Astronomers have been looking for intelligent life in space since 1960, when Frank Drake started Project Ozma, using a radio telescope to listen for signals from two nearby sun-like stars - Drake knew that radio waves travel more easily through the cosmos than light waves. He didn't hear anything back. Since then, our searches have become more thorough thanks to larger radio telescopes and more sophisticated computers that look for fainter signals. But we still have no signal from ET. Should we want to? This is not just a matter for astronomical research involving distant worlds and academic questions. Could it be that, from across the gulf of space, as H. G. Wells put it, there may emerge an alien threat? That only happens in lurid science fiction films, doesn't it? Well, the threat is real enough to worry many scientists, who make a simple but increasingly urgent point: if we don't know what's out there, why on earth are we deliberately beaming messages into space, to try to contact these civilisations about whom we know precisely nothing?

The searchers are undeterred. They argue that because of the vastness of space - even if there are 10,000 transmitting societies nestled in the stellar arms of the Milky Way - we might have to search millions of star systems to find just one. The SETI (Search for Extra-Terrestrial Intelligence) Institute in California is the only such group that searches the cosmos for signs of intelligent alien life. It does so by listening for radio signals. SETI, which was founded in 1984, has 100 scientists, educators and support staff. Its funding from the American government was cut off in 1992 and it now relies on private donations. The institute's Project Phoenix was the most ambitious search for extraterrestrial intelligence ever undertaken. From February 1995 to March 2004, Phoenix conducted three observing campaigns on some of the world's largest radio telescopes, targeting stars within 240 light years of Earth. In more than 11,000 hours of observing, using telescopes in Australia, West Virginia and Puerto Rico, the project "tuned in" to more than 800 stars. No ET signals were detected.

The next stage in the search is the Allen Telescope Array, currently under construction in California. Partly funded by Microsoft billionaire Paul Allen, it will eventually consist of 350 six-metre dishes making synchronised sweeps of the sky looking for alien signals. But rather than just listening, some want to announce our presence to the cosmos. In 1974, the then newly resurfaced Arecibo radio telescope in Puerto Rico (made famous in the James Bond film Goldeneye) reversed its usual role of just listening, and transmitted a series of radio pulses towards the M13 star cluster. It sent 1679 pulses in all, which, when arranged in binary form into 23 columns and 73 rows, would form a message from humanity. It was seen as a symbolic gesture, showing those on Earth that we had the technology to send a signal across our galaxy and - if we were on the other side of the relationship - to receive a signal as well. But some scientists objected. Sir Martin Ryle, Britain's astronomer royal at the time, warned that "any creatures out there (might be) malevolent or hungry". Now, after a long period without deliberate transmissions into space, a new round is about to take place and more are planned. A team led by astronomer Alexander Zaitsev has already beamed forth a series of interstellar messages, including pictorial and musical transmissions, from the Evpatoria radio telescope in Ukraine. Another group in Brazil, the Grupo Independente de Radio Astronomos in Rio de Janeiro, claims to have transmitted as well. Half a dozen commercial companies have also sprung up, among them Cosmic Connexion, a company based near Cape Canaveral in Florida. The Cosmic Connexion website invites you to email your messages to them and they will then beam them, free of charge, into space and "introduce you to extraterrestrials". At the moment, though, this is a low-power initiative whose signals won't get far. Other companies offering the same service for a fee are soon to come online.

Many scientists, frightened by the danger that might lurk out there, have argued against seeking contact with extraterrestrials. Jared Diamond, professor of evolutionary biology and Pulitzer Prize winner, says: "Those astronomers now preparing again to beam radio signals out to hoped-for extraterrestrials are naive, even dangerous." The fact is, and this should have been obvious to all, that we do not know what any extraterrestrials might be like - and hoping that they might be friendly, evolved enough to be wise and beyond violence, is an assumption upon which we could be betting our entire existence. When I was a young scientist 20 years ago at Jodrell Bank observatory in Cheshire, England, I asked Sir Bernard Lovell, founder of Jodrell Bank and pioneering radio astronomer, about it. He had thought about it often, he said, and replied: "It's an assumption that they will be friendly - a dangerous assumption." Lovell's opinion is echoed today by the leading scientists. Physicist Freeman Dyson, of the Institute for Advanced Study in Princeton, has been for decades one of the deepest thinkers on such issues. He insists that we should not assume anything about aliens. "It is unscientific to impute to remote intelligences wisdom and serenity, just as it is to impute to them irrational and murderous impulses," he says. "We must be prepared for either possibility."

