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Let's define $n=\frac{N}{V}$ the number density of communicative civilizations in the Galaxy, where $N$ is the total number of such civilizations existing at any given time in the Milky Way and $V$ is the total galactic volume. Assuming an homogeneous distribution across the Galaxy, the average separation $r$ between two neighbour communicative civilization is $r\sim n^{-1/3}$. In order for two civilization to communicate, their separation needs necessarily to be smaller than their communicative phase lifetime $L$. This is assuming communication technology that is limited by the speed of light. We can then ask what would be the minimum value $L^*$ such that communication can occur. That is we impose $r=L^*$. We then obtain $L^*\approx (N/V)^{-1/3}$. On the other hand our discussion of the Drake equation led to the conclusion that, roughly speaking, $N \sim L = L^*$. And so we can further simplify and write   $L^* = (L^*/V)^{-1/3}$ (\frac{L^*}{V})^{-1/3}$