Zoo hypothesis

<h2 id="assumptions">Assumptions</h2>
The zoo hypothesis assumes, first, that whenever the conditions are such that life can exist and evolve, it will, and secondly, there are many places where life can exist and a large number of extraterrestrial cultures in existence.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> It also assumes that these extraterrestrials have great reverence for independent, natural evolution and development. In particular, assuming that intelligence is a physical process that acts to maximize the diversity of a system's accessible futures,<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> a fundamental motivation for the zoo hypothesis would be that premature contact would "unintelligently" reduce the overall diversity of paths the universe itself could take.
These ideas are perhaps most plausible if there is a relatively universal cultural or legal policy among a plurality of extraterrestrial civilizations necessitating isolation with respect to civilizations at Earth-like stages of development. In a universe without a <a href="/facts/Hegemony/I2SBWNgu">hegemonic</a> power, random single civilizations with independent principles would make contact. This makes a crowded universe with clearly defined rules seem more plausible.<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a>
If there is a plurality of extraterrestrial cultures, however, this theory may break down under the uniformity of motive concept because it would take just a single extraterrestrial civilization, or simply a small group within any given civilisation, to decide to act contrary to the imperative within human range of detection for it to be undone, and the probability of such a violation of hegemony increases with the number of civilizations.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a><a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> This idea, however, becomes more plausible if all civilizations tend to evolve similar cultural standards and values with regard to contact much like <a href="/facts/Convergent_evolution/dd0L8z4R">convergent evolution</a> on Earth has independently evolved eyes on numerous occasions,<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a> or all civilizations follow the lead of some particularly distinguished civilization, such as the <a href="/facts/First_civilization/slYINUco">first civilization</a> among them.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a>
In this hypothesis, the problem of universal ethical homogeneity is solved because the acquisition of a persistent advanced level of civilization requires overcoming many problems, such as self-destruction, war, overpopulation, pollution, and scarcity. Managing to solve these problems could guide a civilization to adopt a responsible and wise behavior, otherwise they would disappear (involving other solutions to the Fermi paradox). In the zoo hypothesis, no contact would be possible until humanity had acquired a certain level of civilization and maturity (responsibility and wisdom), otherwise it would become a potential threat.
One estimate for when humanity might be able to test the zoo hypothesis, essentially by eliminating ways technological extraterrestrials within the galaxy may be able to hide, is some time within the next half century.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>

<h2 id="fermi-paradox">Fermi paradox</h2>
Main article: <a href="/facts/Fermi_paradox/P85yfUo0">Fermi paradox</a>
A modified zoo hypothesis is a possible solution to the Fermi paradox. The time between the emergence of the <a href="/facts/First_civilization/slYINUco">first civilization</a> within the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a> and all subsequent civilizations could be enormous. <a href="/facts/Monte_Carlo_simulation/AkHjY7jc">Monte Carlo simulation</a> shows the first few inter-arrival times between emergent civilizations would be similar in length to <a href="/facts/Epoch_(geology)/RhfBWrP2">geologic epochs</a> on Earth. The zoo hypothesis assumes a civilization may have a ten-million, one-hundred-million, or half-billion-year head start on humanity,<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a> i.e., it may have the capability to completely negate our best attempts to detect it.
