A probabilistic analysis of the Fermi paradox in terms of the Drake formula: the role of the L factor

ABSTRACT In evaluating the number of technological civilizations N in the Galaxy through the Drake formula, emphasis is mostly put on the astrophysical and biotechnological factors describing the emergence of a civilization and much less on its the lifetime, which is intimately related to its demise. It is argued here that this factor is in fact the most important regarding the practical implications of the Drake formula, because it determines the maximal extent of the ‘sphere of influence’ of any technological civilization. The Fermi paradox is studied in the terms of a simplified version of the Drake formula, through Monte Carlo simulations of N civilizations expanding in the Galaxy during their space faring lifetime L. In the framework of that scheme, the probability of ‘direct contact’ is determined as the fraction of the Galactic volume occupied collectively by the ‘spheres of influence’ of N civilizations. The results of the analysis are used to determine regions in the parameter space where the Fermi paradox holds. It is argued that in a large region of the diagram the corresponding parameters suggest rather a ‘weak’ Fermi paradox. Future research may reveal whether a ‘strong’ paradox holds in some part of the parameter space. Finally, it is argued that the value of N is not bound by N = 1 from below, contrary to what is usually assumed, but it may have a statistical interpretation.