On Bayesian inference considerations and other issues concerning Drake's equation of Astrosociobiology

Abstract

Speculation about the existence of advanced forms of life in the Universe and in our galaxy, has been since ever a subject of fascination and discussion in fiction, as well as in astrophysics, biology and philosophy. The well-known Fermi's 1950s challenge, "Where are the aliens?" has acquired more substance with the realisation of the potentialities of radioastronomy, which led to the paradigmatic Drake's equation. The emergence of astrobiology, together with the discovery up to now of more than seven thousand exoplanets, has brought increasing support to the discussion about putative life cradles. However, after more than six decades, the only quantitative tool available to estimate how widespread is life and, in particular, advanced forms of life, is, besides direct searches, which so far provided no evidence, still Drake's equation. In the present work we review the current knowledge about this equation and present new arguments of multiple origin in order to evaluate one of its most critical terms, namely the one associated to the time span that a technological civilisation must search for detectable signs of the existence and for how long a search must be extended to bear fruits. We propose that this term should be replaced by a more specific one which involves critical parameters in the enterprise of gathering information, such as energy expenditure, searching area and entropy generation. These terms can be regarded as the capability that any cosmic civilisation must show in order to face the challenge of going beyond the climate and other crises that its development inevitably ensues. Our considerations suggest that a typical time span is about a couple of decades, meaning that a successful and systematic searching programme around about hundred stars might take around a few thousand years.