Editing Fermi paradox (section)

== Basis ==
[[File:Enrico Fermi 1943-49.jpg|thumb|upright|[[Enrico Fermi]] (1901–1954)]]

The Fermi [[paradox]] is a conflict between the argument that [[scale (spatial)|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<ref>{{cite news |last=Cain |first=Fraser |url=http://www.universetoday.com/102630/how-many-stars-are-there-in-the-universe/ |title=How Many Stars are There in the Universe? |work=Universe Today |date=June 3, 2013 |access-date=2016-05-25 |archive-url=https://web.archive.org/web/20190804214958/https://www.universetoday.com/102630/how-many-stars-are-there-in-the-universe/ |archive-date=August 4, 2019 |url-status=live }}</ref> (2–4 × [[orders of magnitude|10<sup>11</sup>]]) and 70 sextillion (7×10<sup>22</sup>) in the [[observable universe]].<ref>{{cite news|url = http://news.bbc.co.uk/1/hi/sci/tech/3085885.stm|title = Astronomers count the stars|work = BBC News|access-date = April 8, 2010|author = Craig, Andrew|date = July 22, 2003|archive-url = https://web.archive.org/web/20180418172602/http://news.bbc.co.uk/1/hi/sci/tech/3085885.stm|archive-date = April 18, 2018|url-status = live}}</ref> Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of [[wikt:extant#English|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 [[habitat (ecology)|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 significant evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8&nbsp;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.<ref group=note>See Hart for an example of "no aliens are here", and Webb for an example of the more general "We see no signs of intelligence anywhere".</ref> 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&nbsp;million years to colonize the galaxy.<ref name=cr>Crawford, I.A., [https://www.scientificamerican.com/issue/sa/2000/07-01/ "Where are They? Maybe we are alone in the galaxy after all"] {{Webarchive|url=https://web.archive.org/web/20111201003944/http://www.scientificamerican.com/article.cfm?id=where-are-they |date=December 1, 2011 }}, ''Scientific American'', July 2000, 38–43, (2000).</ref> This is relatively brief on a [[geological time|geological scale]], let alone a [[Timeline of the Big Bang|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 [[#Conjectures about interstellar probes|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 [[Observable universe#Size|size of the observable universe]].<ref>{{cite book |last1=Shklovskii |first1=Iosif |author-link=Iosif Shklovsky |last2=Sagan |first2=Carl |author2-link=Carl Sagan |title=Intelligent Life in the Universe |url=https://archive.org/details/intelligentlifei00shkl |url-access=registration |location=San Francisco |isbn=978-1-892803-02-3 |publisher=Holden–Day |date=1966}}</ref> 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.<ref>{{cite book |title=Extraterrestrials; Where Are They? |editor1-first = Ben |editor1-last = Zuckerman |editor2-first=Michael |editor2-last=Hart |author=J. Richard Gott, III |chapter=Chapter 19: Cosmological SETI Frequency Standards|page=180}}</ref>

=== Drake equation ===
{{Main|Drake equation}}

The theories and principles in the [[Drake equation]] are closely related to the Fermi paradox.<ref>Gowdy, Robert H., VCU Department of Physics [http://www.courses.vcu.edu/PHY-rhg/astron/html/mod/019/s5.html SETI: Search for ExtraTerrestrial Intelligence. The Interstellar Distance Problem] {{Webarchive|url=https://web.archive.org/web/20181226013330/https://courses.vcu.edu/PHY-rhg/astron/html/mod/019/s5.html |date=December 26, 2018 }}, 2008</ref> The equation was formulated by [[Frank Drake]] in 1961 in an attempt to find a systematic means to evaluate the numerous probabilities involved in the existence of alien life. The equation is:

:<math>N = R_* \cdot f_\mathrm{p} \cdot n_\mathrm{e} \cdot f_\mathrm{l} \cdot f_\mathrm{i} \cdot f_\mathrm{c} \cdot L</math>