The zoo hypothesis relies in part on applying the concept of <a href="/facts/Hegemony/I2SBWNgu">hegemonic power</a> to the Fermi paradox. Even if a first hegemonic <a href="/facts/Non-interventionism/MLYE6g9K">non-interventionist</a> grand civilization (<a href="/facts/Cradle_of_civilization/slYINUco">first civilization</a>) is long gone, their initial legacy could persist in the form of a passed-down tradition, or perhaps in an artificial lifeform (<a href="/facts/Artificial_superintelligence/VL1NaEPS">artificial superintelligence</a>) dedicated to a non-interventionist hegemonic goal without the risk of death. Thus, the hegemonic power does not even have to be the first civilization, but simply the first to spread its non-interventionist doctrine and control over a large volume of the galaxy. If just one civilization acquired hegemony in the distant past, it could form an unbroken chain of taboo against rapacious colonization in favour of non-interference in any civilizations that follow. The uniformity of motive concept previously mentioned would become moot in such a situation. The main problem would be how a galaxy-wide civilization would block Earth from receiving all intentional or unintentional communications.<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a>
Nonetheless, if the oldest civilization still present in the Milky Way has, for example, a 100-million-year time advantage over the next oldest civilization, then it is conceivable that they could be in the singular position of being able to control, monitor, influence or isolate the emergence of every civilization that follows within their sphere of influence. This is analogous to what happens on Earth within our own civilization on a daily basis, in that everyone born on this planet is born into a pre-existing system of familial associations, customs, traditions and laws that were already long established before our birth and which we have little or no control over.<a class="footnote-ref" id="fnref:18" href="#fn:18">18</a>

<h2 id="meti-messaging-extraterrestrial-intelligence">METI (Messaging Extraterrestrial Intelligence)</h2>
Main article: <a href="/facts/METI_(Messaging_Extraterrestrial_Intelligence)/nI5tU73a">METI (Messaging Extraterrestrial Intelligence)</a>
Overcoming the zoo hypothesis is one of the goals of METI, an organization created in 2015 to <a href="/facts/Active_SETI/FywPqWz5">communicate with extraterrestrials</a>, an active form of the search for extraterrestrials (SETI).<a class="footnote-ref" id="fnref:19" href="#fn:19">19</a> METI, however, has been criticized for not representing humanity's collective will and for potentially endangering humanity.

<h2 id="criticism">Criticism</h2>
Some critics of the hypothesis say that only a single dissident group in an extraterrestrial civilization, or alternatively the existence of galactic cliques instead of a unified galactic club, would be enough to break the pact of no contact.<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a><a class="footnote-ref" id="fnref:21" href="#fn:21">21</a> To Stephen Webb and others, it seems unlikely, taking humans and human intercivilizational politics as reference, that such prohibition would be in effect for millions of years or at least human existence without a single breach thereof.<a class="footnote-ref" id="fnref:22" href="#fn:22">22</a><a class="footnote-ref" id="fnref:23" href="#fn:23">23</a> Others say that the zoo hypothesis, along with its <a href="/facts/Planetarium_hypothesis/XNLtTYVa">planetarium</a> variation, is highly speculative and more aligned with <a href="/facts/Theology/N71Fyw2M">theological</a> theories.<a class="footnote-ref" id="fnref:24" href="#fn:24">24</a> One possible counterargument to the dissident (rogue) group argument is that extraterrestrial <a href="/facts/Artificial_intelligence/lJGauQwX">artificial superintelligences</a> dominate space, including space occupied by biological intelligences; moreoever, separate artificial superintelligences are assumed to tend towards a network of merged superintelligencies, thereby dissuading rogue behaviour.<a class="footnote-ref" id="fnref:25" href="#fn:25">25</a>

<h2 id="appearance-in-fiction">Appearance in fiction</h2>
The zoo hypothesis is a common theme in science fiction.

