  Civilization cannot sustain an exponential growth for long time even when
neglecting numerous laws of physics! In this paper, we examine what are
fundamental obstacles to long term survival of a civilization and its
possibility to colonize the Galaxy. Using the solar system as a reference,
resources available for sustained growth are analyzed. Using this information,
we will explore the probability of discovering a civilization at its different
stage of energy evolution as estimating some possible value of L, the typical
life time of an extra-terrestrial civilization.
