Richard Dawkins suggested that the first measure of an intelligent species, should two chance to meet, would be whether they understood how they came to be. I imagine such a remarkable exchange of cosmic existentialism set on a hovering space dongle, lit only by the faint glow of distant ringed planets:

“Hey, you know about evolution?”

And with these words, the warmth of science would triumph over the cold curve of the ever widening Universe, at least for a moment.

The SETI at Home project distributes the search for intelligent communication across persoonal computers

Will such an exchange ever take place? Why haven’t we met an aliens yet? Are we likely to? I did some reading, and found these three perspectives interesting:

(1) Nick Bostrom

If conditions are right, intelligent life will develop eventually. So why haven’t we encountered any aliens? Bostrom describes the Great Filter (attributed to Robin Hanson, who is really responsible for most of these ideas): Either the right conditions are so extraordinarily rare that we are basically alone, or plenty of intelligent life has evolved but it destroyed itself before it had a chance to contact us.

There seems like a lot of room in between these two extremes--extraterrestrials that are hiding, undetected in a distant galaxy--but Bostrom argues that a vast time scale makes this intermediate possibility much less likely. That is, once you have intelligent life, time is short before it begins broadcasting its presence, colonizing via self-replicating robots. Does this seem far off for our own civilization? Well what about in a million years?

Perhaps the Great Filter is behind us: the conditions required for the evolution of intelligent life are so rare that of 125 billion galaxies and 6x1018 stars with orbiting planets, none besides us managed it in the last 13 billion years. There is considerable speculation about how this could be, but less focus on the other possibility: the Filter is ahead of us. Somehow, all intelligent life finds its way down one dramatically destructive dead end. Nuclear annihilation? Induced black hole? Destructive artificial intelligence?

Bostrom avoids taking sides. Rather, he points out that the longer we go without bumping into any aliens, the more reason to be optimistic that the Filter is behind us. So while most people would be excited about the discovery of life on Mars, Bostrom would be upset: life is not so rare after all; it just destroys itself quickly, and there’s no reason to think we will avoid this fate.

(2) Francis Crick

Francis Crick suggested that life may have originated on Earth through “Directed Panspermia”. That is, seeds of life were spread to various planets by an advanced civilization that faced catastrophic destruction, or perhaps as part of a farming experiment. The seeds--particularly robust microbes, for example--might be attached to meteors or set afloat on solar sails aimed at newly forming stars. Directed Panspermia bypasses the Great Filter: we don’t need to explain the probability of life evolving on Earth relative to other planets because it didn’t happen organically.

Geoffrey Miller

Recent data suggests that the Great Filter is not behind us. Many scientists think there’s a good chance the Universe has plenty of intelligent life. And exponential colonization seems reasonable given enough time. So where is everybody? This perspective is known as Fermi’s Paradox (after Enrico Fermi). Miller suggests that the radio silence is not the result of anything so dramatic as nuclear annihilation. Rather, the culprit is a gradual but inexorable addiction to video games as engineering of artificial stimuli surpasses natural pleasures. In deference to religious fundamentalists the world over, Miller calls this the Great Temptation.


I’ve summarized these three perspectives very succinctly. The original articles are excellent (see references below) What am I missing?

Bostrom: Where Are They? Why I hope that the search for extraterrestrial life finds nothing
Hanson: The Great Filter: Are We Almost Past It?
Miller: Why We Haven't Met Any Aliens
Wikipedia on Panspermia