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Explaining SETI
By Greg Taylor of The Daily Grail

The search for extraterrestrial intelligence is a scientific investigation into possible communication signals originating from an extraterrestrial civilisation. It originated in searches and subsequent discussions between a few interested individuals around forty years ago. These early forays resulted in more systematic searches for microwave signals, and also in the proposal of Drake's Equation, a method for estimating the number of communicating extraterrestrial civilisations. The searches have been centred on a certain part of the microwave spectrum, in ever-narrower bands as technology increases to allow it. They can be either targeted, allowing greater sensitivity, or sky surveys which increase the area searched though with less precision. Other methods have been proposed, some of which consider the technology that an advanced civilisation may have developed, while others are based on our own progression. Search programs may be categorised as either dedicated, directed or shared. Current programs, due to lack of funds, generally fit into the directed or shared category, though they still manage intensive searches. One of the most innovative is the SETI@home program, which utilises volunteer's home computers to process data. Other programs such as Optical SETI are developing into exciting new search methods which may supercede the microwave searches currently underway. The problems facing the search for extraterrestrial intelligence are manifold, but to date they have been managed well and current interest levels suggest a bright future.

The scientific search for extraterrestrial intelligence (SETI) has its origins in a seminal paper by Giuseppe Cocconi and Philip Morrison in 1959, called 'Searching for Interstellar Communications', in which they argued for the use of microwave communication between interstellar civilisations. At much the same time (and independently) Frank Drake, a young radio astronomer, conducted 'Project Ozma': a single-channel scan in the direction of two nearby sun-like stars, using an 85-foot antenna. Interest continued to grow after these initial excursions into 'alien territory', with the Soviet Union and the United States of America taking turns at leading the search for extraterrestrial intelligence. The hopes for success grew in the 1970's after a comprehensive analysis of SETI by the NASA-commissioned 'Project Cyclops' (History of SETI 1999). Continuing advances since then in the technology required for searching has resulted in a rapidly evolving field in which the chances of success seem to be improved by an order of magnitude each year. The various programs currently running are, however, for naught if there is no one out there. Estimates of the number of interstellar civilisations which are capable of broadcasting a signal, are therefore an important element of the search. The accepted equation for estimating this value is called 'Drake's Equation'.

In 1961 Frank Drake proposed an equation for expressing the number (N) of observable civilisations in the Milky Way galaxy (Billingham & Tarter 1993, p. 248):

N = R * fp * ne * fl * fi * fc * L

where

  • R is the rate of star birth in our galaxy,
  • fp is the fraction of stars that have solar systems,
  • ne is the average number of Earth-like planets in each solar system,
  • fl is the fraction of those planets on which life forms,
  • fi is the fraction of life-bearing planets which produce an intelligent species,
  • fc is the fraction of intelligent species that are capable of interstellar communication,
  • L is the average lifetime of a communicating civilisation (in years).

Estimates of N by experts, based upon this equation, vary from just one (we are alone) to thousands, if not millions. The reason for this huge discrepancy is the difficulty inherent in quantifying each element of the equation. While some are now reasonably certain (for example, R is now generally thought to be about 1, although it may have been as high as 5 in the early history of the galaxy (Schilling 1999), others are not so easy. Examining fi exposes the problem - we currently have only one example: ourselves. Some believe that humanity is nothing special, and that the value is therefore close to one; others think that we are the product of numerous coincidences, and that the value is almost 0. Similarly, pessimists may say that in view of our own self-destructive tendencies L may be quite small; others believe that an advanced civilisation may be able to continue for millions of years. Thus it is evident that each 'unknown' in the equation is capable of either creating a bottleneck, which substantially reduces N, or opening a floodgate, which improves the chances that we are not alone. It is important to note that this figure is not an estimation of the number of planets with life on them, or even the number of intelligent species. N is equivalent to the number of intelligent civilisations capable of communicating across interstellar distances (Billingham & Tarter, p. 248). For a recent (2003) debate on the Drake Equation, see this article on Astrobio.net. Though the Drake Equation remains a theoretical quandary, the question of whether we are alone or not may be answered in the meantime by a more practical method: listening.

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