The first serious attempt to listen for possible radio signals from other civilizations was carried out at the National Radio Astronomy Observatory in Greenbank， West Virginia， in 1959 and 1960. It was organized by Frank Drake， now at Cornel University， and was called Project Ozma， after the princess of the Land of Oz， a place very exotic， very distant and very difficult to reach. Drake examined two nearby stars for a few weeks with negative results. Positive results would have been astonishing because as we have seen， even rather optimistic estimates of the number of technical civilizations in the Galaxy imply that several hundred thousand stars must be examined in order to achieve success by random stellar selection.

    Since Project Ozma， there have been six or eight other such programs， all at a rather modest level， in the United States， Canada and the Soviet Union. All results have been negative. The total number of individual stars examined to date in this way is less than a thousand. We have performed something like one tenth of one percent of the required effort.

    However， there are signs that much more serious efforts may be mustered in the reasonably near future. Besides， hand in hand with the recent spectacular advances in radio technology， there has been a dramatic increase in the scientific and public respectability of the entire subject of extraterrestrial life. A clear sign of the new attitude is the Viking missions to Mars， which are to a significant extent dedicated to the search for life on another planet.

    But along with the burgeoning dedication to a serious search， a slightly negative note has emerged which is nevertheless very interesting. A few scientists have lately asked a curious question： If extraterrestrial intelligence is abundant， why have we not already seen its manifestations？ Skeptics also ask why there is no clear evidence of extraterrestrial visits to Earth. We have already launched slow and modest interstellar spacecraft. A society more advance than ours should be able to ply the spaces between the stars conveniently if not effortlessly. Over millions of years such societies should have established colonies， which might themselves launch interstellar expeditions. Why are they not here？ The temptation is to deduce that there are at most a few advanced extraterrestrial civilizations —— either because statistically we are one of the first technical civilizations to have emerged or because it is the fate of all such civilizations to destroy themselves before they are much further along than we.

    It seems to me that such despair is quite premature. All such arguments depend on our correctly surmising the intentions of beings far more advanced than ourselves， and when examined more closely I think these arguments reveal a range of interesting human conceits. Why do we expect that it will be easy to recognize the manifestations of very advanced civilizations？ Is our situation not closer to that of members of an isolated society in the Amazon basin， say， who lack the tools to detect the powerful international radio and television traffic that is all around them？ Also， there is a wide range of incompletely understood phenomena in astronomy. Might the modulation of pulsars or the energy source of quasars， for example， have a technological origin？ Or perhaps there is a galactic ethic of noninterference with backward or emerging civilizations. Perhaps there is a waiting time before contact is considered appropriate， so as to give us a fair opportunity to destroy ourselves first， if we are so inclined. Perhaps all societies significantly more advanced than our own have achieved an effective personal immortality and lose the motivation for interstellar gallivanting， which may， for all we know， be a typical urge only of adolescent civilizations. Perhaps mature civilizations do not wish to pollute the cosmos. There is a very long list of such "perhapses，" few of which we are in a position to evaluate with any degree of assurance.