Fermi paradox

Chain of reasoning

Some of the facts and hypotheses that together serve to highlight the apparent contradiction:

There are billions of stars in the Milky Way similar to the Sun.

With high probability, some of these stars have Earth-like planets orbiting in the habitable zone.

Many of these stars are much older than the Sun. If Earth-like planets are typical, some may have developed intelligent life long ago.

Some of these civilizations may have developed interstellar travel, a step that humans are investigating.

Even at the slow pace of envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.

However, there is no convincing evidence that this has happened.

History

Los Alamos conversation

Enrico Fermi was a Nobel Prize-winning physicist who was known to pose simple but seemingly unanswerable questions—termed "Fermi questions"—such as, "How many atoms of Caesar's last breath do you inhale with each lungful of air?"

In 1950, Fermi visited Los Alamos National Laboratory in New Mexico and, while walking to the Fuller Lodge for lunch, casually talked with fellow physicists Emil Konopinski, Edward Teller, and Herbert York about reports of flying saucers and the feasibility of faster-than-light travel. After the conversation had moved on to other topics, Fermi blurted out his question which has been variously recalled as: "Where is everybody?" (Teller), "Don't you ever wonder where everybody is?" (York), or "But where is everybody?" (Konopinski). According to Teller, "The result of his question was general laughter because of the strange fact that, in spite of Fermi's question coming out of the blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life."

According to York, 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." However, Teller recalled that Fermi did not elaborate on his question beyond "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."

Predecessors

Fermi was not the first to note the paradox. In his 1686 book Conversations on the Plurality of Worlds, Bernard Le Bovier de Fontenelle—later the secretary of the French Academy of Sciences—constructs a dialogue in which Fontenelle's claims of "intelligent beings exist in other worlds, for instance the Moon" are refuted by a character who notes that "If this were the case, the Moon's inhabitants would already have come to us before now." This may have inspired a similar discussion in Jules Verne's 1865 novel Around the Moon, which has also been identified as an early conceptualization of the Fermi paradox.

Another early formulation of the Fermi paradox was presented and dissected in the 1930s writings of Russian rocket scientist Konstantin Tsiolkovsky. Although his rocketry work was embraced by the materialist Soviets, his philosophical writings were suppressed and unknown for most of the 20th century. Tsiolkovsky noted that critics refute the existence of advanced extraterrestrial life as such civilizations would have visited humanity or left some detectable evidence. He posed a solution to the paradox: humanity is quarantined by aliens to protect its independent cultural development, which resembles the zoo hypothesis proposed by John Ball.

Popularization

The Fermi question first appeared in print in a footnote of a 1963 paper by Carl Sagan. Two years later, Stephen Dole noted the dilemma at a symposium—"If there are so many advanced forms of life around, where is everybody?"—but did not attribute it to Fermi. A chapter of Intelligent Life in the Universe, co-authored by Sagan and Iosif Shklovsky, was headlined with the Fermi-attributed "Where are they?" The Fermi question also appeared in NASA's 1970 Project Cyclops report, a 1973 book by Sagan, and a 1975 article in JBIS Interstellar Studies by David Viewing that first described it as a paradox.

Later that year, Michael Hart published a detailed examination of the paradox in the Quarterly Journal of the Royal Astronomical Society. Hart, who concluded that "we are the first civilization in our Galaxy", proposed four broad categories of solutions to the paradox: those that are physical (a space travel limitation), sociological (aliens choose not to visit Earth), temporal (aliens have not had time to travel to Earth), or that extraterrestrials have already visited. His paper sparked significant interest in the paradox among academics and even politicians, with a discussion held in the House of Lords. A seminal response—"Extraterrestrial intelligent beings do not exist"—was written by Frank Tipler, who argued that, if an advanced extraterrestrial civilization existed, their self-replicating spacecraft should have already been detected in the Solar System. The term "Fermi paradox" was coined in a 1977 article by David Stephenson and was widely adopted.

The popularization of the Fermi paradox damaged search for extraterrestrial intelligence (SETI) efforts, and Senator William Proxmire cited Tipler when he spurred the termination of the federally funded NASA SETI program in 1981. According to Robert Gray, the paradox may contribute to a "de facto prohibition on government support for research in a branch of astrobiology".

