Drake [revised]

Broadcasting but not receiving: density dependence considerations for SETI signals

The Drake equation is an attempt to estimate how many extraterrestrial civiliations exist in our galaxy and how many of those we might eventually contact.  It's an interesting idea, and gets a lot of coverage in astrobiology and skeptic publications, but it's essentially pure speculation:

The Drake equation states that:

where:

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

and

R^* is the average rate of star formation in our galaxy

f_p is the fraction of those stars that have planets

n_e is the average number of planets that can potentially support life per star that has planets

f_ℓ is the fraction of the above that actually go on to develop life at some point

f_i is the fraction of the above that actually go on to develop intelligent life

f_c is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space

L is the length of time such civilizations release detectable signals into space.

This paper proposes a change in one of the variables, which is interseting, but still doesn't really lead us anywhere.

"Results show that under certain assumptions, a galaxy can be teeming with civilizations yet not have a guarantee of communication between any of them given either short lifetimes or small maximum distances for communication."