Scientists interested in the search for extraterrestrial intelligence (SETI) have a new challenge. Their pursuit, once viewed with suspicion by most other scientists, is now being accepted into the research mainstream. Important radio listening posts are being made available for SETI.
At the same time, SETI enthusiasts are having to face the fact that their colorful - and often wildly imaginative - speculations about the probability of alien life and the nature of alien beings are merely reflections of fanciful thinking in what Philip Morrison of the Massachusetts Institute of Technology calls ''a great pool of ignorance.'' In the opinion of SETI leaders such as Dr. Morrison, these speculations should now take second place to the search itself, which increasingly is being considered good basic science.
Such was the perspective in which experts viewed their subject at the first International Astronomical Union (IAU) SETI symposium held recently at Boston University
The fact that the IAU held the symposium at all indicates the respect this once far-out subject now commands in the world scientific community. The IAU - the global umbrella organization for astronomers and a conservative institution - now has accorded SETI the status of an IAU commission: Commission 51.
Noting this, Michael D. Papagiannis, head of the BU astronomy department and first chairman of this commission, pointed out that the merely speculative phase of SETI is over. ''We have finally entered the experimental phase of the search for extraterrestrial life,'' he said.
A number of searches have, in fact, been made over the past few decades. But these have generally been small scale and have been piggybacked onto the observing programs of instruments primarily used for other purposes. Now, as Jill Tarter of the NASA-Ames Research Center observed, the scientific community is showing a willingness to build instruments dedicated to SETI. Moreover, these employ radio scanners and signal processors, which were technically and financially infeasible a decade ago and which increase the listening ability of SETI receivers several million fold.
The first of these efforts - Project Sentinel - got under way in March 1983 with support of the Planetary Society. The project, directed by Harvard University astronomer Paul Horowitz, has full use of an 84-foot-diameter radiotelescope in Harvard, Mass. The equipment can scan 131,000 radio channels simultaneously, listening for alien signals that may be beamed toward Earth. The search concentrates on a narrow 2,000-hertz frequency band centered on the 1,420 -megahertz frequency emitted naturally by cosmic hydrogen. This natural frequency would presumably be known to any technically advanced civilization. Thus it might be picked for special signaling.
Project Sentinel equipment is being upgraded. Later this year, it should be able to monitor 8.4 million radio channels at once over a frequency range of 350 ,000 hertz. This should enable Sentinel to listen for signals even if they are not beamed especially toward Earth. Dr. Horowitz notes that this tremendous gain in capacity cost less than $100,000, thanks to progress in microchip technology.
Meanwhile, the US National Aeronautics and Space Administration (NASA) has an even more ambitious project - again on a shoestring budget of about $1.5 million a year. It is developing a system to scan an 8-megahertz frequency band on 8 million channels simultaneously.
This is a joint project of the NASA-Jet Propulsion Laboratory and NASA-Ames Research Center. The two laboratories - respectively in Pasadena and Mountain View, Calif. - hope to have the equipment installed on NASA antennas around the world by 1988. Then they will sweep the entire sky at a frequency range of 1 to 10 gigahertz (1 to 10 billion hertz). Some special coverage will also be given at 10 to 25 gigahertz. After this search, covering virtually all of the microwave radio window, the project hopes to use the 1,000-foot antenna at Arecibo in Puerto Rico to study hundreds of individual stars.
With such major projects under way, a number of symposium participants told their colleagues that SETI has reached too serious a stage for its tone to be set any longer by wild speculation about the probability of life elsewhere.
Leslie E. Orgel of the Salk Institute of Biological Sciences in San Diego - an expert on the early chemistry of primordial organic life - explained that biochemists have only faint notions of how organic life arose on Earth, let alone how it might arise elsewhere. He said he doesn't know how people can think ''their thoughts are reasonably rational'' when they estimate the probability of life elsewhere.
Similar caution was advised on the so-called Fermi paradox, attributed to the late Italian physicist Enrico Fermi. This states that, if other technical civilizations exist, some would be advanced enough to colonize the galaxy. Thus they should have been here by now. Since they don't seem to have been here, where are they?
This supposed paradox has stimulated much discussion and speculation among SETI scientists. Yet Charles Seeger of San Francisco State University warned that, while such speculation may be fun, it is scientifically unproductive. ''Don't call this the Fermi paradox,'' he said. ''There's nothing paradoxical about it at all. It's just plain ignorance.'' He explained that SETI itself is good, basic science and that it is better to do what can be done now than to speculate ignorantly.
Bernard M. Oliver, who directs the SETI project at NASA-Ames Research Center, made a similar point. He said he is comfortable with the proposition that ''what has already happened here (on Earth) may have happened elsewhere.'' But such reasoning can be extrapolated too far. ''I'm entertained by such extrapolations, '' he said, ''but I have trouble when they lead to 'paradox.' ''