英文J.

We are restricted to remote signaling and, of the long-distance techniques available to
our technology, radio is by far the best. Radio telescopes are relatively inexpensive;
radio signals travel at the speed of light, faster than which nothing can go; and the
use of radio for communication is not a short -sighted or anthropocentric activity.
Radio represents a large part of the electromagnetic spectrum and any technical
civilization anywhere in the Galaxy will have discovered radio early -- just as in the
last few centuries we have explored the entire electromagnetic spectrum from short
gamma rays to very long radio waves. Advanced civilizations might very well use
some other means of communication with their peers. But if they wish to communicate
with backward or emerging civilizations, there are only a few obvious methods, the
chief of which is radio. The first serious attempt to listen for possible radiosignals
from other civilizations was carried out at the National Radio Astronomy Observatory
in Greenbank. It was organized by Frank Drake, now at Cornel University, and was
called Project Ozma, 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 successby random stellar selection. Since Project
Ozma, there have been six or eight other such programs, all at a rather modest leve1, in
the United States,Canada and the Soviet Union. A1l 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 oof 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: Ifextraterrestrial 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.