galaxy within 60 million years. Frank J. Tipler, a sceptic with regard to the possibility of contact with ETI, has argued that just one self-copying probe could generate enough probes to have one near each star in the galaxy within 300 million years. Yet, argues Tipler, they have left no sign of their existence near or on the Earth. Even if there was only one other civilization, argued Tipler, albeit a technologically advanced one, they could have built self-replicating machines which could develop space travel and eventually colonize the planets. Even if their transport is less than the speed of light, argued Tipler, they could redevelop and colonize the galaxy within the age of the Earth. So why have they not contacted us already? Tipler concludes that we are alone.
Tipler’s argument is familiar but, as he maintains, its force is under-appreciated. He suggests that if intelligent extraterrestrials ‘did exist and possessed the technology for interstellar communication, they would already be present in our Solar System. Since they are not here, it follows that they do not exist’ (Tipler, 1980: 267). Tipler maintains that ‘an intelligent species with the technology for interstellar communication would necessarily develop the technology for interstellar travel, and this would automatically lead to the exploration and/or colonization of the Galaxy in less than 300 million years’ (ibid.). This assumption that an intelligent, technologically communicative species will necessarily develop the means to explore and colonize the galaxy is based upon questionable grounds. In fact, Tipler appeals to a version of the principle of mediocrity (a belief that our own evolution is typical of life throughout the universe). Yet the thesis that intelligent life is universal is precisely what Tipler ultimately denies. It might be noted that whoever resorts to the principle of mediocrity, it is a weak basis for any argument. There is no evidence other than the development of some, but not all, civilizations on Earth, on which to support the conclusion that we are typical of life-forms – actual or possible – throughout the universe.
Tipler’s argument is nevertheless worth exploration. He begins with the assumption, based on the predictions of certain exponents of Artificial Intelligence, that an intelligent extraterrestrial species ‘will eventually develop a self-replicating universal constructor with intelligence comparable to the human level’ (ibid.: 268). A universal constructor can construct anything that can be constructed, says Tipler (1995: 45) and it can make copies of itself in minute size using nano-technology. These constructors, or von Neumann machines, could then be combined with present-day rocket technology and would replicate themselves out of local resources and ‘eventually colonize the Galaxy in less than 300 million years’ (Tipler, 1980: 268). The cost in energy and expense would be minimal to the sending civilization who would only have to find resources for the first machine, which would be programmed to seek out construction material and replicate itself and then construct similar rockets which would transport copies to the next targets, and so on throughout the galaxy. The cost of such an interstellar probe Tipler estimates to be about US$250 billion at current prices. Improvements in design could be communicated by radio, or by means of self-learning programmes, such that future
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machines would be more sophisticated than their ‘parents’. Tipler also envisages sophisticated machines equipped with artificial womb technology which could synthesize fertilized eggs. The probes could be programmed to construct organic beings, humans, from available raw materials. This would involve beaming human DNA information to the site at the speed of light, so that people could populate an area without having to make the journey. It would, however, have to be accepted that errors in duplication would be likely to occur, and that these would be more likely in a hostile environment, such that the ‘living machines’ envisaged in the original project would have evolved considerably beyond their original state during the 300 million years of galactic colonization.
While Tipler’s suggestion, like the adaptive intelligence claimed on behalf of futuristic von Neumann machines, lies beyond immediate scientific horizons and is indicative of those weak philosophical arguments which appeal to incredible scientific possibilities, there is a plausible basis, at least, for Tipler’s transport programme. The technology required would be little more than conventional rockets designed for one-way journeys to the nearest construction site. This could be accomplished by chemical fuel sources, although nuclear power could be developed for longer journeys. Nevertheless, such speculation is based on wildly optimistic forecasts about machine intelligence and instrument reliability.
According to Tipler, the search for extraterrestrial intelligence would have greater empirical credentials if it were directed towards the location of von Neumann probes. A radio search can miss its target: they might not be broadcasting at the time that we are listening; or we might miss them because we are tuned to the wrong frequency. But a search for probes is more determinate; either they are there or they are not. Tipler’s conclusion that we are alone is based on his argument that, given an earlier start, other extraterrestrial civilizations would have set in motion a colonization process and that by now the galaxy should be teeming with von Neumann probes. Because they have not been observed, he concludes that we must be unique.
How plausible is this argument? Carl Sagan (1983) identified several flaws in Tipler’s thesis and suggested limitations on galactic colonization. According to Sagan, if these machines are programmed to go on replicating, then they will not stop until the entire universe has been converted into von Neumann machines, which would then presumably cannibalise each other. It follows, argues Sagan, that these machines would be a threat to any emerging intelligent civilization and steps would have to be taken to restrict their development long before they proliferate. A prudent policy would be to prevent and disrupt their construction. If Tipler is correct, the entire universe is threatened by them and every intelligent society is in danger. If there are any other intelligent civilizations, they would take steps to limit this threat and the absence of von Neumann probes could well be due to the efforts of intelligent extraterrestrials. Measures to subdue the local population would have to accompany each probe, as it is unlikely that the inhabitants would stand idly by while the colonizers reproduced
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