If there are about 400 billion stars in our Galaxy alone, and about 400 billion galaxies in the universe, it seems reasonable that somewhere out there, in a cosmos 14 billion years old, there are, or there has ever been, at least a civilization as advanced as ours. The sheer immensity of the numbers almost forces us to admit the veracity of this hypothesis. But then why haven't we found any signal, any message, any artifact from those extraterrestrials?
In this thoroughly revised and expanded edition of this well-known work by Stephen Webb, the author breaks down in detail the 75 most convincing and interesting solutions (for the moment) of the famous Fermi paradox: if the figures strongly point to the existence of extraterrestrial civilizations, why have we not found any signs of them?