Fermi paradox - Misplaced Pages

This is an old revision of this page, as edited by Marskell (talk | contribs) at 08:05, 11 May 2007 (→... because God created humans alone: tweaks). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Revision as of 08:05, 11 May 2007 by Marskell (talk | contribs) (→... because God created humans alone: tweaks)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)

This article is about Enrico Fermi's paradox. For the music album, see Fermi Paradox (album).

Template:Redirect5

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.

According to some observers, the extreme age of the universe and its vast number of stars suggest that extraterrestrial life should be common. Discussing this proposition 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 the problem of the probability of extraterrestrial life. Wider examination of the implications of the topic began with Michael Hart in 1975, and it is sometimes referred to as the Fermi-Hart paradox.

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

Related concepts

Drake equation

Main article: 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

Main article: Rare Earth hypothesis

The Rare Earth hypothesis attempts to resolve the Fermi paradox by rejecting the mediocrity principle, and asserting that Earth is not typical, but unusual or even unique. While a unique Earth has had historical support on philosophical or religious grounds, the Rare Earth Hypothesis uses quantifiable and statistical arguments to argue that multicellular life is exceedingly rare in the universe because Earth-like planets are themselves exceedingly rare and 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.

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

Further information: ], Project Ozma, Project Phoenix (SETI), SETI § SERENDIP, and Allen Telescope Array

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

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 their effects on the star they orbit. 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. Astronomers announced the discovery the first planet situated in the habitable zone of a star on April 26, 2007.

Alien constructs

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

Further information: ]

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 as a solution: 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

... 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

... 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 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? Also it is not clear why would not see them as a hunters often sets up beacons to attract their prey. One might suggest that alien intelligences are so far advanced that ours is too insignificant to destroy.

... because human beings were created 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 could be 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.

A related idea is that the perceived universe is a simulated reality in which we appear alone. Webb hypothesizes that aliens may have simulated a universe for us that appears to be empty of other life, by design. (In these cases, other intelligent life does exist but human beings appear to be alone.)

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.

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, 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.

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. 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, while radio transmissions have only been propagated from since 1895. Thus it remains possible that human beings have neither been searching long enough to find other intelligences, nor been in existence long enough to be found.

One million years ago there would have been no humans for any extraterrestrial 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 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.

... 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, and thus 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

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).

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 within our range of detection for it to be abrogated, and the probability of such a violation increases with the number of civilizations.

They are too alien

... 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.

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. Observation could be conducted in a number of 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.

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.

