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\end{itemize}  Finally $L$ is the longevity of a communicative civilization (in years). Humankind has been "communicative" only for a few decades. Interestingly a detection (or even complete silence) would put strong constraints on $L$, in a sense telling something about the usual course of civilizations. In other words about the future of humankind.   $x^\leftrightarrow$ $\stackrel{\leftrightarrow}{x}$