frank drake, 1961. how many civilizations might we hear from?
In 1961, astronomer Frank Drake wrote down a way to estimate the number of technological civilizations in the Milky Way that could potentially communicate with us. Each factor is uncertain — the earlier ones are astronomy (we know them fairly well), the later ones are biology, sociology, and history (we don't). Drag the sliders below to get your estimate.
N = R* · fp · ne · fl · fi · fc · L
estimated communicating civilizations in milky way
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what the fermi paradox adds
Even at pessimistic numbers, the equation usually gives something more than zero — yet we see no signal. "Where is everybody?" — Enrico Fermi, in 1950. Possible answers: we're alone; civilizations destroy themselves; they don't broadcast; the universe is too big; we're being quarantined (zoo hypothesis); we've only been listening for ~70 years and the galaxy is huge. L (lifetime) is the killer term: if civilizations only broadcast for a century before going quiet, we might miss them by millions of years.