The argument claiming that extraterrestrials don’t exist was actually proposed by the astronomer Michael Hart, in a paper he published in 1975. Hart supposed that if an extraterrestrial civilization arose in the galaxy it would develop interstellar travel and launch colonizing expeditions to nearby stars. These colonies would, in turn, launch their own starships spreading a wave of colonization across the galaxy. How long would the wave take to cross the galaxy? Assuming that the starships traveled at one tenth the speed of light and that no time was lost in building new ships upon arriving at the destination, the...