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L_c \sim V^{1/4},  \end{equation}  which states that at any given time the minimum value for the number of galactic communicative civilizations in order to achieve contact is given by the volume of the galaxy to the power 1/4. Note that at the same time this number is also the minimum communicative phase lifetime required for contact. The absenece of a contact then allows to assess that in our Galaxy $L$L $L  That is \textbf{the number of communicative civilizations is smaller than a few thousands, and the lifetime of such civilizations is also usually shorter than 1000...2000 1000\...2000  years.} This does not necessarily mean that the number of intelligent civilizations in the Galaxy is limited to this number, it just implies that their communicative phase is limited to a short time. There could be different reasons for this, including transition to more efficient forms of communication than electromagnetic signals, loss of interest, singularity etc. (many of these possibilities have been extensively discussed in the context of the \href{http://en.wikipedia.org/wiki/Fermi_paradox}{Fermi paradox}). Nevertheless self-annhilation / destruction of natural resources is a realistic possibility. Overall the absence of contact tells us that there should be an important transition in the characteristic of human civilization occuring in a timescale shorter than about 1000\...2000 years.