View source for Astronomy:Fermi paradox

{{Short description|Lack of evidence that aliens exist}}

The '''Fermi paradox''' is the discrepancy between the lack of conclusive evidence of advanced [[Astronomy:Extraterrestrial life|extraterrestrial life]] and the apparently high likelihood of its existence.<ref name="INS-20190921">{{cite news |last=Woodward |first=Avlin |title=A winner of this year's Nobel prize in physics is convinced we'll detect alien life in 100 years. Here are 13 reasons why we haven't made contact yet. |url=https://www.insider.com/why-no-contact-with-aliens-2019-9 |date=September 21, 2019 |work=Insider Inc |access-date=September 21, 2019 }}</ref><ref name="Krauthammer2011">{{cite news |last=Krauthammer |first=Charles |date=December 29, 2011 |title=Are We Alone in the Universe? |url=https://www.washingtonpost.com/opinions/are-we-alone-in-the-universe/2011/12/29/gIQA2wSOPP_story.html |newspaper={{wipe|The Washington Post}} |access-date=January 6, 2015 |archive-url=https://web.archive.org/web/20141210160035/http://www.washingtonpost.com/opinions/are-we-alone-in-the-universe/2011/12/29/gIQA2wSOPP_story.html |archive-date=December 10, 2014 |url-status=live }}</ref> As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."<ref name="NYT-20150803" />

Italian-American [[Physicist|physicist]] [[Biography:Enrico Fermi|Enrico Fermi]]'s name is associated with the paradox because of a casual conversation in the summer of 1950 with fellow physicists [[Biography:Edward Teller|Edward Teller]], [[Biography:Herbert York|Herbert York]], and [[Biography:Emil Konopinski|Emil Konopinski]]. While walking to lunch, the men discussed recent [[Unsolved:UFO|UFO]] reports and the possibility of faster-than-light travel. The conversation moved on to other topics, until during lunch Fermi blurted out, "But where is everybody?" (although the exact quote is uncertain).<ref name="NYT-20150803">{{cite news |last=Overbye |first=Dennis |title=The Flip Side of Optimism About Life on Other Planets |url=https://www.nytimes.com/2015/08/04/science/space/the-flip-side-of-optimism-about-life-on-other-planets.html |date=August 3, 2015 |work={{wipe|The New York Times}} |access-date=October 29, 2015 |archive-url=https://web.archive.org/web/20190919003259/https://www.nytimes.com/2015/08/04/science/space/the-flip-side-of-optimism-about-life-on-other-planets.html |archive-date=September 19, 2019 |url-status=live }}</ref><ref name="Eric-Jones"/>

There have been many attempts to resolve the Fermi paradox,<ref name=":0">[https://books.google.com/books?id=Y111CQAAQBAJ&pg=PP11 ''If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, Second Edition''], Stephen Webb, foreword by Martin Rees, Heidelberg, New York, Dordrecht, London: Springer International Publishing, 2002, 2015.</ref><ref>{{cite news| url = http://www.huffingtonpost.com/wait-but-why/the-fermi-paradox_b_5489415.html| title = The Fermi Paradox| last = Urban| first = Tim| date = June 17, 2014| work = Huffington Post| access-date = January 6, 2015| archive-url = https://web.archive.org/web/20170402042005/http://www.huffingtonpost.com/wait-but-why/the-fermi-paradox_b_5489415.html| archive-date = April 2, 2017| url-status = live}}</ref> such as suggesting that [[Astronomy:Rare Earth hypothesis|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==

The following are some of the facts and hypotheses that together serve to highlight the apparent contradiction:

* There are [[Billion|billion]]s of stars in the [[Astronomy:Milky Way|Milky Way]] similar to the [[Astronomy:Sun|Sun]].<ref>{{cite encyclopedia |title=Star (astronomy) |encyclopedia=Encyclopædia Britannica |url=https://www.britannica.com/topic/star-astronomy |access-date=February 4, 2016 |archive-url=https://web.archive.org/web/20160301055912/https://www.britannica.com/topic/star-astronomy |archive-date=March 1, 2016 |url-status=live}} "With regard to mass, size, and intrinsic brightness, the Sun is a typical star." Technically, the sun is near the middle of the main sequence of the [[Hertzsprung–Russell diagram]]. This sequence contains 80–90% of the stars of the galaxy. [http://astro.unl.edu/naap/hr/hr_background3.html] {{Webarchive|url=https://web.archive.org/web/20110716170751/http://astro.unl.edu/naap/hr/hr_background3.html|date=July 16, 2011}}</ref><ref>{{cite news |last1=Grevesse |first1=N. |last2=Noels |first2=A. |last3=Sauval |first3=A. J. |title=Standard abundances |journal=ASP Conference Series |volume=99 |page=117 |year=1996 |quote=The Sun is a normal star, though dispersion exists.|bibcode=1996ASPC...99..117G }}</ref>
* With high probability, some of these stars have Earth-like planets in a [[Astronomy:Circumstellar habitable zone|circumstellar habitable zone]].<ref>{{cite journal |last1=Buchhave |first1=Lars A. |last2=Latham |first2=David W. |last3=Johansen |first3=Anders |last4=Bizzarro |first4=Martin |last5=Torres |first5=Guillermo |last6=Rowe |first6=Jason F. |last7=Batalha |first7=Natalie M. |last8=Borucki |first8=William J. |last9=Brugamyer |first9=Erik |last10=Caldwell |first10=Caroline |last11=Bryson |first11=Stephen T. |last12=Ciardi |first12=David R. |last13=Cochran |first13=William D. |last14=Endl |first14=Michael |last15=Esquerdo |first15=Gilbert A. |last16=Ford |first16=Eric B. |last17=Geary |first17=John C. |last18=Gilliland |first18=Ronald L. |last19=Hansen |first19=Terese |last20=Isaacson |first20=Howard |last21=Laird|first21=John B. |last22=Lucas |first22=Philip W. |last23=Marcy |first23=Geoffrey W. |last24=Morse |first24=Jon A. |last25=Robertson |first25=Paul |last26=Shporer |first26=Avi |last27=Stefanik |first27=Robert P. |last28=Still |first28=Martin |last29=Quinn |first29=Samuel N. |display-authors=3 
|date=2012 
|title=An abundance of small exoplanets around stars with a wide range of metallicities 
|journal=Nature 
|volume=486 |issue= 7403 |pages=375–377 
|bibcode= 2012Natur.486..375B 
|issn=0028-0836 
|doi=10.1038/nature11121 
|pmid=22722196 
|s2cid=4427321}}</ref> 
* Many of these stars, and hence their planets, are much older than the Sun.<ref>{{cite journal 
|last=Schilling |first=G. |date=June 13, 2012 |url=https://www.science.org/content/article/scienceshot-alien-earths-have-been-around-while |title=ScienceShot: Alien Earths Have Been Around for a While |journal=Science |access-date=January 6, 2015 |archive-url=https://web.archive.org/web/20150809002119/http://news.sciencemag.org/2012/06/scienceshot-alien-earths-have-been-around-while |archive-date=August 9, 2015 |url-status=live
}}</ref><ref>{{Cite journal |author1=Aguirre, V. Silva |author2=G. R. Davies |author3= S. Basu |author4=J. Christensen-Dalsgaard |author5= O. Creevey |author6=T. S. Metcalfe |author7= T. R. Bedding |title=Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology |journal=Monthly Notices of the Royal Astronomical Society |volume=452 |issue=2 |pages=2127–2148 |date=2015 |arxiv=1504.07992|display-authors=0 |doi=10.1093/mnras/stv1388 |bibcode=2015MNRAS.452.2127S |s2cid=85440256}} Accepted for publication in MNRAS. See Figure 15 in particular.</ref> If Earth-like planets are typical, some may have developed [[Philosophy:Human intelligence|intelligent]] life long ago.
* Some of these [[Social:Civilization|civilization]]s may have developed [[Engineering:Interstellar travel|interstellar travel]], a step humans are investigating now.
* Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.<ref name="Hart">{{cite journal
|title = Explanation for the Absence of Extraterrestrials on Earth
|last = Hart
|first = Michael H.
|journal = Quarterly Journal of the Royal Astronomical Society
|volume = 16 |pages = 128–135
|date = 1975
|bibcode=1975QJRAS..16..128H}}</ref>
* 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.<ref>{{cite book |title=The Living Cosmos: Our Search for Life in the Universe |author=Chris Impe |publisher=Cambridge University Press |date=2011 |isbn=978-0-521-84780-3| page=282}}</ref>
* However, there is no convincing evidence that this has happened.<ref name="Hart"/>

==History==
Fermi was not the first to ask the question. An earlier implicit mention was by [[Biography:Konstantin Tsiolkovsky|Konstantin Tsiolkovsky]] in an unpublished manuscript from 1933.<ref>Tsiolkovsky, K. (1933). ''The Planets are Occupied by Living Beings'', Archives of the Tsiolkovsky State Museum of the History of Cosmonautics, Kaluga, Russia. See original text in Russian [[Wikisource]].</ref> He noted "people deny the presence of intelligent beings on the planets of the universe" because "(i) if such beings exist they would have visited Earth, and (ii) if such civilizations existed then they would have given us some sign of their existence." This was not a paradox for others, who took this to imply the absence of extraterrestrial life. But it was one for him, since he believed in extraterrestrial life and the possibility of space travel. Therefore, he proposed what is now known as the [[Astronomy:Zoo hypothesis|zoo hypothesis]] and speculated that mankind is not yet ready for higher beings to contact us.<ref>{{cite journal |title=Tsiolkovsky – Russian Cosmism and Extraterrestrial Intelligence |author=Lytkin, V. |author2=Finney, B. |author3=Alepko, L. |journal=Quarterly Journal of the Royal Astronomical Society |volume=36 |issue=4 |date=December 1995 |pages=369 |bibcode=1995QJRAS..36..369L }}</ref> In turn, Tsiolkovsky himself was not the first to discover the paradox, as shown by his reference to other people's reasons for not accepting the premise that extraterrestrial civilizations exist.

In 1975, Michael H. Hart published a detailed examination of the paradox, one of the first to do so.<ref name="Hart" /><ref name=webb>{{cite book |last1=Webb |first1=Stephen |title=If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life |date=2015 |publisher=Springer International Publishing |isbn=978-3-319-13235-8 |edition=2 |url=https://www.springer.com/gp/book/9783319132358 |language=en |access-date=June 12, 2020 |archive-url=https://web.archive.org/web/20200612035828/https://www.springer.com/gp/book/9783319132358 |archive-date=June 12, 2020 |url-status=live }}</ref>{{rp|27–28}}<ref name=forgan>{{cite book |last1=Forgan |first1=Duncan H. |title=Solving Fermi's Paradox |date=2019 |publisher=Cambridge University Press |isbn=978-1-316-73231-1 |url=https://books.google.com/books?id=5U-PDwAAQBAJ |language=en |access-date=June 12, 2020 |archive-url=https://web.archive.org/web/20200625083852/https://books.google.com/books?id=5U-PDwAAQBAJ |archive-date=June 25, 2020 |url-status=live }}</ref>{{rp|6}} He argued that if intelligent extraterrestrials exist, and are capable of space travel, then the galaxy could have been colonized in a time much less than that of the age of the Earth. However, there is no observable evidence they have been here, which Hart called "Fact A".<ref name=forgan/>{{rp|6}}

Other names closely related to Fermi's question ("Where are they?") include the Great Silence,<ref name="Brin">{{cite journal
| url = https://ui.adsabs.harvard.edu/abs/1983QJRAS..24..283B/abstract
| title = The Great Silence - The Controversy Concerning Extraterrestrial Intelligent Life
| journal = Quarterly Journal of the Royal Astronomical Society
| author= Brin, Glen David 
| volume = 24
| issue = 3
| pages = 283–309
| date = August 1983
| bibcode = 1983QJRAS..24..283B
}}</ref><ref name=Annis>{{cite journal
|arxiv=astro-ph/9901322
|author=Annis, James
|title=An Astrophysical Explanation for the Great Silence
|journal=Journal of the British Interplanetary Society
|volume=52
|issue=1
|pages=19
|date=1999
|bibcode=1999JBIS...52...19A
}}</ref><ref name=Hope>{{cite magazine
| last = Bostrom
| first = Nick
| title = In Great Silence there is Great Hope
| magazine = MIT Technology Review
| date = May 2008
| pages = 72–77
| url = http://www.nickbostrom.com/papers/fermi.pdf
| access-date = September 6, 2010
| archive-url = https://web.archive.org/web/20110228111236/http://www.nickbostrom.com/papers/fermi.pdf
| archive-date = February 28, 2011
| url-status = live
}}</ref><ref name="Cirkovic2009" /> and {{lang|la|silentium universi}}<ref name=Cirkovic2009>{{cite journal
|author1=Milan M. Ćirković
|title=Fermi's Paradox – The Last Challenge for Copernicanism?
|date=2009
|doi=10.2298/SAJ0978001C
|journal=Serbian Astronomical Journal
|issue=178
|pages=1–20
|volume=178
|arxiv=0907.3432
|bibcode = 2009SerAJ.178....1C |s2cid=14038002
}}</ref> (Latin for "silence of the universe"), though these only refer to one portion of the Fermi Paradox, that humans see no evidence of other civilizations.

