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A graphical representation of the Arecibo message - Humanity's first attempt to use radio waves to communicate its existence to alien civilizationsThe Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence of contact with such civilizations.
The extreme age of the universe and its vast number of stars suggest that extraterrestrial life should be common. Considering this with colleagues over lunch in 1950, the physicist Enrico Fermi is said to have asked: "Where are they?" Fermi questioned why, if a multitude of advanced extraterrestrial civilizations exist in the Milky Way galaxy, evidence such as probes, spacecraft or radio transmissions has not been found. The simple question "Where are they?" (alternatively, "Where is everybody?") is possibly apocryphal, but Fermi is widely credited with simplifying and clarifying the problem of the probability of extraterrestrial life.
There have been attempts to resolve the Fermi Paradox by locating evidence of extraterrestrial civilizations, along with proposals that such life could exist without human knowledge. Counterarguments suggest that intelligent extraterrestrial life does not exist or occurs so rarely that humans will never make contact with it.
A great deal of effort has gone into developing scientific theories and possible models of extraterrestrial life and the Fermi paradox has become a theoretical reference point in much of this work. The problem has spawned numerous scholarly works addressing it directly, while various questions that relate to it have been addressed in fields as diverse as astronomy, biology, ecology and philosophy. The emerging field of astrobiology has brought an interdisciplinary approach to the Fermi paradox and the question of extraterrestrial life.
1 Basis of the paradox
2 Related concepts
2.1 Drake equation
2.2 Rare Earth hypothesis
3 Resolving the paradox empirically
3.1 Radio emissions
3.2 Direct planetary observation
3.3 Alien constructs
3.3.1 Probes, colonies, and other artifacts
3.3.2 Advanced stellar scale artifacts
4 Explaining the paradox theoretically
4.1 They do not exist ...
4.1.1 ... and they never did
4.1.2 ... because an inhospitable universe destroys complex intelligent life
4.1.3 ... because it is the nature of intelligent life to destroy itself
4.1.4 ... because it is the nature of intelligent life to destroy others
4.1.5 ... because God created humans alone
4.2 They do exist ...
4.2.1 ... but communication is impossible due to problems of scale
220.127.116.11 Intelligent civilizations are too far apart in space to communicate
18.104.22.168 Intelligent civilizations are too far apart in time to communicate
22.214.171.124 It is too expensive to spread physically throughout the galaxy
126.96.36.199 Human beings have not been searching long enough
188.8.131.52 They haven't got back to us yet
4.2.2 ... but communication is impossible for technical reasons
184.108.40.206 Human beings are not listening properly
220.127.116.11 Civilizations only broadcast detectable radio signals for a brief period of time
18.104.22.168 They tend to experience a technological singularity
4.2.3 ... and they choose not to communicate
22.214.171.124 Earth is purposely isolated (The zoo hypothesis)
126.96.36.199 They are too alien
188.8.131.52 They are not interested
4.2.4 ... and they are here unobserved
184.108.40.206 They hide their presence
220.127.116.11 Human beings refuse to see, or misunderstand, the evidence
5 See also
7 Suggested reading
8 External links
Basis of the paradox
See also: #Drake equation
The Fermi paradox is a conflict between an argument of scale and probability, and a lack of evidence. A more complete definition could be stated thus:
The size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to exist. However, this belief seems logically inconsistent with the lack of observational evidence to support it. Either the initial assumption is incorrect and technologically advanced intelligent life is much rarer than believed, current observations are incomplete and human beings have not detected other civilizations yet, or search methodologies are flawed and incorrect indicators are being sought.
The first aspect of the paradox, "the argument by scale", is a function of the raw numbers involved: there are an estimated 250 billion (2.5 x 1011) stars in the Milky Way and 70 sextillion (7 x 1022) in the visible universe. Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there should still be a great number of civilizations extant in the Milky Way galaxy alone. This argument also assumes the mediocrity principle, which states that Earth is not special, but merely a typical planet, subject to the same laws, effects, and likely outcomes as any other world. Some estimates using the Drake equation support this argument, although the assumptions behind those calculations have themselves been challenged.