Where <math>N</math> is the number of technologically advanced civilizations in the Milky Way galaxy, and <math>N</math> is asserted to be the product of 
* <math>R_*</math>, the rate of formation of stars in the galaxy;
* <math>f_p</math>, the fraction of those stars with planetary systems;
* <math>n_e</math>, the number of planets, per solar system, with an environment suitable for organic life;
* <math>f_l</math>, the fraction of those suitable planets whereon organic life appears;
* <math>f_i</math>, the fraction of life-bearing planets whereon ''intelligent'' life appears;
* <math>f_c</math>, the fraction of civilizations that reach the technological level whereby detectable signals may be dispatched; and
* <math>L</math>, the length of time that those civilizations dispatch their signals.

The fundamental problem is that the last four terms (<math>f_l</math>, <math>f_i</math>, <math>f_c</math>, and <math>L</math>) are entirely unknown, rendering statistical estimates impossible.<ref>{{cite arXiv|last1=Sandberg|first1=Anders|last2=Drexler|first2=Eric|last3=Ord|first3=Toby|date=2018-06-06|title=Dissolving the Fermi Paradox|class=physics.pop-ph|eprint=1806.02404}}</ref>

The Drake equation has been used by both optimists and pessimists, with wildly differing results. The first scientific meeting on the [[search for extraterrestrial intelligence]] (SETI), which had 10 attendees including Frank Drake and [[Carl Sagan]], speculated that the number of civilizations was roughly between 1,000 and 100,000,000 civilizations in the Milky Way galaxy.<ref>{{cite book | last1 = Drake | first1 = F. | last2 = Sobel |first2 = D. | year = 1992 | title = Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence | pages = 55–62 | publisher = Delta | isbn = 978-0-385-31122-9}}</ref> Conversely, [[Frank J. Tipler|Frank Tipler]] and [[John D. Barrow]] used pessimistic numbers and speculated that the average number of civilizations in a galaxy is much less than one.<ref>{{BarrowTipler1986|page=588}}</ref> Almost all arguments involving the Drake equation suffer from the [[overconfidence effect]], a common error of probabilistic reasoning about low-probability events, by guessing specific numbers for likelihoods of events whose mechanism is not understood, such as the likelihood of [[abiogenesis]] on an Earth-like planet, with estimates varying over many hundreds of [[order of magnitude|orders of magnitude]]. An analysis that takes into account some of the uncertainty associated with this lack of understanding has been carried out by [[Anders Sandberg]], [[Eric Drexler]] and [[Toby Ord]],<ref>{{cite arXiv |eprint=1806.02404 |title=Dissolving the Fermi Paradox |author=Anders Sandberg |author2=Eric Drexler |author3=Toby Ord |date=June 6, 2018 |class=physics.pop-ph }}</ref> and suggests "a substantial ''[[ex ante]]'' probability of there being no other intelligent life in our observable universe".

=== Great Filter ===
{{main|Great Filter}}
The Great Filter, a concept introduced by [[Robin Hanson]] in 1996, represents whatever natural phenomena that would make it unlikely for life to evolve from inanimate matter to an [[Kardashev scale|advanced civilization]].<ref name="Hanson">{{cite web|last=Hanson |first=Robin |author-link=Robin Hanson |url=http://hanson.gmu.edu/greatfilter.html |title=The Great Filter – Are We Almost Past It? |date=1998 |archive-url=https://web.archive.org/web/20100507074729/http://hanson.gmu.edu/greatfilter.html |archive-date=2010-05-07 |url-status=dead }}</ref><ref name="NYT-20150803"/> The most commonly agreed-upon low probability event is [[abiogenesis]]: a gradual process of increasing complexity of the first self-replicating molecules by a randomly occurring chemical process. Other proposed great filters are the emergence of [[eukaryotes|eukaryotic cells]]<ref group=note>Eukaryotes also include plants, animals, fungi, and algae.</ref> or of [[meiosis]] or some of the steps involved in the evolution of a brain capable of complex logical deductions.<ref name="Lineweaver, 2009"/>