=== Original conversations ===
[[File:Los Alamos aerial view.jpeg|thumb|Los Alamos National Laboratory, Los Alamos, New Mexico, United States]]
In the summer of 1950 at [[Organization:Los Alamos National Laboratory|Los Alamos National Laboratory]] in New Mexico, Enrico Fermi and co-workers Emil Konopinski, Edward Teller, and Herbert York had one or several lunchtime conversations.<ref name="Eric-Jones">[http://www.fas.org/sgp/othergov/doe/lanl/la-10311-ms.pdf "Where is everybody?": An account of Fermi's question"] {{Webarchive|url=https://web.archive.org/web/20070629174738/http://www.fas.org/sgp/othergov/doe/lanl/la-10311-ms.pdf |date=June 29, 2007 }}, Dr. Eric M. Jones, Los Alamos technical report, March 1985. Jones wrote to Edward Teller on July 13, 1984, Herbert York on Sept. 4, and Emil Konopinski on Sept. 24, 1984.</ref><ref>{{cite web|last=Shostak|first=Seth|date=October 25, 2001|archive-url=https://web.archive.org/web/20060415185347/http://space.com/searchforlife/shostak_paradox_011024.html |url=http://www.space.com/searchforlife/shostak_paradox_011024.html|archive-date=April 15, 2006|title=Our Galaxy Should Be Teeming With Civilizations, But Where Are They?|work=Space.com|access-date=October 14, 2014 |url-status=dead}}</ref> In one, Fermi suddenly blurted out, "Where is everybody?" (Teller's letter), or "Don't you ever wonder where everybody is?" (York's letter), or "But where is everybody?" (Konopinski's letter).<ref name="Eric-Jones" /> Teller wrote, "The result of his question was general laughter because of the strange fact that in spite of Fermi's question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life."<ref name="Eric-Jones" />

In 1984 York wrote that 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."<ref name="Eric-Jones" /> Teller remembers that not much came of this conversation "except 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."<ref name="Eric-Jones" />

Fermi died of cancer in 1954. However, in letters to the three surviving men decades later in 1984, Dr. Eric Jones of Los Alamos was able to partially put the original conversation back together. He informed each of the men that he wished to include a reasonably accurate version or composite in the written proceedings he was putting together for a previously held conference entitled "Interstellar Migration and the Human Experience".<ref name="Eric-Jones" /><ref name="Interstellar Migration and the Human Experience, edited by Finney and Jones, 1985">[https://books.google.com/books?id=iKnaLbRtQasC&pg=PA249 ''Interstellar Migration and the Human Experience''], edited by Ben R. Finney, Eric M. Jones, University of California Press, 1985.</ref> Jones first sent a letter to Edward Teller which included a secondhand account from Hans Mark. Teller responded, and then Jones sent Teller's letter to Herbert York. York responded, and finally, Jones sent both Teller's and York's letters to Emil Konopinski who also responded. Furthermore, Konopinski was able to later identify a cartoon which Jones found as the one involved in the conversation and thereby help to settle the time period as being the summer of 1950.<ref name="Eric-Jones" />

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

The Fermi [[Philosophy:Paradox|paradox]] is a conflict between the argument that scale and [[Probability|probability]] seem to favor intelligent life being common in the universe, and the total lack of [[Philosophy:Evidence|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 × 10<sup>11</sup>) and 70 sextillion (7×10<sup>22</sup>) in the [[Astronomy:Observable universe|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 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 [[Astronomy:Mediocrity principle|mediocrity principle]], by which Earth is a typical [[Astronomy:Planet|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 [[Astronomy:Star system|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 current scientific understanding of the nature of the universe itself 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 [[Astronomy:Solar System|Solar System]]?". If [[Engineering:Interstellar travel|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., [http://www.scientificamerican.com/article.cfm?id=where-are-they "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 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 [[Astronomy:Observable universe#Size|size of the observable universe]].<ref>{{cite book |last1=Shklovskii |first1=Iosif |last2=Sagan |first2=Carl |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|Astronomy:Drake equation}}

The theories and principles in the [[Astronomy:Drake equation|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 presented as follows:

:<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 [[Astronomy:Search for extraterrestrial intelligence|search for extraterrestrial intelligence]] (SETI), which had 10 attendees including Frank Drake and [[Biography:Carl Sagan|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, [[Biography:Frank J. Tipler|Frank Tipler]] and [[Biography:John D. Barrow|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 [[Philosophy:Overconfidence effect|overconfidence effect]], a common error of probabilistic reasoning about low-probability events, by guessing specific numbers for likelihoods of events whose mechanism is not yet understood, such as the likelihood of [[Biology:Abiogenesis|abiogenesis]] on an Earth-like planet, with current likelihood 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 [[Biography:Toby Ord|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|Astronomy: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 [[Astronomy:Kardashev scale|advanced civilization]].<ref name="Hanson">{{cite web|last=Hanson |first=Robin |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 [[Biology:Abiogenesis|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 eukaryotic cells<ref group=note>Eukaryotes also include plants, animals, fungi, and algae.</ref> or of [[Biology:Meiosis|meiosis]] or some of the steps involved in the evolution of a brain capable of complex logical deductions.<ref name="Lineweaver, 2009"/>

Astrobiologists [[Biography:Dirk Schulze-Makuch|Dirk Schulze-Makuch]] and William Bains, reviewing the history of life on Earth, including [[Biology:Convergent evolution|convergent evolution]], concluded that transitions such as oxygenic photosynthesis, the [[Biology:Eukaryote|eukaryotic cell]], multicellularity, and [[Engineering:Tool|tool]]-using [[Philosophy:Intelligence|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>

== Empirical evidence ==
{{Main|Astronomy:Search for extraterrestrial intelligence|Astronomy:Technosignature}}

There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potential [[Astronomy:Planetary habitability|habitable planets]], and that humans see no evidence of life. The first point, that many suitable planets exist, was an assumption in Fermi's time but is now supported by the discovery that [[Astronomy:Exoplanet|exoplanet]]s are common. Current models predict billions of habitable worlds in the Milky Way.<ref>{{cite journal |title=On The History and Future of Cosmic Planet Formation |journal=MNRAS |date=December 1, 2015 |volume=454 |issue=2 |pages=1811–1817 |doi=10.1093/mnras/stv1817|arxiv = 1508.01202 |bibcode = 2015MNRAS.454.1811B |last1=Behroozi |first1=Peter |last2=Peeples |first2=Molly S. |s2cid=35542825 }}</ref>

The second part of the paradox, that humans see no evidence of extraterrestrial life, is also an active field of scientific research. This includes both efforts to find any indication of life,<ref>{{cite journal |journal=Astrophys Space Sci |date=2013 |volume=348 |issue=1 |pages=1–10 |doi=10.1007/s10509-013-1536-9 |title=Final frontiers: the hunt for life elsewhere in the Universe |author=Sohan Jheeta|bibcode = 2013Ap&SS.348....1J |s2cid=122750031 }}</ref> and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing.<ref group=note>See, for example, the [[Organization:SETI Institute|SETI Institute]], [http://seti.harvard.edu/seti/ The Harvard SETI Home Page] {{Webarchive|url=https://web.archive.org/web/20100816170426/http://seti.harvard.edu/seti/ |date=August 16, 2010 }}, or [http://seti.berkeley.edu/ The Search for Extra Terrestrial Intelligence at Berkeley] {{Webarchive|url=https://web.archive.org/web/20190406232538/http://seti.berkeley.edu/ |date=April 6, 2019 }}</ref>

Although astronomers do not usually search for extraterrestrials, they have observed phenomena that they could not immediately explain without positing an intelligent civilization as the source. For example, [[Astronomy:Pulsar|pulsar]]s, when [[Astronomy:PSR B1919+21|first discovered]] in 1967, were called [[Social:Little green men|little green men]] (LGM) because of the precise repetition of their pulses.<ref>{{cite journal |title=Discovery of pulsars: a graduate student's story |author=Wade, Nicholas |journal=Science |volume=189 |issue=4200 |pages=358–364 |year=1975 |url=https://www.science.org/doi/abs/10.1126/science.189.4200.358 |bibcode=1975Sci...189..358W |doi=10.1126/science.189.4200.358 |pmid=17840812 |access-date=July 21, 2015 |archive-url=https://web.archive.org/web/20150924142852/http://www.sciencemag.org/content/189/4200/358.short |archive-date=September 24, 2015 |url-status=live }}</ref> In all cases, explanations with no need for intelligent life have been found for such observations,<ref group=note>Pulsars are now attributed to neutron stars, and Seyfert galaxies to an end-on view of the accretion onto the black holes.</ref> but the possibility of discovery remains.<ref>{{cite web|title=NASA/CP2007-214567: Workshop Report on the Future of Intelligence in the Cosmos |url=http://event.arc.nasa.gov/main/home/reports/CP2007-214567_Langhoff.pdf |publisher=NASA |url-status=dead |archive-url=https://web.archive.org/web/20140811194232/http://event.arc.nasa.gov/main/home/reports/CP2007-214567_Langhoff.pdf |archive-date=August 11, 2014 }}</ref> Proposed examples include [[Astronomy:Asteroid mining|asteroid mining]] that would change the appearance of debris disks around stars,<ref>{{cite journal |title=Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence |author= Duncan Forgan, Martin Elvis |journal=International Journal of Astrobiology |date=March 28, 2011 |arxiv=1103.5369 |bibcode = 2011IJAsB..10..307F |doi = 10.1017/S1473550411000127 |volume=10 |issue=4 |pages=307–313 |last2= Elvis |s2cid= 119111392 }}</ref> or spectral lines from nuclear waste disposal in stars.<ref>{{cite journal |author1=Whitmire, Daniel P. |author2=David P. Wright. |title=Nuclear waste spectrum as evidence of technological extraterrestrial civilizations |journal=Icarus |volume=42 |issue=1 |date=1980 |pages=149–156 |doi=10.1016/0019-1035(80)90253-5 |bibcode=1980Icar...42..149W}}</ref>

Explanations based on [[Astronomy:Technosignature|technosignature]]s, such as radio communications, have been presented.<ref name="AJ-20221222">{{cite journal |last=Wandel |first=Amri |title=The Fermi Paradox Revisited: Technosignatures and the Contact Era |date=22 December 2022 |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=941 |issue=184 |page=184 |doi=10.3847/1538-4357/ac9e00 |arxiv=2211.16505 |bibcode=2022ApJ...941..184W |s2cid=254096277 |doi-access=free }}</ref>

=== Electromagnetic emissions ===
{{Further|Astronomy:Project Phoenix (SETI)|Astronomy:SERENDIP|Astronomy:Allen Telescope Array}}
[[File:parkes.arp.750pix.jpg|thumb|left|[[Astronomy:Radio telescope|Radio telescope]]s are often used by SETI projects.]]
Radio technology and the ability to construct a [[Astronomy:Radio telescope|radio telescope]] are presumed to be a natural advance for technological species,<ref>{{cite web
|last=Mullen
|first=Leslie
|date=2002
|url=http://www.space.com/scienceastronomy/alien_intelligence_021202.html
|archive-url=https://web.archive.org/web/20030212141854/http://www.space.com/scienceastronomy/alien_intelligence_021202.html
|url-status=dead
|archive-date=February 12, 2003
|title=Alien Intelligence Depends on Time Needed to Grow Brains
|work=Astrobiology Magazine
|publisher=Space.com
|access-date=April 21, 2006
}}</ref> theoretically creating effects that might be detected over interstellar distances. The careful searching for non-natural radio emissions from space may lead to the detection of alien civilizations. Sensitive alien observers of the Solar System, for example, would note unusually intense [[Radio wave|radio wave]]s for a [[Astronomy:Star#Classification|G2 star]] due to Earth's television and telecommunication broadcasts. In the absence of an apparent natural cause, alien observers might infer the existence of a terrestrial civilization. Such signals could be either "accidental" by-products of a civilization, or deliberate attempts to communicate, such as the [[Astronomy:Arecibo message|Arecibo message]]. It is unclear whether "leakage", as opposed to a deliberate beacon, could be detected by an extraterrestrial civilization. The most sensitive radio telescopes on Earth, {{asof|2019|lc=y}}, would not be able to detect non-directional radio signals even at a fraction of a [[Astronomy:Light-year|light-year]] away,<ref>{{cite web |title=Radio Leakage: Is anybody listening? |first=Brian |last=von Konsky |date=October 23, 2000 |url=https://www.researchgate.net/publication/242237960}}</ref> but other civilizations could hypothetically have much better equipment.<ref>{{cite journal |author=Scheffer, L. |year=2004 |title=Aliens can watch 'I Love Lucy' |journal=Contact in Context |volume=2 |issue=1}}</ref>

A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through SETI organizations such as the [[Organization:SETI Institute|SETI Institute]] and [[Astronomy:Breakthrough Listen|Breakthrough Listen]]. Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions.<ref>{{cite arXiv |author=Participants, NASA |year=2018 |title=NASA and the Search for Technosignatures: A Report from the NASA Technosignatures Workshop |eprint=1812.08681|class=astro-ph.IM }}</ref>

=== Direct planetary observation ===
[[File:Earthlights dmsp 1994–1995.jpg|thumb|upright=1.35|A composite picture of Earth at night, created using data from the [[Engineering:Defense Meteorological Satellite Program|Defense Meteorological Satellite Program]] (DMSP) Operational Linescan System (OLS). Large-scale artificial lighting produced by human civilization is detectable from space.]]
Exoplanet detection and classification is a very active sub-discipline in astronomy; the first possible [[Astronomy:Terrestrial planet|terrestrial planet]] discovered within a star's [[Astronomy:Habitable zone|habitable zone]] was found in 2007.<ref>{{Cite journal |doi=10.1051/0004-6361:20077612 |arxiv=0704.3841 |title=The HARPS search for southern extra-solar planets XI. Super-Earths (5 and 8&nbsp;{{Earth mass}}) in a 3-planet system |journal=Astronomy & Astrophysics |volume=469 |issue=3 |pages=L43–L47 |last1=Udry |first1=Stéphane |last2=Bonfils |first2=Xavier |last3=Delfosse |first3=Xavier |last4=Forveille |first4=Thierry |last5=Mayor |first5=Michel |last6=Perrier |first6=Christian |last7=Bouchy |first7=François |last8=Lovis |first8=Christophe |last9=Pepe |first9=Francesco |last10=Queloz |first10=Didier |last11=Bertaux |first11=Jean-Loup |year=2007 |bibcode=2007A&A...469L..43U |s2cid=119144195 |url=http://exoplanet.eu/papers/udry_terre_HARPS-1.pdf |url-status=dead |archive-url=https://web.archive.org/web/20101008120426/http://exoplanet.eu/papers/udry_terre_HARPS-1.pdf |archive-date=October 8, 2010 }}</ref> New refinements in exoplanet detection methods, and use of existing methods from space (such as the [[Astronomy:Kepler space telescope|Kepler]] and [[Astronomy:Transiting Exoplanet Survey Satellite|TESS]] missions) are starting to detect and characterize Earth-size planets, to determine whether they are within the habitable zones of their stars. Such observational refinements may allow for a better estimation of how common these potentially habitable worlds are.<ref>From {{cite web |url=http://kepler.nasa.gov/Mission/QuickGuide/ |title=Kepler: About the Mission |publisher=NASA |date=March 31, 2015 |access-date=March 30, 2016 |archive-url=https://web.archive.org/web/20120420015542/http://kepler.nasa.gov/Mission/QuickGuide/ |archive-date=April 20, 2012 |url-status=dead }} "The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets."</ref>