The second cornerstone of the Fermi paradox is a rejoinder to the argument by scale: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems likely that any advanced civilization would seek out new resources and colonize first their star system, and then surrounding star systems. As there is no evidence on Earth or anywhere else of attempted alien colonization after 13 billion years of the universe's history, either intelligent life is rare or assumptions about the general behavior of intelligent species are flawed.
Several writers have tried to estimate how fast an alien civilization might spread through the galaxy. There have been estimates of anywhere from 5 million to 50 million years to colonize the entire galaxy; a relatively small amount of time on a geological scale, let alone a cosmological one. Even if colonization is impractical or undesirable to an alien civilization, large scale exploration of the galaxy is still possible; the means of exploration and theoretical probes involved are discussed extensively below.
While numerous theories and principles attend to the Fermi paradox, the one most closely related is the Drake equation. It was formulated by Dr. Frank Drake in 1960, a decade after the objections raised by Enrico Fermi, in an attempt to find a systematic means to evaluate the numerous probabilities involved in alien life. The speculative equation factors: the rate of star formation in the galaxy; the number of stars with planets and the number that are habitable; the number of those planets which develop life and subsequently intelligent communicating life; and finally the expected lifetimes of such civilizations.
The Drake equation has been used by both optimists and pessimists, with varying results. Dr. Carl Sagan, for example, suggested as many as one million communicating civilizations in the Milky Way in 1966, though he later suggested that the number could be far smaller. Other published estimates from Frank Tipler place the value at just one—i.e., human beings are the only extant intelligent life.
Critics of the Drake equation claim that since the variables cannot yet be determined with any real confidence, estimating the number of extraterrestrial civilizations based on it is methodologically flawed, a criticism which the wide divergence in estimates seems to support. Assigning meaningful values to the Drake equation factors will require empirical data, collection of which is still preliminary.
Rare Earth hypothesis
Further information: Evolving the Alien: The Science of Extraterrestrial Life
The Rare Earth hypothesis attempts to resolve the Fermi paradox by suggesting that Earth is not typical, but unusual, and perhaps even unique. This is a rejection of the mediocrity principle. While a unique Earth has had historical support on philosophical or religious grounds, the Rare Earth Hypothesis deploys quantifiable and statistical arguments in support of the theory that multicellular life is exceedingly rare in the universe because Earth-like planets are themselves exceedingly rare. Supporters argue that many improbable coincidences have converged to make complex life on Earth possible.
Insofar as the Rare Earth Hypothesis privileges Earth-life and its process of formation, it is a variant of the anthropic principle. The variant of the Anthropic Principle states the universe seems uniquely suited towards developing human intelligence. Any variation in any one of a myriad universal constants would make developing similar intelligent life more difficult. This philosophical stance opposes not only mediocrity, but the Copernican principle more generally, which suggests there is no privileged location in the universe.
Opponents dismiss both Rare Earth and the anthropic principle as tautological — if a condition must exist in the universe for human life to arise, then the universe must already meet that condition, as human life exists — and as an unimaginative argument. According to this analysis, the Rare Earth hypothesis confuses a description of how life on Earth arose with a uniform conclusion of how life must arise. While the probability of the specific conditions on Earth being widely replicated may be low, complex life may not require exclusively Earth-like conditions in order to evolve.
Resolving the paradox empirically
One obvious way to resolve the Fermi paradox would be to find conclusive evidence of extraterrestrial intelligence. Various efforts to find such evidence have been made since 1960, and several are ongoing. As human beings do not have interstellar travel capability, such searches are being carried out at great distances and rely on careful analysis of very subtle evidence. This limits possible discoveries to civilizations which alter their environment in a detectable way, or produce effects that are detectable at a distance, such as radio emissions. Non-technological civilizations are very unlikely to be detectable from Earth in the near future (though microbial life may be deduced in the Solar System).
One difficulty in searching is avoiding an overly anthropomorphic viewpoint. Conjecture on the type of evidence likely to be found often focuses on the types of activities that humans have performed, or likely would perform given more advanced technology. Intelligent aliens might avoid these "expected" activities, or perform activities totally novel to humans.
Further information: SETI
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. Sensitive 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 terrestrial civilization.
Therefore, the careful searching of radio emissions from space for non-natural signals may lead to the detection of alien civilizations. Such signals could be either "accidental" byproducts of a civilization, or deliberate attempts to communicate, such as the Communication with Extraterrestrial Intelligence's Arecibo message. A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through the SETI organization, although other approaches, such as optical SETI also exist.
Several decades of SETI analysis has not revealed any main sequence stars with unusually bright, or meaningfully repetitive radio emissions, although there have been several candidate signals: on August 15, 1977 the "Wow! signal" was picked up by The Big Ear radio telescope. It lasted for only 72 seconds, and has not been repeated. In 2003, Radio source SHGb02+14a was isolated by SETI@home analysis, although it has largely been discounted by further study. There are numerous technical assumptions underlying SETI that may cause human beings to miss radio emissions with present search techniques; these are discussed below.
Direct planetary observation
A composite picture of Earth at night. Human civilization is detectable from space.Detection and classification of exoplanets has come about out of recent refinements in mainstream astronomical instruments and analysis. While this is a new field in astronomy—the first published paper claiming to have discovered an exoplanet was released in 1989—it is possible that planets which are likely to be able to support life will be found in the near future. Direct observational evidence for the existence of life may eventually be observable, such as the absorption spectrum of chlorophyll. Other detectable biotic signatures include methane and oxygen and, for advanced civilizations, trace industrial gases such as freon. More obvious evidence of an alien technological civilization requires precise imaging (see right).
Exoplanets are rarely directly observed (the first claim to have done so was in 2005) rather their existence is inferred based on effects caused in their orbiting a star. Currently, the size and orbit of an exoplanet can be deduced. This information, along with the stellar classification of its sun, and educated guesses as to its composition based on its size and comparisons to studied bodies, allows for rough approximations of the planetary environment.
The methods for exoplanet detection are not likely to deduce Earth-like life at present, given that most exoplanets discovered are Jupiter mass or larger. As of 9 October 2006, 210 extrasolar planets have been discovered — 159 in single-planet systems and 51 planets in 21 multiple-planet systems.
Probes, colonies, and other artifacts
Further information: Von Neumann probe and Bracewell probe
As noted, given the size and age of the universe, and the relative rapidity at which dispersion of intelligent life can occur, evidence of alien colonization attempts might plausibly be discovered. Additionally, evidence of "unbeinged" exploration in the form of probes and information gathering devices may await discovery.
Some theoretical exploration techniques such as the Von Neumann probe could exhaustively explore a galaxy the size of the Milky Way in as little as half a million years, with relatively little investment in materials and energy relative to the results. If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Evidence of such probes might be found in the solar system—perhaps in the asteroid belt where raw materials would be plentiful and easily accessed.
Another possibility for contact with an alien probe—one that would be trying to find human beings—is an alien Bracewell probe. Such a 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.
Since the 1950s direct exploration has been carried out on a small fraction of the solar system and no evidence that it has ever been visited by alien colonists, or probes, has been uncovered. Detailed exploration of areas of the solar system where resources would be plentiful—such as the asteroids, the Kuiper belt, the Oort cloud and the various planetary ring systems—may yet produce evidence of alien exploration, though these regions are also massive and difficult to investigate. There have been preliminary efforts in this direction in the form of the SETA and SETV projects to search for extraterrestrial artifacts or other evidence of extraterrestrial visitation within the solar system. There have also been attempts to signal, attract, or activate Bracewell probes in Earth's local vicinity, including by scientists Robert Freitas and Francisco Valdes. Many of the projects that fall under this umbrella are considered "fringe" science by astronomers and none of the various projects have located any artifacts.
Should alien artifacts be discovered, even here on Earth, they may not be recognizable as such. The products of an alien mind and an advanced alien technology might not be perceptible or recognizable as artificial constructs. Exploratory devices in the form of bio-engineered life forms created through synthetic biology would presumably disintegrate after a point, leaving no evidence; an alien information gathering system based on molecular nanotechnology could be all around us at this very moment, completely undetected. Clarke's third law suggests that an alien civilization well in advance of humanity's might have means of investigation that are not yet conceivable to human beings.
