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The galactic volume can be estimated as 10^{12}-10^{13} cubic light years: therefore in our Galaxy $L  It also means that at this time in our Galaxy there should be less than $N\approx 350...625$ communicative civilizations.  Note that the calculation above does not include "relic communications", messages received by one communicative civilization but sent by a no longer communicative one. So the estimate above represent an upper limit to $L$ and $N$.  %and the lifetime of such civilizations is also usually shorter than 1000\...3000 years. This does not necessaril%y 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.wikipedi%a.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.