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\item $f_i$ is the fraction of life-bearing planets that develop an intelligent life-form  \item $f_c$ is the fraction of intelligent civilization that decide to communicate  \end{itemize}  Finally $L$ is the longevity of a communicative civilization (in years). Humankind has been "communicative" only for a few decades. A Interestingly a  detection (or the even  complete absence of signal) silence)  would put strong constraints on $L$, in a sense telling something about the usual course of civilizations. That is the future of humankind.