Where Are All the Aliens?

Saturday, at the request of Jill Tarter (for many years, the leader of the major program to search for radio signals from alien civilizations), I spoke to a group at the California Academy of Sciences about what we call the Fermi Paradox. Perhaps you will agree with me that it's one of the most interesting dilemmas in astronomy.

In the 1950's, physicist Enrico Fermi posed a question to some lunch-time companions, which we can sum up in modern terms this way: There are more than 200 billion stars in our Galaxy and Kepler mission results show that many of them have planets.  The Sun and its family are relatively young compared to the Galaxy (5 billion years old versus 13 billion years old).  So there must have been many stars and planets that developed long before ours did.

If so, life (and intelligent life) should have evolved on some of these worlds long before it did on Earth.  There must therefore be many civilizations out there whose technology is far in advance of ours.  In that case, Fermi and others have asked, where are they?  Why have we not found any signals, artifacts, or visitors from these extra-terrestrial civilizations?

The answer most scientists would give is that the stars are far away, travel among them is slow or expensive, and we have just begun to search for complex signals the aliens might be putting out.  Therefore it's much too soon to ask Fermi's question.

But scientists and science fiction author have delighted in finding other answers as well. Maybe the aliens (like your crotchety uncle) don't like to travel or write letters. Maybe they are so happy playing with their equivalent to Facebook, they don't need to find neighbors among the stars. Maybe they communicate in ways we have yet to dream of.

Or maybe they are here, but are too smart to let us see them watching us (sometimes this is called the "zoo hypothesis.")  Another answer is that while planets are common, perhaps the evolution of technology is very rare, making communication rare too.  What if alien species are more like the dolphins, swimming in a planetary ocean and reciting complex poems to one another?  But they don't mine metals or build telescopes and radio transmitters.

A really depressing proposal is that once aliens develop intelligence and technology, they also develop the ability to destroy their planetary environment through pollution or nuclear war.  A clever science fiction story that develops this further, "The Fermi Paradox is our Business Model" by Charlie Jane Anders, can be found free on the web at: http://www.tor.com/stories/2010/08/the-fermi-paradox-is-our-business-model

Many articles and books have been devoted to the Fermi Paradox.  Scientists enjoy such speculations, but eventually we come back to the notion that in science, the ultimate way to judge what is true is doing an experiment or making an observation.  Those scientists who continue the patient, long-term search for signals or other evidence of extraterrestrial intelligence do so in part because the discovery of another intelligent species in the universe might be the best answer we can give to Fermi's question.