=== Conjectures about interstellar probes ===
{{Further|Astronomy:Hart–Tipler conjecture|Engineering:Von Neumann probe|Astronomy:Bracewell probe}}
The Hart-Tipler conjecture is a form of [[Contraposition|contraposition]] which states that because no interstellar probes have been detected, there likely is no other intelligent life in the universe, as such life should be expected to eventually create and launch such probes.<ref name="Gray2015">{{cite journal |last1=Gray |first1=Robert H. |title=The Fermi Paradox Is Neither Fermi's Nor a Paradox |journal=Astrobiology |date=March 2015 |volume=15 |issue=3 |pages=195–199 |doi=10.1089/ast.2014.1247|pmid=25719510 |arxiv=1605.09187 |bibcode=2015AsBio..15..195G }}</ref><ref name="AliensSpaceTime2020">{{cite book|last1=Dick |first1=Steven J. |title=Space, Time, and Aliens: Collected Works on Cosmos and Culture |chapter=Bringing Culture to Cosmos: Cultural Evolution, the Postbiological Universe, and SETI |date=2020 |pages=171–190 |doi=10.1007/978-3-030-41614-0_12 |chapter-url=https://link.springer.com/chapter/10.1007/978-3-030-41614-0_12 |access-date=18 October 2022 |publisher=Springer International Publishing |isbn=978-3-030-41613-3 |s2cid=219414685 |language=en}}</ref> Self-replicating probes could exhaustively explore a galaxy the size of the Milky Way in as little as a million years.<ref name="Hart"/> If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Another speculation for contact with an alien probe—one that would be trying to find human beings—is an alien [[Astronomy:Bracewell probe|Bracewell probe]]. Such a hypothetical device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying a slow speed-of-light dialogue between vastly distant neighbors. Rather than contending with the long delays a radio dialogue would suffer, a probe housing an [[Artificial intelligence|artificial intelligence]] would seek out an alien civilization to carry on a close-range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialogue could be conducted in real time.<ref>{{cite journal| last1=Bracewell| first1=R. N.| title=Communications from Superior Galactic Communities| journal=Nature| volume=186| issue=4726| pages=670–671| date=1960| doi=10.1038/186670a0| bibcode = 1960Natur.186..670B | s2cid=4222557}}</ref>

Direct exploration of the Solar System has yielded no evidence indicating a visit by aliens or their probes. Detailed exploration of areas of the Solar System where resources would be plentiful may yet produce evidence of alien exploration,<ref name="AsteroidBelt">{{cite journal |author=Papagiannis, M. D. |title=Are We all Alone, or could They be in the Asteroid Belt? |journal=Quarterly Journal of the Royal Astronomical Society |volume=19 |pages=277–281 |date=1978 |bibcode = 1978QJRAS..19..277P }}</ref><ref>{{cite news |title=Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox |author=Robert A. Freitas Jr. |journal=[[Organization:Journal of the British Interplanetary Society|Journal of the British Interplanetary Society]] |volume=36 |date=November 1983 |pages=496–500 |url=http://www.rfreitas.com/Astro/ResolvingFermi1983.htm |bibcode=1983JBIS...36..496F |access-date=November 12, 2004 |archive-url=https://web.archive.org/web/20041208080419/http://www.rfreitas.com/Astro/ResolvingFermi1983.htm |archive-date=December 8, 2004 |url-status=live }}</ref> though the entirety of the Solar System is vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded.<ref>{{cite journal| last1=Freitas| first1=Robert A Jr|last2=Valdes|first2=F|doi=10.1016/0094-5765(85)90031-1|title=The search for extraterrestrial artifacts (SETA)| date=1985 |pages=1027–1034| issue=12| volume=12| journal=Acta Astronautica| bibcode = 1985AcAau..12.1027F | citeseerx=10.1.1.118.4668}}</ref>

=== Searches for stellar-scale artifacts ===
{{further|Astronomy:Dyson sphere|Engineering:Stellar engine|Astronomy:Kardashev scale}}
[[File:Dyson Sphere Diagram-en.svg|thumb|upright=1.2|A variant of the speculative [[Astronomy:Dyson sphere|Dyson sphere]]. Such large-scale artifacts would drastically alter the spectrum of a star.]]
In 1959, [[Biography:Freeman Dyson|Freeman Dyson]] observed that every developing human civilization constantly increases its energy consumption, and he conjectured that a civilization might try to harness a large part of the energy produced by a star. He proposed a hypothetical "Dyson sphere" as a possible means: a shell or cloud of objects enclosing a star to absorb and utilize as much [[Physics:Radiant energy|radiant energy]] as possible. Such a feat of astroengineering would drastically alter the observed [[Physics:Spectroscopy|spectrum]] of the star involved, changing it at least partly from the normal emission lines of a natural [[Astronomy:Stellar atmosphere|stellar atmosphere]] to those of [[Astronomy:Black-body radiation|black-body radiation]], probably with a peak in the [[Physics:Infrared|infrared]]. Dyson speculated that advanced alien civilizations might be detected by examining the spectra of stars and searching for such an altered spectrum.<ref>{{cite journal| journal=Science| date=1960| url=http://www.islandone.org/LEOBiblio/SETI1.HTM| title=Search for Artificial Stellar Sources of Infra-Red Radiation| author=Dyson, Freeman J.| pages=1667–1668| volume=131| doi=10.1126/science.131.3414.1667| pmid=17780673| issue=3414| bibcode=1960Sci...131.1667D| s2cid=3195432|  access-date=August 19, 2010| archive-url=https://web.archive.org/web/20190714215002/http://www.islandone.org/LEOBiblio/SETI1.HTM| archive-date=July 14, 2019| url-status=live}}</ref><ref name="G^I">{{Cite journal| arxiv=1408.1133| last1= Wright| first1= J. T.| title= The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification| journal= The Astrophysical Journal| volume= 792| issue= 1| pages= 26| last2= Mullan| first2= B.| last3= Sigurðsson| first3= S.|last4= Povich| first4= M. S.| date= 2014| doi= 10.1088/0004-637X/792/1/26| bibcode= 2014ApJ...792...26W| s2cid= 119221206}}</ref><ref name="G^II">{{Cite journal| arxiv=1408.1134| last1= Wright| first1= J. T.| title= The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. II. Framework, Strategy, and First Result| journal= The Astrophysical Journal| volume= 792| issue= 1| pages= 27| last2= Griffith| first2= R.| last3= Sigurðsson| first3= S.| last4= Povich| first4= M. S.| last5= Mullan| first5= B.| date= 2014| doi= 10.1088/0004-637X/792/1/27| bibcode= 2014ApJ...792...27W| s2cid= 16322536}}</ref>

There have been some attempts to find evidence of the existence of Dyson spheres that would alter the spectra of their core stars.<ref>{{cite web|url = http://home.fnal.gov/~carrigan/infrared_astronomy/Fermilab_search.htm|title = Fermilab Dyson Sphere search program|publisher = Fermi National Accelerator Laboratory|access-date = February 10, 2008|archive-url = https://web.archive.org/web/20060306222359/http://home.fnal.gov/~carrigan/Infrared_Astronomy/Fermilab_search.htm|archive-date = March 6, 2006|url-status = dead}}</ref> Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications.<ref name="G^I" /><ref name="G^II" /><ref name="G^III">{{Cite journal|first1=J. T. |last1=Wright |first2=B |last2=Mullan |first3=S |last3=Sigurdsson |first4=M. S |last4=Povich |arxiv=1504.03418 |title=The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. III. The Reddest Extended Sources in WISE |journal=The Astrophysical Journal Supplement Series |volume=217 |issue=2 |pages=25 |date=2014|doi=10.1088/0067-0049/217/2/25 |bibcode=2015ApJS..217...25G |s2cid=118463557 }}</ref><ref>{{cite news |title=Alien Supercivilizations Absent from 100,000 Nearby Galaxies |url=http://www.scientificamerican.com/article/alien-supercivilizations-absent-from-100-000-nearby-galaxies/ |work=Scientific American |date=April 17, 2015 |access-date=June 29, 2015 |archive-url=https://web.archive.org/web/20150622003330/http://www.scientificamerican.com/article/alien-supercivilizations-absent-from-100-000-nearby-galaxies/ |archive-date=June 22, 2015 |url-status=live }}</ref> In October 2015, there was some speculation that a dimming of light from star [[Astronomy:KIC 8462852|KIC 8462852]], observed by the [[Astronomy:Kepler space telescope|Kepler space telescope]], could have been a result of Dyson sphere construction.<ref>{{Cite journal|arxiv=1510.04606| last1= Wright| first1= Jason T.|title= The Ĝ Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures| journal= The Astrophysical Journal| volume= 816| issue= 1| pages= 17| last2=Cartier| first2= Kimberly M. S.| last3= Zhao| first3= Ming| last4= Jontof-Hutter| first4= Daniel| last5= Ford| first5= Eric B.| year= 2015| doi= 10.3847/0004-637X/816/1/17|bibcode = 2016ApJ...816...17W | s2cid= 119282226| doi-access= free}}</ref><ref name="ATL-20151013">{{cite web| last1=Andersen| first1=Ross| title=The Most Mysterious Star in Our Galaxy| url=https://www.theatlantic.com/science/archive/2015/10/the-most-interesting-star-in-our-galaxy/410023/| date=October 13, 2015| work=The Atlantic| access-date=October 13, 2015| archive-url=https://web.archive.org/web/20170720013427/https://www.theatlantic.com/science/archive/2015/10/the-most-interesting-star-in-our-galaxy/410023/| archive-date=July 20, 2017| url-status=live}}</ref> However, in 2018, observations determined that the amount of dimming varied by the frequency of the light, pointing to dust, rather than an opaque object such as a Dyson sphere, as the culprit for causing the dimming.<ref>{{Cite journal |title=The First Post-Kepler Brightness Dips of KIC 8462852 |journal=The Astrophysical Journal |volume=853 |issue=1 |at=L8 |arxiv=1801.00732 |first1=Tabetha S. |last1=Boyajian |display-authors=0 |doi=10.3847/2041-8213/aaa405 |year=2018 |bibcode=2018ApJ...853L...8B|s2cid=215751718 |doi-access=free }}</ref><ref>{{cite news |last1=Overbye |first1=Dennis |title=Magnetic Secrets of Mysterious Radio Bursts in a Faraway Galaxy |url=https://www.nytimes.com/2018/01/10/science/neutron-star-fast-radio-bursts.html |access-date=April 2, 2019 |work=The New York Times |date=January 10, 2018 |archive-url=https://web.archive.org/web/20180111001837/https://www.nytimes.com/2018/01/10/science/neutron-star-fast-radio-bursts.html |archive-date=January 11, 2018 |url-status=live }}</ref>

== Hypothetical explanations for the paradox ==
<!-- Before adding new explanations, please read the ones in the article and make sure the proposal does not belong to one explanation already included -->
=== Rarity of intelligent life===

==== Extraterrestrial life is rare or non-existent ====
{{main|Astronomy:Rare Earth hypothesis|Astronomy:Firstborn hypothesis}}
Those who think that intelligent [[Astronomy:Extraterrestrial life|extraterrestrial life]] is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the [[Biology:Evolution of biological complexity|evolution of biological complexity]]—are rare or even unique to Earth. Under this assumption, called the [[Astronomy:Rare Earth hypothesis|rare Earth hypothesis]], a rejection of the [[Astronomy:Mediocrity principle|mediocrity principle]], complex multicellular life is regarded as exceedingly unusual.<ref name="rare-earth">{{cite book |title=Rare Earth: Why Complex Life is Uncommon in the Universe |first1=Peter D. |last1=Ward |first2=Donald |last2=Brownlee |author-link2=Donald Brownlee |year= 2000 |page=368 |publisher=Springer |edition=1st |isbn=978-0-387-98701-9}}</ref>

The rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a [[Astronomy:Galactic habitable zone|galactic habitable zone]], a star and planet(s) having the requisite conditions, such as enough of a [[Astronomy:Circumstellar habitable zone|continuous habitable zone]], the advantage of a giant guardian like Jupiter and a large [[Astronomy:Natural satellite|moon]], conditions needed to ensure the planet has a [[Astronomy:Magnetosphere|magnetosphere]] and [[Earth:Plate tectonics|plate tectonics]], the chemistry of the [[Earth:Lithosphere|lithosphere]], [[Astronomy:Atmosphere|atmosphere]], and oceans, the role of "evolutionary pumps" such as massive glaciation and rare [[Astronomy:Meteoroid#Bolide|bolide]] impacts. Perhaps most importantly, advanced life needs whatever it was that led to the transition of (some) [[Biology:Prokaryote|prokaryotic cells]] to [[Biology:Eukaryote|eukaryotic cells]], [[Biology:Sexual reproduction|sexual reproduction]] and the [[Earth:Cambrian explosion|Cambrian explosion]].

