The Zoo Hypothesis speculates on the assumed behavior and existence of technically-advanced extraterrestrial life and the reasons they refrain from contacting Earth. It is one of many theoretical explanations for the Fermi paradox. The hypothesis is that alien life intentionally avoids communication with Earth, and one of its main interpretations is that it does so to allow for natural evolution and sociocultural development, avoiding interplanetary contamination, similarly to people observing animals at a zoo. The hypothesis seeks to explain the apparent absence of extraterrestrial life despite its generally accepted plausibility and hence the reasonable expectation of its existence. A variant on the zoo hypothesis suggested by Ball is the laboratory hypothesis, where humanity is being subjected to experiments, with the Earth being a giant laboratory.
Aliens might, for example, choose to allow contact once the human species has passed certain technological, political, or ethical standards. They might withhold contact until humans force contact upon them, possibly by sending a spacecraft to planets they inhabit. Alternatively, a reluctance to initiate contact could reflect a sensible desire to minimize risk. An alien society with advanced remote-sensing technologies may conclude that direct contact with neighbors confers added risks to oneself without an added benefit. In the related laboratory hypothesis, the zoo hypothesis is extended such that the 'zoo keepers' are subjecting humanity to experiments, a hypothesis which Ball describes as "morbid" and "grotesque", overlooking the possibility that such experiments may be altruistic, i.e., designed to accelerate the pace of civilization to overcome a tendency for intelligent life to destroy itself, until a species is sufficiently developed to establish contact, as in the zoo hypothesis.
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 (i.e. that there are a large number of alien cultures in existence) . It also assumes that these aliens 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 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 hegemonic power, random single civilizations with independent principles would make contact. This makes a crowded Universe with clearly defined rules seem more plausible.
If there is a plurality of alien cultures, however, this theory may break down under the uniformity of motive concept because it would take just a single extraterrestrial civilization to decide to act contrary to the imperative within our range of detection for it to be undone, and the probability of such a violation of hegemony increases with the number of civilizations. This idea, however, becomes more plausible if all civilizations tend to evolve similar cultural standards and values with regard to contact much like convergent evolution on Earth has independently evolved eyes on numerous occasions, or all civilizations follow the lead of some particularly distinguished civilization, such as the first civilization among them.
With this in mind, a modified zoo hypothesis becomes a more appealing answer to the Fermi paradox. The time between the emergence of the first civilization within the Milky Way and all subsequent civilizations could be enormous. Monte Carlo simulation shows the first few inter-arrival times between emergent civilizations would be similar in length to geologic epochs on Earth. Just what could a civilization do with a ten-million, one-hundred-million, or half-billion-year head start?
Even if this first grand civilization is long gone, their initial legacy could live on in the form of a passed-down tradition, or perhaps an artificial life form dedicated to such a goal without the risk of death. Beyond this, it does not even have to be the first civilization, but simply the first to spread its doctrine and control over a large volume of the galaxy. If just one civilization gained this hegemony in the distant past, it could form an unbroken chain of taboo against rapacious colonization in favour of non-interference in those civilizations that follow. The uniformity of motive concept previously mentioned would become moot in such a situation.
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.
Some critics of the hypothesis say that only a single dissident group in an alien civilization, or alternatively the existence of galactic cliques instead of a unified galactic club, would be enough to break the pact of no contact. To Stephen Webb, it seems unlikely, taking humans as reference, that such prohibition would be in effect for millions of years without a single breach thereof. Others say that the zoo hypothesis, along with its planetarium variation, is highly speculative and more aligned with theological theories.
Appearance in fiction
The zoo hypothesis is a common theme in science fiction.
- 1937: In Olaf Stapledon's 1937 novel Star Maker, 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.
- 1951: Arthur C. Clarke's The Sentinel (first published in 1951) and its later novel adaptation 2001: A Space Odyssey (1968) feature a beacon which is activated when the human race discovers it on the moon. An alien race has apparently visited us in the distant past.
- 1953: In Childhood's End, a novel by Arthur C. Clarke published in 1953, the alien cultures had been observing and registering the Earth's evolution and human history for thousands (perhaps millions) of years. At the beginning of the book, when mankind is about to achieve spaceflight, the aliens reveal their existence and quickly end the arms race, colonialism, racial segregation and the Cold War.
