*We might not be alone in this big wide Universe*

在这广袤无垠的宇宙中，我们或许并不孤单。

Whether it’s giant war-mongering spaceships, green humanoid Martians, or creepy beings slithering across uncharted planets, science fiction has painted a very particular image when it comes to aliens.

But while you might think it takes a tin-foil-wearing conspiracy theorist to believe in such life outside of Earth, many highly distinguished scientists are believers too.

无论是好战的巨大战舰、绿色类人“火星人”，还是在未知行星上蠕动的诡异生物，科幻作品为外星人塑造了非常特定的形象。可你也许会以为，只有“头戴锡纸帽”的阴谋论者（指担心被读心或监控的极端怀疑者的调侃称呼）才会相信地球之外存在这种生命；但事实上，许多卓有成就的科学家也持这种观点。

In fact, when surveyed, 86.6 per cent of astrobiologists recently said they either agreed or strongly agreed that extraterrestrial life likely exists somewhere else in the Universe. When asked the same question but for complex or intelligent life, the agreeing percentage only drops to 58.2 per cent. More believe than don’t.

事实上，最近的一项调查显示，86.6%的天体生物学家表示，他们“同意”或“强烈同意”这样一种看法：在宇宙的其他地方很可能存在地外生命。把同样的问题换成“复杂的或智能的生命”时，表示同意的比例也仅下降到58.2%。相信的人多于不相信的人。

So, what do these scientists know that we don’t know? And why are they so sure we’re not alone in this Universe?

那么，这些科学家知道哪些我们不知道的事？他们为何如此确信在这片宇宙中我们并不孤单？

**Why aliens just might exist**

为什么外星人可能真的存在

There is nothing special or remotely unique about Earth or humanity. That’s not our opinion – that’s the overarching idea of the Copernican principle. This is the belief that, while we often see Earth as superior, it is no more special than any other planet.

地球和人类并没有什么特别之处，更谈不上独一无二。这并非我们的主观看法——这正是“哥白尼原理”的核心思想（即人类与地球不处于宇宙的特殊位置）。该原理认为：尽管我们常把地球视为更高级、更重要，但它并不比任何其他行星更特殊。

But what does that have to do with aliens? Well, many who study extraterrestrial life believe that Earth’s habitable state, combined with the development of diverse intelligent life, is a sign that the same is possible elsewhere. And given the sheer scale of the cosmos, some scientists argue the question isn’t if aliens exist, but where.

这和外星人有什么关系呢？研究地外生命的许多人认为，地球的可居住状态（habitable），再加上多样化智能生命的出现，正说明在别处也可能发生同样的事情。考虑到宇宙的庞大尺度，一些科学家认为，我们要问的问题不该是“外星人是否存在”，而是“他们在哪里”。

For instance, our galaxy contains somewhere between 100 and 400 billion stars. Astronomers have already confirmed around 4,000 planets orbiting other stars – and that’s in our galaxy alone. The Milky Way itself is just a tiny fraction of the observable Universe, about one millionth of it. Scale that up, and you’re looking at roughly 100 sextillion stars (100,000,000,000,000,000,000,000) – each potentially hosting planets.

比如，我们的银河系中大约有1,000亿到4,000亿颗恒星。天文学家已经确认了大约4,000颗绕其他恒星运行的行星——而这还仅限于我们这一个星系。银河系本身只是整个可观测宇宙的极小一部分，约为其百万分之一。按这个比例推算，可观测宇宙中大约有100 sextillion（百秭，100,000,000,000,000,000,000,000，即10的23次方）颗恒星——每一颗都有可能拥有行星。

That’s an incomprehensibly large number. For instance, it’s not that there’s likely more planets in the Universe than grains of sand on Earth, but 13.33 billion times more. With odds like that, it feels almost inevitable that at least one of them has given rise to intelligent alien life.

