themselves. Under such circumstances, probes might be limited to unpopulated regions.
Tipler’s model of colonization, which has also been criticized (Sagan 1983), is based upon the mathematical theory of island colonization developed by R.H. MacArthur and E.O. Wilson (1967). This model is not typical of all Earth populations and hence scarcely typical of populations throughout the galaxy. Sagan considers several conventional models of colonization by biological populations and he calculates that ‘the establishment of galactic hegemony requires a perseverance to the task for a period of a billion years’ (1983: 118). According to Sagan, it is therefore unlikely that any advanced extraterrestrial civilization would remain steadfastly committed to this task for even a fraction of that time. A civilization of merely 1,000 years into our technological future would hardly be interested in strip-mining and colonizing every planet in sight. Presumably, an advanced extraterrestrial civilization would have better things to do than imitate events in recent short-term Earth history.
However, Tipler could reply that his theory of colonization by self-replicating von Neumann machines does not require any long-term commitment. Once the first probe is launched, the process is potentially independent of the sending civilization, which may lose interest in the project or self-destruct. Moreover, it would only require the existence of one technologically capable civilization in the entire galaxy to initiate the process of colonization.
This argument becomes less plausible once it is recognized that developing and maturing civilizations are capable of recognizing past errors and of taking steps to put matters right. A civilization might come to recognize the dangerous folly of sending its probes and take steps to curtail them and even warn others about them. Moreover, there is the problem of determining the actual benefits to the sending civilization. Probes that simply reproduce themselves offer little in the way of increased wealth or security to their creators, nor do they enhance their quality of life or chance of survival.
Perhaps a less sophisticated, but nevertheless relevant response to Tipler’s argument is to question the feasibility of a system of self-replicating probes. Such machines bear little relation to present technology or any predictable future developments. The credibility of von Neumann machines rests on an appeal to the analytic–synthetic dichotomy. Put simply, it involves the belief that if a project is not analytically impossible, that is, self-contradictory, then its imple-mentation is merely a technical problem awaiting a practical, perhaps difficult, but not impossible solution. This kind of reasoning might eliminate logically incoherent proposals, but it can give respectability to a wide range of absurdities. For example, there is nothing illogical about the proposal to take a holiday on the Sun. So it is merely a technical problem regarding an adequate mode of transport and a suitable form of protection from the Sun’s radiation. The absurdity of this proposal is not a matter of logic; it lies in the fact that its implementation would demand an almost infinite number of practical solutions
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to problems we can envisage and no doubt as many solutions to problems we have not even encountered.
Similar objections can be made against the proposals to initiate a series of self-replicating probes. As Munévar (1998: 191–2) shows, the vast physical and environmental differences among rocky planets within a stellar system such as ours, will require an almost infinite combination of factors that might affect the mining and manufacture of the basic components. The initial programme, or set of blueprints, would have to be sufficiently general and all-embracing in order to cope with an almost infinite range of environmental contingencies. Although it is not illogical, it is demonstrably absurd.
According to the arguments, these machines will approach a mineral-rich environment, alone or with other such machines, and begin to extract various materials, manufacture plastics, metals, and so on, without steel mills, processing plants or a sophisticated transport and distribution system, and then begin to construct complicated apparatus including rockets containing more probes. This is nothing short of miraculous. Using the best technology on Earth there is not even the remotest possibility of designing a machine that could go out and locate the necessary materials and manufacture a bicycle. Even if the appropriate technology were available, the problem would lie in the inability to write such general purpose programmes that would exhibit a flexible response to an incredible variety of environmental conditions. It would require an immensely complex computer, and the more complex its software, the more likely that there would be errors built into it.
Those who employ these kind of fictitious examples should be able to specify in considerable detail how such schemes can be practically implemented. Perhaps, for the sake of an experimental demonstration, a probe could be sent to the Sahara Desert or the Antarctic, where it could be given the task of replicating itself. This is extremely unlikely. Arguments which rest on appeals to better technology or hitherto unheard of technological advances, should be treated with caution. Frequently references are made to the narrow-minded prejudice of scientists who fail to appreciate future technological marvels. In this context references are made to the prominent American astronomer, Simon Newcomb, who in 1904, ‘proved’ that heavier-than-air flight would require too much energy to be practical. But this is misleading. Newcomb was not confronting the fantasies of science fiction; he was employing out-dated concepts against theory and observational evidence which indicated considerable knowledge of air flight.
SETI’s response to Fermi’s Paradox
SETI’s response to the ‘silence’ problem has been imaginative, if not entirely convincing. There are both sceptical and pluralist explanations of the Great Silence. Sceptics argue that it means that we are unique and that our emergence is an evolutionary fluke. Sceptics also argue that the factors in the Drake
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