The Great Silence & 1.8 Gigayear Interval

Are we the lone sentient life in the universe? So far, we have no evidence to the contrary, and yet the odds that not one single other planet has evolved intelligent life would appear, from a statistical standpoint, to be quite small. There are an estimated 250 billion (2.5 x 1011) stars in the Milky Way alone, and over 70 sextillion (7 x 1022) in the visible universe, and many of them are surrounded by multiple planets. The shear size of the known universe is staggeringly and inconceivably vast.

The odds of there being only one single planet that evolved life among all that unfathomable vastness seems so incredible, that it is all but completely irrational to believe. But then “where are they?” asked physicist Enrico Fermi while having lunch with his colleagues in 1950.

Fermi questioned, if there are other advanced extraterrestrial civilizations, then why is there no evidence of such, like spacecraft or probes floating around the Milky Way. His question became famously known as the Fermi Paradox. The paradox is the contradiction between the high estimates of the probability of the existence of extraterrestrial civilizations and yet the lack of evidence for, or contact with, any such civilizations.

Given the extreme age of the universe, and its vast number of stars, if planets like Earth are at all typical, then there should be many advanced extraterrestrial civilizations out there, and at least a few in our own Milky Way. Another closely related question is the Great Silence, which poses the question: Even if space travel is too difficult, if life is out there, why don’t we at least detect some sign of civilization like radio transmissions?

Milan Cirkovic of the Astronomical Observatory in Belgrade, points out that the median age of terrestrial planets in the Milky Way is about 1.8 gigayears (one billion years) greater than the age of the Earth and the Solar System, which means that the median age of technological civilizations should be greater than the age of human civilization by the same amount. The vastness of this interval indicates that one or more processes must suppress observability of extraterrestrial communities.

Full article here:

http://www.dailygalaxy.com/my_weblog/2007/12/the-fermi-death.html