外星人在哪里？Where Are the Aliens?
Naval：既然我们在聊外星人，就简单谈谈费米悖论。给不知道的听众解释一下，恩里科·费米（Enrico Fermi）是一位著名物理学家，曾参与曼哈顿计划。他问道："外星人都在哪儿？"
宇宙如此浩瀚，可能存在大量能够以某种形式孕育生命的行星。我们难道不应该早就见到他们了吗？
Brett：几乎每颗恒星周围都有一群行星，就像我们的太阳系一样。像银河系这样的典型星系中，恒星的数量大约有 2000 亿颗，尽管有些估计高达 4000 亿。我们能观测到的星系数量大约在 2000 亿到 3000 亿之间。
可观测宇宙只是整个宇宙的一小部分，这意味着行星的数量绝对是天文数字。
既然如此，按理说宇宙中不仅存在着适合生命生存的行星，而且应该处处都是文明——有的远比我们先进，有的远不如我们，还有一些与我们发展程度相近。
那么，他们都在哪儿呢？
Naval: Let’s talk briefly about the Fermi paradox since we’re talking about aliens. For listeners who don’t know, Enrico Fermi was a famous physicist who was part of the Manhattan Project. He said, “Where are the aliens?”
The universe is so large and there are probably so many planets that are capable of supporting life of some kind or another. Shouldn’t we have seen them by now?
Brett: Around almost every star there is a contingent of planets much like our own solar system. The number of stars that exist within a typical galaxy like the Milky Way is something like 200 billion, although the estimates go up to about 400 billion. The number of galaxies that we can see is around 200 to 300 billion.
The observable universe is just a small fraction of the entire universe, which means that the number of planets is absolutely astronomical.
Surely, given these numbers, it has to be the case that there are not only planets out there that are suitable for life but that the universe should be teeming with civilizations far more advanced than ours, less advanced than ours, and some that are similar in advancement to ours.
So where are they?