<h3>1930s</h3>
<ul><li>1937: In <a href="/facts/Olaf_Stapledon/10z2ssgF">Olaf Stapledon's</a> 1937 novel <a href="/facts/Star_Maker/xBkGHTpB">Star Maker</a>, great care is taken by the Symbiont race to keep its existence hidden from "pre-utopian" primitives, "lest they should lose their independence of mind.” It is only when such worlds become utopian-level space travellers that the Symbionts make contact and bring the young utopia to an equal footing.</li></ul>
<h3>1950s</h3>
<ul><li>1951: <a href="/facts/Arthur_C._Clarke/2XJ6UGDB">Arthur C. Clarke</a>'s <a href="/facts/The_Sentinel_(short_story)/saBnKagy">The Sentinel</a> (first published in 1951) and its later novel adaptation <a href="/facts/2001%3a_A_Space_Odyssey_(novel)/YJj95FzF">2001: A Space Odyssey</a> (1968) feature a beacon which is activated when the human race discovers it on the moon.</li>
<li>1953: In <a href="/facts/Childhood%2527s_End/YdMMEHRM">Childhood's End</a>, a novel by <a href="/facts/Arthur_C._Clarke/2XJ6UGDB">Arthur C. Clarke</a> published in 1953, the alien cultures had been observing and registering Earth's evolution and human history for thousands (perhaps millions) of years. At the beginning of the book, when mankind is about to achieve <a href="/facts/Spaceflight/zPrnATzo">spaceflight</a>, the aliens reveal their existence and quickly end the <a href="/facts/Nuclear_arms_race/2IabKwkf">arms race</a>, <a href="/facts/Colonialism/kvqx2DaV">colonialism</a>, <a href="/facts/Racial_segregation/c7QArpIR">racial segregation</a> and the <a href="/facts/Cold_War/spJdJQ3q">Cold War</a>.</li></ul>
<h3>1960s</h3>
<ul><li>In <a href="/facts/Star_Trek/kKGkVK5y">Star Trek</a>, the <a href="/facts/United_Federation_of_Planets/Bj2r5ysg">Federation</a> (including humans) has a strict <a href="/facts/Prime_Directive/JYulD49p">Prime Directive</a> policy of nonintervention with less technologically advanced cultures which the Federation encounters. The threshold of inclusion is the independent technological development of <a href="/facts/Faster-than-light/tpFg6JNd">faster-than-light</a> propulsion. In the show's canon, the <a href="/facts/Vulcans_(Star_Trek)/VjVrVV6X">Vulcan</a> race limited their encounters to observation until Humans made their first <a href="/facts/Warp_drive_(Star_Trek)/1sczVBj8">warp</a> flight, after which they initiated <a href="/facts/Star_Trek%3a_First_Contact/cGfzKoTI">first contact</a>, indicating the practice predated the Human race's advance of this threshold. Additionally, in the episode "<a href="/facts/The_Chase_(Star_Trek%3a_The_Next_Generation)/5bpAUrxM">The Chase (TNG)</a>", a message from a first (or early) civilization is discovered, hidden in the <a href="/facts/DNA/nB89Iauz">DNA</a> of <a href="/facts/Sentience/LQwKbI6F">sentient</a> species spread across many worlds, something that could only have been fully discovered after a race had become sufficiently advanced.</li>
<li>In <a href="/facts/Hard_to_Be_a_God/uJalNRIy">Hard to Be a God</a> by <a href="/facts/Arkady_and_Boris_Strugatsky/H58yvnbF">Arkady and Boris Strugatsky</a>, the (unnamed) medieval-esque planet where the novel takes action is protected by the advanced civilization of Earth, and the observers from Earth present on the planet are forbidden to intervene and make overt contact. One of the major themes of the novel is the ethical dilemma presented by such a stance to the observers.</li></ul>
<h3>1980s</h3>
<ul><li>1986: In <a href="/facts/Speaker_for_the_Dead/yoRGC7ha">Speaker for the Dead</a> by <a href="/facts/Orson_Scott_Card/9TWKxmoW">Orson Scott Card</a>, the human xenobiologists and xenologers, biologists and anthropologists observing alien life are forbidden from giving the native species, the Pequeninos, any technology or information. When one of the xenobiologists is killed in an alien ceremony, they are forbidden to mention it. This happens again until Ender Wiggin, the main character of <a href="/facts/Ender%2527s_Game/IJXdswaa">Ender's Game</a>, explains to the Pequeninos that humans cannot partake in the ceremony because it kills them. While this is not exactly an example of the zoo hypothesis, since humanity makes contact, it is very similar and the humans seek to keep the Pequeninos ignorant of technology.</li>
