Through all of our history we have pondered the stars and mused whether humanity is
unique or if, somewhere else in the dark of the 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 be very exotic
biologies and technologies and societies. 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 search for
extraterrestrial intelligence 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. In the last few years 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
date. 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 profound light on the biological universe.
Some scientists working on the question of civilizations in the Milky Way Galaxy.
Such estimates are little better than guesses. They require assigning numerical values
to quantities such as the numbers and ages of stars; the abundance of planetary
systems and the likelihood of the origin of life. 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 sorts of numbers we come up with are. characteristically, around a million
technical civilizations. A million civilizations is a breathtakingly large number. and
it is exhilarating to imagine the diversity, lifestyles and commerce of those million
worlds. But the Milky Way Galaxy contains some 250 billion stars. and even with
a million civil izations. less than one star in 200, 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 very large number of them. Such considerations suggest
that the quest for extraterrestrial intelligence may require a significant effort. There
isno firm evidence for past visitation of the Earth by other civilizations.