- In Star Trek, the Federation (including humans) has a strict Prime Directive policy of nonintervention with less technologically advanced cultures which the Federation encounters. The threshold of inclusion is the independent technological development of faster-than-light propulsion. In the show's canon the Vulcan race limited their encounters to observation until Humans made their first warp flight, after which they initiated first contact, indicating the practice predated the Human race's advance of this threshold. Additionally, in the episode "The Chase (TNG)", a message from a first (or early) civilization is discovered, hidden in the DNA of sentient species spread across many worlds, something that could only have been fully discovered after a race had become sufficiently advanced.
- In Hard to Be a God by Arkady and Boris Strugatsky, 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.
- 1986: In Speaker for the Dead by Orson Scott Card, 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 Ender's Game, 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.
- 1987: In Julian May's 1987 novel Intervention, the five alien races of the Galactic Milieu keep the Earth under surveillance, but do not intervene until humans demonstrate mental and ethical maturity through a paranormal prayer of peace.
- 1989: Iain M. Banks' The State of the Art depicts the Culture secretly visiting Earth and then deciding to leave it uncontacted, watching its development as a control group, 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., Inversions), both covertly and overtly, for philosophical or foreign policy purposes.
- 1989: Bill Watterson's Calvin and Hobbes comic strip for November 8, 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."
- 2000: In Robert J. Sawyer's SF novel Calculating God (2000), Hollus, a scientist from an advanced alien civilization, denies that her government is operating under the prime directive.
- 2002: In the Kingdom Hearts series of video games, the main characters interact with several worlds based upon Disney film franchises, separated into independent realms. In order to maintain the illusion that each world is independent of the larger Kingdom Hearts universe, Donald Duck will occasionally use his magic to warp his appearance, as well as the appearances of Goofy and the player character Sora. In addition, revealing any information about the outside world is forbidden within the Disney realms.
- 2003: In South Park's inaugural episode of season seven, "Cancelled", 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 The Truman Show in order to maintain the show's integrity.
- In the 2008 video game Spore, which simulates 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 unify their planets, nor develop spaceflight yet. However, they are allowed to abduct their citizens/members, to create crop circles in their terrain and to place in their planets a tool called "monolith", which accelerates their technological evolution.
- Ball, John A. (Jul 1973). "The Zoo Hypothesis". Icarus. 19 (3): 347–349. Bibcode:1973Icar...19..347B. doi:10.1016/0019-1035(73)90111-5.
- 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.
- 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.
- Horwich, David (2002). "Culture Clash: Ambivalent Heroes and the Ambiguous Utopia in the Work of Iain M. Banks". Strange Horizons. 21 January 2002.
- A. D. Wissner-Gross, "Causal entropic forces", Physical Review Letters 110, 168702 (2013).
- Soter, S. (2005). Astrobiol. Mag. 17 Oct "SETI and the Cosmic Quarantine Hypothesis"
- Crawford, I.A., "Where are They? Maybe we are alone in the galaxy after all", Scientific American, July 2000, 38–43, (2000).
- Forgan, Duncan H. (2011-06-08). "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.
- Kozmik, Z.; Ruzickova, J.; Jonasova, K.; Matsumoto, Y.; Vopalensky, P.; Kozmikova, I.; Strnad, H.; Kawamura, S. et al. (Jul 2008). "Assembly of the cnidarian camera-type eye from vertebrate-like components". Proceedings of the National Academy of Sciences of the United States of America 105 (26): 8989–8993.
- Bracewell, R. (1982). Pre-emption of the Galaxy by the First Advanced Civilization, Pergmon Press, Oxford.
- Kardashev scale Kardashev, N.S. (1964). Soviet Astronomy. 8, 217
- "'Zoo hypothesis' may explain why we haven't seen any space aliens". NBC News. Retrieved 1 April 2019.
- 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.
- Forgan, Duncan H. (2016-11-28). "The Galactic Club or Galactic Cliques? Exploring the limits of interstellar hegemony and the Zoo Hypothesis". International Journal of Astrobiology. 16 (4): 349–354. doi:10.1017/s1473550416000392. hdl:10023/10869. ISSN 1473-5504. S2CID 59041278.
- Webb, Stephen (2002). If the Universe Is Teeming with Aliens... Where Is Everybody? Fifty Solutions to Fermi's Paradox and the Problem of Extraterrestrial Life. New York: Copernicus Books. p. 27–231. ISBN 978-0-387-21739-0.
- Cirković, MM; Vukotić, B. (December 2008). "Astrobiological phase transition: towards resolution of Fermi's paradox". Origins of Life and Evolution of the Biosphere. 38 (6): 535–47. Bibcode:2008OLEB...38..535C. doi:10.1007/s11084-008-9149-y. PMID 18855114. S2CID 23047467.