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Putting these factors into the Drake equation above we get   $N \approx \, \frac{1}{4}\, f_i \, L $.   This states that the number of communicative civilizations in our Galaxy is just the product of the chance of emergence of intelligent life times the average lifetime of a civilization's communicative phase. In It seems hard to be able to put strong theoretical constraints on both these factors. However one can turn the problem around, and in  the next and last post of this series I will show how the current search for radio signals from extraterrestrial intelligence has can be used to  put some important constrains on the two factors $f_i$ and $L$, and how these informations can be used to make some bold predictions about the future of humankind. %Note this is basically the same result that Frank Drake obtained in 1961! The difference is that this has been %obtained only making assumptions on the factors $f_i$,$f_l$ and $f_c$, since the other terms in the equation %are now established.