EVERYBODY OUT
人类大迁徙
Q. Is there enough energy to move the entire current human population off-planet?
Q．我们有足够的能量把现在地球上的全部人口都搬到太空中吗？
A. THERE ARE A BUNCH of science fiction movies where, because of pollution, overpopulation, or nuclear war, humanity abandons Earth.
A．许多科幻电影中都出现了与此类似的情景：人类出于污染、人口过多或者核战争等原因放弃了地球。
But lifting people into space is hard. Barring a massive reduction in the population, is launching the whole human race into space physically possible? Let 8217;s not even worry about where we 8217;re headed-we 8217;ll assume we don 8217;t have to find a new home, but we can 8217;t stay here.
把人弄到太空中并不容易。不考虑人口暴减的可能性，那么把所有人口发射到太空中去在物理上可行吗？甚至假设我们不用关心到了太空之后要去哪儿，我们就是想要离开地球。
To figure out if this is plausible, we can start with an absolute baseline energy requirement: 4 gigajoules per person. No matter how we do it, whether we use rockets or a cannon or a space elevator or a ladder, moving a 65-kilogram person-or 65 kilograms of anything-out of the Earth 8217;s gravity well requires at least this much energy.
要想知道这么做是否可行，我们先要知道一个绝对底线：把一个人送到太空所需要的最低能量是40亿焦耳。不管你是用火箭或者大炮，还是用太空电梯或者梯子，把一个65千克重的人搬运出地球的引力范围至少需要这么多能量。
How much is 4 gigajoules? It 8217;s about a megawatt-hour, which is what a typical US household consumes in electricity in a month or two. It 8217;s equal to the amount of stored energy in 90 kg of gasoline or a cargo van full of AA batteries.
40亿焦耳是多少？这差不多是1兆瓦时（一个普通美国家庭1～2个月的用电量），或者90千克汽油所蕴含的能量，或者一个载满AA电池的货车所携带的电能。
Four gigajoules times seven billion people gives us 2.8×1018 joules, or 8 peta-watt-hours. This is about 5 percent of the world 8217;s annual energy consumption. A lot, but not physically implausible.
40亿焦耳乘以70亿人口就是2.8×1018焦耳，也就是8拍瓦时。这相当于全球年耗能的约5％，总量确实很多，但也不是不可能凑齐这些能量。
However, 4 gigajoules is just a minimum. In practice, everything would depend on our means of transportation. If we were using rockets, for example, it would take a lot more energy than that. This is because of a fundamental problem with rockets: They have to lift their own fuel.
然而这40亿焦耳只是一个下限，在实际操作中还要看我们选用怎样的运输方式。如果我们打算用火箭，需要消耗的能量就远大于此。这是源于火箭科学中一个最为基本的问题：消耗的燃料也需要我们自己携带。
Let 8217;s return for a moment to those 90 kilograms of gasoline (about 30 gallons), because they help illustrate this central problem in space travel.
我们先回到之前提到的90千克汽油的类比上来，因为这样能更好地解释这个宇宙飞行中的关键问题。
If we want to launch a 65-kilogram spaceship, we need the energy of around 90 kilograms of fuel. We load that fuel on board-and now our spaceship weighs 155 kilograms. A 155-kilogram spaceship requires 215 kilograms of fuel, so we load another 125 kilograms on board . . .
如果你想要发射一个65千克的飞船，那么我们需要约90千克的燃料。我们把这90千克燃料搬上飞船，现在飞船的质量变成了155千克。155千克的飞船需要215千克的燃料，因而我们需要再加125千克的燃料……