In 1950, the physicist and Nobel laureate Enrico Fermi famously asked his colleagues: “Where are they?” Fermi had been reflecting upon the vastness of the cosmos, and “they” in his question referred to extraterrestrials. With an almost unfathomable number of stars and planets in the universe, it seemed obvious that intelligent civilizations capable of developing radio astronomy and interstellar travel should speckle the distant stars. Yet, in Fermi’s day, no evidence of such civilizations existed — something that still holds true today.

The Fermi Paradox is the term used to describe the lack of evidence for extraterrestrial life in the face of a universe that should be, by the numbers, bursting with it. But we see no signs of alien technology, and our radio telescopes don’t pick up voices from other worlds.

Many hypotheses have been proposed to resolve the Fermi Paradox, but all of these remain unproven. And in the 1990s, another possible explanation for our apparent aloneness in the universe was formulated by Robin Hanson — a postulate that has become known as the Great Filter.

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