I saw a UFO once. I was eight or nine, playing in the street with a friend who was a couple of years older,
我曾经看过一次UFO。我当时约八、九岁，和一位朋友在街上玩耍，他比我大几岁，
and we saw a featureless silver disc hovering over the houses.
我们看到了一个毫无特色的银色碟形物在房子上方盘旋。
We watched it for a few seconds, and then it shot away incredibly quickly.
我们看了几秒钟，接着，它以非常快的速度瞬间飞走。
Even as a kid, I got angry it was ignoring the laws of physics.
即使当时只是孩子，我很气它竟然可以不遵守物理定律。
We ran inside to tell the grown-ups, and they were skeptical -- you'd be skeptical too, right?
我们跑进屋内告诉大人，大人充满怀疑--换成是你们也会怀疑，对吧？
I got my own back a few years later: one of those grown-ups told me,
几年后我也找回了我的怀疑：其中一位大人告诉我：
"Last night I saw a flying saucer. I was coming out of the pub after a few drinks."
“昨晚我看到了飞碟。 我在酒吧喝了几杯，当时正要离开。”
I stopped him there. I said, "I can explain that sighting."
我在那时打断他。我说：“我能解释你的目击。”
Psychologists have shown we can't trust our brains to tell the truth. It's easy to fool ourselves.
心理学家已经证明了我们不能相信我们的大脑会告诉我们真相。要愚弄自己是很容易的。
I saw something, but what's more likely -- that I saw an alien spacecraft, or that my brain misinterpreted the data my eyes were giving it?
我看到某种东西，但哪种情况比较有可能--是我看到了外星人的宇宙飞船，还是我的大脑错误诠释了我的眼睛给它的资料？
Ever since though I've wondered: Why don't we see flying saucers flitting around?
从那之后，我就一直纳闷：为什么我们不会到处看到飞碟？
At the very least, why don't we see life out there in the cosmos?
至少，为什么我们不会看到在宇宙中有生命？
It's a puzzle, and I've discussed it with dozens of experts from different disciplines over the past three decades.
这是个谜，在过去三十年，我已经和数十位不同领域的专家讨论过。
And there's no consensus. Frank Drake began searching for alien signals back in 1960 -- so far, nothing.
没有达成任何共识。1960年，法兰克·德雷克就开始寻找外星人的线--目前，一无所获。
And with each passing year, this nonobservation,
随着每一年过去，都没人观察到，
this lack of evidence for any alien activity gets more puzzling because we should see them, shouldn't we?
并且缺乏外星人活动的证据，让这一切更令人迷惑不解，因为我们应该会看见他们，对吧？
The universe is 13.8 billion years old, give or take.
宇宙已经138亿岁了，这是概略数字。
If we represent the age of the universe by one year, then our species came into being about 12 minutes before midnight, 31st December.
若我们把宇宙的年龄比喻为一年，那么我们这个物种大约是在午夜前12分钟才出现的，12月31日。
Western civilization has existed for a few seconds. Extraterrestrial civilizations could have started in the summer months.
西方文明才几秒钟而已。外星文明可能在夏季就已经出现。
Imagine a summer civilization developing a level of technology more advanced than ours,
想象一个夏季文明，发展出了比我们更先进的科技水平，
but tech based on accepted physics though, I'm not talking wormholes or warp drives -- whatever
不过，我是指以已经被接受的物理为基础，不是指虫洞或曲速引擎--诸如此类，
just an extrapolation of the sort of tech that TED celebrates.
只是TED所颂扬的那种科技的推断而已。
That civilization could program self-replicating probes to visit every planetary system in the galaxy.
该文明有可能可以设计出会自我复制的探测飞船，去探访银河系中的每一个星系。
If they launched the first probes just after midnight one August day, then before breakfast same day, they could have colonized the galaxy.
如果他们在八月某一天的午夜之后发射了第一艘探测飞船，那么在同一天的早餐时间之前，他们就已经殖民了整个银河系。
Intergalactic colonization isn't much more difficult, it just takes longer.
跨银河系的殖民并不会困难太多，只是比较花时间。
A civilization from any one of millions of galaxies could have colonized our galaxy.
数百万银河系中的任何一个文明都有可能在我们的银河殖民。
Seems far-fetched? Maybe it is, but wouldn't aliens engage in some recognizable activity
似乎很牵强？也许是吧，但外星人不会从事一些可辨识的活动吗？
put worldlets around a star to capture free sunlight, collaborate on a Wikipedia Galactica, or just shout out to the universe, "We're here"?
比如在一颗恒星的周围建立小世界来捕捉免费的阳光、合作撰写银河维基百科，或是对着宇宙大叫：“我们在这里！”
So where is everybody? It's a puzzle because we do expect these civilizations to exist, don't we?