Advanced stellar scale artifacts
A variant of the speculative Dyson sphere. Such large scale artifacts would drastically alter the spectrum of a star.Further information: Dyson sphere
In 1959, Dr. Freeman Dyson observed that every developing human civilization constantly increases its energy consumption, and theoretically, a civilization of sufficient age would require all the energy produced by its sun. The Dyson Sphere was the thought experiment solution that he derived: a shell or cloud of objects enclosing a star to harness as much radiant energy as possible. Such a feat of astroengineering would drastically alter the observed spectrum of the sun, changing it at least partly from the normal emission lines of a natural stellar atmosphere, to that of a black body radiation, probably with a peak in the infrared. Dyson himself speculated that advanced alien civilizations might be detected by examining the spectra of stars, searching for such an altered spectrum.
Since then, several other theoretical stellar-scale megastructures have been proposed, but the central idea remains that a highly advanced civilization — Type II or greater on the Kardashev scale — could alter its environment enough as to be detectable from interstellar distances.
However, such constructs may be more difficult to detect than originally thought. Dyson spheres might have different emission spectra depending on the desired internal environment; life based on high-temperature reactions may require a high temperature environment, with resulting "waste radiation" in the visible spectrum, not the infrared. Additionally, a variant of the Dyson sphere has been proposed which would be difficult to observe from any great distance; a Matrioshka Brain is a series of concentric spheres, each radiating less energy per area than its inner neighbour. The outermost sphere of such a structure could be close to the temperature of the interstellar background radiation, and thus be all but invisible.
There have been some preliminary attempts to find evidence of the existence of Dyson spheres or other large Type-II or Type-III Kardashev scale artifacts that would alter the spectra of their core stars, but optical surveys have not located anything. Fermilab has an ongoing program to find Dyson spheres, but such searches are preliminary and incomplete as yet.
Explaining the paradox theoretically
Certain theoreticians accept that the apparent absence of evidence proves the absence of extraterrestrials and attempt to explain why. Others offer possible frameworks in which the silence may be explained without ruling out the possibility of such life, including assumptions about extraterrestrial behaviour and technology.
They do not exist ...
The simplest explanation is that the human species is alone in the galaxy. Several theories along these lines have been proposed, explaining why intelligent life might be either very rare, or very short lived.
... and they never did
See also: Rare Earth hypothesis
Those who believe that extraterrestrial intelligent life does not exist in the galaxy argue that the conditions needed for life—or at least complex life—to evolve are rare or even unique to Earth. While some have pointed out that complex life may evolve through other mechanisms than those found specifically here on Earth, the fact that in the extremely long history of life on the Earth only one species has developed a civilization to the point of being capable of space flight and radio technology seems to lend more credence to the idea of technologically advanced civilization being a rare commodity in the universe.
For example, the emergence of intelligence may have been an evolutionary accident. Geoffrey Miller proposes that human intelligence is the result of runaway sexual selection, which takes unpredictable directions. Steven Pinker, in his book How the Mind Works, cautions that the idea that evolution of life (once it has reached a certain minimum complexity) is bound to produce intelligent beings, relies on the fallacy of the "ladder of evolution": As evolution does not strive for a goal but just happens, it uses always the adaption most useful for a given ecological niche, and the fact that, on Earth, this lead to language-capable sentience only once so far may suggest that this adaption is only rarely a good choice and hence by no means a sure endpoint of the evolution of a tree of life.
Another theory along these lines is that even if the conditions needed for life might be common in the universe, that the formation of life itself, a complex array of molecules that are capable simultaneously of reproduction, the creation or extraction of all base components that it uses to build itself, from the environment, and of obtaining energy in a form that it can use to maintain the reaction (or the initial abiogenesis on a potential life-bearing planet), might ultimately be very rare even if worlds that might have the proper initial conditions for life might be common.
... because an inhospitable universe destroys complex intelligent life
Another possibility is that life can and does arise elsewhere, but events such as ice ages, impact events, or other catastrophes prevent complex life forms from evolving. Even if initial conditions for the development of life are not unique to Earth, it may be that on most worlds such events routinely and periodically destroy such life. Even if a "benign local environment" might exist on some world long enough for intelligent life to finally arise, such life might also be exterminated by cosmological events (such as supernovae, or gamma ray bursts) suddenly sterilizing previously hospitable regions of space.
