Sometimes, people who are unfamiliar with the scientific picture of cosmic and biological evolution think the equation is highly speculative. In fact, it is just the opposite, since each phenomenon it assumes to take place in the universe is an event that has already taken place at least one.(6)
The original formulation is: \[N = R_{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L\] where $N$ is the number of civilizations in our galaxy with which radio-communication might be possible (i.e. which are on our current past light cone);
$R_{\ast}$ the average rate of star formation in our galaxy;
$f_p$ the fraction of those stars that have planets;
$n_e$ the average number of planets that can potentially support life per star that has planets;
$f_{\ell}$ the fraction of planets that could support life that actually develop life at some point;
$f_i$ the fraction of planets with life that actually go on to develop intelligent life (civilizations);
$f_c$ the fraction of civilizations that develop a technology that releases detectable signs of their existence into space;
$L$ the length of time for which such civilizations release detectable signals into space(8)
In 1966 Shklovskii and Sagan added a new term to the equation, $f_g$, the fraction of stars suitable for supporting life(2, 3, 4).
Now, at the end of January, SETI released the video of an interview to Frank Drake conducted by Andrew Fraknoi (http://fraknoi.blogspot.it/2014/02/an-interview-with-father-of-search-for.html):
In June 2012, the SETI Institute sponsored another three-day public event called SETICon -- a series of talks, panels, and events featuring scientists, authors, futurists, and film-makers. It was a time and place for science and the imagination to meet. One highlight of the program was an interview with the "father of SETI science," astronomer Frank Drake (the first President of the SETI Institute Board. ) It was conducted by SETI Institute Board Member and veteran astronomy educator Andrew Fraknoi. The discussion ranged widely over Dr. Drake's career and current thinking, and included reminiscences of Project OZMA, the very first experiment searching for signals from civilizations among the stars, and his current view of the Drake Equation (estimating the chances of intelligent life out there). He also reflects on a number of modern developments, including the discovery of numerous planets orbiting other stars and new ways of searching for extra-terrestrial civilizations.
More about Drake equation:
The Drake equation revisited
An interactive object about the Drake equation
Thanks to Andrew Fraknoi for sharing the video
(1) Cocconi G. & Morrison P. (1959). Searching for Interstellar Communications, Nature, 184 (4690) 844-846. DOI: 10.1038/184844a0 (pdf)
(2) I.S. Shklovskii, Carl Sagan (1966). Intelligent life in the universe. Holden-Day (San Francisco)
(3) Freeman J. & Lampton M. (1975). Interstellar archaeology and the prevalence of intelligence, Icarus, 25 (2) 368-369. DOI: 10.1016/0019-1035(75)90032-9 (pdf)
(4) Walters C., Hoover R.A. & Kotra R.K. (1980). Interstellar colonization: A new parameter for the Drake equation?, Icarus, 41 (2) 193-197. DOI: 10.1016/0019-1035(80)90003-2 (pdf)
(5) Ćirković M.M. (2004). The Temporal Aspect of the Drake Equation and SETI, Astrobiology, 4 (2) 225-231. DOI: 10.1089/153110704323175160 (pdf, arXiv)
(6) Frank Drake, Dava Sobel (2010). The origin of the Drake equation. Astronomy Beat, 46 pdf)
(7) Frank Drake (en.wiki)
(8) Drake equation (en.wiki)
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