In his book ''Wonderful Life'' (1989), Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings most probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life.<ref>''The Nature of Nature: Examining the Role of Naturalism in Science'', editors Bruce Gordon and William Dembski,
[https://books.google.com/books?id=pe5nAwAAQBAJ&pg=PT491 Ch. 20 "The Chain of Accidents and the Rule of Law: The Role of Contingency and Necessity in Evolution"] by Michael Shemer, published by Intercollegiate Studies Institute, 2010.</ref>

==== Extraterrestrial intelligence is rare or non-existent ====
It is possible that even if complex life is common, intelligence (and consequently civilizations) is not.<ref name="Lineweaver, 2009"/> While there are remote sensing techniques that could perhaps detect life-bearing planets without relying on the signs of technology,<ref>{{cite journal|title=Detecting Life-bearing Extrasolar Planets with Space Telescopes|bibcode=2008ApJ...684.1404B|author1=Steven V. W. Beckwith|doi=10.1086/590466|issue=2|journal=The Astrophysical Journal|volume=684|pages=1404–1415|date=2008|arxiv = 0710.1444|s2cid=15148438}}</ref><ref>{{cite journal|title=Detection of circular polarization in light scattered from photosynthetic microbes|author=Sparks, W.B. |author2=Hough, J. |author3=Germer, T.A. |author4=Chen, F. |author5=DasSarma, S. |author6=DasSarma, P. |author7=Robb, F.T. |author8=Manset, N. |author9=Kolokolova, L. |author10=Reid, N. |display-authors=0|journal=Proceedings of the National Academy of Sciences|volume=106|issue=14–16|pages=1771–1779|date=2009|url=http://www.pnas.org/content/early/2009/04/28/0810215106.full.pdf |archive-url=https://web.archive.org/web/20150924175633/http://www.pnas.org/content/early/2009/04/28/0810215106.full.pdf |archive-date=2015-09-24 |url-status=live|doi=10.1016/j.jqsrt.2009.02.028|hdl=2299/5925 |hdl-access=free }}</ref> none of them have any ability to tell if any detected life is intelligent. This is sometimes referred to as the "algae vs. alumnae" problem.<ref name="Tarter-NYAS" />

Charles Lineweaver states that when considering any extreme trait in an animal, intermediate stages do not necessarily produce "inevitable" outcomes. For example, large brains are no more "inevitable", or convergent, than are the long noses of animals such as [[Biology:Aardvark|aardvark]]s and elephants. Humans, apes, whales, dolphins, octopuses, and squids are among the small group of definite or probable intelligence on Earth. As he points out, "dolphins have had ~20{{nbsp}}million years to build a radio telescope and have not done so".<ref name="Lineweaver, 2009">[https://arxiv.org/abs/0711.1751 Paleontological Tests: Human Intelligence is Not a Convergent Feature of Evolution.] {{Webarchive|url=https://web.archive.org/web/20191220142600/https://arxiv.org/abs/0711.1751 |date=December 20, 2019 }}, Charles Lineweaver, Australian National University, Canberra, published in ''From Fossils to Astrobiology'', edited by J. Seckbach and M. Walsh, Springer, 2009.</ref>

In addition, Rebecca Boyle points out that of all the species who have ever evolved in the history of life on the planet Earth, only one—human beings and only in the beginning stages—has ever become space-faring.<ref name="Rebecca Boyle, Quanta Magazine">[https://www.quantamagazine.org/galaxy-simulations-offer-a-new-solution-to-the-fermi-paradox-20190307/ "Galaxy Simulations Offer a New Solution to the Fermi Paradox"], Quanta Magazine "Abstraction Blog," Rebecca Boyle, March 7, 2019. "The sun has been around the center of the Milky Way 50 times," said Jonathan Carroll-Nellenback, astronomer at the University of Rochester.</ref>

==== Periodic extinction by natural events ====
[[File:Coast Impact.jpg|thumb|An asteroid impact may trigger an extinction event.]]
New life might commonly die out due to runaway heating or cooling on their fledgling planets.<ref>{{cite news |url=http://astrobiology.com/2016/01/the-aliens-are-silent-because-they-are-extinct.html |title=The Aliens Are Silent Because They Are Extinct |work=Australian National University |date=January 21, 2016 |access-date=2016-01-22 |archive-url=http://arquivo.pt/wayback/20160523051724/http://astrobiology.com/2016/01/the%2Daliens%2Dare%2Dsilent%2Dbecause%2Dthey%2Dare%2Dextinct.html |archive-date=May 23, 2016 |url-status=live }}</ref> On Earth, there have been numerous major [[Biology:Extinction event|extinction event]]s that destroyed the majority of complex species alive at the time; the extinction of the non-avian dinosaurs is the best known example. These are thought to have been caused by events such as impact from a large meteorite, massive volcanic eruptions, or astronomical events such as [[Astronomy:Gamma-ray burst|gamma-ray burst]]s.<ref>{{cite journal |doi=10.1017/S1473550404001910 |title=Did a gamma-ray burst initiate the late Ordovician mass extinction? |vauthors=Melott AL, Lieberman BS, Laird CM, Martin LD, Medvedev MV, Thomas BC, Cannizzo JK, Gehrels N, Jackman CH |journal=International Journal of Astrobiology |volume=3 |issue=1 |pages=55–61 |date=2004 |url=http://acdb-ext.gsfc.nasa.gov/People/Jackman/Melott_2004.pdf |arxiv=astro-ph/0309415 |bibcode=2004IJAsB...3...55M |hdl=1808/9204 |s2cid=13124815 |access-date=August 20, 2010 |archive-url=https://web.archive.org/web/20110725132522/http://acdb-ext.gsfc.nasa.gov/People/Jackman/Melott_2004.pdf |archive-date=July 25, 2011 |url-status=live }}</ref> It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life, or at least its civilizations, before the species is able to develop the technology to communicate with other intelligent species.<ref>{{cite book |title=Global catastrophic risks |author1=Nick Bostrom |author2=Milan M. Ćirković |chapter=12.5: The Fermi Paradox and Mass Extinctions}}</ref>

=== Evolutionary explanations ===

==== Intelligent alien species have not developed advanced technologies ====
[[File:Neanderthal Flintworkers (Knight, 1920).jpg|thumb|300px|''[[Place:Le Moustier|Le Moustier]]'' Neanderthals (Charles R. Knight, 1920)]]
It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would also be very difficult to detect.<ref name="Tarter-NYAS">{{cite journal| last1=Tarter| first1=Jill| title=What is SETI?| journal=Annals of the New York Academy of Sciences| volume=950|issue=1|pages=269–275|date=2006| pmid=11797755|doi=10.1111/j.1749-6632.2001.tb02144.x|bibcode = 2001NYASA.950..269T | s2cid=27203660}}</ref> A trip using conventional rockets would take hundreds of thousands of years to reach the nearest stars.<ref>{{cite magazine |title=Are Alien Civilizations Technologically Advanced? |url=https://blogs.scientificamerican.com/observations/are-alien-civilizations-technologically-advanced/ |magazine=Scientific American |first=Abraham |last=Loeb |date=January 8, 2018 |access-date=January 11, 2018 |archive-url=https://web.archive.org/web/20180112100817/https://blogs.scientificamerican.com/observations/are-alien-civilizations-technologically-advanced/ |archive-date=January 12, 2018 |url-status=live }}</ref>

To skeptics, the fact that in the history of life on the Earth only one species has developed a civilization to the point of being capable of [[Engineering:Spaceflight|spaceflight]] and radio technology lends more credence to the idea that technologically advanced civilizations are rare in the universe.<ref>{{cite news |title=The Intelligent-Life Lottery |url=https://www.nytimes.com/2014/08/19/science/in-search-for-intelligent-life-consider-the-lottery.html |newspaper=The New York Times |date=August 18, 2014 |first=George |last=Johnson |access-date=March 1, 2017 |archive-url=https://web.archive.org/web/20170324212404/https://www.nytimes.com/2014/08/19/science/in-search-for-intelligent-life-consider-the-lottery.html |archive-date=March 24, 2017 |url-status=live }}</ref>

Another hypothesis in this category is the "Water World hypothesis". According to author and scientist David Brin: "it turns out that our Earth skates the very inner edge of our sun's continuously habitable—or 'Goldilocks'—zone. And Earth may be anomalous. It may be that because we are so close to our sun, we have an anomalously oxygen-rich atmosphere, and we have anomalously little ocean for a water world. In other words, 32 percent continental mass may be high among water worlds..."<ref name="DB_2012" /> Brin continues, "In which case, the evolution of creatures like us, with hands and fire and all that sort of thing, may be rare in the galaxy. In which case, when we do build starships and head out there, perhaps we'll find lots and lots of life worlds, but they're all like Polynesia. We'll find lots and lots of intelligent lifeforms out there, but they're all dolphins, whales, squids, who could never build their own starships. What a perfect universe for us to be in, because nobody would be able to boss us around, and we'd get to be the voyagers, the ''Star Trek'' people, the starship builders, the policemen, and so on."<ref name="DB_2012">{{cite magazine |url=https://www.wired.com/2012/08/geeks-guide-david-brin/ |title=Why David Brin Hates Yoda, Loves Radical Transparency |archive-url=https://web.archive.org/web/20190406204416/https://www.wired.com/2012/08/geeks-guide-david-brin/ |archive-date=April 6, 2019 |url-status=live |magazine=Wired |date=August 8, 2012}}</ref>

==== It is the nature of intelligent life to destroy itself ====

[[File:Operation Upshot-Knothole - Badger 001.jpg|right|thumb|A 23-kiloton tower shot called BADGER, fired as part of the Operation Upshot–Knothole nuclear test series]]
This is the argument that technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or spaceflight technology. The astrophysicist [[Biography:Sebastian von Hoerner|Sebastian von Hoerner]] stated that the progress of science and technology on [[Astronomy:Earth|Earth]] was driven by two factors—the struggle for domination and the desire for an easy life. The former potentially leads to complete destruction, while the latter may lead to biological or mental degeneration.<ref name="Hoerner">{{cite journal |last=von Hoerner |first=Sebastian |title=The Search for Signals from Other Civilizations |journal=Science |volume=134 |issue=3493 |date=December 8, 1961 |pages=1839–1843 |doi=10.1126/science.134.3493.1839 |pmid=17831111 |bibcode=1961Sci...134.1839V |issn=0036-8075}}</ref> Possible means of annihilation via major global issues, where global interconnectedness actually makes humanity more vulnerable than resilient,<ref>{{Cite book|last1=Hite |first1=Kristen A. |last2=Seitz |first2=John L.|title=Global Issues : an introduction|date=2020|publisher=Wiley-Blackwell|isbn=978-1-119-53850-9|oclc=1127917585}}</ref> are many,<ref>{{cite book |last=Webb |first=Stephen |title=If the Universe Is Teeming with Aliens... Where Is Everybody? Seventy five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life |edition=2nd |publisher=Copernicus Books |date=2015 |isbn=978-3-319-13235-8 |url=https://books.google.com/books?id=QWKyrQEACAAJ |access-date=July 21, 2015 |archive-url=https://web.archive.org/web/20150903224843/https://books.google.com/books?id=QWKyrQEACAAJ |archive-date=September 3, 2015 |url-status=live }} Chapters 36–39.</ref> including war, accidental environmental contamination or damage, the development of [[Engineering:Biotechnology risk|biotechnology]],<ref>{{Cite journal|last=Sotos|first=John G.|date=2019-01-15|title=Biotechnology and the lifetime of technical civilizations|journal=International Journal of Astrobiology|volume=18|issue=5|pages=445–454|doi=10.1017/s1473550418000447|issn=1473-5504|arxiv=1709.01149|bibcode=2019IJAsB..18..445S|s2cid=119090767}}</ref> synthetic life like [[Physics:Mirror life|mirror life]],<ref>{{cite news |url=https://www.wired.com/2010/11/ff_mirrorlife/ |title=Mirror-image cells could transform science – or kill us all |magazine=Wired |date=November 29, 2010 |first=John |last=Bohannon |access-date=March 16, 2019 |archive-url=https://web.archive.org/web/20190513045312/https://www.wired.com/2010/11/ff_mirrorlife/ |archive-date=May 13, 2019 |url-status=live }}</ref> resource depletion, [[Earth:Global warming|climate change]],<ref>{{cite news |url=https://www.nytimes.com/2015/01/18/opinion/sunday/is-a-climate-disaster-inevitable.html |title=Is a Climate Disaster Inevitable? |first=Adam |last=Frank |work=The New York Times |date=January 17, 2015 |access-date=March 1, 2017 |archive-url=https://web.archive.org/web/20170324212811/https://www.nytimes.com/2015/01/18/opinion/sunday/is-a-climate-disaster-inevitable.html |archive-date=March 24, 2017 |url-status=live }}</ref> or [[Unsolved:Existential risk from artificial general intelligence|poorly-designed artificial intelligence]]. This general theme is explored both in fiction and in scientific hypothesizing.<ref>{{cite web |title=Existential Risks Analyzing Human Extinction Scenarios and Related Hazards |last=Bostrom |first=Nick |url=http://www.nickbostrom.com/existential/risks.html |access-date=October 4, 2009 |archive-url=https://web.archive.org/web/20110427030852/http://www.nickbostrom.com/existential/risks.html |archive-date=April 27, 2011 |url-status=live }}</ref>

In 1966, Sagan and [[Biography:Iosif Shklovsky|Shklovskii]] speculated that technological civilizations will either tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales.<ref>{{cite web |url=http://www.bigear.org/vol1no2/sagan.htm |title=Cosmic Search Vol. 1 No. 2 |last=Sagan |first=Carl |work=Cosmic Search Magazine |access-date=2015-07-21 |archive-url=https://web.archive.org/web/20060818144558/http://www.bigear.org/vol1no2/sagan.htm |archive-date=August 18, 2006 |url-status=live }}</ref> Self-annihilation may also be viewed in terms of [[Physics:Thermodynamics|thermodynamics]]: insofar as life is an ordered [[Philosophy:System|system]] that can sustain itself against the [[Physics:Entropy|tendency to disorder]], Stephen Hawking's "external transmission" or interstellar communicative phase, where [[Finance:Knowledge economy|knowledge production]] and [[Knowledge management|knowledge management]] is more important than transmission of information via [[Biology:Evolution|evolution]], may be the point at which the system becomes unstable and self-destructs.<ref>{{cite web|url=http://www.hawking.org.uk/lectures/life.html|title=Life in the Universe|last=Hawking|first=Stephen|work=Public Lectures|publisher=University of Cambridge|archive-url=https://web.archive.org/web/20060421051343/http://www.hawking.org.uk/lectures/life.html|archive-date=April 21, 2006|access-date=May 11, 2006}}</ref><ref>{{Cite book|last=Yudkowsky|first=Eliezer|title=Global catastrophic risks|publisher=Oxford University Press|year=2008|isbn=978-0-19-960650-4|editor-last=Bostrom|editor-first=Nick|location=New York|pages=308–345|chapter=Artificial Intelligence as a Positive and Negative Factor in Global Risk|oclc=993268361|editor-last2=Ćirković|editor-first2=Milan M.}}</ref> Here, Hawking emphasizes self-design of the [[Biology:Human genome|human genome]] ([[Transhumanism|transhumanism]]) or enhancement via machines (e.g., [[Brain–computer interface|brain–computer interface]]) to enhance [[Philosophy:Human intelligence|human intelligence]] and reduce [[Philosophy:Aggression|aggression]], without which he implies human civilization may be too stupid collectively to survive an increasingly unstable system. For instance, the development of technologies during the "external transmission" phase, such as [[Engineering:Weaponization|weaponization]] of [[Artificial general intelligence|artificial general intelligence]] or [[Physics:Antimatter|antimatter]], may not be met by concomitant increases in human ability to manage its own inventions. Consequently, disorder increases in the system: [[Social:Global governance|global governance]] may become increasingly destabilized, worsening humanity's ability to manage the possible means of annihilation listed above, resulting in global [[Social:Societal collapse|societal collapse]].

