a runaway greenhouse, with more and more water vapour (a potent greenhouse gas) evaporating from the oceans, making it even hotter.

太阳热量的增加会加剧岩石的风化，从而引发一系列化学反应，消除空气在碳循环过程中的大量二氧化碳，最终摧毁以二氧化碳进行光合作用的植物。此外，地球会热得像一个失控的温室，从海洋中蒸发出越来越多的水蒸汽，从而使得地球更加炎热。

One approach is to try to protect the biosphere with megascale engineering for as long as possible. We may perform geoengineering by adding reflective aerosols to the stratosphere, build a solar shade between the Earth and Sun, or even move the planet outwards.

其中一个办法是建造超大规模的工程，尽可能长时期保护地球生物圈。那时的人类还可以实施地球工程，在平流层添加能够反射太阳光的悬浮颗粒，或是在地球与太阳之间建造一个太阳遮板，甚至可以把地球往远离太阳的方向移一移。

Another solution is to move life into space, if we have not already done so. Self-sustaining space habitats appear possible, and there is material out there for many billion times the Earth’s surface area. Even if these structures appear hard to build, we should remember that we literally have a billion years to become more skilled, richer, and to work on them.

另一个解决之道是将地球生命转移到太空，当然要是那时人类还没开始太空移民的话。宇宙看来存在适宜生命居住的行星，而且太空中的物质要比地球表面积大好几十亿倍。就算人类移民太空的工程是非常的艰巨，但请别忘了，我们人类还有10亿年的时间来提高技术，增加财富，进行建造。

By this point humanity has to be an actor on the scale of the Solar System if it is to survive.

到了这个关头，人类文明要想延续下去，就必须到太阳系中去建立新的家园。

Problem 3: Survive the end of the Sun’s main sequence lifetime

问题3：人类能活到太阳功能减弱之后吗

In around 5 billion years, the Sun’s brightness will start to increase more rapidly because accumulated helium in the core will heat it up, turning it into an enormous red giant. The surface temperature goes down but the total light output is far larger due to the huge surface area. This likely spells the end for the Earth, since it is likely to be swallowed by the Sun as it expands. If it isn’t, it will be fried to an airless rock. “Soon” after (a billion years or so) the Sun will expel much of its atmosphere as a nebula and becomes a tiny white dwarf.

再过大约50亿年，太阳核心的氢聚变反应加快，氦元素逐渐增大，太阳的明亮程度会快速增长。这时的太阳结束其主序星阶段，膨胀成一颗硕大的红巨星。虽然这时太阳的表面温度会降低，但由于表面积巨大，所辐射的光将远远超过现在的太阳。这时地球的末日也就到，很可能会被不断变大的太阳所吞噬。就算没有被太阳吃掉，也会被空前强烈的太阳光煎烤成一块连一丝风一滴水都没有的大石头。之后太阳“很快”（这是以宇宙的时间而言，但大约也要10亿年）将其绝大部分物质向外抛出，形成星云，这时的太阳会变成一颗小型的白矮星。

To survive this, any intelligent life living in the Solar System will need to move to other solar systems. One can of course adapt but there is not much light and energy from a white dwarf.

太阳系的智慧生命要想躲过此劫，就必须移民到其他星系。那时已达超级智慧的人类当然也可以尝试去适应此时的环境，但变成白矮星的太阳已没有多少的光和能量可以发出，人类不得不离开太阳系。

Reaching other solar systems will either require very fast spacecraft, or will take a long time.

要到达其他星系建立新家园，需要速度极大的航天器，否则就要花上非常漫长的时间。

To people already living on self-sufficient space habitats, setting them in motion towards new destinations might be fairly natural. They would need energy sources that could last for a long time (not to mention to give them a decent speed) and enough material to maintain the habitat for the multi-millennia transit.

对于已经生活在自给自足的太阳系太空生命基地上的未来人类，当太阳死亡，移民外星系，寻找或建立新的生命家园也是很自然的事情。但这需要源源不断维持很长时间的能量供给，而且前往太阳系外的星系，飞行速度也不能太慢。此外还要有足够的物质来维持这场跨越千年的迁徙。

The most likely way to get to the stars might, however, be tiny nanorobot spacecraft. Instead of using vast energy to push giant starships to a modest velocity, it might be better used to send tiny craft fast using a reflective sail and a powerful laser. They are small and redundant: if one does not make it, send a thousand. They could also carry the genetic elements to create life – even human beings. Once they arrive, they land on a suitable asteroid, unfold solar collectors, mine material and build more robots, solar collectors and factories. Eventually they can build habitats and nurture people to live in them.

