In 1950 the great physicist Enrico Fermi (Fig. 1) asked a question that continues to puzzle astronomers, biologists and philosophers to this very day. In simple terms, the question is this: If our Milky Way galaxy and the universe at large (Fig. 2) are indeed teeming with intelligent extraterrestrial (ET) civilizations, why haven't we seen any sign of them yet? In other words, where are they? This mystery has become known as the "Fermi paradox." The paradox is compounded by the fact that estimates suggest that even an ET civilization that is only marginally more advanced than ours could have reached almost all corners of the Milky Way within a few tens of millions of years -- a negligible period of time, considering the age of the Earth and the universe.
Figure 1. Italian-American physicist Enrico Fermi. (Credit: Department of Energy, Office of Public Affairs)
Figure 2. The spiral galaxy NGC 2841. Such galaxies are expected to harbor billions of planets orbiting Sun-like stars. (Credit: NASA, ESA and the Hubble Heritage [STScI/AURA]-ESA/Hubble Collaboration)
Over the past decades numerous attempts have been made to resolve the Fermi paradox. Possible explanations range from suggestions that extraterrestrial intelligent life is actually extremely rare to the idea that intelligent ET life could exist without making its presence detectable to humans.
At the present time it is virtually impossible to determine whether any of the proposed explanations -- or some combination thereof -- is the correct one. However, there are certain classes of explanations that I personally find less attractive than others. These are the ones that find a plethora of reasons that civilizations such as ours should be exceedingly rare. To give a specific example of one of the more exotic arguments of this type, some have maintained that the fact that humans have developed technologies was a direct consequence of the availability of fossil fuels. Without such fuels, this idea continues, even if an intelligent ET civilization develops, it would not become technological and therefore will remain undetectable. The reason that I do not consider such arguments compelling is that so far, the history of science has continuously supported and even strengthened the concept known as the "Copernican principle," the realization that humans are nothing special in the cosmos.
More likely, in my humble opinion, are the explanations based on the enormous gap in knowledge that is expected to exist between two galactic life forms. Chances are that another civilization is either more or less advanced than humanity by something like a billion years. The probability that two such civilizations would be at about the same stage in their scientific development is miniscule (unless technological civilizations are extremely short-lived, in which case they may indeed be rare). However, a huge gap in development means that one civilization compares with the other rather like worms compare with humans. In other words, not only would two such civilizations not be able to communicate, but the more advanced one may have no interest in the other, and it would be easy for it to mask its presence from the inferior life form. If this resolution of the Fermi paradox is indeed correct, the prospects of us ever finding an intelligent form of ET life may be slim. However, this obstacle should not prevent us from finding more primitive life.
All in all, there is no question that the best resolution of the paradox can only come from an aggressive search for ET life, using all the different strategies that are currently being pursued (described in "The Search for Extraterrestrial Life"). As the ancient biblical saying goes, "seek and ye shall find."