[[File:Frank climate model, 2018.svg|thumb|Possible trajectories of anthropogenic climate change in a model by Frank ''et al''., 2018]]
Using extinct civilizations such as Easter Island (Rapa Nui) as models, a study conducted in 2018 by Adam Frank ''et al.'' posited that [[Earth:Climate change|climate change]] induced by "energy intensive" civilizations may prevent sustainability within such civilizations, thus explaining the paradoxical lack of evidence for intelligent extraterrestrial life. According to his model, possible outcomes of climate change include gradual population decline until an equilibrium is reached; a scenario where [[Earth:Sustainability|sustainability]] is attained and both population and surface temperature level decrease; and societal collapse, including scenarios where a [[Earth:Tipping points in the climate system|tipping point]] is crossed.<ref name = "billings">{{cite magazine|last=Billings|first=Lee|date=13 June 2018|title=Alien Anthropocene: How Would Other Worlds Battle Climate Change?|url=https://www.scientificamerican.com/article/alien-anthropocene-how-would-other-worlds-battle-climate-change/|volume=28|issue=3s|magazine=Scientific American|publisher=Springer Nature|access-date=14 August 2019|archive-url=https://web.archive.org/web/20190701233738/https://www.scientificamerican.com/article/alien-anthropocene-how-would-other-worlds-battle-climate-change/|archive-date=July 1, 2019|url-status=live}}</ref>

A less theoretical example might be the resource-depletion issue on Polynesian islands, of which Easter Island is only the best known. David Brin points out that during the expansion phase from 1500 BC to 800 AD there were cycles of overpopulation followed by what might be called periodic cullings of adult males through war or ritual. He writes, "There are many stories of islands whose men were almost wiped out—sometimes by internal strife, and sometimes by invading males from other islands."<ref>"The Great Silence: the Controversy . . . " (15-page paper), ''Quarterly J. Royal Astron. Soc.,'' David Brin, 1983, [http://articles.adsabs.harvard.edu//full/1983QJRAS..24..283B/0000301.000.html page 301 second-to-last paragraph] {{Webarchive|url=https://web.archive.org/web/20200503142635/http://articles.adsabs.harvard.edu//full/1983QJRAS..24..283B/0000301.000.html |date=May 3, 2020 }}. Brin cites, ''The Prehistory of Polynesia'', edited by J. Jennings, Harvard University Press, 1979. See also ''Interstellar Migration and the Human Experience'', edited by Ben Finney and Eric M. Jones, Ch. 13 "Life (With All Its Problems) in Space" by Alfred W. Crosby, University of California Press, 1985.</ref>

==== It is the nature of intelligent life to destroy others ====
Another hypothesis is that an intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear, perhaps by using [[Self-replicating spacecraft|self-replicating probes]]. Science fiction writer Fred Saberhagen has explored this idea in his ''Berserker'' series, as has physicist [[Biography:Gregory Benford|Gregory Benford]]<ref>"The Great Silence: the Controversy . . . " (15-page paper), ''Quarterly J. Royal Astron. Soc.,'' David Brin, 1983, [http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000296.000.html page 296 bottom third] {{Webarchive|url=https://web.archive.org/web/20200204194035/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000296.000.html |date=February 4, 2020 }}.</ref> and, as well, science fiction writer [[Biography:Greg Bear|Greg Bear]] in his ''The Forge of God'' novel,<ref>{{cite web | url=https://www.latimes.com/archives/la-xpm-1987-09-20-bk-8862-story.html | archive-url=https://web.archive.org/web/20220816233413/https://www.latimes.com/archives/la-xpm-1987-09-20-bk-8862-story.html | archive-date=August 16, 2022 | title=Self-Reproducing Machines from Another Planet : THE FORGE OF GOD by Greg Bear (Tor Books : $17.95; 448 pp.) | website={{wipe|Los Angeles Times}} | date=September 20, 1987 }}</ref> and later Liu Cixin in his ''The Three-Body Problem'' series.

A species might undertake such extermination out of expansionist motives, greed, paranoia, or aggression. In 1981, cosmologist Edward Harrison argued that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat.<ref>{{cite web| last = Soter| first = Steven| date = 2005| url = http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1745| title = SETI and the Cosmic Quarantine Hypothesis| work = Astrobiology Magazine| publisher = Space.com| access-date = May 3, 2006| archive-url = https://web.archive.org/web/20070929092545/http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1745| archive-date = September 29, 2007 |url-status=usurped}}</ref> It has also been suggested that a successful alien species would be a [[Biology:Apex predator|superpredator]], as are humans.<ref>{{cite journal| last=Archer |first=Michael| title=Slime Monsters Will Be Human Too| journal=Aust. Nat. Hist| volume=22| pages=546–547| date=1989}}</ref><ref name=webb02>{{cite book |last=Webb |first=Stephen |title=If the Universe Is Teeming with Aliens... Where Is Everybody? Fifty solutions to the Fermi Paradox and the Problem of Extraterrestrial Life |publisher=Copernicus Books |date=2002 |isbn=978-0-387-95501-8 |url=https://books.google.com/books?id=UhzjBwAAQBAJ |access-date=July 21, 2015 |archive-url=https://web.archive.org/web/20150903224843/https://books.google.com/books?id=UhzjBwAAQBAJ |archive-date=September 3, 2015 |url-status=live }}</ref>{{rp|112}} Another possibility invokes the "[[Tragedy of the commons|tragedy of the commons]]" and the [[Astronomy:Anthropic principle|anthropic principle]]: the first lifeform to achieve interstellar travel will necessarily (even if unintentionally) prevent competitors from arising, and humans simply happen to be first.<ref name="ARX-20180327">{{cite arXiv |last=Berezin |first=Alexander |title='First in, last out' solution to the Fermi Paradox |date=March 27, 2018 |class=physics.pop-ph |eprint=1803.08425v2 }}</ref><ref>{{cite news |last=Dockrill |first=Peter |title=A Physicist Has Proposed a Pretty Depressing Explanation For Why We Never See Aliens |url=https://www.sciencealert.com/a-physicist-has-proposed-a-pretty-depressing-explanation-for-why-we-never-see-aliens |date=June 2, 2019 |work=ScienceAlert |access-date=June 2, 2019 |archive-url=https://web.archive.org/web/20190602213542/https://www.sciencealert.com/a-physicist-has-proposed-a-pretty-depressing-explanation-for-why-we-never-see-aliens |archive-date=June 2, 2019 |url-status=live }}</ref>

==== Civilizations only broadcast detectable signals for a brief period of time ====
It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. The usual assumption is that civilizations outgrow radio through technological advancement.<ref>{{Cite journal|title=Calculating the probability of detecting radio signals from alien civilizations |journal=International Journal of Astrobiology |volume=5 |issue=2 |pages=143–149 |author=Marko Horvat |date=2007 |arxiv=0707.0011|doi=10.1017/S1473550406003004 |bibcode = 2006IJAsB...5..143H |s2cid=54608993 }} "There is a specific time interval during which an alien civilization uses radio communications. Before this interval, radio is beyond the civilization's technical reach, and after this interval radio will be considered obsolete."</ref> However, there could be other leakage such as that from microwaves used to transmit power from solar satellites to ground receivers.<ref name=Stephenson1984>{{cite journal |title=Solar Power Satellites as Interstellar Beacons |author=Stephenson, D. G. |bibcode=1984QJRAS..25...80S |journal=Quarterly Journal of the Royal Astronomical Society|volume=25 |issue=1 |page=80 |date=1984 }}</ref>

Regarding the first point, in a 2006 ''Sky & Telescope'' article, [[Biography:Seth Shostak|Seth Shostak]] wrote, "Moreover, radio leakage from a planet is only likely to get weaker as a civilization advances and its communications technology gets better. Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions."<ref name="Seth Shostak, Sky & Telescope, 2006">[https://www.skyandtelescope.com/astronomy-news/the-future-of-seti/ The Future of SETI] {{Webarchive|url=https://web.archive.org/web/20190524193142/https://www.skyandtelescope.com/astronomy-news/the-future-of-seti/ |date=May 24, 2019 }}, ''Sky & Telescope'', Seth Shostak, July 19, 2006. This article also discusses strategy for optical SETI.</ref>

More hypothetically, advanced alien civilizations may evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by technologies not developed or used by mankind.<ref>{{Cite web |last=Scharf |first=Caleb |date=2022-08-10 |title=We Might Already Speak the Same Language As ET |url=https://nautil.us/we-might-already-speak-the-same-language-as-et-22841/ |access-date=2022-08-11 |website=Nautilus Quarterly |language=en-US}}</ref> Some scientists have hypothesized that advanced civilizations may send [[Physics:Neutrino|neutrino]] signals.<ref>{{cite web |url=http://www.bigear.org/vol1no3/neutrino.htm |title=Cosmic Search Vol. 1 No. 3 |publisher=Bigear.org |date=September 21, 2004 |access-date=July 3, 2010 |archive-url=https://web.archive.org/web/20101027210854/http://bigear.org/vol1no3/neutrino.htm |archive-date=October 27, 2010 |url-status=live }}</ref> If such signals exist, they could be detectable by [[Astronomy:Neutrino detector|neutrino detector]]s that are now under construction for other goals.<ref>{{cite journal| last1=Learned| first1=J| last2=Pakvasa| first2=S| last3=Zee| first3=A| title=Galactic neutrino communication| arxiv=0805.2429| journal=Physics Letters B| volume=671| issue=1| pages=15–19| date=2009 |doi=10.1016/j.physletb.2008.11.057| bibcode = 2009PhLB..671...15L | s2cid=118453255}}</ref>

==== Alien life may be too incomprehensible ====
[[File:TerrestrialMicrowaveWindow.jpg|thumb|upright=1.25|Microwave window as seen by a ground-based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence]]

Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Aliens may be psychologically unwilling to attempt to communicate with human beings. Perhaps human mathematics is [[Social:Parochialism|parochial]] to Earth and not shared by other life,<ref>Schombert, James. [http://abyss.uoregon.edu/~js/cosmo/lectures/lec28.html "Fermi's paradox (i.e. Where are they?)"] {{Webarchive|url=https://web.archive.org/web/20111107220554/http://abyss.uoregon.edu/~js/cosmo/lectures/lec28.html |date=November 7, 2011 }} ''Cosmology Lectures'', University of Oregon.</ref> though others argue this can only apply to abstract math since the math associated with physics must be similar (in results, if not in methods).<ref>{{cite journal |title=Mathematics on a distant planet |author=Hamming, RW |journal=The American Mathematical Monthly |volume=105 |issue=7 |pages=640–650 |date=1998 |jstor=2589247|doi=10.2307/2589247 }}</ref>

Physiology might also cause a communication barrier. Carl Sagan speculated that an alien species might have a thought process orders of magnitude slower (or faster) than that of humans.<ref>{{cite book |title=Contact |author=Carl Sagan}} Chapter 3, p. 49.</ref> A message broadcast by that species might well seem like random background noise to humans, and therefore go undetected.

Another thought is that technological civilizations invariably experience a [[Philosophy:Technological singularity|technological singularity]] and attain a post-biological character.<ref>{{cite news |last=Istvan |first=Zoltan |url=https://motherboard.vice.com/en_us/article/vv7bkb/why-havent-we-met-aliens-yet-because-theyve-evolved-into-ai |title=Why Haven't We Met Aliens Yet? Because They've Evolved into AI |work=Motherboard |publisher=[[Company:Vice Media|Vice Media]] |date=March 16, 2016 |url-status=live |archive-url=https://archive.today/20171230231127/https://motherboard.vice.com/en_us/article/vv7bkb/why-havent-we-met-aliens-yet-because-theyve-evolved-into-ai |archive-date=December 30, 2017 |access-date=December 30, 2017 }}</ref> Hypothetical civilizations of this sort may have advanced drastically enough to render communication impossible.<ref>{{cite book| last1=Long| first1=K. F.| title=Deep Space Propulsion: A Roadmap to Interstellar Flight| isbn=978-1-4614-0607-5| page=114| url=https://books.google.com/books?id=EAD2-GrarkEC&q=fermi+paradox+technological+singularity&pg=PA114| access-date=June 23, 2015| year= 2011| publisher=Springer}}</ref><ref>{{cite book| last1=Cook| first1=Stephen P.| title=Life on Earth and other Planetary Bodies| isbn=978-94-007-4966-5| page=54| chapter=SETI: Assessing Imaginative Proposals| year=2012| publisher=Springer}}</ref><ref>{{Cite web|url=https://www.vice.com/en_us/article/jpgvxx/the-language-of-aliens-will-always-be-indecipherable|title=The Language of Aliens Will Always Be Indecipherable|last=Istvan|first=Zoltan|date=2016-08-26|website=[[Company:Vice Media|Vice Media]]|language=en-US|archive-url=https://web.archive.org/web/20190717040808/https://www.vice.com/en_us/article/jpgvxx/the-language-of-aliens-will-always-be-indecipherable|archive-date=2019-07-17|access-date=2019-07-17}}</ref>

In his 2009 book, SETI scientist [[Biography:Seth Shostak|Seth Shostak]] wrote, "Our experiments [such as plans to use drilling rigs on Mars] are still looking for the type of extraterrestrial that would have appealed to [[Biography:Percival Lowell|Percival Lowell]] [astronomer who believed he had observed canals on Mars]."<ref>[https://books.google.com/books?id=jFcJHatXiWIC&dq=%22Our+experiments+are+still+looking+for+the+type+of+extraterrestrial+that+would+have+appealed+to+Percival+Lowell%22&pg=PT212 Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence], Seth Shostak (Senior Astronomer, SETI Institute), Ch. 7 "Beyond Gray and Hairless," p. 264, published by National Geographic, 2009.</ref>

