Rarity of intelligent life Extraterrestrial life is rare or non-existent Those who think that intelligent
extraterrestrial life is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the
evolution of biological complexity—are rare or even unique to Earth. Under this assumption, called the
rare Earth hypothesis, a rejection of the
mediocrity principle, complex multicellular life is regarded as exceedingly unusual. The rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a
galactic habitable zone, a star and planet(s) having the requisite conditions, such as enough of a
continuous habitable zone, the advantage of a giant guardian like Jupiter and a large
moon, conditions needed to ensure the planet has a
magnetosphere and
plate tectonics, the chemistry of the
lithosphere,
atmosphere, and oceans, the role of "evolutionary pumps" such as massive
glaciation and rare
bolide impacts. Perhaps most importantly, advanced life needs whatever it was that led to the transition of (some)
prokaryotic cells to
eukaryotic cells,
sexual reproduction and the
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 probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life. On the other hand, maybe the filter is further back in time: recently, astrophysicist Matt O'Dowd has identified the eukaryogenesis as a reasonable candidate for such a filter in the past given the increasing difficulties in finding new useful proteins for prokaryotes.
Extraterrestrial intelligence is rare or non-existent It is possible that even if complex life is common, intelligence (and consequently civilizations) is not. none of them have the ability to determine if any detected life is intelligent. This is sometimes referred to as the "algae vs. alumnae" problem. 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
aardvarks and elephants. As he points out, "dolphins have had ~20million years to build a radio telescope and have not done so". In addition, Rebecca Boyle points out that of all the species that have 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.
Extraterrestrial intelligence is relatively new Given that the expected
lifespan of the universe is at least one trillion years and the
age of the universe is around 14 billion years, it is possible that humans have emerged at or near the earliest possible opportunity for intelligent life to evolve.
Avi Loeb, an astrophysicist and cosmologist, has suggested that Earth may be a very early example of a life-bearing planet and that life-bearing planets may be more likely trillions of years from now. He has put forward the view that the Universe has only recently reached a state in which life is possible and this is the reason humanity has not detected extraterrestrial life. The
firstborn hypothesis posits that humans are the first, or one of the first, intelligent species to evolve. Therefore, many intelligent species may eventually exist, but few, if any, currently do. Moreover, it is possible that said species, even if they already exist, are developing more slowly, or have more limited resources on their home world, meaning that they may take longer than humans have to achieve spaceflight.
Periodic extinction by natural events may trigger an
extinction event. New life might commonly die out due to runaway heating or cooling on their fledgling planets. On Earth, there have been numerous major
extinction events 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 asteroid, massive volcanic eruptions, or astronomical events such as
gamma-ray bursts. 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. However, the chances of extinction by natural events may be very low on the scale of a civilization's lifetime. Based on an analysis of impact craters on Earth and the Moon, the average interval between impacts large enough to cause global consequences (like the
Chicxulub impact) is estimated to be around 100 million years.
Evolutionary explanations It is the nature of intelligent life to destroy itself , 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
Sebastian von Hoerner stated that the progress of science and technology on
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. Possible means of annihilation via major global issues, where global interconnectedness actually makes humanity more vulnerable than resilient, are many, including war, accidental environmental contamination or damage, the development of
biotechnology,
synthetic life like
mirror life,
resource depletion,
climate change, or
artificial intelligence. This general theme is explored both in fiction and in scientific hypotheses. In 1966, Sagan and
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. Self-annihilation may also be viewed in terms of
thermodynamics: insofar as life is an ordered
system that can sustain itself against the
tendency to disorder, Stephen Hawking's "external transmission" or interstellar communicative phase, where
knowledge production and
knowledge management is more important than transmission of information via
evolution, may be the point at which the system becomes unstable and self-destructs. Here, Hawking emphasizes self-design of the
human genome (
transhumanism) or enhancement via machines (e.g.,
brain–computer interface) to enhance
human intelligence and reduce
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
weaponization of
artificial general intelligence or
antimatter, may not be met by concomitant increases in human ability to manage its own inventions. Consequently, disorder increases in the system:
global governance may become increasingly destabilized, worsening humanity's ability to manage the possible means of annihilation listed above, resulting in global
societal collapse. 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." Using extinct civilizations such as
Easter Island as models, a study conducted in 2018 by
Adam Frank et al. posited that
climate change induced by "energy intensive" civilizations may prevent sustainability within such civilizations, thus explaining the paradoxical lack of evidence for intelligent extraterrestrial life. Based on
