Between the 1970s and early 1980s, two physicists named Michael Hart and Frank Tipler published papers arguing that extraterrestrial intelligence did not exist. They reasoned that if alien civilizations were real, they would have had enough time to develop advanced computers, spaceflight, and self-replicating machines. These machines could travel across the galaxy and build copies of themselves. Since there was no evidence of such visits, they concluded that humans are alone in the universe. This argument became known as the Hart-Tipler Conjecture, or Fermi's Paradox.