A graphical representation of the Arecibo message - Humanity's first attempt to use radio waves 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. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Arecibo Observatory This is the message with color added to highlight its separate parts. ... “Green people” redirects here. ...

The extreme age of the universe and its vast number of stars suggest that if the Earth is typical, extraterrestrial life should be common.^[1] Discussing this proposition with colleagues over lunch in 1950, the physicist Enrico Fermi asked: "Where are they?" (alternatively, "Where is everybody?")^[2][3] 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. 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.^[4] Not to be confused with physician, a person who practices medicine. ... Enrico Fermi (September 29, 1901 – November 28, 1954) was an Italian physicist most noted for his work on the development of the first nuclear reactor, and for his contributions to the development of quantum theory, particle physics and statistical mechanics. ... The Milky Way as seen from Death Valley The Milky Way is the galaxy where the Solar System (and the Earth) is located. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ... A von Neumann probe is a specific example of a hypothetical concept based on the work of Hungarian-born American mathematician and physicist John von Neumann. ... It has been suggested that this article or section be merged into UFO. (Discuss) An alien spacecraft is a hypothetic spacecraft originating from extraterrestrials. ... For other uses, see Radio (disambiguation). ...

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. “Green people” redirects here. ...

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. A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy (also frequently referred to as astrophysics) is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as the cosmic background radiation). ... This article needs additional references or sources for verification. ... This article or section does not cite any references or sources. ... The philosopher Socrates about to take poison hemlock as ordered by the court. ... Pigments other than green might dominate plant life on exoplanets[1] The DNA structure might not be the only nucleic acid in the universe capable of supporting life[2] Astrobiology (from Greek: ἀστρο, astro, constellation; βίος, bios, life; and λόγος, logos, knowledge) is the study of life in space, combining aspects of astronomy...

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: Spatial scale provides a shorthand form for discussing relative lengths, areas, distances and sizes. ... Probability is the likelihood that something is the case or will happen. ... For other senses of this word, see evidence (disambiguation). ... Look up definition in Wiktionary, the free dictionary. ...

The size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to exist. However, this hypothesis seems inconsistent with the lack of observational evidence to support it.

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.^[5] 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. To help compare orders of magnitude this page lists dimensionless numbers between 109 and 1012. ... The Milky Way as seen from Death Valley The Milky Way is the galaxy where the Solar System (and the Earth) is located. ... To help compare orders of magnitude this page lists dimensionless numbers between 1021 and 1024. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ... This article needs additional references or sources for verification. ... This article is about Earth as a planet. ... The eight planets and three dwarf planets of the Solar System. ... The Drake equation (rarely also called the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of exobiology, astrosociobiology and the search for extraterrestrial intelligence. ...

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 conclusive or certifiable evidence on Earth or elsewhere in the known universe of other intelligent life after 13 billion years of the universe's history, it may be assumed that intelligent life is rare or our assumptions about the general behavior of intelligent species are flawed. In economics, scarcity is defined as a condition of limited resources, where society does not have sufficient resources to produce enough to fulfill subjective wants. ... Habitat (which is Latin for it inhabits) is the place where a particular species live and grow. ...

The Fermi paradox can be asked in two ways. The first is "Why are no aliens or their artifacts physically here?". If interstellar travel is possible, even the "slow" kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy.^[6] This is a relatively small amount of time on a geological scale, let alone a cosmological one. Since there are many stars older than the sun, or since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy is not colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large scale exploration of the galaxy is still possible; the means of exploration and theoretical probes involved are discussed extensively below. However, no signs of colonization or exploration have been found. The table and timeline of geologic periods presented here is in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy. ... This article does not cite any references or sources. ...

The argument above may not hold for the universe as a whole, since travel times may well explain the lack of physical presence on Earth for far away galaxies. However, the question then becomes "Why do we see no signs of intelligent life?" as a sufficiently advanced civilization^[7] could potentially be seen over a significant fraction of the size of the observable universe^[8] Even if such civilizations are rare, since they could be detected from far away, many more potential sites for their origin are within our view. However, no signs of such civilizations have been detected. Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. ...

It is currently unclear which version of the paradox is stronger^[9]

Related concepts

Drake equation and the anthropic principle

Main articles: Drake equation and anthropic principle

While numerous theories and principles attend to the Fermi paradox, the two most closely related are the Drake equation and the anthropic principle. The Drake equation (rarely also called the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of exobiology, astrosociobiology and the search for extraterrestrial intelligence. ... In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence of observers (usually assumed to be carbon-based life or even specifically human beings). ... In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence of observers (usually assumed to be carbon-based life or even specifically human beings). ...

