BEFORE August, it would have been easy to believe that life beyond the Earth was rare or even nonexistent. Everything changed, however, with the sensational claim that a meteorite from Mars contains fossil evidence of microorganisms. Whether or not the claim withers under intense scientific scrutiny, few doubt that primitive organisms could have evolved on ancient Mars, where titanic rivers once gushed beneath a thick, gaseous atmosphere.
Finding that life may have started on two neighbouring planets in a single planetary system is remarkable enough. But add to this the recent discovery that many nearby stars are accompanied by planets, and there is the distinct possibility that our Galaxy is teeming with life. Suddenly, there are renewed worries about a long-standing puzzle: if life is widespread, why has the search for extraterrestrial intelligence, or SETI, drawn a blank? Where is everybody?
One person who is convinced he has the answer is Frank Tipler of Tulane University in New Orleans. 鈥淚f the Martian evidence holds up,鈥� he says, 鈥渨e may have to face the fact that primitive life is common in the Universe but that the development of intelligence is vastly improbable.鈥� In fact, he believes it is so fantastically improbable that it has happened only once since the big bang. 鈥淚 believe we are the very first intelligence to arise in our Galaxy,鈥� he says.
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This extraordinary claim is based on a straightforward comparison between the age of our Galaxy and how long it would take a civilisation capable of interstellar travel to explore and colonise it. According to Tipler, such a colonisation would be achieved most efficiently by dispatching self-reproducing probes to the stars. The concept of self-reproducing probes was developed back in the 1950s by John von Neumann, a Hungarian-American mathematician. On arrival at a star, the von Neumann probes would use the available resources to build and launch copies of themselves. 鈥淥ne probe would become two, two would become four, and so on,鈥� says Tipler. 鈥淚n this way, they would proliferate exponentially.鈥�
Of course, you could argue that if probes like these populated the Galaxy, that would be very different from finding life. However, Tipler makes no distinction between the putative extraterrestrials and their robot emissaries. The probes would need to be highly intelligent and capable of using the material and energy resources in their environment to reproduce. In a sense, they would be life forms in their own right, flesh made machine, and the space-faring successors of planet-based life.
According to Tipler, the biggest obstacle to creating von Neumann probes is computer technology. 鈥淭he probes would need to have at least human-level intelligence,鈥� he says. They would also have to be fast. But travelling at 90 per cent of the speed of light would not be beyond the capabilities of an advanced civilisation, says Tipler.
Travelling at such a speed, a probe would take about five years to reach a star 4.3 light years away-the distance between the Sun and its nearest neighbour, Alpha Centauri. If the probe takes, say, 100 years to make a copy of itself, then the average speed at which all probes would spread throughout the Galaxy would be about 1/25th the speed of light. At such a speed, the exploration of the Galaxy, which is roughly 100 000 light years across, would take about 3 million years. Even travelling at the speed of current rockets, it would take only 300 million years to explore every corner of the Galaxy and maintain a base around each star.
Long overdue
鈥淭he time needed to explore the Galaxy is hugely less than the age of the Galaxy, which is around 10 billion years,鈥� says Tipler. 鈥淪o, if extraterrestrials exist, they should be here in the Solar System today. Since they鈥檙e obviously not, they don鈥檛 exist.鈥�
Understandably, other scientists are reluctant to accept that we are alone in the Universe. Some say Tipler is premature in claiming there are no extraterrestrials in our backyard. 鈥淚t鈥檚 impossible to tell,鈥� says Edward Harrison of the University of Massachusetts at Amherst. 鈥淭he evidence of life may be written across the sky and we may simply not recognise it.鈥�
Many others share Harrison鈥檚 view that the absence of evidence is not necessarily evidence of absence. 鈥淭he whole point is we don鈥檛 know whether they鈥檙e out there or not,鈥� says Freeman Dyson of the Institute for Advanced Studies in Princeton.
