The Quest for Extraterrestrial Intelligence
By Carl Sagan

The Quest for Extraterrestrial Intelligence
By Carl Sagan
Through all of our history we have pondered the stars and mused whether mankind is unique
or if, somewhere else out there in the dark of night sky, there are other beings who
contemplate and wonder as we do – fellow thinkers in the cosmos. Such beings might view
themselves and the universe differently. Somewhere else there might exist exotic biologies,
technologies and societies. What a splendid perspective contact with a profoundly different
civilization might provide! In a cosmic setting vast and old beyond ordinary human
understanding we are a little lonely, and we ponder the ultimate significance, if any, of our tiny
but exquisite blue planet, the Earth. The Search for Extra-Terrestrial Intelligence (SETI) is the
search for a generally acceptable cosmic context for the human species. In the deepest sense
the search for extraterrestrial intelligence is a search for ourselves.
Until recently there could be no such search. No matter how deep the concern or how
dedicated the effort, human beings could not scratch the surface of the problem. But in the
last few years – in one millionth of the lifetime of our species on this planet – we have
achieved an extraordinary technological capability which enables us to seek out unimaginably
distant civilizations, even if they are no more advanced than we. That capability is called radio
astronomy and involves single radio telescopes, collections or arrays of radio telescopes,
sensitive radio detectors, advanced computers for processing received data, and the
imagination and skill of dedicated scientists. Radio astronomy has, in the last decade, opened
a new window on the physical universe. It may also, if we are wise enough to make the effort,
cast a brilliant light on the biological universe.
Some scientists working on the question of extraterrestrial intelligence, myself among them,
have attempted to estimate the number of advanced technical civilizations in the Milky Way
galaxy – that is, societies capable of radio astronomy. Such estimates are little better than
guesses. They require assigning numerical values to quantities such as the numbers and ages
of stars, which we know well; the abundance of planetary systems and the likelihood of the
origin of life within them, which we know less well; and the probability of the evolution of
intelligent life and the lifetime of technical civilizations, about which we know very little
indeed. When we do the arithmetic, the number that my colleagues and I come up with is
around a million technical civilizations in our Galaxy alone. That is a breathtakingly large
number, and it is exhilarating to imagine the diversity, lifestyles and commerce of those million
worlds. But there may be as many as 250 billion stars in the Milky Way Galaxy. Even with a
million civilizations, less than one star in 250,000 would have a planet inhabited by an
advanced civilization. Since we have little idea which stars are likely candidates, we will have to
examine a huge number of them.
Thus the quest for extraterrestrial intelligence may require a significant effort.
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