The former 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 fundamental problem is that the last four terms (fraction of planets with life, odds life becomes intelligent, odds intelligent life become communicative, and lifetime of communicating civilizations) are completely unknown. We have only one example, rendering statistical estimates impossible, and even the example we have is subject to a strong anthropic bias. Professor Frank Drake Frank Drake (born May 28, 1930, Chicago, Illinois) is an American astronomer and astrophysicist. ... Enrico Fermi (September 29, 1901 – November 28, 1954) was an Italian physicist most noted for his work on the development of the first nuclear reactor, and for his contributions to the development of quantum theory, particle physics and statistical mechanics. ... Media:Example. ... Anthropic bias is the bias arising when your evidence is biased by observation selection effects, according to philosopher Nick Bostrom. ...

The Drake equation has been used by both optimists and pessimists with wildly differing results. Dr. Carl Sagan, using optimistic numbers, 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. Skeptics, such as Frank Tipler, have put in pessimistic numbers and concluded that the average number of civilizations in a galaxy is much less than one.^[10] (Note that for our galaxy, at the current time, it must be at least one. This is an excellent example of anthropic bias. Any galaxy containing beings who might ask this question will contain at least one civilization, no matter how low the average is. Otherwise there is no one to ask the question.) Insert non-formatted text here Carl Edward Sagan (November 9, 1934 – December 20, 1996) was an American astronomer and astrobiologist and a highly successful popularizer of astronomy, astrophysics, and other natural sciences. ... The Milky Way as seen from Death Valley The Milky Way is the galaxy where the Solar System (and the Earth) is located. ...

Frank Drake himself has commented that the Drake Equation is unlikely to settle the Fermi paradox; instead it is just a way of organizing our ignorance on the subject.

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. Artists depiction of a hypothetical Wormhole Induction Propelled Spacecraft, based loosely on the 1994 warp drive paper of Miguel Alcubierre. ...

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. This article does not cite any references or sources. ... In mathematics, a conjecture is a mathematical statement which appears likely to be true, but has not been formally proven to be true under the rules of mathematical logic. ...

Radio emissions

Further information: SETI, Project Ozma, Project Phoenix (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^[11] 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. This article is about the search for extra-terrestrial intelligence. ... Project Ozma was a pioneering SETI experiment started in 1960 by Cornell University astronomer Frank Drake, at the National Radio Astronomy Observatory at Green Bank, West Virginia. ... Project Phoenix is a SETI project: a search for extraterrestrial intelligence by listening for radio signals. ... SERENDIP or Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations is a UC Berkeley SETI Program. ... ATA Dish Size Scale The Allen Telescope Array (ATA), formerly known as the One Hectare Telescope (1hT), is a joint effort by the SETI Institute and the Radio Astronomy Laboratory at the University of California, Berkeley to construct a radio interferometer that will be dedicated to astronomical and simultaneous search... The 64 meter radio telescope at Parkes Observatory A radio telescope is a form of directional radio antenna most often used in radio astronomy and in tracking and collecting data from satellites and space probes (see Deep Space Network), and are also used in the SETI project. ... STAR is an acronym for: Organizations Society of Ticket Agents and Retailers], the self-regulatory body for the entertainment ticket industry in the UK. Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ...

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. CETI (Communication with Extraterrestrial Intelligence, or METI, Messaging to Extraterrestrial Intelligence) is a branch of SETI research that focuses on composing and deciphering messages that could theoretically be understood by another technological civilization. ... Arecibo Observatory This is the message with color added to highlight its separate parts. ... This article is about the search for extra-terrestrial intelligence. ... This article is about the search for extra-terrestrial intelligence. ...

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. Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... is the 227th day of the year (228th in leap years) in the Gregorian calendar. ... Also: 1977 (album) by Ash. ... The WOW! Signal Credit: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO). ... The Big Ear was a radio telescope located on the grounds of the Ohio Wesleyan Universitys The Perkins Observatory from the 1960s to 1998 when it was disassembled. ... SHGb02+14a is a radio emitted frequency. ... SETI@home logo SETI@home (SETI at home) is a distributed computing project using Internet-connected computers, hosted by the Space Sciences Laboratory, at the University of California, Berkeley, in the United States. ...

Direct planetary observation

A composite picture of Earth at night. Human civilization is detectable from space.

Detection and classification of exoplanets has come 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. Download high resolution version (2400x1200, 534 KB)Composite image of the Earth at night, created by NASA and NOAA. NASA Description: This image of Earth’s city lights was created with data from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS). ... Download high resolution version (2400x1200, 534 KB)Composite image of the Earth at night, created by NASA and NOAA. NASA Description: This image of Earth’s city lights was created with data from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS). ... Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ...

