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Our greatest quest
12 July 2003 news service
Martin Rees

If anyone, or anything, is watching the human activity on Earth from a distance they must notice that we are trying to return the compliment. Using many novel techniques, we are combing through the cosmos for signs of life. The European Space Agency's Mars Express is currently on its way to Mars. On Christmas Day, the Beagle 2 lander will detach from the orbiter, descend to the surface, and start its excavations. A few weeks later, NASA's two Mars Exploration Rovers will follow its lead. In 2004 the Huygens probe, a European contribution to NASA's Cassini mission to Saturn, will parachute into Titan's atmosphere, seeking any indicators of life that might exist in this giant moon's atmosphere. And there are still more places to explore: the most promising site anywhere in our solar system may be Jupiter's frozen moon Europa. In the long term, the space agencies should certainly be aiming to land a submersible probe to seek life in Europa's ice-covered oceans.

This serious quest for alien life is, in my view, entirely justified - indeed it is surely among the most fascinating challenges to science in the 21st century. It would plainly be a momentous event if humans could find signs of extraterrestrial life.

If life had emerged twice within our own solar system, and not simply been transferred from one cosmic habitat to another, it would surely be common on planets around other stars. We would immediately conclude that our universe harbours trillions of habitats where some kind of life, or vestige of it, exists.

We can be confident that if anyone or anything is watching us, they are doing so over interstellar distances. Even if there is life in our solar system - on Mars or Europa, for instance - nobody expects it to be advanced. Indeed, even if primitive life were widespread in our galaxy, the emergence of advanced life might not be. But it is possible that somewhere out beyond our solar system there exist creatures that could be deemed intelligent.

We must ensure we are not prejudiced about what forms that life might take. Searches for life elsewhere in the galaxy will justifiably focus on Earth-like planets orbiting long-lived solar-type stars. In his forthcoming book Life's Solution Simon Conway Morris argues that convergence in evolution is so strong that aliens would look more or less like us, and so it seems reasonable that they would require a similar habitat (New Scientist, 16 November 2002, page 26). But there are more exotic alternatives. In their entertaining book Evolving the Alien, Jack Cohen and Ian Stewart argue that life will take immensely diverse forms, emerging in a variety of environments. They are no more impressed by the "rare Earth" argument that only a very special planet could harbour life than by the fact that our legs are exactly long enough to reach the ground.

Perhaps life can flourish at lower temperatures, on a planet that has been flung into the frozen darkness of interstellar space and whose main warmth comes from internal radioactivity, the process that heats the Earth's core. Could living structures resembling Fred Hoyle's fictional "Black Cloud" be floating through interstellar space? These entities, living and thinking in slow motion, may come into their own in the future.

Any claims that advanced life is widespread must, of course, confront the famous question posed by the great physicist Enrico Fermi: "Why aren't the aliens here?" Why haven't they visited Earth already, or at least manifested their existence in a way that leaves us in no doubt? This argument gains further weight when we realise that some stars are billions of years older than our sun: if life were common, its emergence should have had a head start on planets around these ancient stars.

The cosmologist Frank Tipler, perhaps the most vocal proponent of the view that we are alone, argues that at least one alien civilisation would have developed self-reproducing machines and launched them into space. They would spread through the galaxy within 10 million years - a time far shorter than the head start that other civilisations could have had.

But we don't see any evidence of such machines. Some people of course claim we have been visited. But if aliens really had the brainpower and technology to reach the Earth, would they merely despoil a few cornfields or abduct a few well-known cranks?

The Fermi paradox is not entirely compelling. Indeed, a recent book by Stephen Webb claims to offer 50 different counter-arguments. ET's signal may simply have been missed, for example. Webb concludes the best option may be that we really are alone. But as often in science, opinions are most strongly polarised when evidence is minimal; we know far too little about how life began, even on Earth, to offer confident odds. Moreover, no matter how heavy the odds against finding it, searches for extraterrestrial intelligence are surely a worthwhile gamble because of the huge philosophical import of any detection.

A manifestly artificial signal - an ultra-narrow-band radio transmission, or a message as boring as a set of prime numbers or the digits of in binary notation - would convey the momentous message that intelligence, though not necessarily consciousness, is not unique to the Earth and has evolved elsewhere, and that concepts of logic and physics are not peculiar to the type of biological hardware contained within human skulls.

The SETI Institute at Mountain View, California, is spearheading these searches, supported by hefty private benefactions. Any interested amateur with a home computer can download and analyse some of the institute's data stream from radio telescopes, and comb the cosmos for signs of intelligent life.

More than 3 million people have taken up this offer - each, no doubt, inspired by the hope of being the one who finds ET. In the light of this broad public interest, it seems surprising that SETI searches have had such a hard time getting public funding - even at a level that matches the tax revenue generated by a single science fiction movie. If I were an American scientist testifying to Congress I'd be happier requesting a few million dollars for SETI than seeking funds for conventional space projects or particle accelerators.

Of course, even if intelligence were widespread, we may only ever recognise a small and atypical fraction of it. We may not be able to conceive of the ways in which some brains may package reality. Other brains could be living deep under some planetary ocean and doing nothing to reveal their presence. Even if signals are being transmitted, we may not recognise them as artificial. Cohen and Stewart point out that a radio engineer from 60 years ago, only familiar with amplitude-modulation systems, might have a hard time decoding modern wireless communications. Perhaps the only type of intelligence we could detect would be the small subset using a technology attuned to our own parochial concepts.

It would perhaps be disappointing if searches for alien intelligence were doomed to fail. As long as we have a negative result, we will never make progress in answering the long-standing question of our cosmic significance. Some 250 years ago the cosmologist Thomas Wright was asking the same questions as I am in this article (see "Are we important?") and, in many ways, he knew just as much about the answers as we do today.

On the other hand, maybe it should boost our cosmic self-esteem: if our tiny Earth were the unique abode of intelligence, we could view it in a less humble light than it would merit if, as Wright conjectured, the galaxy already teemed with complex life.

More time lies ahead than has elapsed in the entire course of biological evolution. In those aeons, Earth could be the "seed" from which post-human life spreads through the galaxy. The fate of humanity could then have an importance that is truly cosmic: what happens here might conceivably make the difference between a near eternity filled with ever more complex and subtle forms of life and one filled with nothing but base matter.

From issue 2403 of New Scientist magazine, 12 July 2003, page 25

Are we important?

"In this great Celestial Creation, the Catastrophy of a World, such as ours, or even the total Dissolution of a System of Worlds, may possibly be no more to the great Author of Nature, than the most common Accident in Life with us, and in all Probability such final and general DoomsDays may be as frequent there, as even Birth-Days or Mortality with us upon this Earth."

Cosmologist Thomas Wright of Durham, "An Original Theory of New Hypothesis of the Universe" (1750)

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