The Drake equation

What do we need to know in order to decide whether life exists in other parts of the universe? This is a reasonable question. How might we locate another technologically advanced civilisation? This might not be nearly so reasonable. To assume that "life" implies "technologically advanced civilisation" is to beg a lot of questions. Nevertheless, considerable effort goes into scanning the heavens for radio emissions that might betoken the existence of one or more technologically advanced civilisations in other parts of the galaxy.

In 1961, Frank Drake, a radio astronomer who later became chairman of the SETI Institute, tried to specify the factors involved in the development of a technologically advanced alien civilisation. He encapsulated his ideas in an equation that has guided much subsequent discussion of the subject. The equation can be written:-

N = R* x fp x ne x fl x fi x fc x L where N = the number of civilisations in the galaxy from which electromagnetic emissions are detectable; R* = the rate of formation of stars that are compatible with the development of life; fp = the fraction of those stars that have planetary systems; ne = the number of planets with life-supporting environments orbiting each of these stars; fl = the fraction of these planets on which life appears; fi = the fraction of life-bearing planets on which intelligent life evolves; fc = the fraction of civilisations that develop an advanced technology, emitting detectable radio signals; and L = the length of time during which these civilisations release detectable signals.

The National Research Council of the USA is just one official body that assumes the Drake equation to be a valid guide to research. The equation has the merit of being simple and dimensionally correct, and it focuses our curiosity and interest in a fascinating question. However, it presupposes that intelligent life and the development of advanced technology are natural, inevitable outcomes of some cosmic process, and as we shall see, that is a very dubious supposition. Also, it overlooks a number of important factors.

We shall return to the Drake equation and its connotations at the end of the present chapter. In the interim we shall try to assess the likelihood of life on other planets, using arguments based on earlier parts of this book. This will enable us to identify what is missing from the Drake equation, or questionably assumed in it.

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