Direct evidence for the existence of life may eventually be observable, such as the detection of biotic signature gases (such as methane and oxygen) — or even the industrial air pollution of a technologically advanced civilization — in an exoplanet's atmosphere by means of spectrographic analysis.^[12] With improvements in our observational capabilities, it may eventually even be possible to detect direct evidence such as that which humanity produces (see right). Methane is a chemical compound with the molecular formula CH4. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... Air pollution is a chemical, particulate matter, or biological agent that modifies the natural characteristics of the atmosphere. ... Extremely high resolution spectrogram of the Sun showing thousands of elemental absorption lines (fraunhofer lines) Spectroscopy is the study of the interaction between radiation (electromagnetic radiation, or light, as well as particle radiation) and matter. ...

However, exoplanets are rarely directly observed (the first claim to have done so was made in 2004^[13]); rather, their existence is usually inferred from the effects they have on the star(s) they orbit. This means that usually only the mass 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 (usually based on the mass of the planet, and its distance from its sun), allows for rough approximations of the planetary environment. In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. ...

The current methods for exoplanet detection are not likely to detect life-bearing Earth-like worlds. Methods such as gravitational microlensing can detect the presence of "small" worlds, potentially even smaller than the Earth, but can only detect such worlds for very brief moments of time, and no follow-up is possible. Other methods such as radial velocity and astrometry allow prolonged observations of exoplanet effects, but only work with worlds that are are several times the mass of Jupiter. These seem unlikely candidates to harbor Earth-like life. However, exoplanet detection and classification is a very active sub-discipline in astronomy, with 241 such planets being detected between 1988 and 2007,^[14] and the first possibly terrestrial planet discovered within a star's habitable zone being found in 2007.^[15] It is hoped that refinements in exoplanet detection methods will lead to increased detection and prolonged observation of terrestrial planets. Such observational refinements would allow us to better gauge how common potentially inhabitable worlds are, and thus allow us a much better idea of how common life in the universe might be, which of course has profound influence over the expectations behind the Fermi Paradox itself. Any planet is an extremely faint light source compared to its parent star. ... Gravitational microlensing is an astronomical technique used to detect planets - stellar mass objects in space using the gravitational lens effect. ... Any planet is an extremely faint light source compared to its parent star. ... Illustration of the use of optical wavelength interferometry to determine precise positions of stars. ... Adjectives: Jovian Atmosphere [4] Surface pressure: 20–200 kPa[8] (cloud layer) Scale height: 27 km Composition: Jupiter (IPA: or ) is the fifth planet from the Sun and the largest planet within the solar system. ... The inner planets, Mercury, Venus, Earth, and Mars, their sizes to scale. ... It has been suggested that Goldilocks phenomenon be merged into this article or section. ... Any planet is an extremely faint light source compared to its parent star. ...

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. A von Neumann probe is a specific example of a hypothetical concept based on the work of Hungarian-born American mathematician and physicist John von Neumann. ... A Bracewell probe is a hypothetical concept for an autonomous interstellar probe dispatched for the express purpose for communication with (an) alien civilization(s). ...

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.^[16] A von Neumann probe is a specific example of a hypothetical concept based on the work of Hungarian-born American mathematician and physicist John von Neumann. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ... The Milky Way as seen from Death Valley The Milky Way is the galaxy where the Solar System (and the Earth) is located. ... For details on the physical properties of bodies in the asteroid belt see Asteroid and Main-belt comet. ...

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.^[17] A Bracewell probe is a hypothetical concept for an autonomous interstellar probe dispatched for the express purpose for communication with (an) alien civilization(s). ... A line showing the speed of light on a scale model of Earth and the Moon The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness.[1] It is the speed of all electromagnetic... Garry Kasparov playing against Deep Blue, the first machine to win a chess game against a reigning world champion. ...

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 vast 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.^[18] 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.^[19] 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. 253 Mathilde, a C-type asteroid. ... The Kuiper belt, derived from data from the Minor Planet Center. ... This image is an artists rendering of the Oort cloud and the Kuiper Belt. ... A Bracewell probe is a hypothetical concept for an autonomous interstellar probe dispatched for the express purpose for communication with (an) alien civilization(s). ... Robert A. Freitas Jr. ...

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. Synthetic biology is a new area of research that combines science and engineering in order to design and build novel biological functions and systems. ... Molecular nanotechnology (MNT) is the concept of engineering functional mechanical systems at the molecular scale. ... Arthur C. Clarke formulated the following three laws of prediction: When a distinguished but elderly scientist states that something is possible, he is almost certainly right. ...