Astrobiologists [[Dirk Schulze-Makuch]] and William Bains, reviewing the history of life on Earth, including [[convergent evolution]], concluded that transitions such as [[oxygenic photosynthesis]], the [[eukaryote|eukaryotic cell]], [[multicellularity]], and [[tool]]-using [[intelligence]] are likely to occur on any Earth-like planet given enough time. They argue that the Great Filter may be abiogenesis, the rise of technological human-level intelligence, or an inability to settle other worlds because of self-destruction or a lack of resources.<ref>{{cite book |last1=Schulze-Makuch |first1=Dirk |last2=Bains |first2=William |title=The Cosmic Zoo: Complex Life on Many Worlds |date=2017 |publisher=Springer |isbn=978-3-319-62045-9 |pages=201–206 |url=https://books.google.com/books?id=m7E_DwAAQBAJ |language=en}}</ref> Paleobiologist [[Olev Vinn]] has suggested that the great filter may have universal biological roots related to evolutionary animal behavior.<ref name=vinn2024>{{cite journal|last=Vinn|first=O.|date=2024|title=Potential incompatibility of inherited behavior patterns with civilization: Implications for Fermi paradox|journal=Science Progress|volume=107|issue=3|pages=1–6|doi=10.1177/00368504241272491|pmid=  39105260|s2cid=  |doi-access=free|pmc=11307330}}</ref>

=== Grabby Aliens ===
{{Main|Quiet and loud aliens}}
In 2021, the concepts of quiet, loud, and grabby aliens were introduced by Hanson ''et al.'' The possible "loud" aliens [[Space colonization|expand rapidly]] in a highly detectable way throughout the universe and endure, while "quiet" aliens are hard or impossible to detect and eventually disappear. "Grabby" aliens prevent the emergence of other civilizations in their [[sphere of influence]], which expands at a rate near the speed of light. The authors argue that if loud civilizations are rare, as they appear to be, then quiet civilizations are also rare. The paper suggests that humanity's existing stage of technological development is relatively early in the potential timeline of intelligent life in the universe, as loud aliens would otherwise be observable by astronomers.<ref>{{Cite journal |last1=Hanson |first1=Robin |last2=Martin |first2=Daniel |last3=McCarter |first3=Calvin |last4=Paulson |first4=Jonathan |date=November 30, 2021 |title=If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare |journal=The Astrophysical Journal |language=en |volume=922 |issue=2 |pages=182 |doi=10.3847/1538-4357/ac2369 |doi-access=free |arxiv=2102.01522 |bibcode=2021ApJ...922..182H |issn=0004-637X}}</ref><ref>{{Cite web |title=Grabby Aliens – a simple model by Robin Hanson |url=https://grabbyaliens.com/ |access-date=2024-06-29 |website=grabbyaliens.com}}</ref>

Earlier in 2013, Anders Sandberg and Stuart Armstrong examined the potential for intelligent life to spread [[Intergalactic travel|intergalactically]] throughout the universe and the implications for the Fermi Paradox. Their study suggests that with sufficient energy, intelligent civilizations could potentially colonize the entire Milky Way galaxy within a few million years, and spread to nearby galaxies in a timespan that is cosmologically brief. They conclude that intergalactic colonization appears possible with the resources of a [[Planetary system|single solar system]] and that intergalactic colonization is of comparable difficulty to interstellar colonization, and therefore the Fermi paradox is much sharper than commonly thought.<ref>{{Cite journal |last1=Armstrong |first1=Stuart |last2=Sandberg |first2=Anders |date=2013-08-01 |title=Eternity in six hours: Intergalactic spreading of intelligent life and sharpening the Fermi paradox |url=https://www.sciencedirect.com/science/article/pii/S0094576513001148 |journal=Acta Astronautica |volume=89 |pages=1–13 |doi=10.1016/j.actaastro.2013.04.002 |bibcode=2013AcAau..89....1A |issn=0094-5765}}</ref>