How many intelligent alien civilizations are out there? Adjust the sliders and let the math decide.
In 1961, astronomer Frank Drake proposed a simple equation to estimate the number of active, communicative civilizations in the Milky Way galaxy. The equation multiplies seven factors, from the rate of star formation to the lifespan of technological civilizations. It was never meant to give a precise answer, but rather to organize scientific thinking about the search for extraterrestrial intelligence (SETI).
If the Drake Equation suggests even a few civilizations, then as physicist Enrico Fermi famously asked: "Where is everybody?" This is the Fermi Paradox. Possible explanations include the "Great Filter" (some step in development that almost no civilization survives), the vast distances between stars, civilizations choosing not to communicate, or simply that we have not been looking long enough.