... because it is the nature of intelligent life to destroy itself
See also: Doomsday argument
Technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or space flight technology. Possible means of annihilation include nuclear war, biological warfare or accidental contamination, nanotechnological catastrophe, or a Malthusian catastrophe after the deterioration of a planet's ecosphere. This general theme is explored both in fiction and in mainstream scientific theorizing. Indeed, there are probabilistic arguments which suggest that humanity's end may occur sooner rather than later. In 1966 Sagan and Shklovskii suggested 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, the "external transmission" or interstellar communicative phase may be the point at which the system becomes unstable and self-destructs.
From a Darwinian perspective, self-destruction is a paradoxical outcome of evolutionary success. The evolutionary psychology that developed during the competition for scarce resources over the course of human evolution has left the species subject to aggressive, instinctual drives to consume resources, increase longevity, and to reproduce — in part, the very motives that lead to the development of technological society. It seems likely that intelligent extraterrestrial life would evolve subject to similar conditions and thus face the same possibility of self-destruction. It has been suggested, for instance, that a successful alien species will be a superpredator, as is Homo sapiens.
... because it is the nature of intelligent life to destroy others
See also: technological singularity and Von Neumann probe
Another possibility is that intelligent species beyond a certain point of technological capability will destroy other intelligence as it appears. The idea that someone, or something, is destroying intelligent life in the universe is well explored in science fiction, for instance. The primary motive would be perceived competition for an aggressive, expansionist species. In 1981, cosmologist Edward Harrison also pointed out that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies would view any other species bent on galactic expansion as a kind of virus.
Violent extermination of other civilizations is not an unrealistic goal. The concept of self-replicating spacecraft need not be limited to exploration or communication, but can be applied to aggression. Even if a civilization who created such machines were to disappear, the probes could outlive their creators, destroying civilizations far into the future.
While it appears plausible that intelligent life tends to suppress other intelligent life, the idea can be criticized as continuing to beg the question at the heart of the Fermi Paradox: if intelligence destroys upstart intelligence, why is humanity still here? One might suggest that alien intelligences are so far advanced that ours is too insignificant to destroy.
However it should be noted that hunters often do not hide themselves from their prey. Often they try to make something that will attract their intended prey.
... because God created humans alone
Although not a testable scientific explanation, the belief that a God or gods has placed humanity as the only intelligent life in the universe is widespread through cultures and history. Tenets of the Judeo-Christian tradition can be interpreted to position human beings as unique in the universe and have created ambivalent ideas regarding the question of alien life. While not necessarily an outcome of the Rare Earth Hypothesis, like Rare Earth it is a variant of the strong anthropic principle, which in this case becomes teleological: the universe has to be this way, or it was designed to be this way, for the express purpose of creating human intelligence.
They do exist ...
It may be that technological extraterrestrial civilizations exist, but that human beings cannot communicate with them because of various constraints: problems of scale or of technology; because their nature is simply too alien for meaningful communication; or because human society refuses to admit to evidence of their presence.
... but communication is impossible due to problems of scale
Intelligent civilizations are too far apart in space to communicate
It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication. If two civilizations are separated by several thousand light years, it is very possible that one or both cultures may become extinct before meaningful dialogue can be established. Human searches may be able to detect their existence, but communication will remain impossible because of distance. This problem might be ameliorated somewhat if contact/communication is made through a Bracewell probe. In this case at least one partner in the exchange may obtain meaningful information. This theory is similar to another known as 'Divine Quarantine', which suggests that some form of God purposely sets different instances of life so far from one another that they could never contact each other and thus never destroy one another through purposeful or inadvertent interaction.
Intelligent civilizations are too far apart in time to communicate
See also: Relativity of simultaneity and #...because it is the nature of intelligent life to destroy itself.
