and hunting and eventual control over the environment, the minimum conditions for a scientific culture.
Intelligent human civilizations, such as those in Ancient Greece and Ancient China, flourished and died without any likelihood of discovering Maxwell’s laws of electro-magnetism: the minimum condition for radio communication. A scientific civilization also requires a few lucky breaks from the physical environment. Despite their acknowledged intelligence and communicative skills, dolphins will never invent printing or acquire an adequate knowledge of electro-magnetism as a precursor to radio contact. But even if an ET civilization did develop an understanding of the laws of physics, that would still leave open the intriguing philosophical question of whether they mean the same to them as they do to us.
Generous estimates of the number of planets with intelligent communicative life suffered a serious setback in 1992 following the completion of a radio search conducted by D.G. Blair at the University of Western Australia. Blair led a team of astronomers, using a Parkes radio telescope, at the ‘magic’ frequency of 4.462336275 GHz during two observations in 1991 and 1992. This frequency was selected by multiplying the frequency of hydrogen by pi, the first funda-mental constant considered likely to be discovered by any intelligent civilization. The search covered the neighbourhoods of 176 stars in the F, G and K range, within forty light years of the Earth. No signal was detected. These negative results weaken Drake’s assumption that technological intelligence will inevitably emerge given enough time on an Earth-size planet near a Sun-like star (Blair et al., 1992).
L: the lifetime of an advanced technological civilization capable of interstellar communication
Estimates for this factor have neither theoretical nor empirical support. The only advanced technological society we have knowledge of is our own and it will only be after its extinction that an accurate figure for L for one civilization can be supplied. But to appreciate how consideration of L might reduce the size of N (the number of civilizations currently capable of communicating), it is worth reflecting that it took about 300 million years of the cool planet period for life to evolve and a further 3,500 million years for only one species, out of billions, to invent the radio. And within half a century after that, intelligent life on Earth had produced nuclear weapons and began to pose an ecological threat to the planet, both of which are capable of terminating life within a very short period. It may be a mere fluke that humans developed the potential for interstellar communication before self-destruction.
Reflections on the quantification of L are bound up with a related terrestrial question: can we survive advanced technology? In fact, it was reflection upon L which enabled Carl Sagan to persuade Senator Proxmire that SETI might provide a means of understanding how our society might survive the nuclear age,
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as L indicates civilizations which have avoided self-destruction. It may be that L is little over a hundred years. It would be a matter of cosmic irony if civilizations turn out to be capable of destroying themselves at the very moment they achieve the ability to communicate with each other. Apart from war and ecological disaster, L can be further reduced by collisions with asteroids, exhaustion of natural resources, over-population, genetic degeneration and loss of interest in science, where people turn to religions like Zen Buddhism, new age cults or simply lapse into hedonism.
A pessimistic calculation for L was made by J. Richard Gott III (1993) who estimated an 0.2 million to 8 million years limit to longevity of our species at 95 per cent confidence level. According to Gott, the average longevity for most species is between 1 million and 11 million years, and for mammals it is about 2 million years. Gott insists that intelligence – defined in terms of the possession of self-consciousness and cognitive skills which enable abstract thought, creativity and an ability to think about the future – confers no survival value. He points out that our early ancestor, Homo erectus, lasted 1.4 million years and the Neanderthals lasted 200,000 years. Taking the Copernican principle – there is nothing unique or privileged about the human race – at face value, Gott argues that there is unlikely to be sufficient time for the human race to either colonize space or establish widespread contact through radio communication. Space travel, he suggests, somewhat pessimistically, has run its course. The Cold War, which supported the space race, is over. Moon travel lasted a mere four years. Human activity in space is confined largely to a close-to-Earth orbit. With increasing pollution of radio space and an energy crisis, our capacity for radio communication and space travel will soon expire. The space window may soon be closed, and will only remain open for a limited period before energy supplies fail, space debris (see Simpson, 1994), or light and electromagnetic pollution (McNally, 1994) close it.
It could transpire that the limit to the communicative abilities of a technological civilization is less dramatic than destruction by war or famine; these limits may be determined by its tendency to surround itself with a barrier of pollutants. Thus in 1993 Gott estimated that we only have the space window for another thirty-two years. If we do not use that window we shall stay and become extinct, but if we do use it we have a slim chance of survival via the spread of colonies. But he warns that our chances are slender.
With luck, perhaps, humankind will survive and remain capable of sending messages which can be picked up if there is anyone out there to listen. The only incontestable fact in this mass of speculation is that so far the human race has survived.
The belt of life and the sleeping dragon
Severe limitations upon the first and last factors in the Drake equation have been set by invoking the notions of a belt of life and a sleeping dragon at the centre of
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