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, Kardashev scale, Alderson disk, Matrioshka brain, Stellar engine

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.^[20] Image File history File links Dyson_Sphere_Diagram. ... Image File history File links Dyson_Sphere_Diagram. ... A cut-away diagram of an idealized Dyson shell—a variant on Dysons original concept—1 AU in radius. ... A cut-away diagram of an idealized Dyson shell—a variant on Dysons original concept—1 AU in radius. ... Kardashev scale projections ranging from 1900 to 2100. ... An Alderson disk (named after Dan Alderson, its originator) is an artificial astronomical megastructure, like Nivens Ringworld or a Dyson sphere. ... A Matrioshka Brain is a hypothetical megastructure, based on the Dyson sphere, of immense computational capacity. ... // A stellar engine is a class of hypothetical megastructures which use a stars radiation to create usable energy. ... Freeman John Dyson (born December 15, 1923) is a British-born American physicist and mathematician, famous for his work in quantum mechanics, solid-state physics, nuclear weapons design and policy, and for his serious theorizing in futurism and science fiction concepts, including the search for extraterrestrial intelligence. ... A cut-away diagram of an idealized Dyson shell—a variant on Dysons original concept—1 AU in radius. ... In philosophy, physics, and other fields, a thought experiment (from the German Gedankenexperiment) is an attempt to solve a problem using the power of human imagination. ... Astroengineering is the construction of megastructures in space by technologically advanced beings. ... Extremely high resolution spectrogram of the Sun showing thousands of elemental absorption lines (fraunhofer lines) Spectroscopy is the study of the interaction between radiation (electromagnetic radiation, or light, as well as particle radiation) and matter. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ... Photo taken during the French 1999 eclipse The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone. ... As the temperature decreases, the peak of the black body radiation curve moves to lower intensities and longer wavelengths. ... Image of two girls in mid-infrared (thermal) light (false-color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. ...

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. A megastructure, in science fiction and speculative (or exploratory) engineering, is an enormous self-supporting artificial construct. ... Kardashev scale projections ranging from 1900 to 2100. ...

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.^[21] 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. A Matrioshka Brain is a hypothetical megastructure, based on the Dyson sphere, of immense computational capacity. ...

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,^[22] but such searches are preliminary and incomplete as yet. A cut-away diagram of an idealized Dyson shell—a variant on Dysons original concept—1 AU in radius. ... Kardashev scale projections ranging from 1900 to 2100. ... Aerial view of the Fermilab site. ...

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.

No other civilizations currently 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.

No other civilizations have arisen

See also: Rare Earth hypothesis

Those who believe that extraterrestrial intelligent life does not exist argue that the conditions needed for life—or at least complex life—to evolve are rare or even unique to Earth. This is known as the Rare Earth hypothesis, which 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/or many improbable coincidences have converged to make complex life on Earth possible.^[23] 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. The Rare Earth hypothesis is a hypothesis in planetary astronomy and astrobiology which argues that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. ... For other uses, see Life (disambiguation). ... This article needs additional references or sources for verification. ... Look up Hypothesis in Wiktionary, the free dictionary. ... For Wikipedia statistics, see m:Statistics Statistics is the science and practice of developing human knowledge through the use of empirical data expressed in quantitative form. ... This article is about the tv programme Life on Earth. ...

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 the adaptation most useful for a given ecological niche, and the fact that, on Earth, this led 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. Geoffrey Miller Geoffrey Miller is a widely recognised evolutionary psychologist, whose work is in the tradition of scientists such as Richard Dawkins, Daniel Dennett, and Steven Pinker. ... Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ... Steven Pinker Steven Arthur Pinker (born September 18, 1954) is a prominent Canadian-born American experimental psychologist, cognitive scientist, and popular science writer known for his spirited and wide-ranging advocacy of evolutionary psychology and the computational theory of mind. ... How the Mind Works is a book by American cognitive scientist Steven Pinker, published in 1996. ... This article is about evolution in biology. ... A biological adaptation is an anatomical structure, physiological process or behavioral trait of an organism that has evolved over a period of time by the process of natural selection such that it increases the expected long-term reproductive success of the organism. ... Two lichenes species on a rock, in two different ecological niches In ecology, a niche is a term describing the relational position of a species or population in an ecosystem. ... The Tree-of-Life is a fictional plant (the ancestor of yams, with similar appearance and taste) in Larry Nivens Known Space universe, for which all Hominids have an in-built genetic craving. ...

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. For other uses, see Life (disambiguation). ... This article focuses on the history of thought regarding abiogenesis (the spontaneous generation of life from non-living sources). ...