<li>1987: In <a href="/facts/Julian_May/pb01Uf1a">Julian May</a>'s 1987 novel <a href="/facts/Galactic_Milieu_Series/sdjrRHg9">Intervention</a>, the five alien races of the Galactic Milieu keep Earth under surveillance, but do not intervene until humans demonstrate mental and ethical maturity through a paranormal prayer of peace.</li>
<li>1989: <a href="/facts/Iain_M._Banks/qCumP2SJ">Iain M. Banks</a>' <a href="/facts/The_State_of_the_Art/2Ml78p5X">The State of the Art</a> depicts <a href="/facts/The_Culture/4J7d9x37">the Culture</a> secretly visiting Earth and then deciding to leave it uncontacted, watching its development as a <a href="/facts/Control_group/zeT4uJ2S">control group</a>, to confirm whether their manipulations of other civilizations are ultimately for the best (the laboratory hypothesis). Other works by Banks depict the Culture (or a Culture equivalent) routinely manipulating less advanced civilizations, including pre-industrial ones (e.g., <a href="/facts/Inversions_(novel)/NAym9AED">Inversions</a>), both covertly and overtly, for philosophical or foreign policy purposes.</li>
<li>1989: <a href="/facts/Bill_Watterson/pXLVQWsS">Bill Watterson</a>'s <a href="/facts/Calvin_and_Hobbes/bQdeCLlk">Calvin and Hobbes</a> comic strip for 8 November 1989 alludes to the possibility of an ethical threshold for first contact (or at least for the prudence of first contact) in Calvin's remark "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."</li></ul>
<h3>2000s</h3>
<ul><li>2000: In <a href="/facts/Robert_J._Sawyer/YgN5SetS">Robert J. Sawyer</a>'s SF novel <a href="/facts/Calculating_God/fppUfpru">Calculating God</a> (2000), Hollus, a scientist from an advanced alien civilization, denies that her government is operating under the prime directive.</li>
<li>2003: In <a href="/facts/South_Park/1gvjuzwN">South Park</a>'s inaugural episode of season seven, "<a href="/facts/Cancelled_(South_Park)/zJHyTFrf">Cancelled</a>", aliens refrain from contacting Earth because the planet is the subject and setting of a reality television show. Unlike most variations of the zoo hypothesis where contact is not initiated in order to allow organic socioeconomic, cultural, and technological development, the aliens in this episode refrain from contact for the sole purpose of entertainment. In essence, the aliens treat all of Earth like the titular character in <a href="/facts/The_Truman_Show/U52ID1om">The Truman Show</a> in order to maintain the show's integrity.</li>
<li>2008: In the video game <a href="/facts/Spore_(2008_game)/ELCcHo0b">Spore</a>, which <a href="/facts/Simulation_video_game/Cg6yt1Mx">simulates</a> the evolution and life of species on a fictional galaxy, intelligent species in the "Space Stage" cannot contact those in previous stages, which did not <a href="/facts/World_unification/hg7DoNjA">unify</a> their planets, nor develop <a href="/facts/Spaceflight/zPrnATzo">spaceflight</a> yet. However, they are allowed to <a href="/facts/Alien_abduction/HGN2muTr">abduct</a> their citizens/members, to create <a href="/facts/Crop_circle/AxVIpLn8">crop circles</a> in their terrain and to place in their planets a tool called "<a href="/facts/Monolith_(Space_Odyssey)/LlsDI7HD">monolith</a>", which accelerates their technological evolution.</li></ul>
<h3>2010s</h3>
<ul><li>2012: In the <a href="/facts/Science_fiction/d9hE2Lwt">sci-fi</a> video game <a href="/facts/Star_Citizen/bBqHWpFS">Star Citizen</a>, the zoo hypothesis is vaguely referenced in a lore point and is referred to as the Fair Chance Act. In the document, humans are generally forbidden from <a href="/facts/Terraforming/ny4dMbl1">terraforming</a>, mining, and inhabiting planets if the world is found to harbor lifeforms capable of developing intelligence. Multiple <a href="/facts/Planetary_system/6mcVtrTC">planetary systems</a> are planned to be implemented as the game's active development continues that will feature planets protected under the Fair Chance Act.</li>