那是一个大到难以想象的数字。举例来说，宇宙中的行星数量很可能不只是多于地球上的沙粒，而是多出133.3亿倍。以这样的几率来看，似乎几乎不可避免，其中至少有一颗已经孕育出智能的外星生命。

“We now know that most stars have a planetary system. That means there are loads of places where life can arise. Besides, simple life arose so quickly on Earth. To me, that’s a key sign that it must be happening elsewhere,” says Mike Garrett, chair of astrophysics at the University of Manchester.

“我们现在知道，大多数恒星都有行星系统。这意味着生命可能出现的地方多得很。再者，简单生命在地球上出现得非常快。对我来说，这是一个关键信号，表明这种事情在别处也一定正在发生。”曼彻斯特大学天体物理学讲席教授Mike Garrett说。

And it’s not just the sheer number of planets that makes alien life seem plausible – it’s what they’re made of. The basic ingredients for life are more common than you might think.

让外星生命显得可信的，不仅仅是行星数量之多——更在于这些行星由什么构成。生命的基本“原料”比你想象的要常见。

Carbon, hydrogen, oxygen and nitrogen – the elements that make up living things on Earth – have all been detected on comets, asteroids and distant worlds. Astronomers have also identified many planets where water, another key ingredient, is present.

碳、氢、氧和氮——这些构成地球上生物的元素——都已在彗星、小行星以及遥远的行星上被探测到。天文学家还确认，在许多行星上存在水这种另一种关键成分。

Take KOI-5715.01 – an exoplanet around 3,000 light-years from us. The name might not sound catchy, but this world is one of the most Earth-like we’ve found so far. It’s a similar size to our planet, sits in a habitable temperature range, and shows signs of water – making it a prime candidate in the hunt for alien life.

以 KOI-5715.01 为例——一颗距离我们约3,000光年的系外行星（指绕其他恒星运行的行星）。名字也许不够上口，但它是目前发现的最像地球的世界之一。它的大小与地球相近，位于适居温度范围内，并且出现了水的迹象——这使它成为搜寻外星生命的重点候选目标。

We can also turn to Earth as a sign that life can thrive in places once thought impossible. From acidic hot springs to deep-sea vents and the frozen Antarctic, a host of recently discovered ‘extremophiles’ show that living organisms can adapt to far harsher conditions than a mild, Earth-like planet.

我们也可以把地球本身当作一个信号：生命能够在曾被认为不可能生存的地方茁壮生长。从酸性温泉到深海热液喷口，再到冰封的南极，近年发现的大量“嗜极生物”（能在极端环境中生存的生物）表明，生物体能够适应远比温和、类地行星环境严酷得多的条件。

Giant tube worms, for example, thrive in the pitch-black depths of the ocean, clustered around hydrothermal vents that spew out scalding, mineral-rich water at temperatures of up to 350 °C. Despite the intense heat, crushing pressure and complete absence of sunlight, these worms form vast colonies, relying on symbiotic bacteria to turn the vent’s chemicals into energy.

例如，巨型管状蠕虫（giant tube worms）在漆黑的深海中茁壮生长，成群聚集在海底热液喷口（hydrothermal vents）周围，这些喷口会以高达 350 °C 的温度喷出灼热、富含矿物质的水。尽管那里炽热难耐、压力巨大、完全没有阳光，这些蠕虫仍能形成庞大的群落，依靠共生细菌（symbiotic bacteria）把喷口中的化学物质转化为能量。

Even more extreme is Deinococcus radiodurans, a microbe whose name roughly translates to “terrible radiation-surviving berry”.

更为极端的是“耐辐射奇球菌”（Deinococcus radiodurans），其学名大致可译为“可怖的耐辐射‘浆果’（球菌）”。

This bacterium can endure radiation levels thousands of times higher than what would kill a human, shrugging off conditions that would sterilise most known life. It has been found surviving not only in radioactive waste sites but also in the vacuum of space during experiments on the International Space Station.

这种细菌能够承受比足以致人于死地的辐射强度高出数千倍的剂量，甚至无惧足以让大多数已知生命体被“灭菌”的环境。研究发现，它不仅能在放射性废物场所存活，在国际空间站的实验中还被证实可以在太空的真空环境中生存。

“When the Earth formed 4.5 billion years ago, at first its surface was hellishly hot. Flowing oceans of magma covered the Earth’s surface, creating an inhospitable environment for life. And yet, life arose as soon as the terrain cooled,” says Douglas Vakoch, author and founder of METI (Messaging to Extra-Terrestrial Intelligence), an organisation researching communication with alien life.

