[0901.3863] Broadcasting but not receiving: density dependence considerations for SETI signals
Physicist Reginald Smith publishes what, in retrospect, seems to be an obvious answer to the Fermi Paradox, which states that if the universe is teeming with intelligent life (that is, the Drake Equation has a solution greater than 1), then why haven't we heard from them. Smith takes into account of the exponential degradation of any signal produced by a civilization and works out the minimum civilization density required for civilizations to be able to contact each other. According to his theory, under some conditions you can have a galaxy filled with intelligent civilizations but have none able to communicate with each other. Our problem may, in fact, be that we are a star on the sparsely-populated outer arm of the galaxy so outside the communication range of other civilizations.