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. This philosophical stance opposes not only mediocrity, but the Copernican principle more generally, which suggests there is no privileged location in the universe. In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence of observers (usually assumed to be carbon-based life or even specifically human beings). ... This article needs additional references or sources for verification. ... The Copernican principle is the philosophical statement that no special observers should be proposed. ...

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.^[24] While the probability of the specific conditions on Earth being widely replicated is low, we do not know what complex life may require in order to evolve. In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence of observers (usually assumed to be carbon-based life or even specifically human beings). ... Within the study of logic, a tautology is a statement containing more than one sub-statement, that is true regardless of the truth values of its parts. ... The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. ... The argument from ignorance, also known as argumentum ad ignorantiam (appeal to ignorance [1]) or argument by lack of imagination, is a logical fallacy in which it is claimed that a premise is true only because it has not been proven false, or that a premise is false only 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, ill-advised physics experiments, 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.^[25] 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.^[26] The Doomsday argument (DA) is a probabilistic argument that claims to predict the future lifetime of the human race given only an estimate of the total number of humans born so far. ... Nuclear War is a card game designed by Douglas Malewicki, and originally published in 1966. ... For the use of biological agents by terrorists, see bioterrorism. ... Grey goo refers to a hypothetical end-of-the-world scenario involving molecular nanotechnology in which out-of-control self-replicating robots consume all living matter on Earth while building more of themselves (a scenario known as ecophagy). ... The Relativistic Heavy Ion Collider at Brookhaven National Laboratory. ... A Malthusian catastrophe, sometimes known as a Malthusian check, Malthusian crisis, Malthusian dilemma, Malthusian disaster, Malthusian trap, or Malthusian limit is a return to subsistence-level conditions as a result of agricultural (or, in later formulations, economic) production being eventually outstripped by growth in population. ... Ecosphere has several different meanings: In astronomy an ecosphere is an imaginary shell of space surrounding stars where conditions are such that life might survive. ... Insert non-formatted text here Carl Edward Sagan (November 9, 1934 – December 20, 1996) was an American astronomer and astrobiologist and a highly successful popularizer of astronomy, astrophysics, and other natural sciences. ... Iosif Samuilovich Shklovsky (Ио́сиф Самуи́лович Шкло́вский) (July 1, 1916 – March 3, 1985) was a Russian astronomer and astrophysicist. ...

From a Darwinian perspective, self-destruction would be 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 led 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, yet for destruction to provide a good answer Fermi's Question, it would have to be very nearly universal, that is, have a probability of very nearly 1.0. It has been suggested, for instance, that a successful alien species will be a superpredator, as is Homo sapiens.^[27] This article includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... This snapping turtle is trying to make a meal of a Canada goose, but the goose is too wary. ... Homo sapiens (Latin: wise man) is the scientific name for the human species. ...

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 has been well explored in science fiction^[28] and the scientific literature^[29]. A species might undertake such extermination out of expansionist motives, paranoia or simple aggression. 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 might view any other species bent on galactic expansion as a kind of virus.^[30] When plotted on a logarithmic graph, 15 separate lists of paradigm shifts for key events in human history show an exponential trend. ... A von Neumann probe is a specific example of a hypothetical concept based on the work of Hungarian-born American mathematician and physicist John von Neumann. ... Science fiction is a form of speculative fiction principally dealing with the impact of imagined science and technology, or both, upon society and persons as individuals. ... Edward Hardy Ted Harrison (1926–) is a Canadian artist notable for his paintings of the Yukon. ...

The extermination of other civilizations might be carried out with self-replicating spacecraft. Under such a scenario, even if a civilization that created such machines were to disappear, the probes could outlive their creators, destroying civilizations far into the future. It is hard to imagine any intelligent species actually wanting to develop such probes as they are dangerous to the creators as well as to all other life. Furthermore if their aim is to colonize the galaxy first they can fulfill their aim simply by being the first to colonize as the colonization time for the galaxy is much less than the age of the galaxy.^[31]

Human beings were created alone

Religious and philosophical speculation about extraterrestrial intelligent life long predates the modern scientific inquiry into the subject. Some religious thinkers, including the Jewish rationalist commentator Rabbi Hasdai Crescas (c. 1340–1410/1411)^[32] and the Christian mystic Nicholas of Cusa (1401–1464), posited the possibility of such extraterrestrial intelligence. On the other hand, at least some strains within the various Western religious traditions suggest the uniqueness of human beings in the divine plan and would counsel against belief in intelligent life on other worlds.^[33] Hasdai ben Abraham Crescas (c. ... Nicholas of Cusa Nicholas of Cusa (1401– August 11, 1464) was a German cardinal of the Roman Catholic Church, a philosopher, jurist, mathematician, and an astronomer. ...