dynamical systems theory, the study examined how technological civilizations (exo-civilizations) consume resources and the
feedback effects this consumption has on their planets and its
carrying capacity. According to Adam Frank "[t]he point is to recognize that driving climate change may be something generic. The laws of physics demand that any young population, building an energy-intensive civilization like ours, is going to have feedback on its planet. Seeing climate change in this cosmic context may give us better insight into what's happening to us now and how to deal with it." Generalizing the
Anthropocene, their model produces four different outcomes: • Die-off: A scenario where the population grows quickly, surpassing the planet's carrying capacity, which leads to a peak followed by a
rapid decline. The population eventually stabilizes at a much lower equilibrium level, allowing the planet to partially recover. •
Sustainability: A scenario where civilizations successfully transition from high-impact resources (like fossil fuels) to sustainable ones (like solar energy) before significant environmental degradation occurs. This allows the civilization and planet to reach a stable equilibrium, avoiding catastrophic effects. •
Collapse Without Resource Change: In this trajectory, the population and environmental degradation increase rapidly. The civilization does not switch to sustainable resources in time, leading to a total collapse where a
tipping point is crossed and the population drops. • Collapse With Resource Change: Similar to the previous scenario, but in this case, the civilization attempts to transition to sustainable resources. However, the change comes too late, and the environmental damage is irreversible, still leading to the civilization's collapse.
Only one intelligent species can exist in a given region of space 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 probes. Science fiction writer
Fred Saberhagen has explored this idea in his
Berserker series, as has physicist
Gregory Benford and also, science fiction writer
Greg Bear in his
The Forge of God novel, 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. It has also been suggested that a successful alien species would be a
superpredator, as are humans. Another possibility invokes the "
tragedy of the commons" and the
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.
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. However, there could be other leakage such as that from microwaves used to transmit power from solar satellites to ground receivers. Regarding the first point, in a 2006
Sky & Telescope article,
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." 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. Some scientists have hypothesized that advanced civilizations may send
neutrino signals. If such signals exist, they could be detectable by
neutrino detectors that are under construction for other goals.
Alien life may be too incomprehensible 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
parochial to Earth and not shared by other life, 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). In his 2009 book, SETI scientist
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
Percival Lowell [astronomer who believed he had observed canals on Mars]." Physiology might also be 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. A message broadcast by that species might seem like random background noise to humans, and therefore go undetected.
Paul Davies stated 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 level[...] 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[...] levels."
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". Another thought is that technological civilizations invariably experience a
technological singularity and attain a post-biological character.
Sociological explanations Expansionism 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."
Alien species may have only settled part of the galaxy According to a study by Frank
et al., advanced civilizations may not colonize everything in the galaxy due to their potential adoption of steady states of expansion. This hypothesis suggests that civilizations might reach a stable pattern of expansion where they neither collapse nor aggressively spread throughout the galaxy. 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." An alternative scenario is that long-lived civilizations may only choose to colonize stars during closest approach. As low mass
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.
Alien species may isolate themselves in virtual worlds Avi Loeb suggests that one possible explanation for the Fermi paradox is
virtual reality technology. Individuals of extraterrestrial civilizations may prefer to spend time in
virtual worlds or
metaverses that have different physical law constraints as opposed to focusing on colonizing planets. Nick Bostrom suggests that some advanced beings may divest themselves entirely of physical form, create massive artificial virtual environments, transfer themselves into these environments through
mind uploading, and exist totally within virtual worlds, ignoring the external physical universe. It may be that intelligent alien life develops an "increasing disinterest" in their outside world. 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.
Artificial intelligence may not be aggressively expansionist While
artificial intelligence supplanting its creators could only deepen the Fermi paradox, such as through enabling the colonizing of the galaxy through
self-replicating probes, it is also possible that after replacing its creators, artificial intelligence either doesn't expand or endure for a variety of reasons.
Michael A. Garrett has suggested that biological civilizations may universally underestimate the speed that AI systems progress, and not react to it in time, thus making it a possible great filter. He also argues that this could make the longevity of advanced technological civilizations less than 200 years, thus explaining the great silence observed by SETI.