Given the length of time that intelligent life has existed on Earth or is likely to exist, the "window of opportunity" for detection or contact might be quite small. Advanced civilizations may periodically arise and fall throughout our galaxy, but this may be such a rare event, relatively speaking, that the odds of two or more such civilizations existing at the same time are low. There may have been intelligent civilizations in the galaxy before the emergence of intelligence on Earth, and there may be intelligent civilizations after its extinction, but it is possible that human beings are the only intelligent civilization in existence now. The term "now" is somewhat complicated by the finite speed of light and the nature of spacetime under relativity. Assuming that an extraterrestrial intelligence is not able to travel to our vicinity at faster-than-light speeds, in order to detect an intelligence 1,000 light-years distant, that intelligence will need to have been active 1,000 years ago.
There is a possibility that archaeological evidence of past civilizations may be detected through deep space observations — especially if they left behind large artifacts such as Dyson spheres — but this seems less likely than detecting the output of a thriving civilization.
A special case of this is that humans may well be the first intelligent life in the galaxy. This is often dismissed as a result of two factors: the idea that if there was going to be any other intelligent life, it would exist by now, and the tendency on the part of scientists to try to avoid anthropocentrism. Unlikely though it may be, it is still a possibility.
It is too expensive to spread physically throughout the galaxy
See also: Project Daedalus, Project Orion (nuclear propulsion), and Project Longshot
Many assumptions about the ability of an alien culture to colonize other stars are based on the idea that interstellar travel is technologically feasible. While the current 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. This idea underlies the concept of the Von Neumann probe and the Bracewell probe as evidence of extraterrestrial intelligence.
It is possible, however, that present scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood and the cost of materials and energy for such ventures may be so high as to make it unlikely that any civilization could afford to attempt it. This possibility has been examined in terms of percolation theory: colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the fact that colonies will inevitably develop a culture and civilization of their own. Colonization will thus occur in "clusters," with large areas remaining uncolonized at any one time.
This possibility is a reasonable explanation as to why a past civilization hasn't already attempted colonization of the earth and other nearby stars. As the origin civilization colonies from its home planet, with current understanding of natural limits on speed of communication and travel taken into account, colonies will invariably build their own civilizations and break away from the original. The expensive self-sustaining mechanism of expansion might naturally break down after a natural limit has been reached. Since the entire colonizing effort will likely have been within a short cosmological time frame, entire colonizing "bubbles" may have died off or have yet to be developed. Due to the large scale of our galaxy there may be dozens if not hundreds of these colonizing bubbles in existence today and not one is in direct contact with another.
Human beings have not been searching long enough
Humanity's ability to detect and comprehend 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 (about 200,000 years) is a very brief period on a cosmological scale, a position which changes little even if the species survives for hundreds of thousands of years more. Thus it remains possible that human beings have neither been searching long enough to find other intelligences, nor existed long enough to be found.
One million years ago there would have been no humans for alien emissaries to meet. For each further step back in time, there would have been increasingly fewer indications to such emissaries that intelligent life would develop on Earth. In a large and already ancient universe, a space-faring alien species may well have had many other more promising worlds to visit and revisit. Even if alien emissaries visited in more recent times, they may have been misinterpreted by early human cultures as supernatural entities.
This hypothesis is more plausible if alien civilizations tend to stagnate or die out, rather than expand. However, "the probability of a site never being visited, even [with an] infinite time limit, is a non-zero value". Thus, even if intelligent life expands elsewhere, it remains statistically possible that terrestrial life will go undiscovered.
They haven't got back to us yet
Artificial radio waves emitting from Earth have only propagated since first broadcasts made by Popov, Marconi and Tesla in 1895. This would mean that as of 2006, only intelligent extraterrestrial life within 55 light years would have been able to receive the signal and manage to send a reply back to Earth. Or perhaps first and subsequent signals were too weak to be received, and detection may have only been possible at the beginning of the space age, in 1957. In that case, only aliens within 24 light years would have been able to communicate back to earth. With time, the number of potential alien planets within reach increases.
... but communication is impossible for technical reasons
Human beings are not listening properly
There are some assumptions that underlie the SETI search programs that may cause searchers to miss signals that are present. For example, the radio searches to date would completely miss highly compressed data streams (which would be almost indistinguishable from "white noise" to anyone who did not understand the compression algorithm). Extraterrestrials might also use frequencies that scientists have decided are unlikely to carry signals, or use modulation strategies that are not being looked for. "Simple" broadcast techniques might be employed, but sent from non-main sequence stars which are searched with lower priority; current programs assume that most alien life will be orbiting Sun-like stars.