Criticism

Fermi did not publish anything regarding the paradox, with Sagan once suggesting the quote to be apocryphal. Scientists like Robert Gray have criticized its attribution to Fermi, and alternative terms like the "Hart–Tipler argument" or "Tsiolkovsky–Fermi–Viewing–Hart paradox" have been proposed. According to Gray, the current understanding of the paradox misinterprets Fermi's question and subsequent discussion, which was challenging the feasibility of interstellar travel rather than the existence of advanced extraterrestrial life.

Basis

The Fermi paradox is a conflict between the argument that scale and probability seem to favor intelligent life being common in the universe, and the total lack of evidence of intelligent life having ever arisen anywhere other than on Earth.

The first aspect of the Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way (2–4 × 1011) and 70 sextillion (7×1022) in the observable universe. 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 mediocrity principle, by which Earth is a typical planet.

The second aspect of the Fermi paradox is the argument of probability: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems possible that at least some civilizations would be technologically advanced, seek out new resources in space, and colonize their star system and, subsequently, surrounding star systems. Since there is no known evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8 billion years of the universe's history, there is a conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than is thought, that assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that the scientific understanding of the nature of the universe is quite incomplete.

The Fermi paradox can be asked in two ways. The first is, "Why are no aliens or their artifacts found on Earth, or in the Solar System?". If interstellar travel is possible, even the "slow" kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy. 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.

Drake equation

The theories and principles in the Drake equation are closely related to the Fermi paradox. 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:

N

=

R

∗

⋅

f

p

⋅

n

e

⋅

f

l

⋅

f

i

⋅

f

c

⋅

L

,

{\displaystyle N=R_{*}\cdot f_{\mathrm {p} }\cdot n_{\mathrm {e} }\cdot f_{\mathrm {l} }\cdot f_{\mathrm {i} }\cdot f_{\mathrm {c} }\cdot L,}

where

N

{\displaystyle N}

is the number of technologically advanced civilizations in the Milky Way galaxy, and is asserted to be the product of

R

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 advanced civilization. The most commonly agreed-upon low probability event is abiogenesis: a gradual process of increasing complexity of the first self-replicating molecules by a randomly occurring chemical process. Other proposed great filters are the emergence of eukaryotic cells or of meiosis or some of the steps involved in the evolution of a brain-like organ capable of complex logical deductions.

Astrobiologists Dirk Schulze-Makuch and William Bains, reviewing the history of life on Earth, including convergent evolution, concluded that transitions such as oxygenic photosynthesis, the eukaryotic cell, multicellularity, and tool-using intelligence are likely to occur on any Earth-like planet given enough time. They argue that the Great Filter may be abiogenesis, the rise of technological human-level intelligence, or an inability to settle other worlds because of self-destruction or a lack of resources. Paleobiologist Olev Vinn has suggested that the great filter may have universal biological roots related to evolutionary animal behavior.

Grabby Aliens

In 2021, the concepts of quiet, loud, and grabby aliens were introduced by Hanson et al. The proposed "loud" aliens expand rapidly in a highly detectable way throughout the universe and endure, while "quiet" aliens are hard or impossible to detect and eventually disappear. "Grabby" aliens prevent the emergence of other civilizations in their sphere of influence, which expands at a rate near the speed of light. The authors argue that if loud civilizations are rare, as they appear to be, then quiet civilizations are also rare. The paper suggests that humanity's existing stage of technological development is relatively early in the potential timeline of intelligent life in the universe, as loud aliens would otherwise be observable by astronomers.

Earlier in 2013, Anders Sandberg and Stuart Armstrong examined the potential for intelligent life to spread intergalactically throughout the universe and the implications for the Fermi Paradox. Their study suggests that with sufficient energy, intelligent civilizations could potentially colonize the entire Milky Way galaxy within a few million years, and spread to nearby galaxies in a timespan that is cosmologically brief. They conclude that intergalactic colonization appears possible with the resources of a single planetary system and that intergalactic colonization is of comparable difficulty to interstellar colonization, and therefore the Fermi paradox is much sharper than commonly thought.

