amount of failure in SETI endeavours constitutes proof that we are alone. Only the existence of life is demonstrable.’ Despite the fact that SETI resembles a search for a very small needle in a very large haystack there must, nevertheless, be criteria which set a limit on failure. At present, however, the search has barely begun. Even a massive search, lasting hundreds of years, could miss technological civilizations which emerge with a capability for radio communications during the search period. It is difficult to draw the conclusion from a negative search that the target areas are devoid of life. One can only conclude that during the time of the search no signals were being directed at us. It is worth noting that the Earth has only been radio-detectable for a century. Thus a radio search of Earth before the late nineteenth century would not reveal ‘intelligent life’ whereas a search during this century would reveal to any extraterrestrial civilization that there is intelligent life here.
The fact that we are now detectable may stimulate our galactic neighbours into communicative efforts. The fact that we are of potential interest should stimulate us to further efforts to listen. Suppose that, like us, ET civilizations are listening and have powerful receivers which can pick up our leakage signals from radio and TV broadcasts, bearing in mind that radio and TV signals are weak and are not broadcast on any of the magic frequencies. If the nearest receiving civilization is 40 light years away they will have picked up our leakage signals beginning in the 1930s which have continued since then and we could expect their reply from the year 2010 onwards. On this view the passage of time increases the prospect of contact. But it also means that there is a reduced likelihood of a receiving communicating civilization nearer than 40 light years away and that this distance will increase with the passage of time. Meanwhile, better technology and bigger searches using some of the world’s largest radio telescopes are currently underway.
Why should they wish to communicate?
Most SETI researchers assume that ETs are willing to communicate and share their superior knowledge with us. But why should they wish to communicate? What likely benefit would accrue to them? One suggestion is that they realize that their sun is nearing its end and with it their whole civilization. They transmit, giving details of their history, thus expressing a desire that some memory of their existence will survive them, with no hope of receiving a reply. A second suggestion is that a civilization begins space exploration and discovers reasonable proof that life has evolved on another nearby planet, thus giving them optimistic reasons to initiate a communicative network with other civilizations. A third suggestion is that two advanced civilizations may have evolved close to each other so that first efforts are immediately successful. This would generate an intense search from the beginning. Once communication is established, it would then be likely to be widespread, as success would encourage more beacons. Thus, so the arguments go, if interstellar communication exists, it
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is probably a reality for countless races who are participating in a galactic network. The assumption that ETs have a desire to communicate has meant that searches have been designed to identify a deliberate signal. This may limit the prospect of detecting intelligent life as there may be technological civilizations who have no conscious desire to attract our attention. A case can therefore be made for eavesdropping on their radio and TV leakage. Thus Woodruff T. Sullivan (1980) argues that a well-designed search system must allow for radio leakage as well as an intended message. This may be more difficult to detect, but leakage can be separated from natural emissions, so long as it is narrow band and periodic.
What kind of intelligence is sought?
The technology employed to conduct the search for ETI has imposed its own definition of intelligence. SETI researchers are looking for a technological intelligence appropriate to the late twentieth-century scientific civilization on Earth. Given the potential for a vast diversity of intelligent cultures, this might appear as a rather narrow and restricted objective. But this is all that existing SETI technology can detect. There is an anecdote told by SETI scientists: a man looks for a key that he has lost in a dark street. There are a few lights, so he concentrates his search beneath the light because that is all he can do. The light provides the reasonable range of the search. Likewise SETI technology provides the reasonable range of the search, and confines it to the search for similar technology. There is, however, a further reason to suppose that ETI will share an understanding with terrestrial technological intelligence. Given that curiosity is a related feature of intelligence and the same physical laws operate throughout the universe, it is likely that curious ETIs will have discovered what we have discovered.
Many of the assumptions of our scientific civilization are accordingly built into the SETI programme, such as the assumption that nature itself is not intelligent. Data that have a natural origin are excluded. SETI research is primarily a search for evidence of technology. Some SETI scientists have sought evidence of interstellar nuclear waste dumping, looking for evidence of leakage from ET nuclear plants by means of the detection of tritium in the vicinity of a solar-type star. This radioactive isotope is a particularly good indicator of nuclear technology; it has a half-life of 12.5 years and its presence would indicate leakage from a nuclear fusion plant. Presumably any evidence of an ET Chernobyl-type disaster would be an indication of intelligent life! If detected in our solar system tritium could indicate the presence of a nearby ET probe.
Current radio astronomical searches are based on a concept of high technology intelligence and are unlikely to make contact with any life-form that has not evolved such an intelligence. They will not make contact with any equivalent of ancient Greece or ancient China. SETI scientists define intelligence in terms of the power of abstract thought, the ability to construct at least partly successful
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