Debating Drake

Carl Sagan. (ed) Communication with Extra Terrestrial Intelligence (CETI). MIT Press.
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This is the proceedings of a conference on the subject indicated in the title, sponsored jointly by the American and Soviet Academies of Sciences, held in Russia in September 1971. The conference was mainly concerned with two questions: the probability of the existence of extra-terrestrial civilizations, and possible techniques which could be employed in an attempt to detect them or contact them. Sagan states that there are a number of ways of formulating the problem posed by the first question and the simplest, devised by F. D. Drake , is to write the number of extant communicating civilizations in our galaxy as the product of seven factors:

N = R * fp * ne * fl * fi * fc * L

where R* is the rate of star formation averaged over the lifetime of the galaxy, in units ot numbers of stars per year; fp is the fraction of stars which have planetary systems which are suitable for life; tl is the fraction of such planets on which life actually occurs; fi is the fraction of such planets on which, after the origin of life, intelligence evolves; to is the fraction of such planets on which the intelligent beings develop develop a communicative phase, and L is the mean lifetime of each technical civilisations.

The scientists at the Conference considered each of these factors in great detail. The most interesting factor for most people will be concerning the origin of life. It seems to be generally agreed that there must be many other planets where conditions are suitable for the evolution of life as we understand it on Earth. It has also been established that on such planets the formation of the complex organic compounds necessary to living organisms is an inevitable natural process. It is also inevitable that combinations of these compounds will get together, by a process known as 'coacervation' so that they can react with one another and form even more complex molecules. What is not at present understood, however, is how such conditions can lead to the evolution of self-maintaining, self-replicating systems which we call living organisms. Given the right conditions, is the emergence of life highly improbable, quite likely, or virtually certain? Or is it so extremely improbable that life on this planet is unique? Biochemistry has not yet reached the stage where there can be any reasonable assessment of this probability, so we can only speculate.

Most of the other factors in the equation are also highly speculative, as we have only one example - our own planet and civilisation - on which to base our estimates. However, readers may find it instructive to play around with the equation, using their own scientific knowledge and common-sense to assign reasonable values to the various factors. Even is we assume the existence of extraterrestrial civilisations we have the problem of how to detect them or contact them. Radio or even lasers are considered the main possibilities, but the scientists appreciate that other civilisations may employ more advanced or more subtle means of communication which are beyond our present ability to detect. This book is recommended to all who like to speculate on the possibility of intelligent life elsewhere in the universe. - John Harney. MUFOB New Series 3, Summer 1976.