2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/141936Civilization 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.4 pages, to appear in the proceedings of "Bioastronomy 2007", ed. Meech et alPopular PhysicsAstrophysicstext