Nobel Prize-winning American biologist George Wald agrees: he could think of no nightmare so terrifying as establishing communication with a superior technology in outer space. The late Carl Sagan, the American astronomer who died a decade ago, also worried about so-called "first contact". He recommended that we, the newest children in a strange and uncertain cosmos, should listen quietly for a long time, patiently learning about the universe and comparing notes. He said there was no chance that two galactic civilisations would interact at the same level. In any confrontation, one would always dominate. Australian astronomer Ronald Bracewell, now of Stanford University, warns that other species would place an emphasis on cunning and weaponry, as we do, and that an alien ship sent our way is likely to be armed. Indeed, evolution on Earth is, as they say, red in tooth and claw. And it's likely that any creature we contact will also have had to claw its way up its own evolutionary ladder and may possibly be every bit as nasty as we are - or worse. Imagine an extremely adaptable, extremely aggressive super-predator with superior technology.

So should we stay quiet and ban these transmissions into space? When, as a newly minted young scientist, I was discussing this with the (late) influential astronomer Zdenek Kopal, he grabbed me by the arm and said in a tone of seriousness: "Should we ever hear the space-phone ringing, for God's sake let us not answer. We must avoid attracting attention to ourselves." Others have put it more graphically, saying the civilisation that blurts out its existence might be like some early hominid descending from the trees and calling "here kitty" to a sabre-toothed tiger. But not all scientists are worried. Frank Drake, who devised Project Ozma and was behind the Arecibo transmission says, "As I thought in 1974, the objections to sending interstellar messages were naive and carried no weight. The argument then, as now, is that humanity has been, and is making, its presence known through our TV and radio and military radars which, in many cases, release most of their radiated power into interstellar space." Radio waves from Earth, from TV and radio broadcasts and from powerful intercontinental military radars are leaking into space. Some believe they could be detected, but should we go beyond this and actively announce our presence to the cosmos? Drake points out that our present terrestrial radio telescopes, if placed on nearby worlds, would be unable to detect these transmissions at distances beyond a few light years. However, aliens would be more advanced, he says, and it is quite within the abilities of current terrestrial technology to build telescopes, using the array approach, which could detect these transmissions from great distances in the galaxy.

"The point here is that Earth has made its presence known by sending a multitude of signals. It is too late - we have made ourselves visible," he adds. But scientist and science-fiction author David Brin thinks those in charge of drafting policy about transmissions from Earth - ostensibly a body called the International Astronomical Union, which would make recommendations to the United Nations - are being complacent, if not irresponsible. Whatever has happened in the past, he doesn't want any new deliberate transmissions adding to the risk. "In a fait accompli of staggering potential consequence," he says, "we will soon see a dramatic change of state. One in which Earth civilisation may suddenly become many orders of magnitude brighter across the Milky Way - without any of our vaunted deliberative processes having ever been called into play."

Michael Michaud, a former US diplomat and chairman of the Transmissions from Earth Working Group (a subdivision of the International Astronomical Union's Search for Extraterrestrial Intelligence Study Group established in 2001) is on the verge of resigning in frustration at the lack of discussion about the problem. He believes it is being confined to a narrow group of scientists who share the same limited astronomical views and he wants the study group widened beyond to include planetary scientists, philosophers, historians and so on. He sees it as a problem that affects all humanity - and one that should be debated as such. But despite these concerns, for the moment, the plans for deliberate transmissions from Earth go ahead and there is nothing anyone can do to stop them - or even demand a discussion beforehand. One thing is clear from our searches for ET - there is nobody transmitting strong interstellar beacons in our local vicinity. If "they" are out there, they are keeping quiet, prompting the question that they might know something we don't.

Perhaps the aliens already know about us and are on their way. Or perhaps not. Intelligences - possibly vast, cool and unsympathetic - could be sweeping their skies looking for us. At the moment when they point their instruments in the direction of our sun - a commonplace yellow-dwarf star - they may well find nothing unusual, if no one's sending messages in the other direction. Should we keep it that way? - THE INDEPENDENT