The Fermi paradox (or Fermi’s paradox) is the 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. The basic points of the argument are:
- The sun is a young star. There are billions of stars in the galaxy that are billions of years older;
- If the Earth is typical, some of these stars likely have planets with intelligent life;
- Presumably some of these civilizations will develop interstellar travel, as Earth seems likely to do;
- At any practical pace of interstellar travel, the galaxy can be completely colonized in just a few tens of millions of years.
- The observable universe is currently believed to have at least 80 billion galaxies.
According to this line of thinking, the Earth should have already been colonized, or at least visited. But no convincing evidence of this exists. Hence Fermi’s question “Where is everybody?”.The age of the universe and its vast number of stars suggest that unless the Earth is very atypical, extraterrestrial life should be common. 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, and it is sometimes referred to as the Fermi–Hart paradox. Other common names for the same phenomenon areFermi’s question (“Where are they?”), the Fermi Problem, the Great Silence, and silentium universi (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 or briefly that humans will never make contact with it.
Starting with Hart, a great deal of effort has gone into developing scientific theories 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.
However, this hypothesis seems inconsistent with the lack of observational evidence to support it.
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 (2–4 ×1011) stars in the Milky Way and 70 sextillion (7×1022) in the visible universe. 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.
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 13.7 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, or that our assumptions about the general behavior of intelligent species are flawed.
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. 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 could potentially be observable over a significant fraction of the size of the observable universe. 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 which version of the paradox is stronger