References

^ Sagan, Carl. Cosmos, Ballantine Books 1985
Shostak, Seth (25 October 2001). ""Our Galaxy Should Be Teeming With Civilizations, But Where Are They?"". Space.com. Space.com. {{cite web}}: Check date values in: |year= (help); External link in |publisher= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)CS1 maint: year (link)
Wesson, Paul (1992). "Cosmology, extraterrestrial intelligence, and a resolution of the Fermi-Hart paradox". Royal Astronomical Society, Quarterly Journal. 31: 161–170. ISSN 0035-8738. Retrieved 2007-05-06. {{cite journal}}: Unknown parameter |month= ignored (help)
Craig, Andrew (2003). ""Astronomers count the stars"". BBC News. BBC. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
Crawford, I.A., "Where are They? Maybe we are alone in the galaxy after all", Scientific American, July 2000, 38-43, (2000).
Tipler, Frank. The Most Advanced Civilization in the Galaxy is Ours, Mercury, vol. 5, pg. 5, 1982.
Peter Ward and Donald Brownlee. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books. January 2000. ISBN 0-387-98701-0.
Athena Andreadis. "E. T., Call Springer-Verlag!" SETI League Publications, 2000.
Mullen, Leslie (2002). ""Alien Intelligence Depends on Time Needed to Grow Brains"". Astrobiology Magazine. Space.com. {{cite web}}: External link in |publisher= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
Habitable Planet Signposts, Astrobiology magazine.
Schneider, Jean (2006-10-09). "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopaedia. Retrieved 2006-10-11. {{cite web}}: Check date values in: |date= (help)
Udry et al. (2007). "The HARPS search for southern extra-solar planets, XI. An habitable super-Earth (5 M⊕) in a 3-planet system". Astronomy and Astrophysics preprint: preprint.
Papagiannis, M. D. "Are We Alone or Could They be in the Asteroid Belt?," Quarterly Journal of the Royal Astronomical Society, 19, 277-281 (1978)
Bracewell, R. N. "Communications from Superior Galactic Communities," Nature, 186, 670-671 (1960). Reprinted in A.G. Cameron (ed.), Interstellar Communication, W. A. Benjamin, Inc., New York, pp. 243-248, 1963.
SETV projects
Freitas Jr., Robert A. and Valdes, Francisco. "The Search for Extraterrestrial Artifacts," Acta Astronautica, 12, No. 12, 1027-1034 (1985).
Dyson, Freeman, "Search for Artificial Stellar Sources of Infra-Red Radiation", Science, June 1960.
Niven, Larry, "Bigger than Worlds", Analog, March 1974.
"Fermilab Dyson Sphere search program". Fermi National Accelerator Laboratory. {{cite web}}: External link in |publisher= (help); Italic or bold markup not allowed in: |publisher= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
Bonnell, Jerry (2002). ""A Bad Day in the Milky Way"". NOVA Online. PBS. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
Darling, David. ""Extraterrestrial intelligence, hazards to"". The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Worlds of David Darling. {{cite web}}: External link in |work= (help); Italic or bold markup not allowed in: |work= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
Hawking, Stephen. ""Life in the Universe"". Public Lectures. University of Cambridge. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
Archer, Michael. "Slime Monsters Will Be Human Too," Nature Australia, vol. 22, 1989.
Soter, Steven (2005). ""SETI and the Cosmic Quarantine Hypothesis"". Astrobiology Magazine. Space.com. {{cite web}}: External link in |publisher= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
Wiker, Benjamin D. "Christianity and the Search for Extraterrestrial Life". crisismagazine.com.
cf. Gonzalez, Guillermo and Richards, Jay W. The Privileged Planet, Regnery, 2004.
Webb, pp 51-60.
Webb, Stephen. If the Universe Is Teeming With Aliens...Where Is Everybody?, Springer, 2002, pp. 62-71
Landis, Geoffrey. "The Fermi Paradox: An Approach Based on Percolation Theory", Journal of the British Interplanetary Society, London, vol 51, page 163-166, 1998.
Kinouchi, Osame. "Persistence solves Fermi Paradox but challenges SETI projects," Condensed Matter, 0112137 v1, December 2001.
Margaret C. Turnbull and Jill C. Tarter. "Target selection for SETI: A catalog of nearby habitable stellar systems," The Astrophysical Journal Supplement Series, 145: 181-198, March 2003.
SETI's FAQ, Sec 1.2.3
SETI's FAQ, Sec 1.6
History of Solar Energy, www.solarenergy.com
Smart, John, "Answering the Fermi Paradox: Exploring the Mechanisms of Universal Transcension", Journal of Evolution and Technology, June 2002.
John A. Ball. "The Zoo Hypothesis," Icarus, vol 19, issue 3, pp 347-349, July 1973.
Crawford, July 2000.
Schombert, James. "Fermi's paradox (i.e. Where are they?)" Lectures, University of Oregon.
Webb, pp. 27-60

External links

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
Beyond Kardaschev: Possible Answer to Fermi's Paradox by Paul Hughes
Fermi Paradox debate Astrobiology Magazine July 2002. Michael Meyer, Frank Drake, Christopher McKay, Donald Brownlee, & David Grinspoon.
Introduction an Drake equations for the Fermi's paradox

Template:Link FA

Categories: