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The Fermi paradox is the seeming inconsistency between the lack of evidence of extraterrestrial civilizations and the apparently high likelihood of their existence.

In simple terms, the Fermi paradox asks why, given the vast number of stars and potentially habitable planets in our observable universe, there is no clear evidence of extraterrestrial civilizations. The paradox is named after physicist Enrico Fermi who blurted out the question—remembered by Emil Konopinski as "But where is everybody?"—during lunchtime after the conversation had already moved on to other topics. The paradox first appeared in print in a 1963 paper by Carl Sagan, and the paradox has since been more fully described by scientists. Early formulations of the paradox have also been identified in writings by Bernard Le Bovier de Fontenelle (1686) and Jules Verne (1865), and by Soviet rocket scientist Konstantin Tsiolkovsky (1933). There have been many attempts to resolve the Fermi paradox, such as suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) humans see no evidence.

Since many of the Sun-like stars are billions of years older than the Sun, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes.

After the conversation had moved on to other topics, Fermi blurted out his question which has been variously recalled as: "Where is everybody?" (Teller), "Don't you ever wonder where everybody is?" (York), or "But where is everybody?" (Konopinski). According to York, Fermi "followed up with a series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on.

Fermi was not the first to note the paradox. Another early formulation of the Fermi paradox was presented and dissected in the 1930s writings of Russian rocket scientist Konstantin Tsiolkovsky. Tsiolkovsky noted that critics refute the existence of advanced extraterrestrial life as such civilizations would have visited humanity or left some detectable evidence.

Two years later, Stephen Dole noted the dilemma at a symposium—"If there are so many advanced forms of life around, where is everybody?"—but did not attribute it to Fermi. A chapter of Intelligent Life in the Universe, co-authored by Sagan and Iosif Shklovsky, was headlined with the Fermi-attributed "Where are they?" The Fermi question also appeared in NASA's 1970 Project Cyclops report, a 1973 book by Sagan, and a 1975 article in JBIS Interstellar Studies by David Viewing that first described it as a paradox.

Scientists like Robert Gray have criticized its attribution to Fermi, and alternative terms like the "Hart–Tipler argument" or "Tsiolkovsky–Fermi–Viewing–Hart paradox" have been proposed.

The Fermi paradox is a conflict between the argument that scale and probability seem to favor intelligent life being common in the universe, and the total lack of evidence of intelligent life having ever arisen anywhere other than on Earth. The second aspect of the Fermi paradox is the argument of probability: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems possible that at least some civilizations would be technologically advanced, seek out new resources in space, and colonize their star system and, subsequently, surrounding star systems.

They conclude that intergalactic colonization appears possible with the resources of a single planetary system and that intergalactic colonization is of comparable difficulty to interstellar colonization, and therefore the Fermi paradox is much sharper than commonly thought.

There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potentially habitable planets, and that humans see no evidence of life.

Perhaps the Fermi paradox itself, however aliens may conceive of it, is the reason for any civilization to avoid contact with other civilizations, even if no other obstacles existed.

Quick Facts

• In simple terms, the Fermi paradox asks why, given the vast number of stars and potentially habitable planets in our observable universe, there is no clear evidence of extraterrestrial civilizations.
• There have been many attempts to resolve the Fermi paradox, such as suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) humans see no evidence.
• The paradox is named after physicist Enrico Fermi who blurted out the question—remembered by Emil Konopinski as "But where is everybody?"—during lunchtime after the conversation had already moved on to other topics.
• The paradox first appeared in print in a 1963 paper by Carl Sagan, and the paradox has since been more fully described by scientists.
• Fermi was not the first to note the paradox.

Source material: Wikipedia - "Fermi paradox". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.