deletions | additions       diff --git a/Equation.tex b/Equation.tex  index 32dc085..d38e070 100644  --- a/Equation.tex  +++ b/Equation.tex 
...
\section*{Counting Civilizations} In order to estimate the number of technological civilizations that might exist among the stars, in 1961 Frank Drake proposed the following simple equation $N = R \times f_p \times n_e \times f_l \times f_i \times f_c \times L$ \begin{itemize} \item $N$ is the number of civilizations in the Milky Way Galaxy with whom we could make contact. \item $R$ is the rate of star formation, which tells how many stars are born every year in our Galaxy. \item $n_e$ is the average number of habitable planets in any planetary system \end{itemize} All the $f$ terms are factors $\le 1$: \begin{itemize} \item $f_p$ is the fraction of stars that have planets \item $f_l$ is the fraction of planets where life actually begins \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 detection (or the complete absence of signal) would put strong constraints on $L$, in a sense telling something about the usual course of civilizations. That is the future of humankind.