Fermi Paradox/Drake Equation
http://www.newton.dep.anl.gov/askasci/phy99/phy99024.htmDrake's equation attempts to estimate the number N of currently extant civilizations in our galaxy that would be able to contact each other.
Submitted in section: Extraterrestrial Life: Fermi's Paradox: Fermi Paradox: Drake Equation