Some scientists point out that extraterrestrials could be here without ever letting on. 鈥淪ay there were nanoprobes abroad in the Solar System,鈥� says Seth Shostak of the SETI Institute in Mountain View, California. 鈥淗ow would we ever know?鈥�
However, Tipler claims that if the Solar System had been visited the signs would be unmistakable. 鈥淭here would be no Oort Cloud of comets and no asteroid belt,鈥� he says. 鈥淎ll the available resources would have been turned into structures.鈥�
Tipler鈥檚 idea is that, on arriving at a new star system, a von Neumann probe would not just make copies of itself to send to other stars, but it would also exploit all the available mineral and energy resources of the star system. In our Solar System, for instance, the comets in the Oort Cloud and the asteroids in the asteroid belt would be obvious sources of minerals, which is why Tipler believes they would be long gone if extraterrestrials had ever visited. He has no idea what kind of technological artefacts such resources would be used to create, but that鈥檚 not unreasonable. After all, the Romans would have had no idea that future civilisations would turn sand into computers, or bauxite into aeroplanes. The disappearance of resources is a logical consequence of Tipler鈥檚 鈥渂iological鈥� model for interstellar colonisation, in which life鈥檚 success in filling all available niches and exploiting all available resources on Earth is repeated by intelligent life in the greater arena of the Universe.
Others, like Harrison, believe there is a fundamental error in Tipler鈥檚 biological model of the expansion of intelligent life. 鈥淚t assumes that extraterrestrials share the motivations of humans,鈥� he says. 鈥淗owever, life may evolve to a level beyond imagination and its motivations may be utterly incomprehensible to us.鈥�
Tipler counters that he assumes only that intelligent life forms behave like all known life forms on Earth, all of which go through a dispersal phase. He says it is time astronomers admitted that intelligent life is subject to the same evolutionary laws as other life.
Assuming that Tipler鈥檚 argument is valid, those who oppose his contention that we are alone must explain how the Galaxy can be teeming with colonising civilisations without any ending up in our neck of the woods. One possibility, according to Shostak, is that the Solar System is too dull. 鈥淗umans, despite colonising the Earth, are not everywhere on the planet-they are concentrated in cities,鈥� says Shostak. 鈥淚n the same way, extraterrestrials may be concentrated in the interesting places in the Milky Way-the Galactic centre, giant molecular clouds, giant star clusters, and so on.鈥� Even if humans are not everywhere on Earth, says Tipler, the effects of their activities are seen everywhere on Earth, and microorganisms are ubiquitous.
Other possible explanations for the absence of extraterrestrials in our neighbourhood include the 鈥渟elf-destruction hypothesis鈥� and the 鈥渃ontemplation hypothesis鈥�. According to the self-destruction hypothesis, civilisations blow themselves up or otherwise commit suicide before they can travel to other stars. The contemplation hypothesis states that mature civilisations grow out of the adolescent urge to colonise, preferring instead to stay at home and explore the frontiers of art, perhaps, or contemplate the meaning of life.
However, all these possibilities suffer from the same flaw. 鈥淭echnological civilisations are likely to be diverse just like living organisms,鈥� says Tipler. 鈥淪o, even if most self-destruct or stay at home to gaze at their navels, there will always be the exceptions. And the exceptions, by the logic of Darwinian evolution, are bound to come our way.鈥�
In the long term, says Tipler, even extraterrestrials with a tendency to be couch potatoes will have to up sticks and move on. 鈥淛ust as the Sun will turn into a red giant and force us to leave the Earth, the stars of alien races will eventually force them to go forth and colonise,鈥� he says.
