Between the mid-70s and early 80s, two physicists (Michael Hart and Frank Tipler) published a controversial series of papers arguing that extraterrestrial intelligence didn't exist. As they argued, the likelihood that extraterrestrial civilizations (ETCs) would have had enough time to develop advanced computing, spaceflight, and self-replicating machines (Von Neumann probes) means they would have colonized the galaxy (and come to Earth) long ago. Since there was no evidence of this, they...