# Carl Sagan on the Drake Equation

Professor Frank Drake proposed an equation that could be used to estimate the number of *detectable* extraterrestrial civilizations in the Milky Way galaxy. The equation was deemed important for his work at the National Radio Astronomy Observatory in Green Bank, West Virginia (which I've driven by many times -- their huge telescope is quite a sight!). In essence, Drake decided to define a series of limiting factors, so that we could take the total number of observable stars, then scope way down to get to some estimate for how many might have civilizations that we could contact. The resulting Drake Equation is one of the most exciting bits of math I've ever seen. Wikipedia explains it like so:

The Drake equation states that:

where:

N= the number of civilizations in our galaxy with which communication might be possible;and

R^{*}= the average rate of star formation per year in our galaxyf_{p}= the fraction of those stars that have planetsn_{e}= the average number of planets that can potentially support life per star that has planetsf_{?}= the fraction of the above that actually go on to develop life at some pointf_{i}= the fraction of the above that actually go on to develop intelligent lifef_{c}= the fraction of civilizations that develop a technology that releases detectable signs of their existence into spaceL= the length of time for which such civilizations release detectable signals into space.

If that's too mathy for you, just watch Carl Sagan explain it in this eight-minute video:

Where it really gets interesting (and frustrating) is when you start to figure how many of these detectable civilizations are actually *currently broadcasting* during a time period when *we* might actually contact them or receive their broadcast (adjusted, of course, for the massive lag time to get the broadcast from point A to point B). Sagan touches on part of this problem in his discussion, but doesn't get into the details. Read more about all this at Wikipedia.