Religious reasons for doubting the existence of extraterrestrial intelligent life, resemble some forms of the Rare Earth Hypothesis. The argument here would be a teleological form of the strong anthropic principle: the universe was designed for the express purpose of creating human (and only human) intelligence.^[34] Teleology is the philosophical study of purpose (from the Greek teleos, perfect, complete, which in turn comes from telos, end, result). ... In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence of observers (usually assumed to be carbon-based life or even specifically human beings). ...

They do exist, but we see no evidence

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.

Communication is impossible due to problems of scale

See also: Relativity of simultaneity

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 or time The relativity of simultaneity is the dependence of the notion of simultaneity on the observer. ... Image File history File links Download high resolution version (1900x1425, 138 KB) Terrestrial Planet Finder - Infrared interferometer concept source: http://photojournal. ... Image File history File links Download high resolution version (1900x1425, 138 KB) Terrestrial Planet Finder - Infrared interferometer concept source: http://photojournal. ... The National Aeronautics and Space Administration (NASA) is an agency of the United States government, responsible for the nations public space program. ... Terrestrial Planet Finder - Infrared interferometer concept The Terrestrial Planet Finder (TPF) is a plan by NASA for a telescope system that would be capable of detecting extrasolar terrestrial planets. ...

It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication.^[35] 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. Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present.^[36] A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one year. ... A Bracewell probe is a hypothetical concept for an autonomous interstellar probe dispatched for the express purpose for communication with (an) alien civilization(s). ...

The problem of distance is compounded by the fact that timescales affording a "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. NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ... A line showing the speed of light on a scale model of Earth and the Moon The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness.[1] It is the speed of all electromagnetic... In physics, spacetime is any mathematical model that combines space and time into a single construct called the space-time continuum. ... Faster-than-light (also superluminal or FTL) communications and travel are staples of the science fiction genre. ...

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 cut-away diagram of an idealized Dyson shell—a variant on Dysons original concept—1 AU in radius. ...

    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. An artists conception of the British Interplanetary Society design for Project Daedalus Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible interstellar unmanned spacecraft. ... An artists conception of the NASA reference design for the Project Orion spacecraft powered by nuclear propulsion. ... Project Longshot is a design for an interstellar spaceship, an unmanned probe intended to fly to Alpha Centauri powered by nuclear pulse propulsion. ... For other uses, see Faster than the speed of light (disambiguation). ... A von Neumann probe is a specific example of a hypothetical concept based on the work of Hungarian-born American mathematician and physicist John von Neumann. ... A Bracewell probe is a hypothetical concept for an autonomous interstellar probe dispatched for the express purpose for communication with (an) alien civilization(s). ...

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.^[37] In mathematics, percolation theory describes the behavior of connected clusters in a random graph. ... It has been suggested that this article or section be merged with Colonialism. ...

A similar argument holds that interstellar physical travel may be possible, but is much more expensive than interstellar communication. Furthermore, to an advanced civilization, travel itself may possibly replaced by communication, through mind uploading and similar technologies^[38]. Therefore the first civilization may have physically explored or colonized the galaxy, but subsequent civilizations find it cheaper, faster, and easier to travel and get information through contacting existing civilizations rather than physically exploring or traveling themselves. In this scenario, since there is little or no physical travel, and directed communications are hard to see except to the intended receiver, there could be many technical and interacting civilizations with few signs visible across interstellar distances. In transhumanism and science fiction, mind uploading (also occasionally referred to by other terms such as mind downloading, mind transfer, whole brain emulation, whole body emulation, or electronic transcendence) refers to the hypothetical transfer of a human mind to an artificial substrate, such as a computer simulation. ...

    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. The 64 meter radio telescope at Parkes Observatory A radio telescope is a form of directional radio antenna most often used in radio astronomy and in tracking and collecting data from satellites and space probes (see Deep Space Network), and are also used in the SETI project. ... Homo sapiens (Latin: wise man) is the scientific name for the human species. ... // For the history of humans on Earth, see History of the world. ...

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. Look up Supernatural in Wiktionary, the free dictionary. ...

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".^[39] Thus, even if intelligent life expands elsewhere, it remains statistically possible that such terrestrial life could eventually be discovered.

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 do not penetrate our atmosphere, or use modulation strategies that are not being looked for. The signals might be at a datarate that is too fast for our electronics to handle. "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.^[40] This article is about the search for extra-terrestrial intelligence. ... Calculated spectrum of a generated approximation of white noise White noise is a random signal (or process) with a flat power spectral density. ... In telecommunications, modulation is the process of varying a periodic waveform, i. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... A solar twin is the informal name for a star with characteristics similar to our own Sun. ...

