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The Drake equation is a probabilistic argument used to arrive at an estimate of the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.〔〔http://arxiv.org/ftp/arxiv/papers/1112/1112.1506.pdf〕 The number of such civilizations, ''N'', is assumed to be equal to the mathematical product of (i) the average rate of star formation, ''R *'', in our galaxy, (ii) the fraction of formed stars, ''fp'', that have planets, (iii) the average number of planets per star that has planets, ''ne'', that can potentially support life, (iv) the fraction of those planets, ''fl'', that actually develop life, (v) the fraction of planets bearing life on which intelligent, civilized life, ''fi'', has developed, (vi) the fraction of these civilizations that have developed communications, ''fc'', i.e., technologies that release detectable signs into space, and (vii) the length of time, ''L'', over which such civilizations release detectable signals, for a combined expression of: : The equation was written in 1961 by Frank Drake, not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI). The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life.〔 Criticism of the Drake equation follows from the fact that several of its terms are conjectural, the net result being that the error associated with any derived value is very large such that the equation cannot be used to draw firm conclusions. ==History== In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an article in the journal ''Nature'' with the provocative title "Searching for Interstellar Communications."〔 〕〔 〕 Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 centimeters (1,420.4 megahertz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe, and they reasoned that other intelligences might see this as a logical landmark in the radio spectrum. Two months later, Harvard University astronomy professor Harlow Shapley speculated on the number of inhabited planets in the universe, saying "The universe has 10 million, million, million suns (10 followed by 18 zeros) similar to our own. One in a million has planets around it. Only one in a million million has the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution."〔 〕 Seven months after Cocconi and Morrison published their article, Drake made the first systematic search for signals from extraterrestrial intelligent beings. Using the 25 meter dish of the National Radio Astronomy Observatory in Green Bank, West Virginia, Drake monitored two nearby Sun-like stars: Epsilon Eridani and Tau Ceti. In this project, which he called Project Ozma, he slowly scanned frequencies close to the 21 cm wavelength for six hours a day from April to July 1960.〔 The project was well designed, inexpensive, and simple by today's standards. It was also unsuccessful. Soon thereafter, Drake hosted a "search for extraterrestrial intelligence" meeting on detecting their radio signals. The meeting was held at the Green Bank facility in 1961. The equation that bears Drake's name arose out of his preparations for the meeting.〔 〕 As I planned the meeting, I realized a few day() ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it's going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms. —Frank Drake. The ten attendees were conference organizer J. Peter Pearman, Frank Drake, Philip Morrison, businessman and radio amateur Dana Atchley, chemist Melvin Calvin, astronomer Su-Shu Huang, neuroscientist John C. Lilly, inventor Barney Oliver, astronomer Carl Sagan and radio-astronomer Otto Struve.〔 〕 These participants dubbed themselves "The Order of the Dolphin" (because of Lilly's work on dolphin communication), and commemorated their first meeting with a plaque at the observatory hall.〔 (【引用サイトリンク】url=http://www.setileague.org/photos/miscpix/drakeqn.jpg )〕〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Drake equation」の詳細全文を読む スポンサード リンク
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