Signals to space worth a shout

I REALLY liked the furious debate that broke out recently among astronomers about whether we should send out signals to the universe saying "we're here". It implicitly assumes that somehow, if your science is really advanced, then interstellar travel is possible.

I like it because I hate the idea that the human race will never be able to go beyond this little planetary system "far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy", as Douglas Adams put it in his Hitch-Hiker's Guide to the Galaxy.

We need somebody to do to Einstein's physics what Einstein did to Newton's. But while we're waiting for that, it's good to know that some quite grown-up scientists (astronomers, not physicists, admittedly, but I'll take whatever I can get) think it's worth having a debate about whether we should take the risk of letting all the aliens know we are here.

I missed the debate when it took place at the American Association for the Advancement of Science's annual conference in San Jose last month, because I was on Mars at the time (well, somewhere that felt quite like Mars, anyway). But here's a couple of quotes to give the flavour of it:

"Any society that could come here and ruin our whole day by incinerating the planet already knows we are here," said Dr Seth Shostak, senior astronomer and director at the Centre for SETI (Search for Extra-Terrestrial Intelligence) Institute in California.

Not so fast, said space scientist and science-fiction writer David Brin. "The arrogance of shouting into the cosmos without any proper risk assessment defies belief. It is a course that would put our grandchildren at risk," he said. If we send them messages, they may come here and enslave us." Now, the traditional way to shut this debate down is to point out that we've already been sending out radio and television signals for 100 years. Therefore, any intergalactic pirates within a 100 light-years of here already know where we are. But it turns out that this isn't actually true - our radio and television signals begin to fade into the background radio static beyond about one light-year away.

Since the nearest star is more than four light-years away, there's not much chance that the Klingons or Vogons or whoever you're worried about knows we're here yet (and there goes the plot of Galaxy Quest.)

On the other hand, powerful radar signals of the kind we have been using to map the surface of other planets in our own system travel a very long way, and we've already been sending them out for more than 20 years. They don't carry much information - they just say "somebody here can generate microwave radiation" - but that just might be enough to attract unwelcome attention.

This new debate is actually about "active SETI". We have been doing "passive SETI" - listening for messages from civilisations around other stars - for more than 40 years already, using large radio telescopes that can pick up very faint signals. But there are quite strict rules about who should reply if they do get a message.

The First Protocol, drafted by the International Academy of Astronautics SETI Panel in 1989, says that "no transmission in response to a signal or other evidence of extraterrestrial intelligence should be sent until appropriate international consultations have taken place". The advocates of "active SETI" want to scrap that and send out an "all call" to the universe.

One of the reasons the debate has got more heated is we now know planets are as common as dirt. It's only 20 years since the first confirmed discovery of an "exoplanet", but now we know of 1906 of them, mostly orbiting relatively nearby stars and a very small proportion showing Earth-like characteristics. (But the actual number of Earth-like planets may be much higher, since it's a lot easier to find gas-giants like Jupiter or Saturn.)

There are probably hundreds of thousands of planets in our vicinity (there are 260,000 stars within 250 light-years). If even a mere few thousand of them are Earth-like, then it is imaginable that somebody might come calling in response to the messages we send - if, and only if, it is possible to travel at near- or trans-light speeds.

Nobody knows how light-speed travel could be done, and our understanding of physics says that it can't be done. But this would be a very silly debate if scientists were all convinced there is no possibility of getting around the current speed limit.

They will never say that it might be possible, because they cannot suggest how it might be done and the risk to their reputations would therefore be extreme. But they are quite happy to engage in a debate that would be totally irrelevant if they didn't think there is a chance that we - or some other civilisation in our galactic vicinity - will eventually figure out how to do it. And that cheers me up considerably.

Gwynne Dyer is an independent journalist whose articles are published in 45 countries.