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.^[41] 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.^[42] 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. The Arecibo Observatory is located approximately 9 miles south-southwest from Arecibo, Puerto Rico (near the extreme southwestern corner of Arecibo pueblo). ... A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one year. ... Narrowband (narrow bandwidth) refers to a signal which occupies only a small amount of space on the radio spectrum -- the opposite of broadband or wideband. ... Arecibo Observatory This is the message with color added to highlight its separate parts. ...

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. Arecibo Observatory This is the message with color added to highlight its separate parts. ...

    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. Optical fibers An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length by confining as much light as possible in a propagating form. ... Broadcasting is the distribution of audio and/or video signals which transmit programs to an audience. ... In communications, transmission is the act of transmitting electrical messages (and the associated phenonomena of radiant energy that pass through media). ... This article does not cite any references or sources. ... Microwaves are electromagnetic waves with wavelengths longer than those of terahertz (THz) frequencies, but relatively short for radio waves. ... Experiment with a laser (US Military) In physics, a laser is a device that emits light through a specific mechanism for which the term laser is an acronym: Light Amplification by Stimulated Emission of Radiation. ... Several mobile phones A mobile telephone or cellular telephone (commonly mobile phone or cell phone) is a long-range, portable electronic device used for mobile communication. ... Official Wi-Fi logo Wi-Fi was originally a brand licensed by the Wi-Fi Alliance to describe the embedded technology of wireless local area networks (WLAN) based on the IEEE 802. ... Digital television (DTV) is a telecommunication system for broadcasting and receiving moving pictures and sound by means of digital signals, in contrast to analog signals used by analog (traditional) TV. DTV uses digital modulation data, which is digitally compressed and requires decoding by a specially designed television set, or a... Spread-spectrum telecommunications is a technique in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information. ...

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. As first expressed in Hubbert peak theory, peak oil is the point or timeframe at which the maximum global petroleum production rate is reached. ...

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.^[43] 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. Fossil fuels are hydrocarbons, primarily coal and petroleum (fuel oil or natural gas), formed from the fossilized remains of dead plants and animals[1] by exposure to heat and pressure in the Earths crust over hundreds of millions of years[2]. The theory that hydrocarbons were formed from these... Solar power describes a number of methods of harnessing energy from the light of the sun. ... The end of civilization or the end of the world are phrases used in reference to human extinction scenarios, doomsday events, and related hazards which occur on a global scale. ... World renewable energy in 2005 (except 2004 data for items marked* or **). Enlarge image to read exclusions. ...

    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. The Sentience Quotient concept was invented by Robert A. Freitas Jr. ... A Matrioshka Brain is a hypothetical megastructure, based on the Dyson sphere, of immense computational capacity. ... When plotted on a logarithmic graph, 15 separate lists of paradigm shifts for key events in human history show an exponential trend. ... Posthuman Future by Michael Gibbs A posthuman or post-human is a hypothetical future being whose capabilities so radically exceed those of present humans as to be no longer human by current standards. ...

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 Borogoves (1943), suggest a migration of advanced beings out of the presently known physical universe into a different and presumably more agreeable alternate one. In transhumanism and science fiction, mind transfer (also referred to as mind uploading or mind downloading, depending on ones point of reference), whole body emulation, or electronic transcendence refers to the hypothetical transfer of a human mind to an artificial substrate. ... Lewis Padgett was the joint pseudonym of the science-fiction authors and spouses Henry Kuttner and C. L. Moore. ... Mimsy Were The Borogoves is a short story (now being made into a feature-length film titled The Last Mimzy) by Lewis Padgett originally published in 1943. ...

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.^[44] John Smart is a developmental systems theorist whose interests include accelerating change, computational autonomy, evolutionary development, and the technological singularity. ...

They choose not to interact with us

    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".^[45] The zoo hypothesis is one of a number of suggestions that have been advanced in response to the Fermi paradox, regarding the apparent absence of evidence in support of the existence of advanced extraterrestrial life. ... Look up fallacy in Wiktionary, the free dictionary. ...

Many other reasons that an alien race might avoid contact have been proposed. Aliens might only choose to allow contact once the human race has passed certain ethical, political or technological standards. They may not want to interfere with our natural independent progress^[46]. The Earth may have been set as an explicit experiment that contact would ruin, or it may be felt that it is too dangerous to make contact (for us or them). Perhaps advanced civilizations actively hide, not only from Earth but from everyone else, since the galaxy is a dangerous place.