“当地球在约45亿年前形成时，起初其表面炽热如地狱。汹涌流动的岩浆之海覆盖着地表，创造出一个对生命极不友好的环境。可即便如此，一旦地形冷却，生命便随之出现。”Douglas Vakoch 如是说。他是 METI（Messaging to Extra-Terrestrial Intelligence，向地外智能发送信息）的作者与创始人，该组织致力于研究与外星生命的交流。

“Once life arose on Earth, it spread everywhere. The same evolutionary principles that allow life to adapt on Earth should operate throughout the Universe, creating exotic alien lifeforms unlike anything that we can imagine.”

“一旦生命在地球上诞生，它便扩散到各处。让生命得以在地球上适应环境的同样进化原理，理应在整个宇宙中同样发挥作用，从而孕育出我们难以想象的奇异外星生命形态。”

Even if alien life is out there, part of the challenge is finding it. For example, if you dedicated each member of the human race to search for aliens, every person would have to explore 13,000 planets. While that has remained challenging for years, scientists believe that it is becoming easier to find the signs that would show an alien world is inhabited.

即使外星生命确实存在，难题的一部分在于如何把它找出来。比如说，假如把全人类都投入到搜寻外星人的任务中，平均到每个人也得探索13,000颗行星。尽管多年来这一直极其艰难，但科学家认为，要发现能表明某个外星世界“有生命居住”的迹象，正变得越来越容易。

“We’re in a much better position to discover it now than we were five years ago. Everything is going in the right direction for detection. Especially for the detection of techno-signatures (a measurable signature that shows evidence of past or present technology on a planet),” says Garrett.

“与五年前相比，我们现在发现它的条件好得多。一切都在朝着有利于探测的方向发展。尤其是在探测‘技术签名’方面（即能表明某颗行星上存在过去或当前技术活动的、可被测量的信号），”加勒特（Garrett）说。

“Advancements in computing, digitisation, and artificial intelligence mean that we are moving very rapidly towards alien detection because the technology improves so fast now.”

计算、数字化和人工智能方面的进步，意味着我们正以极快的速度逼近对外星生命的探测，因为相关技术如今发展得非常迅速。

Vakoch agrees. “In the next 20 years, we’ll have telescopes that can scan the atmospheres of planets that orbit stars as we search for signs of life,” he says.

瓦科奇表示同意。“在未来20年里，我们将拥有能够扫描围绕恒星运行的行星大气的望远镜，用以寻找生命迹象，”他说。

“When the European Space Agency’s ARIEL mission (a plan to observe and study 1000 exoplanets) launches in 2029, we’ll have much greater capabilities to discover life through the chemical byproducts that life inevitably gives off.”

“当欧洲航天局（European Space Agency）的ARIEL任务（一个观测并研究1000颗系外行星的计划）在2029年发射时，我们将更有能力通过生命不可避免释放的化学副产物来发现生命。”

**Why haven’t we found them yet?**

为什么我们至今还没有发现它们（外星人）？

So if aliens likely exist out there, why haven’t we discovered them yet? And why have we never received any extraterrestrial visitors? Welcome to the Fermi Paradox.

那么，如果外星人很可能确实存在于某处，为什么我们至今还没有发现他们？又为什么从未接待过任何来自外星的访客？欢迎来到“费米悖论”（Fermi Paradox）。

Over lunch way back in 1950, physicist Enrico Fermi threw out an intriguing question to his colleagues about extraterrestrials: “Where are they?” If intelligent life is common in the galaxy, Fermi argued, then Earth should already have been visited many times over.

