  In this article an analysis of the fundamentals used to search for
extraterrestrial artificial signals in the galaxy, which have been developing
for more than five decades, is presented. It is shown that the key factor for
the success of these research projects is given by the technological
civilizations lifetimes. Assuming the Principle of Mediocrity, estimations are
made to determine the minimum number of civilizations that may co-exist in the
galaxy and the probability of detecting a signal from them.
