Can an equation tell us how many alien civilizations exist?
Frank Drake wrote it as a meeting agenda. Fed with different guesses, it says the galaxy holds fifty million civilizations or almost none.
▶ Start the storyNot really, and its inventor never meant it to. Frank Drake formulated the equation in 1961 as an agenda for the first scientific meeting on SETI, to stimulate dialogue rather than to quantify the number of civilizations. As Drake put it, planning the meeting he realised a few days ahead that it needed an agenda. The ten attendees called themselves "The Order of the Dolphin", after John Lilly's work on dolphin communication.
The agenda became a multiplication. N, the number of civilizations in the Milky Way with which communication might be possible, equals the rate at which stars form, times the fraction with planets, times the number of life-friendly planets per star, times the fractions that develop life, intelligence and detectable technology, times L, the length of time such civilizations release detectable signals into space.
Feed in Drake's 1961 educated guesses and you get a minimum N of 20 and a maximum of 50,000,000. The meeting concluded that N is roughly L, probably between 1000 and 100,000,000 civilizations. Feed in pessimistic guesses and you can get a number far below one. That is the criticism: the factors are so conjectural that the equation cannot be used to draw firm conclusions. Its defenders say the usefulness is not in the solving but in contemplating the concepts, a map of what we need to learn.
20 → 50,000,000
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Recap
The Drake equation is a map of what we need to learn, not a calculator of how many aliens there are.
💡 A trick to remember it · Seven keys on one ring: if even one of them does not turn, the door stays closed, and we cannot yet tell which key is the stuck one.
Surprising fact · Drake wrote it as a meeting agenda in 1961, and the group that met called itself 'The Order of the Dolphin'.
Sources (1)
No source, no claim. Every fact in this lesson (24 claims) cites at least one of these.