Are we alone, or is ET out there?

Now primitive life has been found on Mars, does that mean we can expect to find alien intelligence in our galaxy? Is Planet Earth unique - or does ET know all about us, but choose to stay at home?

If ET is out there, why hasn't he come our way? It's a question which has long worried scientists, but which in recent months has begun to haunt them. The reason is, of course, the dramatic claim back in August that a rock from Mars contained fossil bacteria. If the red planet once harboured primitive life - or harbours life today - then evidence for life will have been found on both the planets - Earth and Mars - whose rocks we have examined. Add to this the recent discovery that planets are common around nearby stars, and it is hard to avoid the conclusion that life is widespread in our Milky Way galaxy.

One possible explanation why we have not been visited by ET is that primitive life is common, but that intelligent life is not. This is the view of Frank Tipler, professor of physics at Tulane University in New Orleans. "I believe the development of intelligence is fantastically improbable," he says. "In fact, I would go as far as to say we are the very first intelligence to appear in our galaxy."

Professor Tipler arrives at this startling conclusion after comparing the age of our galaxy with the time it would take a race capable of travelling between the stars to explore and/or colonise it. The best way to achieve such a goal, he says, is by sending out interstellar probes capable of reproducing themselves. Such probes would use the available energy and mineral resources around a star to build copies of themselves. "In this way, they would spread exponentially throughout the galaxy like bacteria in a culture," he explains.

The professor believes that an advanced civilisation could launch probes at speeds as high as 90 per cent of that of light - that's 600 million miles an hour. At this speed, a probe would take about five years to travel between stars in the neighbourhood of the sun. If it takes a century, say, to make a copy of itself, then the speed at which all probes spread throughout the galaxy is about one-25th of the speed of light. It would take a mere 3 million years to explore the Milky Way, which is about 100,000 light years across.

"The crucial thing to realise is that the time needed to explore the galaxy is hugely less than the age of the galaxy, which is around 10,000 million years," says Professor Tipler. "If extraterrestrials exist, they should therefore be here in the solar system today. Since they're obviously not, they don't exist."

Not surprisingly, other scientists dispute Professor Tipler's conclusion. "The absence of evidence is not necessarily evidence of absence," says Professor Edward Harrison of the University of Massachusetts in Amherst. "The evidence of life may be written across the sky and we may simply not recognise it."

Tipler, on the other hand, claims that the signs would be very easy to see. "All the mineral resources of the solar system - those tied up in comets and asteroids - would have long ago been exploited, for the same reason that we exploit the mineral resources of the Earth," he says.

It's all a consequence of Professor Tipler's "biological" picture of interstellar colonisation, in which life spreads throughout the galaxy, inexorably filling all available niches and exploiting all available resources.

It's a picture which some, like Professor Harrison, believe is fundamentally wrong. "It assumes that extraterrestrials have the same motivations as human beings," he says. "But life may evolve to a level beyond imagination, and its motivations may be utterly incomprehensible to us."

If Professor Tipler's argument is right, however, those who oppose his claim that we are alone must explain how the galaxy can be full of civilisations without any coming our way. Possible explanations include the Self Destruction Hypothesis - that civilisations blow themselves up before they can build starships - and the Contemplation Hypothesis - that civilisations simply grow out of the immature urge to colonise and prefer to stay at home, perhaps to contemplate the meaning of life.

However, Professor Tipler points out, all such explanations have the same flaw. "Technological civilisations are likely to come in as many different forms as living organisms," he says. "Even if most self-destruct or stay at home, there will always be the exceptions which will come our way."

Another possible explanation for the absence of extraterrestrials in our backyard is the Zoo Hypothesis - that infant civilisations such as ours are left alone by the star-faring civilisations of the galaxy, which obey a Star Trek-like non-interference policy. But, according to Tipler, this idea also has its flaw. "It's a universal truth in the world that if you have three members of a society, you will have three opinions," says Tipler. "There will inevitably be a diversity of opinion among galactic societies. Some will want to leave us alone, others will not."

Nothing can apparently deflect Tipler from his conviction that we are alone in the galaxy. And he goes farther, much farther. For the irresistible logic of exponential expansion implies that if an intelligence race had arisen anywhere in space, it would have arrived in our backyard by now. "We are not only the first intelligence to evolve in our galaxy," says Tipler, "we are the first intelligence to evolve in the whole universe."

The incredible improbability of such a situation does not worry him in the least. "Somebody has to be first," he says. "Why not us?"

And Tipler is not depressed by the prospect of being alone in the cosmos. On the contrary, he is excited by it. "We're going to be the parents of the universal biosphere," he says. "It will be our descendants' privilege to spread and colonise the entire universe"n