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This article is about the absence of clear evidence of extraterrestrial life. For a type of estimation problem, see Fermi problem.
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence.[1][2][3]
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 informally posed the question—remembered by Emil Konopinski as “But where is everybody?”—during a 1950 conversation at Los Alamos with colleagues Konopinski, Edward Teller, and Herbert York. The paradox first appeared in print in a 1963 paper by Carl Sagan and the paradox has since been fully characterized 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,[4][5] 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.
Chain of reasoning
Some of the facts and hypotheses that together serve to highlight the apparent contradiction:
- There are billions of stars in the Milky Way similar to the Sun.[6][7]
- With high probability, some of these stars have Earth-like planets orbiting in the habitable zone.[8]
- Many of these stars, and hence their planets, are much older than the Sun.[9][10] If Earth-like planets are typical, some may have developed intelligent life long ago.
- Some of these civilizations may have developed interstellar travel, a step that humans are investigating.[11]
- Even at the slow pace of envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.[12]
- 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.[13]
- However, there is no convincing evidence that this has happened.[12]
History
Los Alamos conversation
Enrico Fermi was a Nobel Prize-winning physicist who predicted the existence of neutrinos and helped create the first artificial nuclear reactor, an early feat of the Manhattan Project.[14] He was known to pose simple but seemingly unanswerable questions—termed “Fermi questions“—to his colleagues and students, like “How many atoms of Caesar’s last breath do you inhale with each lungful of air?”[15]
In 1950,[note 1] Fermi visited Los Alamos National Laboratory in New Mexico and, while walking to the Fuller Lodge for lunch, conversed with fellow physicists Emil Konopinski, Edward Teller, and Herbert York about reports of flying saucers and the feasibility of faster-than-light travel.[18] When the conversation shifted to unrelated topics at the lodge, Fermi blurted a question variously recalled as: “Where is everybody?” (Teller), “Don’t you ever wonder where everybody is?” (York), or “But where is everybody?” (Konopinski).[19] According to Teller, “The result of his question was general laughter because of the strange fact that, in spite of Fermi’s question coming out of the blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life.”[20]
Enrico Fermi posed the paradox to fellow physicists Emil Konopinski (left), Edward Teller (middle), and Herbert York (right) at Los Alamos in 1950.
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. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over.”[21] However, Teller recalled that Fermi did not elaborate on his question beyond “perhaps a statement that the distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center.”[20][note 2]
Predecessors
Fermi was not the first to note the paradox. In his 1686 book Conversations on the Plurality of Worlds, Bernard Le Bovier de Fontenelle—later the secretary of the French Academy of Sciences—constructs a dialogue in which Fontenelle’s claims of “intelligent beings exist in other worlds, for instance the Moon” are refuted by a character who notes that “If this were the case, the Moon’s inhabitants would already have come to us before now.”[24] This may have inspired a similar discussion in Jules Verne‘s 1865 novel Around the Moon, which has also been identified as an early conceptualization of the Fermi paradox.[25]
Another early formulation Fermi paradox was presented and dissected in the 1930s writings of Russian rocket scientist Konstantin Tsiolkovsky.[26] Although his rocketry work was embraced by the materialist Soviets, his philosophical writings were suppressed and unknown for most of the 20th century.[27] Tsiolkovsky noted that critics refute the existence of advanced extraterrestrial life as such civilizations would have visited humanity or left some detectable evidence.[28] He posed a solution to the paradox: humanity is quarantined by aliens to protect its independent cultural development, which resembles the zoo hypothesis proposed by John Ball.[29]
Popularization
The Fermi question first appeared in print in a footnote of a 1963 paper by Carl Sagan.[30] 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.[31] A chapter of Intelligent Life in the Universe, co-authored by Sagan and Iosif Shklovsky, was headlined with the Fermi-attributed “Where are they?”[31] 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.[32][31]
Later that year, Michael Hart published a detailed examination of the paradox in the Quarterly Journal of the Royal Astronomical Society.[28] Hart, who concluded that “we are the first civilization in our Galaxy”, proposed four broad categories of solutions to the paradox: those that are physical (a space travel limitation), sociological (aliens choose not to visit Earth), temporal (aliens have not had time to travel to Earth), or that extraterrestrials have already visited.[28][33] His paper sparked significant interest in the paradox among academics and even politicians, with a discussion held in the House of Lords.[34] A seminal response—”Extraterrestrial intelligent beings do not exist”—was written by Frank Tipler, who argued that, if an advanced extraterrestrial civilization existed, their self-replicating spacecraft should have already been detected in the Solar System.[35] The term “Fermi paradox” was coined in a 1977 article by David Stephenson and was widely adopted.[30]
The popularization of the Fermi paradox damaged search for extraterrestrial intelligence (SETI) efforts, and Senator William Proxmire cited Tipler when he spurred the termination of the federally funded NASA SETI program in 1981.[33] According to Robert Gray, the paradox may contribute to a “de facto prohibition on government support for research in a branch of astrobiology”.[30]
Criticism
Fermi did not publish anything regarding the paradox, with Sagan once suggesting the quote to be apocryphal.[33][28][note 3] 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.[37][38] According to Gray, the current understanding of the paradox misinterprets Fermi’s question and subsequent discussion, which was challenging the feasibility of interstellar travel rather than the existence of advanced extraterrestrial life.[39]
Basis
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 first aspect of the Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way[40] (2–4 × 1011) and 70 sextillion (7×1022) in the observable universe.[41] Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of extant civilizations, and if the percentage were high enough it would produce a significant number of extant civilizations in the Milky Way. This assumes the mediocrity principle, by which Earth is a typical planet.
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. Since there is no known evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8 billion years of the universe’s history, there is a conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than is thought, that assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that the scientific understanding of the nature of the universe is quite incomplete.
The Fermi paradox can be asked in two ways.[note 4] The first is, “Why are no aliens or their artifacts found on Earth, or in the Solar System?”. If interstellar travel is possible, even the “slow” kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy.[42] This is relatively brief on a geological scale, let alone a cosmological one. Since there are many stars older than the Sun, and since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large-scale exploration of the galaxy could be possible by probes. These might leave detectable artifacts in the Solar System, such as old probes or evidence of mining activity, but none of these have been observed.
The second form of the question is “Why are there no signs of intelligence elsewhere in the universe?”. This version does not assume interstellar travel, but includes other galaxies as well. For distant galaxies, travel times may well explain the lack of alien visits to Earth, but a sufficiently advanced civilization could potentially be observable over a significant fraction of the size of the observable universe.[43] Even if such civilizations are rare, the scale argument indicates they should exist somewhere at some point during the history of the universe, and since they could be detected from far away over a considerable period of time, many more potential sites for their origin are within range of human observation. It is unknown whether the paradox is stronger for the Milky Way galaxy or for the universe as a whole.[44]