【英语时差8,16】为什么外星人不想吃我们

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Dear A Moment of Science. Are aliens going to eat us? Signed, Frightened.  Well, Frightened, that's more of a science fiction question. But science does have something to say here, and sometimes purely¹ imaginative questions are great tools for helping² us think about how the real world works.

First, it's not strictly³ impossible. Many scientists agree that the Milky⁴ Way galaxy⁵ 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.

The point of eating is to take in usable energy. A lion will expend⁶ a lot of energy running down a zebra because, once it is caught, the calories contained in the meal far outweigh⁷ those expended⁸ 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.

Any alien predators⁹ 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 munching¹⁰ on people. It would be like climbing Mount Everest in order to pick a few berries.

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 exhausted¹¹.   

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