[[Biography:Paul Davies|Paul Davies]] states that 500 years ago the very idea of a computer doing work merely by manipulating internal data may not have been viewed as a technology at all. He writes, "Might there be a still {{em|higher}} level{{Nbsp}}[...] If so, this 'third level' would never be manifest through observations made at the informational level, still less the matter level. There is no vocabulary to describe the third level, but that doesn't mean it is non-existent, and we need to be open to the possibility that alien technology may operate at the third level, or maybe the fourth, fifth{{Nbsp}}[...] levels."<ref>[https://archive.org/details/eeriesilencerene00davi/page/145 <!-- quote="Might there be a still higher level" inauthor:Davies. --> ''The Eerie Silence: Renewing Our Search for Alien Intelligenc''e], Paul Davies (Beyond Center for Fundamental Concepts in Science, Arizona State University), Boston, New York: Houghton Mifflin Harcourt, 2010, pp. 144–145.</ref>

[[Biography:Arthur C. Clarke|Arthur C. Clarke]] hypothesized that "our technology must still be laughably primitive; we may well be like jungle savages listening for the throbbing of tom-toms, while the ether around them carries more words per second than they could utter in a lifetime".<ref>{{cite book |title=Living Philosophies The Reflections of Some Eminent Men and Women of Our Time |date=1990 |publisher=Doubleday |page=50}}</ref>

=== Sociological explanations ===

==== Colonization is not the cosmic norm ====
In response to Tipler's idea of self-replicating probes, Stephen Jay Gould wrote, "I must confess that I simply don't know how to react to such arguments. I have enough trouble predicting the plans and reactions of the people closest to me. I am usually baffled by the thoughts and accomplishments of humans in different cultures. I'll be damned if I can state with certainty what some extraterrestrial source of intelligence might do."<ref name="Beyond Fermi's Paradox II, Stephen Jay Gould quote, April 2015">[https://www.universetoday.com/tag/carl-sagan/ Beyond “Fermi’s Paradox” II: Questioning the Hart-Tipler Conjecture] {{Webarchive|url=https://web.archive.org/web/20190322205106/https://www.universetoday.com/tag/carl-sagan/ |date=March 22, 2019 }} (middle of page), ''Universe Today'', April 8, 2015.</ref><ref name="If the Universe Is Teeming . . . 75 Solutions, Stephen Webb, 2015">''If the Universe Is Teeming...'', Stephen Webb, [https://books.google.com/books?id=Y111CQAAQBAJ&pg=PA28 p. 28].</ref>

==== Alien species may have only settled part of the galaxy ====
A February 2019 article in ''Popular Science'' states, "Sweeping across the Milky Way and establishing a unified galactic empire might be inevitable for a monolithic super-civilization, but most cultures are neither monolithic nor super—at least if our experience is any guide."<ref name="Struggling and hustling, just like us, Popular Science, Feb. 2019"/> Astrophysicist Adam Frank, along with co-authors such as astronomer Jason Wright, ran a variety of simulations in which they varied such factors as settlement lifespans, fractions of suitable planets, and recharge times between launches. They found many of their simulations seemingly resulted in a "third category" in which the Milky Way remains partially settled indefinitely.<ref name="Struggling and hustling, just like us, Popular Science, Feb. 2019">{{cite web|url=https://www.popsci.com/where-are-aliens-new-model |title=Where are all the aliens? Struggling and hustling, just like us |archive-url=https://web.archive.org/web/20190222214111/https://www.popsci.com/where-are-aliens-new-model |archive-date=February 22, 2019 |url-status=live |work=Popular Science |first=Charlie |last=Wood |date=February 22, 2019}}</ref> The abstract to their 2019 paper states, "These results break the link between Hart's famous 'Fact A' (no interstellar visitors on Earth now) and the conclusion that humans must, therefore, be the only technological civilization in the galaxy. Explicitly, our solutions admit situations where our current circumstances are consistent with an otherwise settled, steady-state galaxy."<ref>{{cite journal |last1=Carrol-Nellenback |first1=Jonathan |last2=Frank |first2=Adam |last3=Wright |first3=Jason |last4=Scharf |first4=Caleb |title=The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion, and Steady States |journal=The Astronomical Journal |year=2019 |publication-date=August 20, 2019 |volume=158 |issue=3 |page=117 |doi=10.3847/1538-3881/ab31a3 |arxiv=1902.04450 |bibcode=2019AJ....158..117C |s2cid=119185080 |doi-access=free }}</ref>

An alternative scenario is that long-lived civilizations may only choose to colonize stars during closest approach. As low mass [[Astronomy:K-type main-sequence star|K-]] and M-type dwarfs are by far the most common types of main sequence stars in the Milky Way, they are more likely to pass close to existing civilizations. These stars have longer life spans, which may be preferred by such a civilization. Interstellar travel capability of 0.3 light years is theoretically sufficient to colonize all M-dwarfs in the galaxy within 2 billion years. If the travel capability is increased to 2 light years, then all K-dwarfs can be colonized in the same time frame.<ref>{{cite journal | title=Galactic Settlement of Low-mass Stars as a Resolution to the Fermi Paradox | last1=Haqq-Misra | first1=Jacob | last2=Fauchez | first2=Thomas J. | journal=The Astronomical Journal | volume=164 | issue=6 | id=247 | date=December 2022 | page=247 | doi=10.3847/1538-3881/ac9afd | arxiv=2210.10656 | bibcode=2022AJ....164..247H | s2cid=252992620 | doi-access=free }}</ref>

==== Alien species may not live on planets ====
Some colonization scenarios predict spherical expansion across star systems, with continued expansion coming from the systems just previously settled. It has been suggested that this would cause a strong [[Biology:Natural selection|selection]] process among the colonization front favoring cultural or biological [[Biology:Adaptation|adaptation]]s to living in starships or space habitats. As a result, they may forgo living on planets.<ref name="David Brin, 1983, abandonment of planet-dwelling"/>

This may result in the destruction of terrestrial planets in these systems for use as building materials, thus preventing the development of life on those worlds. Or, they may have an ethic of protection for "nursery worlds", and protect them in a similar fashion to the [[Astronomy:Zoo hypothesis|zoo hypothesis]].<ref name="David Brin, 1983, abandonment of planet-dwelling">"The Great Silence: the Controversy Concerning Extraterrestrial Intelligent Life" (15-page paper), ''Quarterly J. Royal Astron. Soc.,'' David Brin, 1983, [http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000300.000.html p. 300 " …. abandonment of planet-dwelling …. "] {{Webarchive|url=https://web.archive.org/web/20190406165950/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000300.000.html |date=April 6, 2019 }}.</ref>

==== Alien species may isolate themselves from the outside world ====
It has been suggested that some advanced beings may divest themselves of physical form, create massive artificial virtual environments, transfer themselves into these environments through [[Philosophy:Mind uploading|mind uploading]], and exist totally within virtual worlds, ignoring the external physical universe.<ref>{{cite magazine| last1=Bostrom| first1=Nick| title=Where Are They?|url=https://www.technologyreview.com/2008/04/22/220999/where-are-they/| magazine=MIT Technology Review| access-date=October 5, 2020| date=April 22, 2008}}</ref>

It may also be that intelligent alien life develops an "increasing disinterest" in their outside world.<ref name=webb02/>{{rp|86}} Possibly any sufficiently advanced society will develop highly engaging media and entertainment well before the capacity for advanced space travel, with the rate of appeal of these social contrivances being destined, because of their inherent reduced complexity, to overtake any desire for complex, expensive endeavors such as space exploration and communication. Once any sufficiently advanced civilization becomes able to master its environment, and most of its physical needs are met through technology, various "social and entertainment technologies", including virtual reality, are postulated to become the primary drivers and motivations of that civilization.<ref>{{cite book |last=Webb |first=Stephen |title=If the Universe Is Teeming with Aliens... Where Is Everybody? Seventy five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life |edition=2nd |publisher=Copernicus Books |date=2015 |isbn=978-3-319-13235-8 |url=https://books.google.com/books?id=QWKyrQEACAAJ |access-date=July 21, 2015 |archive-url=https://web.archive.org/web/20150903224843/https://books.google.com/books?id=QWKyrQEACAAJ |archive-date=September 3, 2015 |url-status=live }} Chapter 15: "They Stay at Home and Surf the Web"</ref>

=== Economic explanations ===

==== Lack of resources needed to physically spread throughout the galaxy ====
The ability of an alien culture to colonize other star systems is based on the idea that interstellar travel is technologically feasible. While the current understanding of physics rules out the possibility of [[Physics:Faster-than-light|faster-than-light]] travel, it appears that there are no major theoretical barriers to the construction of "slow" interstellar ships, even though the engineering required is considerably beyond present human capabilities. This idea underlies the concept of the Von Neumann probe and the Bracewell probe as a potential evidence of extraterrestrial intelligence.

It is possible, however, that present scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood, and the resources needed may be so great as to make it unlikely that any civilization could afford to attempt it. Even if interstellar travel and colonization are possible, they may be difficult, leading to a colonization model based on [[Percolation theory|percolation theory]].<ref name=landis>{{cite journal|author=Landis, Geoffrey|url=http://www.sff.net/people/Geoffrey.Landis/percolation.htp|title=The Fermi Paradox: An Approach Based on Percolation Theory|journal=Journal of the British Interplanetary Society|volume=51|pages=163–166|date=1998|issue=5|bibcode=1998JBIS...51..163L|access-date=June 6, 2004|archive-url=https://web.archive.org/web/20060927154115/http://www.sff.net/people/Geoffrey.Landis/percolation.htp|archive-date=September 27, 2006|url-status=live}}</ref><ref name=kinouchi>{{Cite journal| last1=Galera| first1=E.| last2=Galanti| first2=G. R.| last3=Kinouchi| first3= O.|title = Invasion Percolation Solves Fermi Paradox but Challenges SETI Projects|journal = International Journal of Astrobiology |volume= *| issue=4|pages= 316–322|date=2018| doi=10.1017/S1473550418000101| s2cid=126238563}}</ref>

Colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the expectation that colonies will inevitably develop a culture and civilization of their own. Colonization may thus occur in "clusters", with large areas remaining uncolonized at any one time.<ref name="landis" /><ref name="kinouchi" />

==== Information is cheaper to transmit than matter is to transfer ====
If a human-capability machine construct, such as via [[Philosophy:Mind uploading|mind uploading]], is possible, and if it is possible to transfer such constructs over vast distances and rebuild them on a remote machine, then it might not make strong economic sense to travel the galaxy by spaceflight. After the first civilization has physically explored or colonized the galaxy, as well as sent such machines for easy exploration, then any subsequent civilizations, after having contacted the first, may find it cheaper, faster, and easier to explore the galaxy through intelligent mind transfers to the machines built by the first civilization, which is cheaper than spaceflight by a factor of 10<sup>8</sup>–10<sup>17</sup>. However, since a star system needs only one such remote machine, and the communication is most likely highly directed, transmitted at high-frequencies, and at a minimal power to be economical, such signals would be hard to detect from Earth.<ref>{{cite journal |author=Scheffer, L.K. |date= 1994 |title=Machine Intelligence, the Cost of Interstellar Travel and Fermi's Paradox |bibcode=1994QJRAS..35..157S|journal=Quarterly Journal of the Royal Astronomical Society |volume=35 |pages= 157 }}</ref>

=== Discovery of extraterrestrial life is too difficult ===

==== Humans have not listened properly ====
There are some assumptions that underlie the SETI programs that may cause searchers to miss signals that are present. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional (in human terms) [[Physics:Frequency|frequencies]], which would make them hard to distinguish from background noise. Signals might be sent from non-[[Astronomy:Main sequence|main sequence]] star systems that humans search with lower priority; current programs assume that most alien life will be orbiting Sun-like stars.<ref>{{cite journal| last1=Turnbull| first1=Margaret C.| last2=Tarter| first2=Jill C.| doi=10.1086/345779| title=Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems| url=http://www.projectrho.com/HabCat.pdf| date=2003| pages=181–198| issue=1| volume=145| journal=The Astrophysical Journal Supplement Series| access-date=August 19, 2010| bibcode=2003ApJS..145..181T| arxiv=astro-ph/0210675| s2cid=14734094| archive-url=https://web.archive.org/web/20100614030445/http://www.projectrho.com/HabCat.pdf| archive-date=June 14, 2010| url-status=live}}</ref>

The greatest challenge is the sheer size of the radio search needed to look for signals (effectively spanning the entire observable universe), the limited amount of resources committed to SETI, and the sensitivity of modern instruments. SETI estimates, for instance, that with a radio telescope as sensitive as the [[Astronomy:Arecibo Observatory|Arecibo Observatory]], Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star. A signal is much easier to detect if it consists of a deliberate, powerful transmission directed at Earth. Such signals could be detected at ranges of hundreds to tens of thousands of light-years distance.<ref>{{cite journal |journal=Icarus |volume=26 |issue=4 |date=December 1975 |pages=462–466 |title=The Arecibo message of November, 1974 |author=The Staff at the National Astronomy and Ionosphere Center |bibcode = 1975Icar...26..462. |doi = 10.1016/0019-1035(75)90116-5 }} "A radio telescope in M13 operating at the transmission frequency, and pointed toward the Sun at the time the message arrives at the receiving site will observe a flux density from the message which will exceed the flux density of the Sun itself by a factor of roughly 10<sup>7</sup>. Indeed, at that unique time, the Sun will appear to the receptors to be by far the brightest star of the Milky Way."</ref> However, this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent. Many SETI searches assume that extraterrestrial civilizations will be broadcasting a deliberate signal, like the Arecibo message, in order to be found.

Thus, to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortunate circumstances: that the broadband radio emissions of alien radio technology are much stronger than humanity's own; that one of SETI's programs is listening to the correct frequencies from the right regions of space; or that aliens are deliberately sending focused transmissions in Earth's general direction.

