Whose paradox seeks to answer the question: "Where are the aliens?"?

Given that our solar system is quite young compared to the rest of the universe — roughly 4.5 billion years old, compared to 13.8 billion — and that interstellar travel might be fairly easy to achieve given enough time, Earth should have been visited by aliens already, the idea goes.

N = R* × fp × ne × fl × fi × fc × L

N, the number of civilizations currently transmitting signals, depends on seven factors:

R* is the yearly formation rate of stars hospitable to planets where life could develop

fp is the fraction of those stars with planets

ne is the number of planets per solar system with conditions suitable for life

fl is the fraction of planets suitable for life on which life actually appears

fi is the fraction of planets with life on which intelligent life emerges

fc is the fraction of planets with intelligent life that develops technologies such as radio transmissions that we could detect

L is the average length of time in years that civilizations produce such signs