Fermi paradox

Wikipedia - The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for or contact with such civilizations.

The extreme age of the universe and its vast number of stars suggest that if the Earth is typical, extraterrestrial life should be common. Discussing this proposition with colleagues over lunch in 1950, the physicist Enrico Fermi asked: "Where are they?" (alternatively, "Where is everybody?") Fermi questioned why, if a multitude of advanced extraterrestrial civilizations exist in the Milky Way galaxy, evidence such as probes, spacecraft, or radio transmissions has not been found. Fermi is widely credited with simplifying the problem of the probability of extraterrestrial life. Wider examination of the implications of the topic began with Michael Hart in 1975, and it is sometimes referred to as the Fermi-Hart paradox.

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 that humans will never make contact with it.

A great deal of effort has gone into developing scientific theories 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 various 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 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 250 billion (2.5 x 1011) stars in the Milky Way and 70 sextillion (7 x 1022) in the visible universe. Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there should 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. Some estimates using the Drake equation support this argument, although the assumptions behind those calculations have themselves been challenged.

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 any advanced civilization would seek out new resources and colonize first their star system, and then surrounding star systems. As there is no conclusive or certifiable evidence on Earth or elsewhere in the known universe of other intelligent life after 13 billion years of the universe's history, it may be assumed that intelligent life is rare or 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 allowed by our physics and 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 is not 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 colonization or exploration have been found.

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 for far away galaxies. However, the question then becomes "Why do we see no signs of intelligent life?" as a sufficiently advanced civilization could potentially be seen over a significant fraction of the size of the observable universe[8] Even if such civilizations are rare, since they could be detected from far away, many more potential sites for their origin are within our view. However, no signs of such civilizations have been detected.

Fermi paradox - Wikipedia

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