飞往太阳之外的星系最有可能的方式，是使用小型的纳米机器人太空器。大型星际飞船需要巨大能量才能达到一定速度，使用小型太空器可以利用反光帆板和强劲射线推动快速行进，效果会更好。纳米机器人太空器不仅体积小，而且可大量使用。如果发射一个失败，那就发射一千个。纳米太空器还可以搭载着遗传基因以创造生命，甚至搭载人类。到达其他星系后，这些小型太空器可以在适合的小行星上降落，打开太阳能收集器，开采物质，并制造更多的机器人、太阳能收集器以及工厂。最终，建成一个个太空栖息地，让人类能够在这些机器人建造的生命基地上繁衍人类文明。

It might well be that no biological human will ever physically leave the Solar System. At this point we might question whether it is actually our humanity or a new species that is spreading. But if our descendants survive the red giant Sun they will now be living among the stars of the galaxy.

称之为人类的这一地球物种可能永远也无法离开太阳系。此刻，我们可以提出这样的问题，向太阳系外的星系发展的智慧生命到底是我们人类自己，还是一个新的生命物种。但无论如何，人类的后代如果真的能熬过太阳变成红巨星这个灾难，活了下来，那一定应该已生活在银河系的其他的恒星之中。

Problem 4: Survive the end of stars

问题4：能存活到恒星消亡之后吗

Star formation in the Universe has already peaked and in the next few tens of billions of years we will reach “peak star”. As the bright and short-lived stars burn out, we will be left with a staid but long-lived population of red dwarf stars. They can shine for trillions of years. But star formation will decline, and in 10-100 trillion years even the red dwarfs will sputter and go out. To survive, life will need energy sources other than starlight.

宇宙的恒星形成的巅峰时期已经来临，在未来的几百亿年里，我们将迎来最高“恒星巅峰”期。那时明亮且寿命较短的恒星将会燃烧殆尽，剩下的都是稳定且寿命较长的红矮星，发光时间可达数万亿年。但是恒星形成的数量将会下降，再过几千几万亿年，红矮星也将坍塌消散。生命要想存续，不能依靠恒星的光热，而是需要其他能量来源。

There are actually many possibilities: fusion using hydrogen from brown dwarfs and gas planets, dumping matter into black hole accretion disks and gathering the released energy, or even tapping black hole rotation directly using so-called super-radiant scattering (known as “black hole bombs”). In any case it will require engineering on a vast scale. What about normal nuclear power? Fission power will end when there are no new radioisotopes produced by merging neutron stars and supernovas, which by now are long in the past. Geothermal energy also runs out when the isotopes inside planets decay away and they cool down.

可能性其实很多，可以使用褐矮星以及气态行星上的氢制造核聚变产生能量，或是将物质投入黑洞的吸积盘中，收集释放出的能量。甚至利用一种被称为超辐射的东西（也就是所谓"黑洞炸弹"）按一定角度抛入黑洞强大的旋转盘，然后带着新获取的能量逃逸出来。不论哪种方法，都是巨大无比的工程。那么使用普通的核能可以吗？如果没有超新星爆炸合并旁边的中子星而产生可裂变同位素，也就不会再有核裂变的能量。现在超新星爆炸已是非常久远的事了。当行星内部的同位素逐渐衰变冷却后，行星的地热能也会消耗一空。

“Life” may also adapt to low temperatures and exotic environments. Artificial intelligence and silicon-based organisms would probably thrive in the near absolute zero temperature environment. It might well be that as the stars go out, carbon-based life and intelligence retreats into comfortable virtual worlds far bigger and more complex than the external Universe ever was.

“生命”或许也可以适应低温和极特异的环境。人工智能及硅基生命或许也能在近乎绝对零度的环境中生存。在恒星陆续死亡后，也有可能是，碳基生命及其智慧体会退居到舒适的虚拟世界中，而这个虚拟世界比外部实体的宇宙还要广阔和复杂。

If humanity survives the end of the stars, it will be the largest energy source in the Universe.

如果人类能够活到恒星消亡之后，人类就会成为宇宙中最大的能量源。

Problem 5: Survive the end of galaxies

问题5：能活到银河系消亡之后吗？

Random stellar motions eventually cause galaxies to dissolve: from time to time stars pass by each other and change velocities randomly. Sometimes this gives a star an escape velocity from the galaxy and it disappears out into the great emptiness, leaving the rest of the galaxy slightly more condensed. Eventually – in about 100 million trillion years – all of the galaxy scatters or falls into the central black hole. Planets around the stars will also be flung away in the close encounters.