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 existing understanding of physics rules out the possibility of
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 existing 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 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 more gradual pace of colonization based on
percolation. Colonization efforts may not occur as an unstoppable hyper-aggressive 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, and planets only restarting the colonization process when their populations begin to outstrip their world's carrying capacity. By contrast, in
economics the counter-intuitive
Jevons paradox implies that higher
productivity results in higher
demand. In other words, increased economic efficiency results in increased economic growth. For example, increased renewable energy has the risk of not directly resulting in declining fossil fuel use, but rather additional economic growth as fossil fuels instead are directed to alternative uses. Thus, technological innovation makes human civilization more capable of higher levels of
consumption, as opposed to its existing consumption being achieved more efficiently at a stable level.
Other species' home planets cannot support industrial economies Amedeo Balbi and Adam Frank propose the concept of an "
oxygen bottleneck" for the emergence of the industrial production necessary for spaceflight. The "oxygen bottleneck" refers to the critical level of atmospheric oxygen necessary for
fire and
combustion. Earth's
atmospheric oxygen concentration is about 21%, but has been much lower in the past and may also be on many exoplanets. The authors argue that while the threshold of oxygen required for the existence of complex life and
ecosystems is relatively low, industrial processes which are necessary precursors to spaceflight, particularly
metal smelting and many forms of
electricity generation, require higher oxygen concentrations of at least some 18%. A planet with oxygen sufficient to support intelligent life but not to develop advanced metallurgy would be technologically gated by its extremely limited industrial capabilities at a level likely incapable of supporting spaceflight. Thus, the presence of high levels of oxygen in a planet's atmosphere is not only a potential biosignature but also a critical factor in the emergence of detectable technological civilizations. Another hypothesis in this category is the "waterworlds 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..." 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." To skeptics, the fact that over the history of life on the Earth, only one species has developed a civilization to the point of being capable of
spaceflight, and this only in the early stages, lends credence to the idea that technologically advanced civilizations are rare in the universe.
Developing practical spaceflight technology is very difficult or expensive The rapid increase of scientific and technological progress seen in the 18th to 20th centuries (the
Industrial Revolution), compared to earlier eras, led to the common assumption that such progress will continue at exponential rates as time goes by, eventually leading to the progress level required for space exploration. The "universal limit to technological development" (ULTD) hypothesis proposes that there is a limit to the potential growth of a civilization, and that this limit may be placed well below the point required for space exploration. Such limits may be based on the enormous strain spaceflight may put on a planet's resources, physical limitations (such as
faster-than-light travel being impossible), and even limitations based on the species' own biology.
Discovering extraterrestrial life is very difficult Humans are not listening properly There are some assumptions that underlie the
SETI programs that may cause searchers to miss signals that exist. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional (in human terms)
frequencies, which would make them hard to distinguish from background noise. Signals might be sent from non-
main sequence star systems that humans search with lower priority; our programs assume that most alien life will be orbiting
Sun-like stars.
Radio signals cannot be straightforwardly detected at interstellar distances 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
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. 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. Moreover, as human communication technology has advanced, humans have reduced the use of broadband radio transmissions in favor of more efficient and higher-bandwidth methods such as
satellite communication and
fibre optics. It may be that alien civilizations, having, as we have, largely moved past high-power radio broadcasting, producing very few, if any, detectable transmissions. Thus, to detect alien civilizations through their radio emissions, Earth observers need very sensitive instruments, and moreover must hope that: 1) Aliens have developed radio technology, and, 2) Aliens use radio as a primary means of communication, and, 3) For reasons unknown, their transmitters are orders of magnitude more powerful than ours, or they are deliberately broadcasting high-power radio signals towards Earth as part of their own efforts to contact other civilizations, and, 4) We are listening at the right frequency, at the right time, and, 5) We recognize their transmission as an attempt at communication.
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
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.
Intelligent life may be too far away '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. 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. and humanity has undertaken similar activities like the
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. 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, i.e. that Humans are a relatively early civilization. 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. In 2021, Hanson et al. reconsidered this likelihood and concluded it is indeed plausible when assuming that many civilizations are "grabby", i.e. displace other civilizations. Under this assumption there is a
selection effect of the sort that provided we exist and are not (yet) destroyed by grabby aliens, we are very unlikely to observe aliens. Specifically, grabby aliens imply a typical civilizational expansion rate at nearly the speed of light because otherwise many other civilizations would be visible. The transition time between detection of an alien
technosignature and extinction would be vanishingly short in cosmological timeframes, making it likely we are before that time period. Moreover, such a civilization may have great difficulty getting to space, insofar as even getting to the surface of their world could present a considerable engineering challenge involving tunneling through many kilometres of ice. This may severely hamper their ability to communicate with us.
