外星人也许只是太远了Aliens Might Just Be Too Far Away
Brett：我们仍然面临这样一个问题：在大约二十五亿年的时间里——那是地球生命史上占据绝对多数的岁月——DNA 到底在做什么？为什么在那段时间里它完全没有进化？这到底是怎么回事？
有一本书叫 Rare Earth，作者是彼得·沃德（Peter Ward）和唐纳德·布朗利（Donald Brownlee），他们在书里谈到了地球进化史上发生的各种古怪事件。我特意抓住这样一个事实：我们这些通用解释者（universal explainers）似乎是侥幸出现的，而且似乎只出现了一次；但你可以回头想想，从单细胞细菌进化到多细胞生物这件事本身就怪异而不寻常，而且在实验室里一直无法重现。
再从多细胞生物到类似植物的东西，再到类似动物的东西——每一环似乎都是出于我们尚不理解的原因而发生的。
Naval：可能有多种因素在同时起作用。你的论证可以是统计意义上的，而不必是绝对的。宇宙中也许并非只有我们，但成为通用解释者这件事可能极其罕见；当你再乘以辽阔的星际距离，我们也许只是彼此相隔太远。
我还认为，费米（Fermi）有一个不合理的假设：他假定星际外星人总能想出突破光速的办法，而我们对这种可能性如何实现，没有任何假说。关于如何突破光速，我们连稍微沾边的东西都没有。
所以，如果你受光速的限制，而跃升为通用解释者又十分罕见，那么我们也许真的只是相隔太远，相遇也许只是需要更长的时间。
宇宙非常大，但（至少就行星和恒星而言）它又几乎完全是空的。即便如此，我们仍然很有理由说：人类以及类似人类的解释者相当罕见；在宇宙的尺度上，它们仍处于形成的早期阶段；它们只是被极其辽阔的距离分隔开来，以至于我们从未相遇。
如果我们真的相遇了，我想我们会知道的。
比如说，等一艘外星飞船抵达地球的时候，它们的无线电波早就先到了。在文明的历史上，从它发明无线电、开始向外广播电波，到它发明星际旅行、把火箭和文明送往宇宙各处，中间隔着相当长的一段时间。
Brett: We still have this problem of what DNA was doing for that approximately two and a half billion years—the overwhelming majority of the history of life on Earth. Why didn’t it evolve at all during that time? What’s going on?
There’s a book, Rare Earth, by Peter Ward and Donald Brownlee, and these guys talk about all the quirky things that happened in the evolutionary history of the Earth. I just picked on the fact that we universal explainers evolved seemingly fortuitously, seemingly once; but you can go back and realize that evolving from single-cell bacteria to a multicellular organism was weird and unusual and hasn’t been able to be repeated in a laboratory setting.
Then to go from the multicellular organism to something that’s like a plant and then something that’s like an animal—each of these things seems to have occurred for reasons that we don’t understand.
Naval: There could be a combination of things going on. Your argument can be statistical rather than absolute. We may not be alone in the universe, but becoming universal explainers might be so rare that when you start multiplying that by interstellar distances, which are quite vast, we might just be too far apart.
I think Fermi also had the unreasonable assumption that interstellar aliens would figure out how to get past the speed of light, when we have no hypothesis whatsoever as to how that might be possible. We have nothing even vaguely in the category of how to get past the speed of light.
So if you’re limited by the speed of light, and if the jump to universal explainers is rare, then we might just be too far apart. And it might just take a lot longer.
The universe is very big, but it’s also almost entirely empty, at least as far as planets and the stars are concerned. Given that, it’s still quite reasonable to say that humans and human-like explainers are quite rare; they’re still early in their formation across the universe; and they’re just spread out by such incredibly vast distances that we haven’t encountered each other.
If we did encounter each other, I think we’d know.
For example, by the time an alien spacecraft got here, their radio waves would have arrived long before. There’s a pretty long period in a civilization’s history when it invents the radio and starts to broadcast radio waves out, before it invents interstellar travel and it’s sending rockets and civilizations around the universe.