Yet another possible explanation for the absence of extraterrestrials is the 鈥渮oo hypothesis鈥�. According to this hypothesis, emerging civilisations such as ours are cordoned off by star-faring civilisations of the Galaxy as part of a Star Trek-like non-interference policy. But, according to Tipler, this idea also has its Achilles heel. 鈥淚t鈥檚 a universal truth in human society that if you have three members of a society, you will have four opinions,鈥� he says. 鈥淭here will inevitably be a diversity of opinion among Galactic societies about whether we should be contacted or not.鈥�
Tipler also believes it would be impossible to enforce such a non-interference policy. 鈥淚t would be necessary to patrol the perimeter of the Solar System,鈥� he says. 鈥淓ven light beams would have to be stopped from entering.鈥�
It is hard to imagine the existence of a coherent Galaxy-wide society when it takes 100 000 years for a communications signal to cross from one side to the other. But if the extraterrestrials could communicate faster than the speed of light, then perhaps the society could enforce a non-interference policy, as pointed out last year by Ian Crawford of University College London. ( Quarterly Journal of the Royal Astronomical Society, vol 36, p 205). However, there remains the serious problem of maintaining an unwavering policy over millions or maybe hundreds of millions of years, when the lesson from Earth is that no society stays unchanged forever.
Off limits
But if the zoo hypothesis and other suggestions to explain why the Solar System has stayed resolutely off the interstellar tourist map are all undermined by the logic of diversity and exponential expansion, what about other, more fundamental reasons for the absence of extraterrestrials?
One possibility, according to Shostak, is that it simply takes too much energy to propel an interstellar probe between the stars at an appreciable fraction of the speed of light. Tipler dismisses this as a weak argument. 鈥淚 would remind everyone of the prominent American astronomer Simon Newcomb, who in 1904 `proved鈥� that heavier-than-air vehicles would require too much energy to be practical,鈥� he says.
Tipler believes that an advanced civilisation would find interstellar travel easy. 鈥淚n fact, we鈥檝e already launched interstellar probes-Pioneer 10 and 11,鈥� he says. 鈥淭hey鈥檝e already passed Pluto and could reach the nearest star in about 80 000 years.
This has controversial implications for SETI, which involves looking for intelligent radio or optical broadcasts from nearby stars. 鈥淚f extraterrestrials sent a radio signal, they would have to trust that there would be someone at the other end with the necessary equipment and patience to listen,鈥� says Tipler. 鈥淎 far more efficient strategy would be to send a spaceship.鈥�
Tipler says the same logic would apply if, instead of radio waves, extraterrestrials signalled with gravity waves or neutrinos, a suggestion made by Walter Simmons and his colleagues at the University of Hawaii in 1994 ( Quarterly Journal of the Royal Astronomical Society, vol 35, p 321). 鈥淲hatever way you look at it, SETI is a waste of time,鈥� he says.
Nothing, it appears, can deflect Tipler from his almost messianic conviction that we are alone in the Galaxy. And he goes further. For the irresistible logic of exponential expansion implies that if an intelligent race had arisen anywhere in creation, it would have arrived in the Solar System by now. 鈥淣ot only are we the first intelligence to evolve in our Galaxy, we are the first intelligence to evolve in the whole Universe,鈥� says Tipler. 鈥淲e are totally alone.鈥�
If Tipler is right, it will be an end to the human race鈥檚 centuries-long slide into cosmic obscurity. First, Copernicus taught us that the Earth went around the Sun and not vice versa. Then we learnt that the Sun was a very ordinary star in the Milky Way. Finally, we learnt that the Milky Way was a run-of-the-mill galaxy in a Universe containing billions of others. Now Tipler wants to put the human race firmly at the centre of creation. 鈥淲e鈥檙e special,鈥� he says. 鈥淚f we extinguish ourselves, the Universe will remain lifeless.鈥�
But other astronomers, traditionally wary of theories that depend on the Earth being special, are still sceptical. 鈥淚 don鈥檛 buy it,鈥� says Harrison. 鈥淭he view that we are alone is primitive and bereft of imagination.鈥�
Assuming that we do not extinguish ourselves, Tipler sees a golden future for the human race. 鈥淲e鈥檙e going to be the parents of the universal biosphere,鈥� he says. For this is the ultimate end point of Tipler鈥檚 mind-boggling logic. 鈥淛ust as the descendants of the first cell spread and colonised the Earth,鈥� he says, 鈥渙ur descendants will spread and colonise the entire Universe.鈥