==== Humans have not listened for long enough ====
Humanity's ability to detect intelligent extraterrestrial life has existed for only a very brief period—from 1937 onwards, if the invention of the [[Astronomy:Radio telescope|radio telescope]] is taken as the dividing line—and ''Homo sapiens'' is a geologically recent species. The whole period of modern human existence to date is a very brief period on a cosmological scale, and radio transmissions have only been propagated since 1895. Thus, it remains possible that human beings have neither existed long enough nor made themselves sufficiently detectable to be found by extraterrestrial intelligence.<ref>{{cite journal |author1=Seth D. Baum |author2=Jacob D. Haqq-Misra |author3=Shawn D. Domagal-Goldman |title=Would contact with extraterrestrials benefit or harm humanity? A scenario analysis |journal=Acta Astronautica |volume=68 |issue=11 |date=2011 |pages=2114–2129 |arxiv=1104.4462 |bibcode=2011AcAau..68.2114B |doi=10.1016/j.actaastro.2010.10.012 |url=http://sethbaum.com/ac/2011_ET-Scenarios.pdf |citeseerx=10.1.1.592.1341 |s2cid=16889489 |access-date=August 1, 2018 |archive-url=https://web.archive.org/web/20180721225331/http://sethbaum.com/ac/2011_ET-Scenarios.pdf |archive-date=July 21, 2018 |url-status=live }} "If ETI search for us just as we search for them, i.e. by scanning the sky at radio and optical wavelengths [...] the radiation that has been unintentionally leaking and intentionally transmitted from Earth may have already alerted any nearby ETI to our presence and may eventually alert more distant ETI. Once ETI become alerted to our presence, it will take at least as many years for us to realize that they know."</ref>

==== Intelligent life may be too far away ====
[[File:Terrestrial Planet Finder PIA04499.jpg|thumb|[[Organization:NASA|NASA]]'s conception of the Terrestrial Planet Finder]]
It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication.<ref name=webb02/>{{rp|62–71}} Sebastian von Hoerner estimated the average duration of civilization at 6,500 years and the average distance between civilizations in the Milky Way at 1,000 light years.<ref name="Hoerner"/> If two civilizations are separated by several thousand light-years, it is possible that one or both cultures may become extinct before meaningful dialogue can be established. Human searches may be able to detect their existence, but communication will remain impossible because of distance. It has been suggested that this problem might be ameliorated somewhat if contact and communication is made through a [[Astronomy:Bracewell probe|Bracewell probe]]. In this case at least one partner in the exchange may obtain meaningful information. Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present,<ref>{{cite web|author=Vakoch, Douglas |title=Decoding E.T.: Ancient Tongues Point Way To Learning Alien Languages |publisher=SETI Institute |date=November 15, 2001 |access-date=August 19, 2010 |url=http://www.space.com/searchforlife/seti_decode_011115.html |url-status=dead |archive-url=https://web.archive.org/web/20090523030456/https://www.space.com/searchforlife/seti_decode_011115.html |archive-date=May 23, 2009 }}</ref> and humanity has undertaken similar activities like the [[Astronomy:Arecibo message|Arecibo message]], which could transfer information about Earth's intelligent species, even if it never yields a response or does not yield a response in time for humanity to receive it. It is possible that observational signatures of self-destroyed civilizations could be detected, depending on the destruction scenario and the timing of human observation relative to it.<ref>{{cite journal |author1=Adam Stevens |author2=Duncan Forgan |author3=Jack O'Malley James |date=2015 |title=Observational Signatures of Self–Destructive Civilisations |journal=International Journal of Astrobiology |volume=15 |issue=4 |pages=333–344 |doi=10.1017/S1473550415000397 |arxiv=1507.08530 |s2cid=118428874 }}</ref>

A related speculation by Sagan and Newman suggests that if other civilizations exist, and are transmitting and exploring, their signals and probes simply have not arrived yet.<ref>{{cite journal |author1=Newman, W.T. |author2=Sagan, C. |date=1981 |title=Galactic civilizations: Population. dynamics and interstellar diffusion |journal=Icarus |volume=46 |issue=3 |pages=293–327 |doi=10.1016/0019-1035(81)90135-4 |bibcode=1981Icar...46..293N|hdl=2060/19790011801 |hdl-access=free }}</ref> However, critics have noted that this is unlikely, since it requires that humanity's advancement has occurred at a very special point in time, while the Milky Way is in transition from empty to full. This is a tiny fraction of the lifespan of a galaxy under ordinary assumptions, so the likelihood that humanity is in the midst of this transition is considered low in the paradox.<ref name="David Brin, 1983, pages 287 and 298">"The Great Silence: the Controversy . . " (15-page paper), ''Quart. Journ. Royal Astronomical Soc.,'' David Brin, 1983, [http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000287.000.html page 287, sixth paragraph, "Equilibrium is another concept which weaves through the new SETI debate … . "] {{Webarchive|url=https://web.archive.org/web/20190411223152/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000287.000.html |date=April 11, 2019 }}, as well as [http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000298.000.html page 298, third paragraph, "Newman & Sagan ('''4''') have suggested that population pressure is not . . . "] {{Webarchive|url=https://web.archive.org/web/20190411223717/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000298.000.html |date=April 11, 2019 }}.</ref>

Some SETI skeptics may also believe that humanity is at a very special point of time. Specifically, a transitional period from no space-faring societies to one space-faring society, namely that of human beings.<ref name="David Brin, 1983, pages 287 and 298"/>

==== Intelligent life may exist hidden from view ====
Planetary scientist Alan Stern put forward the idea that there could be a number of worlds with subsurface oceans (such as Jupiter's [[Astronomy:Europa (moon)|Europa]] or Saturn's [[Astronomy:Enceladus|Enceladus]]). The surface would provide a large degree of protection from such things as cometary impacts and nearby supernovae, as well as creating a situation in which a much broader range of orbits are acceptable. Life, and potentially intelligence and civilization, could evolve. Stern states, "If they have technology, and let's say they're broadcasting, or they have city lights or whatever—we can't see it in any part of the spectrum, except maybe very-low-frequency [radio]."<ref>{{Cite web |url=https://www.space.com/38577-fermi-paradox-alien-life-buried-oceans.html |title=Where Are All the Intelligent Aliens? Maybe They're Trapped in Buried Oceans |website=Space.com |first=Mike |last=Wall |date=October 26, 2017}}</ref><ref>[https://ui.adsabs.harvard.edu/abs/2017DPS....4920203S/abstract An Answer to Fermi’s Paradox In the Prevalence of Ocean Worlds] {{Webarchive|url=https://web.archive.org/web/20191221201926/https://ui.adsabs.harvard.edu/abs/2017DPS....4920203S/abstract |date=December 21, 2019 }}, S. Alan Stern, American Astronomical Society, Division for Planetary Sciences Meeting Abstracts #49, October 2017. "... We suggest another—namely that the great majority of worlds with biology and civilizations are interior water ocean worlds (WOWs)..."</ref>

==== Advanced civilizations may limit their search for life to technological signatures ====
If life is abundant in the universe but the cost of space travel is high, an advanced civilization may choose to focus its search not on signs of life in general, but on those of other advanced civilizations, and specifically on [[Social:Radio|radio]] signals. Since humanity has [[History:History of radio|only recently]] began to use radio communication, its signals may have yet to arrive to other inhabited planets, and if they have, [[Engineering:Space probe|probes]] from those planets may have yet to arrive on Earth.<ref>{{Cite journal |last=Wandel |first=Amri |date=2022-12-01 |title=The Fermi Paradox Revisited: Technosignatures and the Contact Era |journal=The Astrophysical Journal |volume=941 |issue=2 |pages=184 |doi=10.3847/1538-4357/ac9e00 |arxiv=2211.16505 |bibcode=2022ApJ...941..184W |s2cid=254096277 |issn=0004-637X|doi-access=free }}</ref>

=== Willingness to communicate ===

==== Everyone is listening but no one is transmitting ====
Alien civilizations might be technically capable of contacting Earth, but could be only listening instead of transmitting.<ref name=webbwhere>{{cite book| last1=Webb| first1=Stephen| title=If the Universe Is Teeming with Aliens … WHERE IS EVERYBODY?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life| isbn=978-0-387-95501-8|url=https://books.google.com/books?id=Y111CQAAQBAJ&q=fermi+paradox+everybody+listening+nobody+sending&pg=PA148| access-date=June 21, 2015| date=2015| publisher=Springer}}</ref> If all or most civilizations act in the same way, the galaxy could be full of civilizations eager for contact, but everyone is listening and no one is transmitting. This is the so-called ''SETI Paradox''.<ref>{{cite arXiv |eprint=physics/0611283 |title=The SETI paradox |author=Alexander Zaitsev |date=2006}}</ref>

The only civilization known, humanity, does not [[Astronomy:Active SETI|explicitly transmit]], except for a few small efforts.<ref name=webbwhere /> Even these efforts, and certainly any attempt to expand them, are controversial.<ref>{{cite news |title=Should We Call the Cosmos Seeking ET? Or Is That Risky? |url=https://www.nytimes.com/aponline/2015/02/13/science/ap-us-sci-calling-the-cosmos.html |newspaper=The New York Times |author=The Associated Press |date=February 13, 2015 |access-date=March 1, 2017 |archive-url=https://web.archive.org/web/20150906144552/http://www.nytimes.com/aponline/2015/02/13/science/ap-us-sci-calling-the-cosmos.html |archive-date=September 6, 2015 |url-status=dead }}</ref> It is not even clear humanity would respond to a detected signal—the official policy within the SETI community<ref>{{cite web |url=http://www.seti.org/post-detection.html |title=Protocols for an ETI Signal Detection: Concerning Activities Following the Detection of Extraterrestrial Intelligence |publisher=SETI Institute |access-date=July 12, 2015 |archive-url=https://web.archive.org/web/20150718122415/http://www.seti.org/post-detection.html |archive-date=July 18, 2015 |url-status=dead }}</ref> is that "[no] response to a signal or other evidence of extraterrestrial intelligence should be sent until appropriate international consultations have taken place". However, given the possible impact of any reply,<ref>{{cite journal | last1 = Michaud | first1 = M. | title = Ten decisions that could shake the world | doi = 10.1016/S0265-9646(03)00019-5 | journal = Space Policy | volume = 19 | issue = 2 | pages = 131–950 | year = 2003 | bibcode = 2003SpPol..19..131M }}</ref> it may be very difficult to obtain any consensus on who would speak and what they would say.

==== Communication is dangerous ====
An alien civilization might feel it is too dangerous to communicate, either for humanity or for them. It is argued that when very different civilizations have met on Earth, the results have often been disastrous for one side or the other, and the same may well apply to interstellar contact.<ref>{{cite web |url=https://opinionator.blogs.nytimes.com/2011/10/05/will-the-aliens-be-nice-dont-bet-on-it/ |title=Will the Aliens Be Nice? Don't Bet On It |first=Gary |last=Gutting |date=October 5, 2011 |work={{wipe|The New York Times}} |access-date=May 29, 2020 |archive-url=https://web.archive.org/web/20191001065131/https://opinionator.blogs.nytimes.com/2011/10/05/will-the-aliens-be-nice-dont-bet-on-it/ |archive-date=October 1, 2019 |url-status=live }}</ref> Even contact at a safe distance could lead to infection by computer code<ref>{{cite journal|title=Do potential SETI signals need to be decontaminated? |last=Carrigan |first=Richard A. |journal=Acta Astronautica |volume=58 |issue=2 |pages=112–117|date=2006 |doi=10.1016/j.actaastro.2005.05.004 |bibcode=2006AcAau..58..112C }}</ref> or even ideas themselves.<ref>{{cite journal| last=Marsden| first=P.| title=Memetics and social contagion: Two sides of the same coin| journal=Journal of Memetics-Evolutionary Models of Information Transmission| date=1998| volume=2| issue=2| pages=171–185| url=http://cfpm.org/jom-emit/1998/vol2/marsden_p.html| access-date=October 20, 2011| archive-url=https://web.archive.org/web/20111012004902/http://cfpm.org/jom-emit/1998/vol2/marsden_p.html| archive-date=October 12, 2011| url-status=live}}</ref> Perhaps prudent civilizations actively hide not only from Earth but from everyone, out of fear of other civilizations.<ref name="HawkingFearTheologyAndScience">{{cite journal |last1=A. Vakoch |first1=Douglas |title=Hawking's fear of an alien invasion may explain the Fermi Paradox |journal=Theology and Science |date=3 April 2017 |volume=15 |issue=2 |pages=134–138 |doi=10.1080/14746700.2017.1299380 |s2cid=219627161 |url=https://www.tandfonline.com/doi/pdf/10.1080/14746700.2017.1299380 |access-date=18 October 2022}}</ref>

Perhaps the Fermi paradox itself—or the alien equivalent of it—is the reason for any civilization to avoid contact with other civilizations, even if no other obstacles existed. From any one civilization's point of view, it would be unlikely for them to be the first ones to make first contact. Therefore, according to this reasoning, it is likely that previous civilizations faced fatal problems with first contact and doing so should be avoided. So perhaps every civilization keeps quiet because of the possibility that there is a real reason for others to do so.<ref name="Brin" />

In Liu Cixin's 2008 novel ''The Dark Forest'', the author proposes a literary explanation for the Fermi paradox in which many multiple alien civilizations exist, but are both silent and paranoid, destroying any nascent lifeforms loud enough to make themselves known.<ref name="ChinaFileCixinLiu">{{cite news |author=Kun Kun |others=Translated by Lucy Johnston |title=But Some of Us Are Looking at the Stars |url=https://www.chinafile.com/reporting-opinion/culture/some-us-are-looking-stars |access-date=18 October 2022 |work=ChinaFile |date=4 June 2012 |language=en}}</ref> This is because any other intelligent life may represent a future threat. As a result, Liu's fictional universe contains a plethora of quiet civilizations which do not reveal themselves, as in a "dark forest"...filled with "armed hunter(s) stalking through the trees like a ghost".<ref>{{cite book |last1=Liu |first1=Cixin |title=The dark forest |date=2015 |location=New York |isbn=9780765377081 |page=484 |edition=First}}</ref><ref name="Dark Forest Answer to Star Wars Optimism">{{Cite web |url=https://www.discovermagazine.com/the-sciences/chinas-dark-forest-answer-to-star-wars-optimism |title=China's 'Dark Forest' Answer to 'Star Wars' Optimism |work=Discover Magazine |first=Jeremy |last=Hsu |date=October 31, 2015}}</ref><ref name="Technology's dark forest">{{Cite web |url=https://techcrunch.com/2019/01/20/technologys-dark-forest/ |title=Technology's dark forest |work=TechCrunch |first=Jon |last=Evans |date=January 20, 2019}}</ref> This idea has come to be known as the [[Astronomy:Dark forest hypothesis|dark forest hypothesis]].<ref name="UnivTodayDarkForest">{{cite news |last1=Williams |first1=Matt |title=Beyond "Fermi's Paradox" XVI: What is the "Dark Forest" Hypothesis? |url=https://www.universetoday.com/149410/beyond-fermis-paradox-xvi-what-is-the-dark-forest-hypothesis/ |access-date=18 October 2022 |work=Universe Today |date=7 January 2021}}</ref><ref name="NHPR">{{cite news |last1=Paradis |first1=Justine |title=Outside/In[box]: What is the Dark Forest Theory? |url=https://www.nhpr.org/environment/2022-02-18/what-is-the-dark-forest-theory-outside-in |access-date=18 October 2022 |work=New Hampshire Public Radio |date=18 February 2022 |language=en}}</ref><ref>{{cite news |last1=Kevra |first1=Derek |title=Dark Forest theory: should we try to contact aliens? |url=https://www.fox2detroit.com/news/dark-forest-theory-should-we-try-to-contact-aliens |access-date=18 October 2022 |work=FOX 2 Detroit |date=11 October 2022}}</ref>

