《那些古怪又讓人憂心的問題》第64期:人類大遷徙(1)
EVERYBODY OUT
Q. Is there enough energy to move the entire current human population off-planet?
Q．我們有足夠的能量把現在地球上的全部人口都搬到太空中嗎？A. THERE ARE A BUNCH of science fiction movies where, because of pollution, overpopulation, or nuclear war, humanity abandons Earth.
A．許多科幻電影中都出現了與此類似的情景：人類出於污染、人口過多或者核戰爭等原因放棄了地球。
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's not even worry about where we're headed-we'll assume we don't have to find a new home, but we can't stay here.
把人弄到太空中並不容易。不考慮人口暴減的可能性，那麼把所有人口發射到太空中去在物理上可行嗎？甚至假設我們不用關心到了太空之後要去哪兒，我們就是想要離開地球。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's gravity well requires at least this much energy.
要想知道這麼做是否可行，我們先要知道一個絕對底線：把一個人送到太空所需要的最低能量是40億焦耳。不管你是用火箭或者大炮，還是用太空電梯或者梯子，把一個65千克重的人搬運出地球的引力範圍至少需要這麼多能量。How much is 4 gigajoules? It's about a megawatt-hour, which is what a typical US household consumes in electricity in a month or two. It's equal to the amount of stored energy in 90 kg of gasoline or a cargo van full of AA batteries.
40億焦耳是多少？這差不多是1兆瓦時（一個普通美國家庭1～2個月的用電量），或者90千克汽油所蘊含的能量，或者一個載滿AA電池的貨車所攜帶的電能。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's annual energy consumption. A lot, but not physically implausible.
40億焦耳乘以70億人口就是2.8×1018焦耳，也就是8拍瓦時。這相當於全球年耗能的約5％，總量確實很多，但也不是不可能湊齊這些能量。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.
然而這40億焦耳只是一個下限，在實際操作中還要看我們選用怎樣的運輸方式。如果我們打算用火箭，需要消耗的能量就遠大於此。這是源於火箭科學中一個最爲基本的問題：消耗的燃料也需要我們自己攜帶。Let's return for a moment to those 90 kilograms of gasoline (about 30 gallons), because they help illustrate this central problem in space travel.
我們先回到之前提到的90千克汽油的類比上來，因爲這樣能更好地解釋這個宇宙飛行中的關鍵問題。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 . . .
如果你想要發射一個65千克的飛船，那麼我們需要約90千克的燃料。我們把這90千克燃料搬上飛船，現在飛船的質量變成了155千克。155千克的飛船需要215千克的燃料，因而我們需要再加125千克的燃料……