所以，他们在哪里？这是个谜，因为我们确实预期会有这类文明存在，对吧？
After all, there could be a trillion planets in the galaxy -- maybe more.
毕竟，银河系中可能有一万亿个星球--甚至更多。
You don't need any special knowledge to consider this question, and I've explored it with lots of people over the years.
不需要任何特殊知识，你也会去思考这个问题，这些年来，我已经和许多人一起探索了这个问题。
And I've found they often frame their thinking in terms of the barriers
我发现，他们通常在表述他们的想法时，
that would need to be cleared if a planet is to host a communicative civilization.
会用需要被清除的障碍来思考一个星球是否会有能够沟通的文明存在。
And they usually identify four key barriers.
他们通常会去确认四项关键障碍。
Habitability -- that's the first barrier.
可居住性--那是第一个障碍。
We need a terrestrial planet in that just right "Goldilocks zone," where water flows as a liquid. They're out there.
我们需要有一个刚好属于“适居带”的外星星球，也就是说，水要以液体形式流动。外面有这种星球存在。
In 2016, astronomers confirmed there's a planet in the habitable zone of the closest star, Proxima Centauri
2016年，航天员确认了在最近的恒星适居带上有一个星球，这个恒星是比邻星，
so close that Breakthrough Starshot project plans to send probes there.
它非常近，近到“突破摄星”计划打算要送探测飞船过去。
We'd become a starfaring species. But not all worlds are habitable.
我们成了在星际旅行的物种。但并非所有世界都可居住。
Some will be too close to a star and they'll fry, some will be too far away and they'll freeze.
有些会太靠近恒星，会被烤熟，有些则太遥远，会被冰冻。
Abiogenesis -- the creation of life from nonlife -- that's the second barrier.
无生源论--从无生命到创造生命--那是第二个障碍。
The basic building blocks of life aren't unique to Earth: amino acids have been found in comets,
生命的基础建材并非地球独有：已经发现慧星上有氨基酸，
complex organic molecules in interstellar dust clouds, water in exoplanetary systems.
行星际尘云中有复杂的有机分子，行星系统中有水。
The ingredients are there, we just don't know how they combine to create life,
原料已经存在，我们只是不知道它们要如何结合起来创造生命，
and presumably there will be worlds on which life doesn't start.
很有可能，这些世界的生命迹象尚未开始。
The development of technological civilization is a third barrier.
第三个障碍是科技文明的发展。
Some say we already share our planet with alien intelligences.
有些人说，我们已经在和外星智慧共享我们的星球。
A 2011 study showed that elephants can cooperate to solve problems.
一篇2011年的研究指出，大象能够合作来解决问题。
A 2010 study showed that an octopus in captivity can recognize different humans.
一篇2010年的研究指出，被囚禁的章鱼能够认出不同的人类。
2017 studies show that ravens can plan for future events -- wonderful, clever creatures
2017年的研究则指出，渡鸦会规划未来事件--很美好、聪明的生物，
but they can't contemplate the Breakthrough Starshot project, and if we vanished today,
但它们无法去思考“突破摄星”计划，如果今天我们都消失了，
they wouldn't go on to implement Breakthrough Starshot -- why should they?
它们也不会去执行“突破摄星”计划—--为什么它们要去执行？
Evolution doesn't have space travel as an end goal. There will be worlds where life doesn't give rise to advanced technology.
演化的最终目标并不是太空旅行。有一些世界中的生命不会去发展先进科技。
Communication across space -- that's a fourth barrier.
跨越太空的沟通--那是第四个障碍。
Maybe advanced civilizations choose to explore inner space rather than outer space, or engineer at small distances rather than large.
也许先进文明会选择探索内太空，而不是外太空，或做短距离的工程建设，而非长距离的。
Or maybe they just don't want to risk an encounter with a potentially more advanced and hostile neighbor.
或许，他们只是单纯不想冒险，以免遇到可能更先进且有敌意的邻居。
There'll be worlds where, for whatever reason, civilizations either stay silent or don't spend long trying to communicate.
在某些世界，不论是什么理由，文明可能会保持沉默，或不会花费心力去试图与远方沟通。
As for the height of the barriers, your guess is as good as anyone's.
至于障碍有多高，大家都只能猜测。
In my experience, when people sit down and do the math, they typically conclude there are thousands of civilizations in the galaxy.
依我的经验，当大家坐下来算数学时，他们的结论通常是，在银河系中有数千个文明存在。
But then we're back to the puzzle: Where is everybody? By definition, UFOs -- including the one I saw -- are unidentified.
但我们又回到原本的谜：他们在哪里？就定义来说，UFO--包括我看见的那一个--是“不明”的。
We can't simply infer they're spacecraft. You can still have some fun playing with the idea aliens are here.
