Astronomers are always on the lookout for any sign of an alien civilisation in space. But what would we do if we found one?

HUMANKIND'S first contact with an alien civilisation, if it ever happens, will be the biggest milestone to that date in the history of civilisation. First contact will almost certainly not involve the sudden arrival of "little green men" but will probably be the detection of a clearly engineered radio signal from outer space. In that event, what will happen next is discussed by Tim Folger in the January 11th edition of Scientific American.

Science assumes that life spontaneously arose on Earth about 3.8 billion years ago from lifeless chemicals. Our sun is only one star among 200 billion other stars in our galaxy, a significant fraction of which are also orbited by planets. There is no reason to assume that what happened on earth couldn’t also happen elsewhere and, on that basis, it can be calculated that there could be 10,000 detectable civilisations in our Milky Way galaxy.

The organisation Search for Extraterrestrial Intelligence (Seti) has been scanning the skies since 1985 (sporadic searches by others have been conducted since 1960) looking for radio signals from alien civilisations, so far without success. The rate of scanning to date has been slow but advances in computer power over the next 20 years will allow astronomers to scan enough stars in our galaxy to give a very good chance of detecting a signal from an alien civilisation. If no signals are detected, then the basic assumptions behind Seti will have to be re-examined.

What will happen if a signal is detected? The detecting observatory will first ask a sister observatory to check the signal and, if the validity of the signal is confirmed, observatories around the world will be alerted through the International Astronomical Union network. If widespread observations continue to verify the signal, the United Nations and world leaders will be informed.

Seti scientists expect that such a signal will be narrow-band, with a lot of energy packed into a few frequencies, as otherwise it wouldn’t stand out from background cosmic radio emissions. Natural phenomena such as pulsars and gas clouds send out radio emissions at many different frequencies, so a narrow-band signal coming from deep space would almost certainly be artificial.

Detecting the signal should be relatively easy, but decoding the message in the signal would be an entirely different matter. The message would be buried deep within the signal in the form of amplitude and frequency modulations. (The signal is a wave – if you think of a water wave, amplitude is its height and frequency is the number of times a boat bobs up and down on it per second). Resolving and processing the fine detail in the signal would require an antenna far more powerful than the world’s largest, the 305m dish at Arecido, Puerto Rico – perhaps 10,000 times larger. Tim Folger thinks this will probably be constructed in the form of many hundreds of antennae spaced over a large area and linked electronically together.

It is assumed that the logic of physics and mathematics is the same everywhere in the universe and that the commonality of this underlying science could be used to decode the message. However, we cannot be sure about this. It may transpire that the message will be impossible to decode.

Then there is the matter of whether or not we should respond by sending back a reply signal. This would signpost our location in the universe and allow the alien civilisation to visit us if it felt so inclined and had the technological capacity to undertake the trip. It is likely that the aliens would be far more technologically advanced than us.

Stephen Hawking has pointed out that our earthly history of technologically advanced cultures discovering simpler cultures is a litany of unhappy experiences for the simpler cultures, eg Native American history subsequent to the discovery of the New World by Columbus. On the other hand, we might do the aliens more harm than they would inflict on us!

In fact, we have already broadcast a number of messages into space. In 1974 scientists from Cornell University broadcast the first deliberate message, using the giant radio telescope in Arecibo. The message contained some crude details in mathematical code about who we are, what we look like and what we are made from. The message was beamed at a cluster of 300,000 stars in the constellation of Hercules, 25,000 light years away. So, we may get a reply 50,000 years from now!

One reasonable deduction we could make if a signal is detected is vividly described by Seti astronomer Alan Shostak: “If Seti succeeds, then intelligence happened in at least one other place. So it probably happened in lots of places. In astronomy the only numbers are one, two and infinity. So if you get two, there are probably lots more. It’s like finding two elephants.”

William Reville is professor of biochemistry and public awareness of science officer at University College Cork: understandingscience.ucc.ie