Critics such as David Kipping have contended that the "Grabby Aliens" model is reliant on unproven assumptions, lacking enough scientific rigor to be empirically falsifiable, and suggested other explanations for the proposed earliness of humans such as planets in M-dwarf systems being uninhabitable. Robin Hanson has responded to these criticisms.

Anthropics

Anthropic reasoning and the question of why we happen to find ourselves as humans creates a number of potential problems for astrobiology. Walter Barta argues that Hanson's grabby aliens model creates an anthropic dilemma. According to Hanson's model, most observers in our reference class should be grabby aliens themselves. This leads to the question of why we do not find ourselves as grabby aliens, but rather as a species confined to a single planet.

Empirical evidence

There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potentially 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 since supported by the discovery that exoplanets are common. Existing models predict billions of habitable worlds in the Milky Way.

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, and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing.

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, pulsars, when first discovered in 1967, were called little green men (LGM) because of the precise repetition of their pulses. In all cases, explanations with no need for intelligent life have been found for such observations, but the possibility of discovery remains. Proposed examples include asteroid mining that would change the appearance of debris disks around stars, or spectral lines from nuclear waste disposal in stars.

Electromagnetic emissions

Radio technology and the ability to construct a radio telescope are presumed to be a natural advance for technological species, 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 waves for a 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 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, as of 2019, would not be able to detect non-directional radio signals (such as broadband) even at a fraction of a light-year away, but other civilizations could hypothetically have much better equipment.

A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through SETI organizations such as the SETI Institute and Breakthrough Listen. Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions.

Direct planetary observation

Exoplanet detection and classification is a very active sub-discipline in astronomy; the first candidate terrestrial planet discovered within a star's habitable zone was found in 2007. New refinements in exoplanet detection methods, and use of existing methods from space (such as the Kepler and TESS missions) have detected and characterized Earth-size planets, and determined whether they are within the habitable zones of their stars. Such observational refinements have allowed better estimates of how common these potentially habitable worlds are, typically in the range of 0.5-1.0 potentially habitable planets per star.

Conjectures about interstellar probes

The Hart–Tipler conjecture is a form of 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. Self-replicating probes could exhaustively explore a galaxy the size of the Milky Way in as little as a million years. 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 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 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.

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, though the entirety of the Solar System is relatively vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded.

Searches for stellar-scale artifacts

In 1959, 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 means: a shell or cloud of objects enclosing a star to absorb and utilize as much radiant energy as possible. Such a feat of astroengineering would drastically alter the observed spectrum of the star involved, changing it at least partly from the normal emission lines of a natural stellar atmosphere to those of black-body radiation, probably with a peak in the infrared. Dyson speculated that advanced alien civilizations might be detected by examining the spectra of stars and searching for such an altered spectrum.

There have been attempts to find evidence of Dyson spheres that would alter the spectra of their core stars. Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications. In October 2015, there was speculation that a dimming of light from star KIC 8462852, observed by the Kepler space telescope, could have been a result of such a Dyson sphere under construction. However, in 2018, further 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 cause of the dimming.

Hypothetical explanations for the paradox

Rarity of intelligent life

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.

It is possible that even if complex life is common, intelligence (and consequently civilizations) is not. While there are remote sensing techniques that could perhaps detect life-bearing planets without relying on the signs of technology, 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 ~20 million 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.

Evolutionary explanations

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."

Sociological explanations

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."

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. 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." 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. 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.

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.

Economic explanations

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.

If a human-capability machine intelligence 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. Louis K. Scheffer calculates the cost of radio transmission of information across space to be cheaper than spaceflight by a factor of 108–1017. For a machine civilization, the costs of interstellar travel are therefore enormous compared to the more efficient option of sending computational signals across space to already established sites. 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. 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.

Discovering extraterrestrial life is very difficult

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.

SETI estimates 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 like us largely moved past high-power radio broadcasting, produce very few, if any, detectable transmissions.

Thus, to detect alien civilizations through their radio emissions, Earth observers need sensitive instruments, and must hope that:

Aliens have developed radio technology, and

Aliens use radio as a primary means of communication, and

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

Willingness to communicate

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.

Even these limited efforts, and certainly any attempt to expand them, are controversial. 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.

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.

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."