<li>2016: In the video game <a href="/facts/Stellaris_(video_game)/BeTnYy3L">Stellaris</a>, players control an interstellar empire that can encounter less technologically advanced, non-space faring civilizations. Depending on player choices and their empire's organization, they can observe such "pre <a href="/facts/Faster-than-light/tpFg6JNd">faster-than-light</a>" planets in a manner similar to the zoo hypothesis, using science stations with missions that can include passive observation, technological enlightenment, covert infiltration and <a href="/facts/Indoctrination/v1Va4ner">indoctrination</a>. Players can also discover "pre-sapient" species, which can be uplifted to <a href="/facts/Sentience/LQwKbI6F">sentience</a> using scientific research projects.</li></ul>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Uncontacted_peoples/831fBt1w">Uncontacted peoples</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Shostak, Seth (31 March 2019). "'Zoo hypothesis' may explain why we haven't seen any space aliens". NBC News. Retrieved 1 April 2019. <a href="https://www.nbcnews.com/mach/science/zoo-hypothesis-may-explain-why-we-haven-t-seen-any-ncna988946" target="_blank">https://www.nbcnews.com/mach/science/zoo-hypothesis-may-explain-why-we-haven-t-seen-any-ncna988946</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Crawford, Ian A.; Schulze-Makuch, Dirk (2024). "Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?". Nature Astronomy. 8 (1): 44–49. Bibcode:2024NatAs...8...44C. doi:10.1038/s41550-023-02134-2. ISSN 2397-3366. <a href="https://www.nature.com/articles/s41550-023-02134-2" target="_blank">https://www.nature.com/articles/s41550-023-02134-2</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Ball, John A. (July 1973). "The Zoo Hypothesis". Icarus. 19 (3): 347–349. Bibcode:1973Icar...19..347B. doi:10.1016/0019-1035(73)90111-5. <a href="/wiki/International_Journal_of_Solar_System_Studies" target="_blank">/wiki/International_Journal_of_Solar_System_Studies</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Ball, John A. (July 1973). "The Zoo Hypothesis". Icarus. 19 (3): 347–349. Bibcode:1973Icar...19..347B. doi:10.1016/0019-1035(73)90111-5. <a href="/wiki/International_Journal_of_Solar_System_Studies" target="_blank">/wiki/International_Journal_of_Solar_System_Studies</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Lippens, Ronnie (2002). "Imachinations of Peace: Scientifictions of Peace in Iain M. Banks's The Player of Games". Utopian Studies. 13 (1): 135–147. ISSN 1045-991X. OCLC 5542757341. <a href="/wiki/ISSN_(identifier)" target="_blank">/wiki/ISSN_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Brown, Chris (2001). "'Special Circumstances': Intervention by a Liberal Utopia". Millennium: Journal of International Studies. 30 (3): 625–633. doi:10.1177/03058298010300031601. ISSN 0305-8298. S2CID 143328512. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Horwich, David (2002). "Culture Clash: Ambivalent Heroes and the Ambiguous Utopia in the Work of Iain M. Banks". Strange Horizons. 21 January 2002. <a href="http://strangehorizons.com/non-fiction/articles/culture-clash-ambivalent-heroes-and-the-ambiguous-utopia-in-the-work-of-iain-m-banks/" target="_blank">http://strangehorizons.com/non-fiction/articles/culture-clash-ambivalent-heroes-and-the-ambiguous-utopia-in-the-work-of-iain-m-banks/</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Ball, John A. (July 1973). "The Zoo Hypothesis". Icarus. 19 (3): 347–349. Bibcode:1973Icar...19..347B. doi:10.1016/0019-1035(73)90111-5. <a href="/wiki/International_Journal_of_Solar_System_Studies" target="_blank">/wiki/International_Journal_of_Solar_System_Studies</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">A. D. Wissner-Gross, "Causal entropic forces Archived 11 January 2020 at the Wayback Machine", Physical Review Letters 110, 168702 (2013). <a href="/wiki/Alexander_Wissner-Gross" target="_blank">/wiki/Alexander_Wissner-Gross</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Soter, Steven (17 October 2005). "SETI and the Cosmic Quarantine Hypothesis". Astrobiology Magazine. Archived from the original on 12 November 2020. <a