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The paradox originated in 1950, during a lunchtime conversation at Los Alamos National Laboratory. Enrico Fermi, a prominent nuclear physicist who worked on the Manhattan Project, and his colleagues were discussing UFOs, perhaps prompted by the 1947 flying disc craze that had shaken the country just a few years earlier. Given the vast number of potentially habitable planets and myriad plausible methods for interstellar communication or travel, they wondered why humans hadn’t yet encountered evidence of alien civilizations. Fermi famously summed up the dilemma in a single question: “Where is everybody?”

The apocryphal story has transformed into a popular thought experiment. A common explanation for the apparent absence of extraterrestrial neighbors is what economist Robin Hanson termed the “Great Filter”—the idea that there exists a major obstacle preventing civilizations from reaching a stage at which they have the capability to send messages or crewed voyages to other star systems. The Great Filter may lie behind us, meaning life on Earth already beat the odds in overcoming some catastrophe, allowing our civilization to develop. Or else we might yet face some challenge that’s hard to survive. Though the term itself is fairly new, it builds on cold war–era concepts, particularly those tied to the Kardashev scale—a framework developed in the 1960s that speculated on how extraterrestrial civilizations might progress.

The Kardashev scale has become a key influence on some technologists. Proposed in 1964 by Soviet astronomer Nikolai Kardashev, the scale classifies extraterrestrial civilizations based on their energy use: Type I civilizations harness all the energy available on their home planet; Type II civilizations capture the total energy output of their star; and Type III civilizations command energy on the scale of their entire galaxy.

He once wrote: “Any self-respecting civilization should at least reach Kardashev Type II.”

Originally a thought experiment, the scale is now often treated as a literal roadmap—implying a desirable, even inevitable, trajectory toward greater energy consumption and interstellar expansion.

The cold war, which gave us both the Fermi paradox and the Kardashev scale, was defined by existential anxiety. Nuclear weapons ushered in the possibility of humanity’s rapid self-destruction, and scientists were acutely aware of their enabling role in our species’ potential demise. This fear deeply influenced early SETI scientists, shaping their ideas about the civilizations they hoped to find in the galaxy. Often their imagined civilizations mirrored their own anxieties and aspirations.