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Space Topics: Search for Extraterrestrial IntelligenceThe Drake EquationThe Drake equation was SETI pioneer Frank Drake's roadmap for answering SETI's ultimate riddle: how many communicating civilizations are there in our galaxy? This grand question, Drake thought, could be reduced to seven more manageable ones: The rate of star formation in our galaxy at the time our Solar System was formed (R*); The fraction of stars that have planets around them (fp); The number of planets per star that are capable of sustaining life (ne); The fraction of planets in ne where life evolves (fl); The fraction of fl where intelligent life evolves (fi); The fraction of fi that communicate (fc); The lifetime of a communicating civilization (L); The product of these seven factors should provides a good estimate of total number of communicating civilization (N). In mathematical shorthand this gives the classical Drake equation: N= R* fp ne fl fi fc L
Drake formulated his equation in 1960 in preparation for the Green Bank meeting that established SETI as a scientific discipline. The historic meeting, whose participants became known as the "Order of the Dolphin," brought together leading astronomers, physicists, biologists, social scientists, and industry leaders to discuss the possibility of detecting intelligent life among the stars. To bring some order and coherence to such an eclectic gathering, Drake came up with the equation. It was a way to order the different issues to be discussed, and bring them to bear on the central question of intelligent life in the galaxy. The equation served well at the Green Bank conference, but soon it became much more than an organizational tool. To Drake's amazement, it proved irresistible to SETI promoters. The short mathematical formula reduced a huge and almost unmanageable speculative question to a neat series of seemingly specific questions. While the larger question seemed too large and speculative, its seven components appeared to lend themselves to scientific inquiry. And so, what for Drake was essentially an organizational tool, became a shorthand for the entire scientific field of the Search for Extraterrestrial Intelligence. The problem with the Drake equation, many have noted, is that estimates of the values of each of its seven factors vary enormously. In 1960, when the equation was first proposed, some empirical evidence existed only for the first term (R*) - the rate of star formation. Today, thanks to the discoveries of extrasolar planets in the past 10 years, scientists can begin to offer estimates of the second factor as well - the fraction of stars that have planets around them (fp). With recent work on "habitable zones" around planets, based on the characteristics of known planetary systems and the dynamics of planet formation, the third factor - the number of planets per star that are capable of sustaining life (ne) - is also beginning to be the subject of empirical scientific study. All this shows the enormous amount of progress made over the past half century in astronomy and planetary science. Nevertheless, the values of the four remaining factors, from "the fraction of planets where life evolves" to "the lifetime of a communicating civilization," remain pure guess work. As a result, estimates for "N" based on the Drake equation can vary by orders of magnitude. It is in reference to this, and to the fact that estimates for most of the components of the equation are based on pure speculation, that some have referred to the Drake equation as "a shorthand for our ignorance." --Amir Alexander |
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