and Frank Drake have suggested that if a species developed life expectancy rates approaching immortality, they would be reluctant to take risks and would avoid interstellar contact out of timidity. One of Sagan’s answers to Tipler is that  there are two types of advanced society: aggressive and peaceful. The peaceful do not take risks, do not colonize, so we are unlikely to hear from them. But the aggressive are intrinsically unstable and are unlikely to pass through the dangerous phase that we have  reached on Earth with nuclear weapons. So either they die off as the result of nuclear war and nuclear winter or they pass through the dangerous phase and become non-colonizing  pacifists  (Sagan, 1983). We thus arrive at the following paradox:  only societies that can survive the critical nuclear weapons stage are those with the potential for colonization. But in order to survive as an advanced technological society, aggressive behaviour of this kind would have to disappear. Hence: ‘the only societies long-lived enough to perform significant colonization of the Galaxy are precisely those least likely to engage in aggressive galactic imperialism’ (ibid.: 120).

There is a further reason why a very advanced civilization is likely to develop pacifist tendencies. Suppose they have developed methods of combating the ageing process or have found ways of indefinitely repairing damage caused by ageing and only encounter death through accident or murder, and throughout their long lives have preserved, as Drake (Drake and Sobel, 1993: 160) suggests, ‘a growing set of memories of individual experience’. At one level we can comprehend this, but the difference between them and us would be massive. They would have, says Drake, a fantastic obsession with safety, avoiding wars, accidents and even risky contacts with other worlds. It is possible that silence would be their preferred option for survival. On the other hand, they could realize that spreading their knowledge about immortality – and hence encouraging others to be equally concerned with safety – would be an alternative strategy for survival.

Intelligent extraterrestrials might very well remain silent for the same reasons as radio astronomers on Earth. Apart from an occasional brief signal, and inadvertent signals from radio and TV networks, which are very weak, the SETI radio searches are for listening, not transmitting. This is partly because intentional broadcasts are more expensive and also because of a fear that a hostile civilization might learn too much. It is possible that everyone is listening and no one is communicating. It is also possible that our failure to observe ET communication is because they have some better means of communication than electromagnetic waves.

David Brin (1990) offers a range of explanations of the Great Silence. He points out that the 250-million-year orbit around the galaxy poses major survival problems for various solar systems. When they pass the spiral arms where new stars are formed in superexplosions they are likely to be destroyed. But a very advanced civilization (for example, a Kardaschev–Dyson Type II or III, see Chapter 7) may simply leave this dangerous place taking their solar system far away. Consequently, the very advanced civilizations would be further away and










less likely to make contact; only the less advanced would remain and they might not have evolved the ability to communicate over long distances. Brin also notes that there may be many unforeseen disasters in the galaxy, such as huge black holes, which could destroy potentially colonizing civilizations. There may be holocausts caused by the effects of colonization such that the colonizers leave nothing behind them. But on a more friendly note, Brin suggests that it is likely that the most habitable planets – not too hot and not too cold – with ample water and oxygen, will be far less dry than ours. Hence land creatures would barely develop. In this respect we are unique. But intelligent life, such as dolphins and whales, will develop in the water, without our technology derived from the use of hand and fire, and hence an intelligence with no likelihood of reaching the stars.

A novel twist to Fermi’s Paradox can be derived from a consideration of what ETIs may wish to obtain from making contact with us. This approach was outlined by T.B.H. Kuiper and M. Morris (1977) who suggested that a more highly developed civilization may place a high value on forms of knowledge or experiences that we have not even learnt to appreciate, and that this resource actually grows with time. Suppose, say Kuiper and Morris, that there is a certain threshold phase to this process before which we could produce nothing of value to an advanced civilization. If they were to make contact too soon, before we reach this threshold, and expose us to the superior knowledge held by the galactic community, ‘instead of enriching the Galactic store of knowledge we would merely absorb it’ (Kuiper and Morris, 1977: 620). Our capacity to produce new ideas and make a contribution to the galactic heritage would be restrained if the best human minds were ‘occupied for generations digesting the technology and cultural experiences of a society advanced far beyond our own’ (ibid.). Thus silence is maintained because early contact would remove the very reason for making contact.

Insofar as there are parallels between the silentium dei of the theologians and silentium universe, then a variation of the role of the Devil can be introduced. Just as the Devil is responsible for preventing communication with God, so a paranoid advanced species may be active in disrupting communication. Every few million years or so they will send down deadly probes to wipe out intelligent life. But maybe they do not act out of devilish motives. Maybe our kind of intelligence, with its destructive potential,  is perceived as a deadly cancer that any really intelligent species will attempt to contain lest it spreads unchecked destroying whatever eco-system it inhabits. A criterion for super intelligence might be based on the capacity to prevent the spread of highly destructive advanced technological civilizations like ours. In that case, the fraction for L would be very small indeed.

These explanations are the stuff of science fiction. But much of science fiction exhibits an impoverished concept of intelligence, which is revealed in one of the most commonly employed explanations of the Great Silence: we have not heard from them because we have not developed the right technology. Perhaps it




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