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Fermi paradox

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   A graphical representation of the Arecibo message - Humanity's first
   attempt to communicate its existence to alien civilizations
   Enlarge
   A graphical representation of the Arecibo message - Humanity's first
   attempt to communicate its existence to alien civilizations

   The Fermi paradox is the apparent contradiction between high estimates
   of the probability of the existence of extraterrestrial civilizations
   and the lack of evidence for or 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.

Basis of the paradox

   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:


   Fermi paradox

     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.


   Fermi paradox

   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
   10^11) stars in the Milky Way and 70 sextillion (7 x 10^22) 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 (see below) 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 behaviour 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.

Related concepts

Drake equation

   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

   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
   Anthropic Principle notes that the universe seems uniquely suited to
   the development of human intelligence and that any variation in any one
   of a myriad of universal constants would make the development of
   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 (see Evolving the Alien: The
   Science of Extraterrestrial Life and Alternative biochemistry for
   further information).

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.

Radio 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. 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.
   Enlarge
   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.

Alien constructs

Probes, colonies, and other artifacts

   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. The SETA ( Search for
   Extraterrestrial Artifacts) and SETV ( Search for Extraterrestrial
   Visitation) projects have attempted to locate evidence 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

   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 (see links above), 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

   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 (see Rare Earth
   Hypothesis section above). 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.

   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

   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 (see Doomsday argument). 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

   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 behaviour 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 (see
   Berserker probe). 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?

... because God created humans alone

   Although not generally considered a testable scientific explanation,
   the belief that a creator deity has placed humanity at the unique focus
   of creation is widespread through cultures and history. Tenets of the
   Judeo-Christian religions can be interpreted to position human beings
   as unique in the universe and the only physical creatures with
   intelligence and free will. However, proponents of this belief fail to
   consider the possibility of such a deity creating other forms of life
   in other places in the universe, and thus may view the issue from a
   human-centric point of view.

   Although this belief is not a necessary outcome of the Rare Earth
   Hypothesis, like Rare Earth it is a variant of the anthropic principle.
   In this case, the principle 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

   NASA's conception of the Terrestrial Planet Finder. Is it possible
   alien civilizations are too far away for meaningful communication?
   Enlarge
   NASA's conception of the Terrestrial Planet Finder. Is it possible
   alien civilizations are too far away for meaningful communication?

   Intelligent civilizations are too far apart in space to communicate

   It may be that 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, the other, 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.

   Intelligent civilizations are too far apart in time to communicate

   Given the length of time that intelligent life has existed on Earth or
   is likely to exist (see discussion above), 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 (see Relativity of simultaneity). 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 archeological 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 good evidence 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

   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 (see Project Daedalus, Project Orion, and Project
   Longshot). 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.

   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 though 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 "fibre 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. 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 anyone other.

   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

   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 (see Matrioshka
   brain), 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

... and they choose not to communicate

   Earth is purposely isolated (The 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.

   This idea is most plausible if there is a single alien civilization
   within contact range, or there is a homogenous 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.

   They are too alien

   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 (see: They're Made Out Of Meat). 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). See also technological singularity above.

   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.

   They do not believe in life elsewhere

   Finally, there is the possibility that an intelligent alien species may
   believe itself to be alone in the universe. This is hardly out of the
   question, as there are many here on Earth who believe the same thing.
   Of course, such speculations about alien psychology and culture are far
   beyond the ability of science to evaluate.

... 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. Scenarios such as these
   have been depicted in popular culture for decades.

   A related series of views consider that alien entities have been
   communicating with humans throughout history, but utilizing methods and
   technologies that are outside most people's experience or imagination.
   "Signals" are arriving, but only a few individuals perceive them, and
   then rarely and possibly in a distorted manner. Accounts of
   communication have perhaps been reported in ancient religious texts
   (accounting for the wide variety of anecdotal reports of angels,
   demons, and so on) but have been dismissed or overlooked.

   As an example: if the human brain utilizes quantum mechanical processes
   in its operation (as theorized by Roger Penrose, Stuart Hameroff, and
   others) then it may be open to receiving some form of nonlocal "
   psychic" communication, perhaps using quantum entanglement. It has been
   proposed that some accounts of mystics, shamans, schizophrenics, and
   channelers may be such "garbled" communications, transmitted by
   non-human intelligences in this manner. According to quantum mechanics
   the transfer of information in the context of information theory is not
   possible using quantum nonlocal correlations. However, supporters of
   the idea of this form of communication idea believe that this may
   explain the "garbled", associative, and inspirational nature of the
   "messages" recorded in the world's religious and anthropological
   history. This idea also explains the evident absence of space travel,
   which is unnecessary to the community of alien intelligence
   communicating via this medium. However, the theories of Penrose,
   Hameroff, and others are not universally accepted, and have met with
   skepticism, nor has it yet been shown that quantum mechanical effects
   are required for consciousness to occur.

Suggested reading

     * Savage, Marshall T. (1992). The Millennial Project: Colonizing the
       Galaxy in 8 Easy Steps. Denver: Empyrean Publishing. ISBN
       0-9633914-8-8.
     * Webb, Stephen (2002). If the Universe Is Teeming with Aliens...
       Where Is Everybody?. Copernicus Books. ISBN 0-387-95501-1.

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