The greatest problem is the sheer size of the radio search needed to look for signals, 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. Clearly detecting an Earth type civilization at great distances is difficult. A signal is much easier to detect if the signal energy is focused in either a narrow range of frequencies (Narrowband transmissions), and/or directed at a specific part of the sky. Such signals can 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 go so far as to assume that extraterrestrial civilizations will be broadcasting a deliberate signal (like the Arecibo message), in order to be found.
Thus to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortuitous circumstances: that the broadband radio emissions of alien radio technology are much stronger than our own; that one of SETI's programs is listening to the correct frequencies from the right regions of space; or that aliens are sending focused transmissions such as the Arecibo message in our general direction.
Civilizations only broadcast detectable radio signals for a brief period of time
It may be that alien civilizations are detectable through their radio emissions only for a short time period, reducing the likelihood of spotting them. There are two possibilities in this regard: civilizations outgrow radio through technological advance or, conversely, resource depletion cuts short the time in which a species broadcasts.
The first idea, that civilizations advance beyond radio, is based in part on the "fiber optic objection": the use of broadcast technologies for the long-distance transmission of information is fundamentally wasteful of energy and bandwidth, as broadcasts typically radiate in all directions evenly and large amounts of power are needed. Human technology is currently moving away from broadcast for long-distance communication and replacing it with wires, optical fibers, narrow-beam microwave and laser transmission. Most recent technologies that employ radio, such as cell phones and Wi-Fi networks, use low-power, short-range transmitters to communicate with numerous fixed stations that are themselves connected by wire or narrow beam radio. Television, as developed in the mid-twentieth century, employs transmitters with strong narrow-band carrier signals that are perhaps the most detectable human signals at stellar range; however digital television is replacing this technology and uses wide-band spread spectrum modulation with much lower carrier power. It is argued that these trends will make the Earth much less visible in the radio spectrum within a few decades. Hypothetically, advanced alien civilizations evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by principles of physics we don't yet understand. Thus it seems plausible that other civilizations would only be detectable for a relatively short period of time between the discovery of radio and the switch to more efficient technologies.
A different argument is that resource depletion will soon result in a decline in technological capability. Human civilization has been capable of interstellar radio communication for only a few decades and is already rapidly depleting fossil fuels and grappling with the problem of peak oil. It may only be a few more decades before energy becomes too expensive, and the necessary electronics and computers too difficult to manufacture, for societies to continue the search. If the same conditions regarding energy supplies hold true for other civilizations, then radio technology may be a short-lived phenomenon. Unless two civilizations happen to be near each other and develop the ability to communicate at the same time it would be virtually impossible for any one civilization to "talk" to another.
Critics of the resource depletion argument point out that an energy-consuming civilization is not dependent solely on fossil fuels. Alternate energy sources exist, such as solar power which is renewable and has enormous potential relative to technical barriers. For depletion of fossil fuels to end the "technological phase" of a civilization some form of technological regression would have to invariably occur, preventing the exploitation of renewable energy sources.
They tend to experience a technological singularity
See also: Sentience Quotient and Matrioshka brain
Another possibility is that technological civilizations invariably experience a technological singularity and attain a posthuman (or postalien) character. Theoretical civilizations of this sort may have altered drastically enough to render communication impossible. The intelligences of a post-singularity civilization might require more information exchange than is possible through interstellar communication, for example. Or perhaps any information humanity might provide would appear elementary. Because of this they do not try to communicate, any more than human beings attempt to talk to ants.
Even more extreme forms of post-singularity have been suggested, particularly in fiction: beings that divest themselves of physical form, create massive artificial virtual environments, transfer themselves into these environments through mind transfer, and exist totally within virtual worlds, ignoring the external physical universe. Surprisingly early treatments, such as Lewis Padgett's short story Mimsy were the Borogroves (1943), suggest a migration of advanced beings out of the presently known physical universe into a different and presumably more agreeable alternate one.
One version of this perspective, which makes predictions for future SETI findings of transcension "fossils" and includes a variation of the Zoo hypothesis below, has been proposed by singularity scholar John Smart They also may be as far superior to us as we are to ants, not being able to communicate meaningfully.
