The Fermi paradox (or Fermi's paradox) is apparent contradiction between high
estimates of the probability of the existence of extraterrestrial civilization and
humanity's lack of contact with, or evidence for, such civilizations.[1]
The basic points of the argument, made by physicists Enrico Fermi and Michael H. Hart, are:
The age of the universe and its vast number of stars suggest that unless the Earth is very
atypical, extraterrestrial life should be common.[4] In an informal discussion in 1950, the physicist Enrico Fermi questioned why, if a multitude of advanced extraterrestrial civilizations exists in the Milky Way galaxy, evidence such as spacecraft or probes is not seen. A more detailed examination of the implications of the topic began with a paper by Michael H. Hart in 1975 ,[5] and it is sometimes referred to as the Fermi–Hart paradox.[6] Other common names for the same phenomenon are Fermi's question ("Where are they?"), the Fermi Problem, the Great Silence,[7][8][9][10][11] and silentium universi[11][12] (Latin for "silence of the universe").
There have been attempts to resolve the Fermi paradox by locating evidence of extraterrestrial civilizations, along with proposals that such life could exist without human knowledge. Counterarguments suggest that intelligent extraterrestrial life does not exist or occurs so rarely and/or briefly that humans will never make contact with it.
Starting with Hart, a great deal of effort has gone into developing scientific hypotheses about, and possible models of, extraterrestrial life, and the Fermi paradox has become a theoretical reference point in much of this work. The problem has spawned numerous scholarly works addressing it directly, while questions that relate to it have been addressed in fields as diverse as astronomy, biology, ecology, and philosophy. The emerging field of astrobiology has brought an interdisciplinary approach to the Fermi paradox and the question of extraterrestrial life.
The Fermi paradox is a conflict between an argument of scale and probability and a lack of evidence. A more complete definition could be stated thus:
The second cornerstone of the Fermi paradox is a rejoinder to the argument by scale: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems likely that at least some civilizations would be technologically advanced, seek out new resources in space and then colonize first their own star system and subsequently the surrounding star systems. Since there is no conclusive or certifiable evidence on Earth or elsewhere in the known universe of other intelligent life after 13.8 billion years of the universe's history, we have the conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than we think, that our assumptions about the general behavior of intelligent species are flawed, or, more radically, that our current scientific understanding of the nature of the universe or reality itself is seriously incomplete.
The Fermi paradox can be asked in two ways. The first is, "Why are no aliens or their artifacts physically here?" 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.[15] This is a relatively small amount of time on a geological scale, let alone a cosmological one. Since there are many stars older than the Sun, or 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 is still possible; the means of exploration and theoretical probes involved are discussed extensively below. However, no signs of either colonization or exploration have been generally acknowledged.
The argument above may not hold for the universe as a whole, since travel times may well explain the lack of physical presence on Earth of alien inhabitants of far away galaxies. However, the question then becomes "Why do we see no signs of intelligent life?" since a sufficiently advanced civilization[Note 1] could potentially be observable over a significant fraction of the size of the observable universe.[16] 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 our observation. However, no incontrovertible signs of such civilizations have been detected.
It is unclear whether the paradox is stronger for our galaxy or for the universe as a whole.[Note 2]
One participant recollects that Fermi then made a series of rapid calculations using estimated figures. (Fermi was known for his ability to make good estimates from first principles and minimal data, see Fermi problem.) According to this account, he then concluded that Earth should have been visited long ago and many times over.[18][19]
estimates of the probability of the existence of extraterrestrial civilization and
humanity's lack of contact with, or evidence for, such civilizations.[1]
The basic points of the argument, made by physicists Enrico Fermi and Michael H. Hart, are:
- The Sun is a young star. There are billions of stars in the galaxy that are billions
- of years older;
- Some of these stars likely have Earth-like planets[2] which, if the Earth is typical, may develop intelligent life;
- Presumably some of these civilizations will develop interstellar travel, a technology Earth is investigating even now;
- At any practical pace of interstellar travel, the galaxy can be completely colonized in a few tens of millions of years.