早在1950年的一次午餐上，物理学家Enrico Fermi向同事抛出了一个关于外星生命的耐人寻味的问题：“他们在哪儿？”Fermi认为，如果银河系里智能生命很常见，那么地球本该早已被造访过许多次了。

It was a question that cut straight to the heart of the matter – and one scientists are still wrestling with today. Entire libraries of theories have been written to explain it, but some think the real answer could be surprisingly simple: a Universe big enough to host life is also big enough to hide it.

这是一个直指问题核心的发问——而且科学家们至今仍在为之苦苦思索与争论。为了解释它，人们提出了汗牛充栋的理论，但也有人认为真正的答案也许出奇简单：一个足够容纳生命的宇宙，也足以将生命隐藏起来。

“Life on Earth – especially the rise of macroscopic, complex life – required a series of extremely unlikely coincidences regarding the composition and nature of Earth and its surface,” says Jason Wright, a professor of astronomy and astrophysics at Pennsylvania State University.

“地球上的生命——尤其是宏观、复杂生命的出现——需要在地球及其表面的组成与性质方面发生一连串极其不可能的巧合，”宾夕法尼亚州立大学天文学与天体物理学教授Jason Wright（贾森·赖特）说。

“After all of that, Earth then required a series of extremely unlikely evolutionary innovations to occur. Those odds are already pretty small, but then you throw in the fact that the nearest aliens could be very far away.”

在此之后，地球还必须经历一连串极不可能的进化创新（即关键性的新特征与能力的演化突破）。这种几率本已微乎其微，更何况还要把这样一个事实算进去：距离我们最近的外星文明也可能非常遥远。

In other words, if life did exist elsewhere, we might just never be able to find it. Humans have inhabited Earth for a couple of hundred thousand years, which, in the grand scheme of the Universe, is short. We’ve only been around for 0.000002 per cent of its entire history.

换句话说，即便宇宙中别处确实存在生命，我们也可能永远无法找到它。人类在地球上生存不过几十万年，从宇宙的宏大时间尺度来看，这极其短暂。放在宇宙的整个历史中计算，我们只存在了0.000002%（约为二亿分之一）的时间。

Other intelligent civilisations could have come and gone, or are in the early stages of development. For us to discover life outside of Earth, many events need to occur at the same time.

其他智能文明可能已经兴衰更替，或者还处于发展的早期阶段。要让我们发现地外生命，许多条件需要在同一时期同时满足。

“Science is inherently sceptical. We want proof – and so far, there’s no direct proof of life beyond Earth,” says Vakoch. “But a few decades ago, the same was true for planets orbiting other stars. We thought they might exist, but we couldn’t see them.”

Vakoch 说：“科学本质上是怀疑的。我们需要证据——而到目前为止，还没有地球之外生命的直接证据。不过，几十年前，围绕其他恒星运行的行星情况也一样。我们以为它们可能存在，但我们看不见它们。”

That changed in the 1990s, when the first exoplanets were confirmed. Today, astronomers have discovered thousands, proving that other worlds are not only possible but abundant.

这种情况在20世纪90年代发生了改变——第一批系外行星（围绕太阳以外恒星运行的行星）得到了确认。如今，天文学家已经发现了数千颗，证明其他世界不仅可能存在，而且还非常普遍。

“To find them, astronomers first had to believe they were out there,” Vakoch adds. “Then they did the hard work of uncovering them.”

“要想找到它们，天文学家首先得相信它们确实在那里，”Vakoch补充道。“然后再投入辛勤的工作，把它们发现出来。”

For many researchers, alien life feels like it’s at the same stage exoplanets once were: strongly suspected, but still unseen. The tools to detect it are improving, but the breakthrough could take decades – or longer.

对许多研究者而言，外星生命如今所处的阶段，正如当年的系外行星：大家高度怀疑它存在，却仍未见其真容。用于探测它的工具在不断进步，但实现突破可能还需要几十年——甚至更久。

But who knows, maybe somewhere billions of light-years away, on an incredibly distant planet, an intelligent lifeform is sitting contemplating the thought: “Are we alone in the Universe?”

但谁又能确定呢？也许在数十亿光年之外的某颗极其遥远的行星上，某种智能生命正坐在那里沉思："我们在宇宙中是孤独的吗？"