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
radio signals. Since humanity has
only recently began to use radio communication, its signals may have yet to arrive to other inhabited planets, and if they have,
probes from those planets may have yet to arrive on Earth.
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. 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. The only civilization known, humanity, does not
explicitly transmit, except for a few small efforts. It is not even clear humanity would respond to a detected signal—the official policy within the SETI community 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, it may be very difficult to obtain any consensus on whether to reply, and if so, who would speak and what they would say. It is therefore quite possible that an alien civilization led by cautious decision-makers might conclude that not responding is the soundest option. Moreover, as the only observed civilization does not have a
planetary central government capable of making a binding decision about a response, alien civilizations, themselves divided into various political units without a central decision-making authority, may be aware of our existence and technically capable of responding, but cannot agree on whether and/or how to do so.
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. Even contact at a safe distance could lead to infection by computer code or even ideas themselves. Perhaps prudent civilizations actively hide not only from Earth but from everyone, out of
fear of other civilizations. Perhaps the Fermi paradox itself, however aliens may conceive 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. According to this reasoning, it is likely that previous civilizations faced fatal problems upon 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. In 1987, science fiction author
Greg Bear explored this concept in his novel
The Forge of God. In
The Forge of God, humanity is likened to a baby crying in a hostile forest: "There once was an infant lost in the woods, crying its heart out, wondering why no one answered, drawing down the wolves." One of the characters explains, "We've been sitting in our tree chirping like foolish birds for over a century now, wondering why no other birds answered. The galactic skies are full of hawks, that's why. Planetisms that don't know enough to keep quiet, get eaten." In
Liu Cixin's 2008 novel
The Dark Forest, the author proposes a literary explanation for the Fermi paradox in which countless alien civilizations exist, but are both silent and paranoid, destroying any nascent lifeforms loud enough to make themselves known. 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". This idea has come to be known as the
dark forest hypothesis.
Earth is deliberately being avoided The
zoo hypothesis states that intelligent extraterrestrial life exists and does not contact life on Earth to allow for its natural evolution and development as a sort of cosmic
closed nature reserve. A variation on the zoo hypothesis is the laboratory hypothesis, where humanity has been or is being subject to experiments, tending not towards a galactic league with a single policy towards Earth, but towards multiple competing factions. However, if
artificial superintelligences are paramount in galactic politics, and such intelligences tend towards consolidation behind a central authority, then this would at least partially address the uniformity of motive flaw by dissuading rogue behavior. 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, inasmuch as it has had time to assert control over resources, and settle the best planets (assuming similar biological needs to competitors). 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
founder effect with or without the continued activity of the founder. 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
selection process among colonists, favoring cultural, biological, or political
adaptation to living aboard spacecraft or space habitats for long periods of time; as a result, they may only settle a very limited number of the highest-quality planets, or simply stay aboard their ships and forgo planets entirely. This may result in a lack of interest in colonization, instead focusing on planets only as a destructible source of
non-renewable resources. Alternatively, they may have an ethic of protection for "nursery worlds", and protect them without intervening. Following this logic, and building on arguments that other proposed solutions to the Fermi paradox may be implausible,
Ian Crawford and
Dirk Schulze-Makuch have argued that technological civilisations are either very rare in the Galaxy or are deliberately hiding from us.
Earth is deliberately being isolated A related idea to the zoo hypothesis is that, beyond a certain distance, the perceived universe is a
simulated reality. The planetarium hypothesis speculates that beings may have created this simulation so that the universe appears to be empty of other life.
Conspiracy theories: alien life is already here, unacknowledged and/or deliberately concealed A significant fraction of the population believes that at least some UFOs (Unidentified Flying Objects) are spacecraft piloted by aliens. While most of these are unrecognized or mistaken interpretations of mundane phenomena, some occurrences remain puzzling even after investigation. The scientific consensus is that although they may be unexplained, they do not rise to the level of convincing evidence. 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. Claims that this has already happened are common in the popular press, 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. 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." == See also ==