我们不能很简单地说它们是宇宙飞船。你仍然可以为了好玩而想象有外星人来到这里。
Some say a summer civilization did colonize the galaxy and seeded Earth with life ...
有人说，夏季文明的确已经殖民了银河系，并且在地球播下生命的种子……
others, that we're living in a cosmic wilderness preserve -- a zoo.
其他人说，我们是住在宇宙的荒野保护区--一个动物园。
Yet others -- that we're living in a simulation. Programmers just haven't revealed the aliens yet.
还有其他人--说我们住在模拟世界中。只是程序设计师尚未让外星人现身而已。
Most of my colleagues though argue that E.T. is out there, we just need to keep looking, and this makes sense.
不过，我大部分的同事主张有E.T.存在，只是我们得要继续寻找，这是合理的。
Space is vast. Identifying a signal is hard, and we haven't been looking that long.
太空很大。找到一个信号是很困难的，并且我们才开始找没有很久。

Without doubt, we should spend more on the search.
无疑，我们应该投入更多心力去搜寻。
It's about understanding our place in the universe. It's too important a question to ignore.
重点在于了解我们在宇宙中的定位。这个问题太重要了，不能无视它。
But there's an obvious answer: we're alone. It's just us.
但有一个很明显的答案：我们是孤单的。只有我们。
There could be a trillion planets in the galaxy.
在银河系可能有一万亿个星球。
Is it plausible we're the only creatures capable of contemplating this question?
若说我们是唯一会思考这个问题的生物，听起来合理吗？
Well, yes, because in this context, we don't know whether a trillion is a big number.
是的，因为在这个情况中，我们不知道一万亿是否算是很大的数目。
In 2000, Peter Ward and Don Brownlee proposed the Rare Earth idea.
2000年时，彼得·沃德和唐·布朗利提出了“地球殊异”的想法。
Remember those four barriers that people use to estimate the number of civilizations? Ward and Brownlee said there might be more.
还记得人们用来估计文明数目的那四个障碍吗？沃德和布朗利说，可能还有更多。
Let's look at one possible barrier. It's a recent suggestion by David Waltham, a geophysicist.
咱们来看看一个可能的障碍。这是地球物理学家大卫·沃尔瑟姆在近期提出来的。
This is my very simplified version of Dave's much more sophisticated argument.
这是我的极简化版，大卫的原版是更精密许多的主张。
We are able to be here now because Earth's previous inhabitants enjoyed four billion years of good weather
我们现在能够在这里，是因为地球先前的居民享受了40亿年的好天气，
ups and downs but more or less clement.
虽然起起落落，但总的来说仍然是温和的。
But long-term climate stability is strange, if only because astronomical influences can push a planet towards freezing or frying.
但长期的稳定气候是很奇特的，因为天文气候的影响是有可能将一个星球推向结冰或烤熟状况的。
There's a hint our moon has helped, and that's interesting because the prevailing theory is that the moon came into being when Theia,
我们的月球似乎是有帮上忙，那很有趣，因为普遍流行的理论是月球会出现是因为忒伊亚，
a body the size of Mars, crashed into a newly formed Earth.
和火星一样大的天体，撞上了刚形成的地球。
The outcome of that crash could have been a quite different Earth-Moon system.
撞击的结果本来有可能是个很不同的地球—月球系统。
We ended up with a large moon and that permitted Earth to have both a stable axial tilt and a slow rotation rate.
我们最后得到的是一个大月球，那让地球有一个稳定的轴向倾斜角度和缓慢的转速。
Both factors influence climate and the suggestion is that they've helped moderate climate change. Great for us, right?
这两个因子影响了气候，意味着它们协助调节了气候改变。对我们很好，对吧？
But Waltham showed that if the moon were just a few miles bigger, things would be different.
但沃尔瑟姆指出，如果月球再大个几英里，一切就会不同。
Earth's spin axis would now wander chaotically. There'd be episodes of rapid climate change -- not good for complex life.
地球的转轴现在就会混乱地偏离正轨。就会发生快速气候变迁的情形--对复杂的生命来说不是好事。
The moon is just the right size: big but not too big.
月球的大小刚刚好：够大但不会太大。
A "Goldilocks" moon around a "Goldilocks" planet -- a barrier perhaps.
在一个“适居”的星球周围有个“适居”的月球--也许也是个障碍。
You can imagine more barriers. For instance, simple cells came into being billions of years ago ...
你们可以想象出更多障碍。比如，数十亿年前，简单细胞就出现了……
but perhaps the development of complex life needed a series of unlikely events.
但也许要发展出复杂的生命，还需要一连串不太可能发生的事件。
Once life on Earth had access to multicellularity and sophisticated genetic structures,
一旦地球上的生命能够有多细胞以及精密的基因结构，
and sex, new opportunities opened up: animals became possible.