==== Earth is deliberately being avoided ====
{{main|Astronomy:Zoo hypothesis}}
The [[Astronomy:Zoo hypothesis|zoo hypothesis]] states that intelligent extraterrestrial life exists and does not contact life on Earth to allow for its natural evolution and development.<ref name=":1">{{cite journal| last1=Ball| first1=J| title=The zoo hypothesis| journal=Icarus| volume=19|issue=3|pages=347–349|date=1973| doi=10.1016/0019-1035(73)90111-5| bibcode=1973Icar...19..347B}}</ref> A variation on the zoo hypothesis is the laboratory hypothesis, where humanity has been or is being subject to experiments,<ref name=":1" /><ref name=":0" /> with Earth or the Solar System effectively serving as a laboratory. The zoo hypothesis may break down under the [[Biology:Uniformity of motive|uniformity of motive]] flaw: all it takes is a single culture or civilization to decide to act contrary to the imperative within humanity's range of detection for it to be abrogated, and the probability of such a violation of hegemony increases with the number of civilizations,<ref name=cr /><ref>{{Cite journal|last=Forgan|first=Duncan H.|date=2011-06-08|title=Spatio-temporal constraints on the zoo hypothesis, and the breakdown of total hegemony|journal=International Journal of Astrobiology|volume=10|issue=4|pages=341–347|doi=10.1017/s147355041100019x|arxiv=1105.2497|bibcode=2011IJAsB..10..341F|s2cid=118431252|issn=1473-5504}}</ref> tending not towards a 'Galactic Club' with a unified foreign policy with regard to life on Earth but multiple 'Galactic Cliques'.<ref>{{Cite journal|last=Forgan|first=Duncan H.|date=2016-11-28|title=The Galactic Club or Galactic Cliques? Exploring the limits of interstellar hegemony and the Zoo Hypothesis|journal=International Journal of Astrobiology|volume=16|issue=4|pages=349–354|doi=10.1017/s1473550416000392|hdl=10023/10869|s2cid=59041278|issn=1473-5504|hdl-access=free}}</ref> However, if artificial superintelligences dominate galactic life, and if it is true that such intelligences tend towards merged hegemonic behavior, then this would address the uniformity of motive flaw by dissuading rogue behavior.<ref>{{Cite journal |last=Visscher |first=Alex De |date=2020 |title=Artificial versus biological intelligence in the Cosmos: clues from a stochastic analysis of the Drake equation |url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/artificial-versus-biological-intelligence-in-the-cosmos-clues-from-a-stochastic-analysis-of-the-drake-equation/7BC4932CD94887E338B2DA55488DE8C6 |journal=International Journal of Astrobiology |language=en |volume=19 |issue=5 |pages=353–359 |doi=10.1017/S1473550420000129 |arxiv=2001.11644 |bibcode=2020IJAsB..19..353D |s2cid=211003646 |issn=1473-5504}}</ref>

Analysis of the inter-arrival times between civilizations in the galaxy based on common astrobiological assumptions suggests that the initial civilization would have a commanding lead over the later arrivals. As such, it may have established what has been termed the ''zoo hypothesis'' through force or as a galactic or universal norm and the resultant "paradox" by a cultural [[Biology:Founder effect|founder effect]] with or without the continued activity of the founder.<ref>{{cite journal |last1=Hair |first1=Thomas W. |title=Temporal dispersion of the emergence of intelligence: an inter-arrival time analysis |journal=International Journal of Astrobiology |date=February 25, 2011 |volume=10 |issue=2 |pages=131–135 |doi=10.1017/S1473550411000024|bibcode=2011IJAsB..10..131H |s2cid=53681377 }}</ref>

It is possible that a civilization advanced enough to travel between solar systems could be actively visiting or observing Earth while remaining undetected or unrecognized.<ref>{{cite journal |title=What Role Will Extraterrestrials Play in Humanity's Future? |author=Tough, Allen |journal=Journal of the British Interplanetary Society |volume=39 |issue=11 |pages=492–498 |year=1986 |url=http://ww.w.ieti.org/articles/future.pdf |bibcode=1986JBIS...39..491T |access-date=June 27, 2015 |archive-url=https://web.archive.org/web/20150630051853/http://ww.w.ieti.org/articles/future.pdf |archive-date=June 30, 2015 |url-status=live }}</ref>

==== Earth is deliberately being isolated ====
{{Main|Astronomy:Planetarium hypothesis}}
A related idea to the zoo hypothesis is that, beyond a certain distance, the perceived universe is a [[Philosophy:Simulated reality|simulated reality]]. The planetarium hypothesis<ref>{{cite journal|author=Baxter, Stephen|date=2001 |title=The Planetarium Hypothesis: A Resolution of the Fermi Paradox| journal=Journal of the British Interplanetary Society| volume= 54|issue=5/6|pages=210–216|bibcode = 2001JBIS...54..210B }}</ref> speculates that beings may have created this simulation so that the universe appears to be empty of other life.

=== Alien life is already here, unacknowledged ===
{{Main|Unsolved:Extraterrestrial UFO hypothesis}}
{{Further|Unsolved:UFO conspiracy theories}} 
A significant fraction of the population believes that at least some UFOs (Unidentified Flying Objects) are spacecraft piloted by aliens.<ref>{{cite web |url=http://news.discovery.com/space/why-do-people-believe-in-ufos-120810.htm |title=Why Do People Believe in UFOs? |date=August 10, 2012 |author=Ray Villard |publisher=Discovery News |access-date=March 18, 2016 |archive-url=https://web.archive.org/web/20160328235420/http://news.discovery.com/space/why-do-people-believe-in-ufos-120810.htm |archive-date=March 28, 2016 |url-status=live }}</ref><ref>{{cite news |url=http://www.huffingtonpost.com/2012/10/15/alien-believers-outnumber-god_n_1968259.html |title=More Believe in Space Aliens Than in God According To U.K. Survey |author=Paul Speigel |work=Huffington Post |date=October 18, 2012 |access-date=April 8, 2017 |archive-url=https://web.archive.org/web/20170409112145/http://www.huffingtonpost.com/2012/10/15/alien-believers-outnumber-god_n_1968259.html |archive-date=April 9, 2017 |url-status=live }}</ref> While most of these are unrecognized or mistaken interpretations of mundane phenomena, some occurrences remain puzzling even after investigation. The consensus scientific view is that although they may be unexplained, they do not rise to the level of convincing evidence.<ref>{{cite journal |title=UFOs, UAPs and CRAPs |author=Shermer, Michael |journal=Scientific American |volume=304 |issue=4 |pages=90 |year=2011 |doi=10.1038/scientificamerican0411-90|pmid=21495489 |bibcode = 2011SciAm.304d..90S }}</ref>

Similarly, it is theoretically possible that SETI groups are not reporting positive detections, or governments have been blocking signals or suppressing publication. This response might be attributed to security or economic interests from the potential use of advanced extraterrestrial technology. It has been suggested that the detection of an extraterrestrial radio signal or technology could well be the most highly secret information that exists.<ref>{{cite journal |author=A. Tough |title=A critical examination of factors that might encourage secrecy |journal=Acta Astronautica |issue=2 |date=1990 |pages=97–102 |doi=10.1016/0094-5765(90)90134-7 |volume=21|bibcode = 1990AcAau..21...97T }}</ref> Claims that this has already happened are common in the popular press,<ref>{{cite web |url=https://www.theregister.co.uk/2006/07/31/signals_seti/ |title=SETI urged to fess up over alien signals |website=The Register |author=Ashlee Vance |date=July 31, 2006 |access-date=August 10, 2017 |archive-url=https://web.archive.org/web/20070402080142/https://www.theregister.co.uk/2006/07/31/signals_seti/ |archive-date=April 2, 2007 |url-status=live }}</ref><ref>{{cite news |url=http://www.huffingtonpost.com/2011/12/06/new-round-of-ufo-white-house-fight_n_1125873.html |title=UFO Hunters Keep Pressing White House For Answers Through 'We The People' Petitions |work=The Huffington Post |date=December 6, 2011 |first=Lee |last=Speigel |access-date=April 16, 2013 |archive-url=https://web.archive.org/web/20130415232659/http://www.huffingtonpost.com/2011/12/06/new-round-of-ufo-white-house-fight_n_1125873.html |archive-date=April 15, 2013 |url-status=live }}</ref> but the scientists involved report the opposite experience—the press becomes informed and interested in a potential detection even before a signal can be confirmed.<ref>{{cite book |author=G. Seth Shostak |title=Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence |publisher=National Geographic |isbn=978-1-4262-0392-3 |date=2009 |page=[https://archive.org/details/confessionsofali00shos/page/17 17] |url-access=registration |url=https://archive.org/details/confessionsofali00shos/page/17 }}</ref>

Regarding the idea that aliens are in secret contact with governments, David Brin writes, "Aversion to an idea, simply because of its long association with crackpots, gives crackpots altogether too much influence."<ref>"The Great Silence: the Controversy . . " (15-page paper), ''Quarterly J. Royal Astron. Soc.,'' David Brin, 1983, [http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000299.000.html p. 299 bottom] {{Webarchive|url=https://web.archive.org/web/20190411223152/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000299.000.html |date=April 11, 2019 }}.</ref>

== See also ==
{{div col|colwidth=30em}}
* {{annotated link|Astronomy:Aestivation hypothesis}}
* {{annotated link|Astronomy:Anthropic principle}}
* {{annotated link|Astronomy:Astrobiology}}
* ''Calculating God'' — 2000 novel by Robert J. Sawyer
* {{annotated link|Fermi problem}}
* {{annotated link|Engineering:Interstellar travel}}
* {{annotated link|Biology:Panspermia}}
* {{annotated link|Astronomy:Quiet and loud aliens}}
* {{annotated link|Astronomy:Rare Earth hypothesis}}
* {{annotated link|Astronomy:Wow! signal}}
{{div col end}}

==Notes==
{{reflist|group=note}}

== References ==
{{Reflist}}

== Further reading ==
{{Library resources box|onlinebooks=yes}}
* {{Cite web |author=Rebecca Boyle and Quanta Magazine |date=March 10, 2019 |url=https://www.theatlantic.com/science/archive/2019/03/star-movement-fermi-paradox-alien-intelligence/584518/ |title=Moving Stars Might Speed the Spread of Alien Life |work=The Atlantic}}
* Ćirković, Milan [http://nautil.us/issue/63/horizons/our-attitude-toward-aliens-proves-we-still-think-were-special] {{Webarchive|url=https://web.archive.org/web/20210505080417/http://nautil.us/issue/63/horizons/our-attitude-toward-aliens-proves-we-still-think-were-special |date=May 5, 2021 }} Why we downplay Fermi's paradox. Nautilus
* Ćirković, Milan [https://global.oup.com/academic/product/the-great-silence-9780199646302?cc=gb&lang=en& The Great Silence: Science and Philosophy of Fermi's Paradox] The Great Silence [[Organization:Oxford University Press|Oxford University Press]]
* {{cite book |title=The Extraterrestrial Life Debate, Antiquity to 1915 |last=Crowe |first=Michael J. |publisher=University of Notre Dame Press |date=2008 |isbn=978-0-268-02368-3 }}
* {{Cite book |last=Forgan |first=Duncan H. |year=2019 |title=Solving Fermi's Paradox |publisher=Cambridge University Press |isbn=9781107163652}}
* {{cite book|last = Michaud|first = Michael|title = Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials|publisher = Copernicus Books|date = 2006|isbn = 978-0-387-28598-6|url-access = registration|url = https://archive.org/details/contactwithalien0000mich}}
* {{cite book|author1=Zuckerman, Ben |author2=Hart, Michael H. |title= Extraterrestrials: Where Are They? |isbn=978-0-521-44803-1 |year= 1995 |publisher=Cambridge University Press}}

== External links ==
* Kestenbaum, David. [https://www.thisamericanlife.org/617/fermis-paradox "Three people grapple with the question, 'Are we alone?'"], ''This American Life'' radio show, hosted by Ira Glass. This episode's first 22 minutes discusses the Fermi Paradox. See also the show's [https://www.thisamericanlife.org/617/transcript May 19, 2017 transcript].
* {{cite book|url=http://www.setileague.org/press/silence.htm |edition=Translation of the documentary |title=Overcome the Great Silence|translator= [[Biography:Aleksandr Leonidovich Zaitsev|Leonidovich Zaitsev, Aleksandr]]}}
* [https://www.youtube.com/watch?v=sNhhvQGsMEc The Fermi Paradox — Where Are All The Aliens? (2015), Kurzgesagt – In a Nutshell]
* [https://www.ted.com/talks/stephen_webb_where_are_all_the_aliens Webb, Stephen (video; 13:09): ''Where Are All The Aliens?''] ([[Software:TED (conference)|TED talk – 2018]]) ([https://www.ted.com/talks/stephen_webb_where_are_all_the_aliens/transcript transcript])
* {{YouTube|qaIghx4QRN4|Webb, Stephen (video; 13:18): ''Where Are All The Aliens?''}} ([[Software:TED (conference)|TED Talk – 2018]])
{{Astrobiology}}
{{Extraterrestrial life}}
{{Interstellar messages}}
{{subject bar|Astronomy|Stars|Spaceflight|Solar System|Science|d=y|auto=1}}

[[Category:Fermi paradox| ]]
[[Category:Astrobiology]]
[[Category:Extraterrestrial life]]
[[Category:Interstellar messages]]
[[Category:Search for extraterrestrial intelligence]]
[[Category:Unsolved problems in astronomy]]

{{Sourceattribution|Fermi paradox}}