Why Aliens Won't Eat Us
为什么外星人不吃我们呢？

Are aliens going to eat us?
外星人会来吃我们吗？
That's more of a science fiction question. But science does have something to say here, and sometimes purely imaginative questions are great tools for helping us think about how the real world works.
这更像是科幻小说里的才会出现的问题。但是在这个问题上我们还是可以寻求科学的答案。很多时候像这种纯粹异想天开的问题却能够帮助我们更好地思考，更好地认识这个世界。

First, it's not strictly impossible. Many scientists agree that the Milky Way galaxy is probably brimming with living things. That doesn't mean any of them are smart enough to build interstellar spaceships, but let's assume they are. Still, why are they unlikely to eat us? One reason lies in energy itself.
首先，严格地说，这也不无可能。许多科学家都认为银河系里可能到处都是生物，但这并不是说它们就有能力建造星际间飞船。退一万步说，就算它们有这个能力来建造星际间飞船，可是，它们又为什么要吃我们呢？原因之一就在于能量本身。

The point of eating is to take in usable energy. A lion will expend a lot of energy running down a zebra because, once it is caught, the calories contained in the meal far outweigh those expended in the chase. If this weren't true–say, if the zebra's body only had the caloric content of a grape–then the energy expended would outweigh the energy gained, and the lion would lose out.
吃东西最主要就是为了摄入可用的能量。一头狮子可以耗掉大量的体力去追捕一匹斑马，那是因为一旦抓住了那匹斑马，它能摄入的斑马肉里的能量大大超过了他在追捕过程中消耗的能量。如果以上都不成立，假如斑马肉里所含能量很少——少到根本就无法补充狮子追捕所耗能量，那么这头狮子也只能是白白耗掉体力了。

Any alien predators would face the same problem. The amount of energy expended in building a space program, fueling rockets, to say nothing of getting from one star to another, would be huge. For any realistic technology it would far outweigh the benefit of just munching on people. It would be like climbing Mount Everest in order to pick a few berries.
既然有这样的说法，那么食肉的外星人也会面对同样的问题。建造一个空间计划，制造燃料火箭都需要消耗很多能量。更不用说从一个星球飞至另一个星球需要消耗更多的能量！与其猎食人类，还不如将那些能量用于更有实际意义的科技开发。其实猎食人类就像是辛辛苦苦攀上珠穆朗玛却只为摘些果子一样大材小用，劳民伤财。

Some day we ourselves may travel to other planets. But any species that goes to such lengths merely to find food will find itself quickly exhausted.
也许某一天人类能够踏上其他星球的土地。但是，任何一个为寻觅食物而踏上其他星球的物种终将认识到如此行径只会消耗掉自己的能量——自取灭亡。