The age of the universe and its vast number of stars suggest that unless the Earth is very
atypical, extraterrestrial life should be common.[4] In an informal discussion in 1950, the physicist Enrico Fermi questioned why, if a multitude of advanced extraterrestrial civilizations exists in the Milky Way galaxy, evidence such as spacecraft or probes is not seen. A more detailed examination of the implications of the topic began with a paper by Michael H. Hart in 1975 ,[5] and it is sometimes referred to as the Fermi–Hart paradox.[6] Other common names for the same phenomenon are Fermi's question ("Where are they?"), the Fermi Problem, the Great Silence,[7][8][9][10][11] and silentium universi[11][12] (Latin for "silence of the universe").
There have been attempts to resolve the Fermi paradox by locating evidence of extraterrestrial civilizations, along with proposals that such life could exist without human knowledge. Counterarguments suggest that intelligent extraterrestrial life does not exist or occurs so rarely and/or briefly that humans will never make contact with it.
Starting with Hart, a great deal of effort has gone into developing scientific hypotheses about, and possible models of, extraterrestrial life, and the Fermi paradox has become a theoretical reference point in much of this work. The problem has spawned numerous scholarly works addressing it directly, while questions that relate to it have been addressed in fields as diverse as astronomy, biology, ecology, and philosophy. The emerging field of astrobiology has brought an interdisciplinary approach to the Fermi paradox and the question of extraterrestrial life.
The Fermi paradox is a conflict between an argument of scale and probability and a lack of evidence. A more complete definition could be stated thus:
The apparent size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to exist.The first aspect of the paradox, "the argument by scale", is a function of the raw numbers involved: there are an estimated 200–400 billion[13] (2–4 ×1011) stars in the Milky Way and 70 sextillion (7×1022) in the visible universe.[14] Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of civilizations extant in the Milky Way galaxy alone. This argument also assumes the mediocrity principle, which states that Earth is not special, but merely a typical planet, subject to the same laws, effects, and likely outcomes as any other world.
However, this hypothesis seems inconsistent with the lack of observational evidence to support it.
The second cornerstone of the Fermi paradox is a rejoinder to the argument by scale: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems likely that at least some civilizations would be technologically advanced, seek out new resources in space and then colonize first their own star system and subsequently the surrounding star systems. Since there is no conclusive or certifiable evidence on Earth or elsewhere in the known universe of other intelligent life after 13.8 billion years of the universe's history, we have the conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than we think, that our assumptions about the general behavior of intelligent species are flawed, or, more radically, that our current scientific understanding of the nature of the universe or reality itself is seriously incomplete.
The Fermi paradox can be asked in two ways. The first is, "Why are no aliens or their artifacts physically here?" 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.[15] This is a relatively small amount of time on a geological scale, let alone a cosmological one. Since there are many stars older than the Sun, or 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 is still possible; the means of exploration and theoretical probes involved are discussed extensively below. However, no signs of either colonization or exploration have been generally acknowledged.
The argument above may not hold for the universe as a whole, since travel times may well explain the lack of physical presence on Earth of alien inhabitants of far away galaxies. However, the question then becomes "Why do we see no signs of intelligent life?" since a sufficiently advanced civilization[Note 1] could potentially be observable over a significant fraction of the size of the observable universe.[16] 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 our observation. However, no incontrovertible signs of such civilizations have been detected.
It is unclear whether the paradox is stronger for our galaxy or for the universe as a whole.[Note 2]
Name[edit]
In 1950, while working at Los Alamos National Laboratory, Fermi had a casual conversation while walking to lunch with colleagues Emil Konopinski, Edward Teller and Herbert York. The men discussed a recent spate of UFO reports and an Alan Dunn cartoon[17] facetiously blaming the disappearance of municipal trashcans on marauding aliens. They then had a more serious discussion regarding the chances of humans observing faster-than-light travel by some material object within the next ten years, which Teller put at one in a million, but Fermi put closer to one in ten. The conversation shifted to other subjects, until during lunch Fermi suddenly exclaimed, "Where are they?" (alternatively, "Where is everybody?").One participant recollects that Fermi then made a series of rapid calculations using estimated figures. (Fermi was known for his ability to make good estimates from first principles and minimal data, see Fermi problem.) According to this account, he then concluded that Earth should have been visited long ago and many times over.[18][19]
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