... and they choose not to communicate
This idea is most plausible if there is a single alien civilization within contact range, or there is a homogeneous culture or law amongst alien civilizations which dictates that the Earth be shielded. If there is a plurality of alien cultures, however, this theory may break down under the uniformity of motive flaw: all it takes is a single culture or civilization to decide to act contrary to the imperative for it to be abrogated, and the probability of such a violation increases with the number of civilizations.
Earth is purposely isolated (The zoo hypothesis)
Main article: Zoo hypothesis
It is possible that the belief that alien races would communicate with the human species is a fallacy, and that alien civilizations may not wish to communicate, even if they have the technical ability. A particular reason that alien civilizations may choose not to communicate is the so-called Zoo hypothesis: the idea that Earth is being monitored by advanced civilizations for study, or is being preserved in an isolated "zoo or wilderness area".
The motivation may be ethical (encouraging humanity's independent development) or strategic (aliens wish to avoid detection and possible destruction at the hands of other civilizations). These ideas are similar to the Prime Directive of the "United Federation of Planets" in the fictional Star Trek television series. This possibility has caused some to speculate that humanity needs to pass a certain ethical, technological or social boundary before being allowed to make contact with existing advanced alien civilizations.
They are too alien
See also: technological singularity
Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Alien psychologies may simply be too different to communicate with, and realizing this, they do not make the attempt. It is also possible that the very concept of communication with other species is one which they cannot conceive. Human mathematics, language, tool use, and other cornerstones of technology and communicative capacity may be parochial to Earth and not shared by other life. Using Earth as an example, it is possible to conceive of dolphins evolving intelligence, but such an intelligence might have difficulty developing technology (and particularly key aspects of our sort of technology, for example fire and electricity).
They are not interested
Most scenarios for communication with other civilizations rest upon the assumption that these other races share our scientific curiosity and our desire to make contact. This may be incorrect. It is entirely possible that, for cultural reasons of its own (such as a religious taboo, xenophobia, or simple indifference), an alien society may have no desire to talk to others even if it has the technical capability.
... and they are here unobserved
It may be that intelligent alien life forms not only exist, but are already present here on Earth. They are not detected because they do not wish it, human beings are technically unable, or because societies refuse to admit to the evidence.
They hide their presence
It is not unreasonable that a life form intelligent enough to travel to Earth would also be sufficiently intelligent to exist here undetected. In this view, the aliens have arrived on Earth, or in our solar system, and are observing the planet, while concealing their presence. While it seems unlikely that alien observers could move amongst the general population undetected for any great length of time, such observation could be conducted in a number of other ways that would be very difficult to detect. For example, a complex system of microscopic monitoring devices constructed via molecular nanotechnology could be deployed on Earth and remain undetected, or sophisticated instruments could conduct passive monitoring from elsewhere.
Human beings refuse to see, or misunderstand, the evidence
Many UFO researchers and watchers argue that society as a whole is unfairly biased against claims of alien abduction, sightings, and encounters, and as a result may not be fully receptive to claims of proof that aliens are visiting our planet. Others use complex conspiracy theories to allege that evidence of alien visits is being concealed from the public by political elites who seek to hide the true extent of contact between aliens and humans. For example, it is also possible that human beings do in fact interact with many alien cultures, but that Earth functions as a kind of prison colony, with the aid of memory wipes. Scenarios such as these have been depicted in popular culture for decades.
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They're Made Out Of Meat
So much space, so little time: why aliens haven't found us yet by Ian Sample,The Guardian January 18, 2007
The Possibilities of FTL: Or Fermi's Paradox Reconsidered by F.E. Freiheit IV
Fermi's Paradox (i.e. Where are They?) by James Schombert
Life in the Universe, by Eric Schulman, Mercury Magazine (May/June to November/December 2000)
Answering the Fermi Paradox: Exploring the Mechanisms of Universal Transcension by John Smart
The Great Filter — Are We Almost Past It? by Robin Hanson
Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox, which argues that our observations are incomplete, and There Is No Fermi Paradox, arguing that the paradox is based on a logical flaw, both by Robert Freitas