还有性，就会有新的机会产生：可能会出现动物。
But maybe it's the fate of many planets for life to settle at the level of simple cells.
但也许许多星球的命运就是只会有简单细胞等级的生命存在。
Purely for the purposes of illustration,
纯粹只是为了说明清楚，
let me suggest four more barriers to add to the four that people said blocked the path to communicative civilization.
让我再提出四个可能的障碍，来补充前面已提到大家认为会阻挡与外星文明沟通的四个障碍。
Again, purely for the purposes of illustration, suppose there's a one-in-a-thousand chance of making it across each of the barriers.
我再说一遍，纯粹只是为了补充说明清楚，假设跨越每一个障碍的机会是千分之一。
Of course there might be different ways of navigating the barriers, and some chances will be better than one in a thousand.
当然，可能会有不同的方式来通过障碍，有些机会可能会高于千分之一。
Equally, there might be more barriers and some chances might be one in a million. Let's just see what happens in this picture.
同样的，可能会有更多障碍并且当中某些的机会只有百万分之一。咱们来看看在这状况下会发生什么事。
If the galaxy contains a trillion planets,
如果银河系有一万亿个星球，
how many will host a civilization capable of contemplating like us projects such as Breakthrough Starshot?
有多少个星球上会有文明存在，并且像我们一样能够去思考像“突破摄星”这样的计划？
Habitability -- right sort of planet around the right sort of star -- the trillion becomes a billion.
可居住性--在对的恒星周围有对的星球--一万亿剩下了十亿。
Stability -- a climate that stays benign for eons -- the billion becomes a million.
稳定性--气候要长久都能很宜人--十亿剩下了一百万。
Life must start -- the million becomes a thousand.
生命必须要能开始--一百万剩下了一千。
Complex life forms must arise -- the thousand becomes one.
必须要有复杂的生命形式出现--一千剩下了一个。
Sophisticated tool use must develop -- that's one planet in a thousand galaxies.
必须要能够去使用精密的工具-- 一千个银河中只有一个星球能符合。
To understand the universe, they'll have to develop the techniques of science and mathematics -- that's one planet in a million galaxies.
要了解宇宙，他们就得要发展出科学和数学的技术--一百万个银河中只有一个星球能符合。
To reach the stars, they'll have to be social creatures,
要去接触其他恒星，他们必须要是社交型的生物，
capable of discussing abstract concepts with each other using complex grammar -- one planet in a billion galaxies.
有能力和彼此讨论抽象的概念，使用复杂的文法--十亿个银河中只有一个星球能符合。
And they have to avoid disaster -- not just self-inflicted but from the skies, too.
他们还得要避开灾难--不只是自己造成的灾难，还有来自天空的灾难。
That planet around Proxima Centauri, last year it got blasted by a flare.
绕着比邻星的那个星球，在去年被太阳耀斑给轰击了。
One planet in a trillion galaxies, just as in the visible universe.
一万亿个银河中只有一个星球能符合，这是在可见宇宙中的假设。
I think we're alone. Those colleagues of mine who agree we're alone often see a barrier ahead -- bioterror, global warming, war.
我想我们是孤单的。我那些认同我们是孤单的同事通常能够先看到障碍--生物灾难、全球变暖、战争。
A universe that's silent because technology itself forms the barrier to the development of a truly advanced civilization. Depressing, right?
一个宇宙是沉默的，因为科技本身就会形成障碍，让一个真正先进的文明很难发展。让人沮丧，是吧？
I'm arguing the exact opposite. I grew up watching "Star Trek" and "Forbidden Planet,"
我的主张正好相反。我是看《星舰迷航记》与《禁忌星球》长大的，
and I saw a UFO once, so this idea of cosmic loneliness I certainly find slightly wistful.
并且我见过一次UFO，所以我肯定对这个宇宙孤独的想法感到有点愁闷。
But for me, the silence of the universe is shouting, "We're the creatures who got lucky."
但对我来说，宇宙的沉默正在吶喊：“我们是走运的生物。”
All barriers are behind us. We're the only species that's cleared them -- the only species capable of determining its own destiny.
我们越过了所有的障碍。我们是唯一清除了所有障碍的物种--唯一能够决定自身命运的物种。
And if we learn to appreciate how special our planet is, how important it is to look after our home and to find others,
如果我们学会带着感激欣赏我们的星球有多特别、照顾我们的家园并寻找其他的是多么重要的事、
how incredibly fortunate we all are simply to be aware of the universe, humanity might survive for a while.
单单知道宇宙的存在我们就有多么幸运，那么人类也许还能生存一阵子。
And all those amazing things we dreamed aliens might have done in the past, that could be our future. Thank you very much.
所有那些不可思议的事，我们幻想外星人在过去可能做过的事，都可能成为我们的未来。非常感谢。