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ASTRONOMY ( HERCHEL) A SHORT HISTORY OF SETI: THE SEARCH FOR EXTRA-TERRESTRIAL INTELLIGENCE Dr Stephen Parkinson, on 19 February 2001 In 1959 a paper in Nature outlined the possibility of interstellar communication with other life forms at our level of technology and beyond. This talk described what has been happening in the last thirty years and the guiding scientific rationale behind the work. Life on earth has certain characteristics that we assume are necessary in any other life form that could develop the technology to communicate across space. These are: • carbon-based in order to provide the complexity of living bodies • chemically unstable so that bodies decompose on dying • able to grow • able to replicate • able to be repaired • highly ordered (negative entropic) • having a continuous demand for energy • evolving What chance is there that other civilisations, with adequate technology to communicate, exist in the galaxy? Drake produced an equation to calculate this probability. By applying `best' and `worst' conditions for each factor, it seems that either (at `worst') we are alone in the galaxy, or there are between one million and one hundred million other civilisations with intelligent, technically-advanced life forms on habitable planets in the galaxy. So it is worth searching for a contact with one of them. We can both send signals and await a reply or listen for signals and them send a reply. For sending there are two `windows' in the sun's radiation, an optical and a radio window, of which the radio is favoured. This window, from 0.1 to 10 GHz frequencies, includes the frequencies of the hydrogen ion (1.423 GHz) and of the hydroxyl ion (~1.8 GHz), which are likely to be recognised. It might be clever to send on pi times the hydrogen frequency (3.1416 x 1.423GHz) to show how advanced we were in mathematics. For listening, we have to scan 10 billion frequencies in bandwidths of 0.1 Hz to cover the reception window and the signals may be intermittent! We can either search target planets of near-by sun-like stars or try to examine every signal from anywhere. It is not surprising that no confirmed (repeated) contacts have been detected in thirty years. Only in the last few years have computers been fast enough to operate an adequate search programme. Now we have such computers we might, in the next hundred years, receive a recognisable and repeated signal. If we do, an agreed reply is awaiting transmission.
SETI started before life, in the form of bacteria and viruses, was discovered in the cosmos, and is searching for intelligent forms. It is appreciated that such life might take longer to communicate than the life of our species. After all, it has taken life on earth 3.5 million years to reach a state where it can communicate intelligently. We have to confine our definitions of `civilisations' and `intelligent' to characteristics we can recognise. We are searching for patterns in random `pictures'; with sets of random numbers this can be done easily with today's computers, so what is so difficult for SETI? The large number of signals received; `waterfall plots' may be able to pick out a `beacon' signal as a line but then we have to find out whether that signal has any message in it. Dr Parkinson provided much information about the many programmes used to carry out 70 searches over 30 years and gave details about the way in which computer users could take part in analysing signals as part of the ARGUS distributed network. Don Lovell |
