According to the Drake Equation, there should be a small but substantial number of intelligent, communicating alien civilizations dispersed throughout our own Milky Way Galaxy, as well as similar civilizations in other galaxies as well. However, we have no scientifically conclusive evidence that these civilizations exist. These two irreconcileable statements, together, are known as the Fermi Paradox.
- Origins -
Technically, the Fermi Paradox actually predates the Drake Equation, having first been presented by Italian-American physicist Enrico Fermi in 1930. By that time, Fermi was already a Nobel laureate and a specialist in nuclear research, having worked on the Manhattan Project during World War II. As he observed, even if life is relatively rare, over the vast stretches of the galaxy it should be common - and so we should be able to hear radio transmissions from them, even if we can't actually see any spaceships.
Like Drake, Fermi is assuming that Earth is not special: in other words, that there are many planets like ours, in many solar systems in ours, and that life did not arise through some sort of dramatic external action, like divine creation or intelligent design. If so, and if our observations are correct that there are no massive spacefaring civilizations out there, the conclusion, Fermi pointed out, must be either that we're completely wrong about how alien civilizations would travel and communicate in space, or we're completely wrong about how common intelligent life is.
The argument is particularly strong with respect to large-scale, interstellar civilizations (what one sees in Star Trek and similar science fiction, for instance). A large interstellar civilization would presumably be highly visible - and yet we lack evidence of such civilizations. Ergo, according to the Fermi Paradox, either no such civilizations exist, or we lack the ability to detect them.
- The Search for Extraterrestrial Intelligence -
SETI and other attempts to identify alien civilizations arose out of an attempt to resolve the Fermi Paradox through greater information collection. There are, essentially, two basic ways that we could plausibly detect alien civilizations using current scientific instruments. First, and most probably, we could hear them through their radio transmissions. This was the basis of most SETI activity, including the successful [email protected] program. However, so far no such transmissions have yet been received.
The second plausible method of finding alien civilizations would be direct observation by astronomers. It is implausible to see spacecraft at interplanetary distances, but perhaps a sufficiently advanced civilization would do things which could be picked up with telescopes, like manipulate pulsars. We are now approaching the point where we can estimate the atmospheric composition of large extrasolar planets, although this cannot say where life is, merely where it might be.
- Explanations for the Paradox -
There are two basic resolutions for the Fermi Paradox. The first is that life truly is extremely rare. Human life may be uncommon, even unique, either because intelligence just isn't likely to evolve (we're an evolutionary fluke, in other words), or because the conditions in our solar system allowed a long-term evolutionary process that just can't happen elsewhere. Without Jupiter's influence, for example, Earth would be bombarded by large meteors far more often than every few dozen million years, and life would therefore be severely disrupted far more often.
It's also possible, if somewhat depressing, that intelligent life tends to destroy itself over time. Global nuclear war would effectively have ended human dreams of space travel; resource exhaustion and global warming still might do the same. Perhaps such problems are common to all intelligent civilizations, not just on Earth, so that civilizations older than ours have already caused their own destruction.
The second general explanation of the Fermi Paradox is that intelligent life exists, but we are unable to see it. Perhaps they are simply too far away: light that takes centuries or millennia to reach us would mean that radio signals from an alien world would have been sent a long time ago. And we, for example, weren't actually sending any radio signals at all until a century ago. Or perhaps they're out there, and we simply haven't been looking long enough to chance upon them.
There are any number of even more exotic explanations, all of them entirely speculative. For example, perhaps there are one or a few civilizations out there that are predatory or xenophobic, and destroy other alien civilizations. If that were the case, then except for the predators themselves, other civilizations would either be extinct or in hiding - in which case, we would not see them. Or perhaps more advanced civilizations retreat into computer-simulations of life, rather than exploring the universe. Perhaps - and this is the most disturbing explanation, in many ways - other civilizations are so advanced that they've set the Earth aside as a sort of zoo, or lab experiment.