Why beaming messages to aliens in space could destroy our planet

Thanks to the foolish antics of a downmarket TV company and a website favoured by self-obsessed teenagers, planet Earth could be in for a nasty shock towards the end of this century.

For if, in the decades to come, a fleet of flying saucers arrives with malicious intent, they will be the culprits.

This week it was announced that documentary-maker RDF and Bebo, a 'social networking site' for dippy youngsters, are to use a big radio telescope in Ukraine to send a powerful focused beam of information - 500 messages from the public in the form of radiowaves - to a nearby star called Gliese 581.

A 'mere' 20 light years (120 trillion miles) away, Gliese not only lies in our cosmic backyard but astronomers think it is also home to one or possibly two Earth-like planets which could be home to life.

The Gliese 581 solar system is, in other words, probably the likeliest home for our cosmic next-door neighbours.

Surely this is a harmless piece of nonsense? What danger could there possibly be from sending a big 'Hello' from Earth to a nearby star system?

After all, aliens are probably just a myth and if they are out there they will come in peace. That's the idea anyway.

Let's get one thing straight: I am not part of the UFO brigade. I have seen no convincing evidence whatsoever that aliens have yet visited the Earth in person.

I know The X Files is a work of fiction, not a documentary, and I accept that those who claim to have seen flying saucers and even to have been abducted by strange little aliens are either sincerely mistaken, mendacious or mad.

I have no truck with crop circle mystics and those who believe the pyramids were built by little green men from the Planet Tharg.

We have been looking for alien life for several decades now, sending probes to Mars and Venus, and listening out for radio messages from the stars.

And, so far, we have found nothing. Not even a microbe.

And yet, I also accept that absence of evidence is not evidence of absence. We have been looking for extraterrestrials for only a very short time.

We have really considered the possibility of their existence for a few centuries at most - a tiny proportion of the time that we humans have been around.

Most of all, I accept that although we have not found aliens yet, the statistical probability that there are intelligent lifeforms somewhere out there must be very high.

There are, after all, a hundred thousand million stars in our galaxy - and more than a hundred billion galaxies besides.

Astronomers now think that most of these stars - more stars than there are grains of sand on all the beaches of Earth - have retinues of planets around them.

So far, they have discovered only about 250 of these potentially life-bearing worlds or 'exoplanets' but there are many hundreds of thousands of millions more out there.

Not all of them will be habitable, but many millions will probably be 'Goldilocks worlds' - not too cold, not too hot, not too big and not too small, but just right, in theory, for life to have evolved.

Indeed at least one of the planets orbiting Gliese 581 is maybe such a Goldilocks world.

And even if we accept that life will not evolve everywhere it can, we must also accept that on the one world we know life has emerged - our own - it did so with startling rapidity.

Life on Earth is very nearly as old as Earth itself, and this suggests that if conditions are right then biology of some form will come into being.

Of course 'life' is not the same as 'intelligent life'. The galaxy could be swarming with microbes and algae, shrubs and lichen, even rabbits, lizards and fish (or their alien equivalents).

Discovering that this is so would be interesting and marvellous, but it would not be the same as discovering fellow intelligent beings out there.

And even intelligent life does not mean 'spacefaring life'; it took us thousands of years to get from the invention of the wheel to the first space rockets and radio telescopes.

But the sheer size and age of our galaxy suggests that at least some other worlds should be inhabited by creatures at least as bright as us - creatures able to build radio telescopes and pick up messages and think about doing something about it when they do.

And herein lies the problem. One of the most plausible reasons that we have seen of no signs of aliens may be simply that they haven't found us yet.

Which brings us to the fact that the most likely means of our discovery by alien life is by sending radio waves announcing our presence through space.

Indeed, there are those who point out that radio and TV signals from Planet Earth have been leaking inadvertently into space since the dawn of the radio age 86 years ago.

Surely intelligent alien lifeforms - if they are out there - would have detected these Earthly signals by now, they argue.

After all, the signals travel on through space at the speed of light, so all star systems closer than about 80 light years away could, in theory, have picked up hints of our transmissions by now.

But in reality, picking up these signals will be hard, even for the most advanced civilisation.

Weak and undirected, ordinary television and radio transmissions become almost undetectable at cosmic distances. Yet powerful, focused signals such as the one to be sent by RDF/Bebo are different - they are far easier to detect.

So far, just a handful of such signals have been sent, the first message fired out by the Arecibo radio telescope in 1974. And these have generated immense controversy.

Some scientists, notably the physicist and writer David Brin, have pointed out the danger of shouting 'we are here' to a potentially hostile cosmos.

The fact is that if a civilisation even a few centuries in advance of ours (in technological terms) were to get wind of our existence then the results could be catastrophic.

For what if Gliese is home to a belligerent lifeform with infinitely superior technology to ours? After all, the history of Earth tells us that when advanced civilisations meet technologically backward ones, the results have been, almost without exception, disastrous for the people with bows and arrows.

If we are unlucky, the inhabitants of Gliese could send an invasion fleet. Since they are 20 light years away, the signal will not reach them until 2028 and it will be some decades after that before the fleet arrives here.

It is important to remember that any aliens capable of flying across the great voids between the stars will be in possession of technology so advanced that fighting them would be like taking on a modern army with spears.

We would have no chance. So the best thing may be to keep shtum.

Or to hope that the inevitable self-obsessed triviality that is bound to comprise any message sent by the Bebo community will be enough to convince any purple-tentacled aliens who are on Gliese 581 that there is no intelligent life on Earth whatsoever - and to leave us well alone.