One equation tries to estimate how many alien civilizations we could actually talk to

Ahead of a 1961 conference on extraterrestrial life, astronomer Frank Drake sketched an equation multiplying every factor that might matter: star formation rate, the fraction of stars with planets, the fraction of those habitable, down to how long a civilization keeps communicating. Almost every term is still a guess, so the equation organizes the uncertainty more than it predicts a number. It remains the standard framework for thinking about the odds of contact with intelligent life elsewhere.

— Frank Drake, The Drake Equation — Formulated for a 1961 SETI meeting at Green Bank Observatory