没有规律的恒星运动最终会导致银河系的解体。恒星彼此间不时擦肩而过，速度会随机发生改变。有时，恒星因此获得的速度能令它逃离银河系，消失在无尽的虚无之中，而余下的银河系则聚合得更加紧密。最终，在大约一万亿亿年之后，整个银河系的星体或者会四散于宇宙，或者全部掉入银河系核心的黑洞之中。围绕恒星运转的行星也会在这一巨变中被抛离运行轨道。

To survive this, intelligent beings need to steer stars to put them into orbits that are long-term stable.

智慧生命要想逃过此大灾难，就要将恒星引到长期稳定的轨道上来。

This looks physically possible! At least in the present era, one could nudge stars by placing reflectors so their radiation acts as very weak rocket engines, getting them to pass by each other in a controlled fashion. This is similar to how we humans used gravity assists to redirect and accelerate the Voyager probes, but now on a vast scale. As the stars change orbits they could be used to further nudge each other in the largest billiard game ever conceived.

从物理学的角度来说这是有可能的！就目前而言，至少可以用电磁波发射器微调恒星的运行。发射器发出的辐射就像小马力的火箭引擎一样能产生动力，以控制恒星相遇时的速度。这就好比人类利用其他天体的引力为两枚外太阳系太空探测器旅行者号调整运行方向以及提升速度一样，只不过要干扰恒星运行的电磁波发射器要巨大得多。这就像是一局最大型的台球游戏，恒星在改变轨道时也可以进一步相互推动。

It would take big structures around every star and massive planning ahead, but the total amount of matter needed is about a big asteroid per solar system and the physics is relatively straightforward. The issue is more about coordinating projects on literally billion year timescales. Which by this time might be everyday planning for a humanity that has already handled the past problems.

这需要在每一个恒星周围建造巨型工程，事前要进行巨大的规划，而工程所需要的物质总量也很大，相当于每个恒星系大约就要一颗大型的小行星。而建造的物理原理则相对清晰明了。关键是要在十亿年的时间跨度规划协调这些工程项目。不过到那个时候，人类已经克服过种种难关，这样的太空工程对他们来说可能不过是稀松平常事。

Problem 6: Survive the end of matter

问题6：存活到宇宙物质消散之后

Our kind of matter is built out of atoms composed of protons, neutrons and electrons. Protons and electrons are normally said to be perfectly stable (the neutrons are stabilised by the protons; on their own they decay with a half-life of a few minutes).

我们所谓的物质全部是由质子、中子和电子这些粒子所组成的原子构成的。质子和电子一般来说非常稳定。中子靠质子来稳定，其自身的半衰期只有几分钟。

However, many physical theories predict that protons are not truly stable and will decay over enormously long timespans. Proton decay has never been observed so far despite some heroic research efforts. But this merely tells us that it takes trillions of years, if it happens.

但许多物理学理论都认为，质子并非真的稳定，放到漫长的时间维度上看最终也将发生衰变。虽然科学界有过一些艰苦卓绝的研究，但目前还没有观察到质子的衰变。不过这只能说明如果质子真的会衰变，可能需要几万亿年的时间。

This decay will spell the end of matter as we know it. Stars and planets will slowly turn into radiation plus free electrons and positrons, unable to form habitable systems. The last cold black dwarf stars would gradually turn into helium and hydrogen crystals that quietly evaporate in stillness. The only thing left would be radiation and black holes in an otherwise empty Universe.

质子的衰变将会终结我们所认知的物质世界。宇宙中的恒星与行星物质都会慢慢解体变成电磁波及自由电子和正电子，不能再形成能供生命栖息的天体。宇宙中最后一批寒冷的黑矮星（恒星残骸）会逐渐成为静止不动的氦晶体及氢晶体，直至灰飞烟灭。这时空旷的宇宙中只有一些基本粒子和黑洞。

Can we get around it? As the great computer in Isaac Asimov’s masterful short story “The Last Question” said, "THERE IS AS YET INSUFFICIENT DATA FOR A MEANINGFUL ANSWER."

人类能够幸免于难吗？答案可以参考阿西莫夫（Isaac Asimov）优秀的短篇小说《最后的问题》（The Last Question）。小说中一台大型计算机的回答是：“数据不足，无法作答。”