href="https://web.archive.org/web/20201112025928/https://www.astrobio.net/alien-life/seti-and-the-cosmic-quarantine-hypothesis/" target="_blank">https://web.archive.org/web/20201112025928/https://www.astrobio.net/alien-life/seti-and-the-cosmic-quarantine-hypothesis/</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Crawford, Ian (July 2000). "Where are They? Maybe we are alone in the galaxy after all". Scientific American. pp. 38–43. Archived from the original on 1 December 2011. Retrieved 20 December 2011. <a href="https://web.archive.org/web/20111201003944/http://www.scientificamerican.com/article.cfm?id=where-are-they" target="_blank">https://web.archive.org/web/20111201003944/http://www.scientificamerican.com/article.cfm?id=where-are-they</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">Forgan, Duncan H. (8 June 2011). "Spatio-temporal constraints on the zoo hypothesis, and the breakdown of total hegemony". International Journal of Astrobiology. 10 (4): 341–347. arXiv:1105.2497. Bibcode:2011IJAsB..10..341F. doi:10.1017/s147355041100019x. ISSN 1473-5504. S2CID 118431252. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Kozmik, Zbynek; Ruzickova, Jana; Jonasova, Kristyna; et al. (1 July 2008). "Assembly of the cnidarian camera-type eye from vertebrate-like components". Proceedings of the National Academy of Sciences. 105 (26): 8989–8993. doi:10.1073/pnas.0800388105. ISSN 0027-8424. PMC 2449352. PMID 18577593. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2449352" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2449352</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Bracewell, R (1982). "Pre-emption of the Galaxy by the First Advanced Civilization". Extraterrestrials – Where are they?. Oxford: Pergmon Press. pp. 34–40. Bibcode:1982ewat.book...34B. <a href="http://adsabs.harvard.edu/abs/1982ewat.book...34B" target="_blank">http://adsabs.harvard.edu/abs/1982ewat.book...34B</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Crawford, Ian A.; Schulze-Makuch, Dirk (2024). "Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?". Nature Astronomy. 8 (1): 44–49. Bibcode:2024NatAs...8...44C. doi:10.1038/s41550-023-02134-2. ISSN 2397-3366. <a href="https://www.nature.com/articles/s41550-023-02134-2" target="_blank">https://www.nature.com/articles/s41550-023-02134-2</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Kardashev scale Kardashev, N.S. (1964). Soviet Astronomy. 8, 217 <a href="/wiki/Kardashev_scale" target="_blank">/wiki/Kardashev_scale</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">Shostak, Seth (31 March 2019). "'Zoo hypothesis' may explain why we haven't seen any space aliens". NBC News. Retrieved 1 April 2019. <a href="https://www.nbcnews.com/mach/science/zoo-hypothesis-may-explain-why-we-haven-t-seen-any-ncna988946" target="_blank">https://www.nbcnews.com/mach/science/zoo-hypothesis-may-explain-why-we-haven-t-seen-any-ncna988946</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
<li id="fn:18">Hair, T. W. (2011). "Temporal dispersion of the emergence of intelligence: An inter-arrival time analysis". International Journal of Astrobiology. 10 (2): 131–135. Bibcode:2011IJAsB..10..131H. doi:10.1017/S1473550411000024. S2CID 53681377. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">Shostak, Seth (31 March 2019). "'Zoo hypothesis' may explain why we haven't seen any space aliens". NBC News. Retrieved 1 April 2019. <a href="https://www.nbcnews.com/mach/science/zoo-hypothesis-may-explain-why-we-haven-t-seen-any-ncna988946" target="_blank">https://www.nbcnews.com/mach/science/zoo-hypothesis-may-explain-why-we-haven-t-seen-any-ncna988946</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">Forgan, Duncan H. (8 June 2011). "Spatio-temporal constraints on the zoo hypothesis, and the breakdown of total hegemony". International Journal of Astrobiology. 10 (4): 341–347. arXiv:1105.2497. Bibcode:2011IJAsB..10..341F. doi:10.1017/s147355041100019x. ISSN 1473-5504. S2CID 118431252. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
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</ol>

Zoo hypothesis open-in-new

Zoo hypothesis