Back in 1950, the Nobel prize-winning physicist Enrico Fermi washaving lunch with friends.
早在1950年，诺贝尔奖获得者恩里克费尔米正与他的朋友吃午餐。
Conversation turned to the possibilityof extraterrestrial life.
他们的话题转到是否可能存在外太空生命上。
According to the popular story, Fermi fell silent for a moment and thenasked the simple but profound question: Where Is Everybody?
按照那个受欢迎故事的说法，当时费米尔沉默了一会，然后问到了这个简单而又深刻的问题：他们都去哪了？
That question is now known as the Fermi Paradox.
现在，那个问题被称为是费米尔悖论。
A version goes like this.
有个版本是这样的：
There are a fewhundred billion stars in the Milky Way Galaxy.
在银河系里，有几千亿颗恒星。
Suppose even half those stars have planets.
假设说那些恒星半数周围有行星。
Then suppose only one or two of, say, ten planets per star is conducive to the formation of life, asis the case with our own solar system.
然后假设和我们的太阳系一样，每颗恒星周围每十颗行星有一颗或两颗有利于形成生命。
Suppose life takes a billion years to get started, ashappened on earth.
假设那的生命和地球上的一样，要花费十亿年才开始形成。
Suppose the evolution of higher life forms takes another couple billion years,as it did here.
再假设更高级形式的生命要再花几十亿年才能进化，就如地球上所经历的那样。
None of that would matter.
没有一项有问题。
The Milky Way galaxy is around ten billion years old.
银河系已经近一百亿光年了。
Even with veryconservative estimates, it should still be swarming with life-even intelligent life.
即使很保守的估计，它应该还是挤满了生命—甚至是智能生命。
The late Carl Sagancalculated that there should be something like a million advanced civilizations out there right now.
后来的卡尔萨根计算了一下，目前应该大约有100万先进文明在那了。
So where are they?
那么他们在哪呢？
Tabloid stories aside, there's no scientific evidence from any corner to suggestwe aren't alone in the Milky Way.
把小报故事放一边看，现在还没有任何角度的科学证据表明我们不是银河系中唯一存在的人类。
But why not? The evidence should be everywhere.
但是为什么不是呢？证据应该到处都是。
That's Fermi's paradox.
那就是费米尔悖论。
It has nothing to do with whether you believe in flying saucers or not.
你是否相信有飞碟都不重要。
It'sa simple estimation: given what we know about our star, our planet, and how life evolved here, wecan extrapolate to what probably happens elsewhere.
这只是一个简单的估计：假如让我们知道有关我们所在的恒星，行星的知识，还有生命如何进化的，那就可以推断别处可能发生什么。
And yet: there's no sign of alien civilizations.
可是，还没有外星文明的迹象呢。
Something must have gone wrong in our thinking.
我们的思想一定是哪出了问题。
But what? More next time.
但是是什么问题呢？下次再说吧。