These ideas are 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.^[47] In astrobiology, the Uniformity of Motive theory suggests that any civilization in the universe would go through similar technological steps in their development. ...

A related idea is that the perceived universe is a simulated reality. The planetarium hypothesis^[48] holds that beings may have simulated a universe for us that appears to be empty of other life, by design. The simulation argument^[49] by Bostrom holds that although such a simulation may contain other life, such life cannot be far in advance of us since a far more advanced civilization would be correspondingly hard to simulate. This explains why we see nothing since our own technology is not good enough to see other civilizations at close to our own level. This article or section needs copy editing for grammar, style, cohesion, tone and/or spelling. ...

    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 human beings, and they are unable or unwilling to make the attempt. Human mathematics, language, tool use, and other concepts and communicative capacity may be parochial to Earth and not shared by other life.^[50] When plotted on a logarithmic graph, 15 separate lists of paradigm shifts for key events in human history show an exponential trend. ... Euclid, Greek mathematician, 3rd century BC, as imagined by by Raphael in this detail from The School of Athens. ... A modern hammer is directly descended from ancient hand tools A tool or device is a piece of equipment which typically provides a mechanical advantage in accomplishing a physical task, or provides an ability that is not naturally available to the user of a tool. ...

    They are non-technological

It is not clear that a civilization of intelligent beings must be technological. If an alien species does not develop technology, because it is difficult in its environment, because it chooses not to, or for any other reason, it will be very hard for human beings to detect.^[51] Intelligence alone, as opposed to life, is not necessarily visible across interstellar distances. While there are various remote sensing techniques which could perhaps detect life-bearing planets, none of them has any ability to distinguish intelligent but non-technical life from non-intelligent life. Not even any theoretical methods for doing so have been proposed, short of an actual physical visit by an astronaut or probe. This is sometimes referred to as the "algae vs. alumnae" problem.^[51]

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.^[52]

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 conceivably 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. Molecular nanotechnology (MNT) is the concept of engineering functional mechanical systems at the molecular scale. ...

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. UFO can mean: Unidentified flying object United Future Organization, a Japanese-Brazilian electronic jazz band UFO, the rock band that previously featured Michael Schenker UFO, the Gerry Anderson TV series United Farmers of Ontario, a political party that formed the government in Ontario from 1919 to 1923 U.F.O... The Abduction Phenomenon is as umbrella term used to describe a number of kidnap individuals--sometimes called abductees--usually for medical testing or for sexual reproduction procedures. ... This is a list of UFO sighting flaps and well-known individual sightings. ... Roswell Daily Record, July 8, 1947, announcing the capture of a flying saucer. ... A conspiracy theory attempts to attribute the ultimate cause of an event or chain of events (usually political, social, or historical events), or the concealment of such causes from public knowledge, to a secret, and often deceptive plot by a covert alliance of powerful or influential people or organizations. ... Popular culture, sometimes called pop culture, consists of widespread cultural elements in any given society. ...

See also

• Astrosociobiology
• Fermi problem
• Anthropic principle

Astrosociobiology (also referred to as exosociobiology and xenosociology) is the speculative scientific study of extraterrestrial civilizations and their possible social characteristics and developmental tendencies. ... In physics, particularly in physics education, a Fermi problem or Fermi question is an estimation problem designed to teach dimensional analysis, approximation, and the importance of clearly identifying ones assumptions. ... In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence of observers (usually assumed to be carbon-based life or even specifically human beings). ...

References

October 25 is the 298th day of the year (299th in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... ISSN, or International Standard Serial Number, is the unique eight-digit number applied to a periodical publication including electronic serials. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era. ... is the 126th day of the year (127th in leap years) in the Gregorian calendar. ... Soviet redirects here. ... An astronomer or astrophysicist is a person whose area of interest is astronomy or astrophysics. ... This article does not cite its references or sources. ... The Arecibo Observatory in Puerto Rico, currently used by the SETI project in the search for extraterrestrial life Extraterrestrial life is life that may exist and originate outside the planet Earth, the only place in the universe currently known to support life. ... Kardashev scale projections ranging from 1900 to 2100. ... John David Barrow FRS (born November 29, 1952, London) is an English cosmologist, theoretical physicist, and mathematician. ... Frank J. Tipler (born in 1947 in Andalusia, Alabama) is a professor of mathematical physics at Tulane University in New Orleans, Louisiana. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era. ... is the 115th day of the year (116th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era. ... is the 115th day of the year (116th in leap years) in the Gregorian calendar. ... Science is the journal of the American Association for the Advancement of Science (AAAS). ... This article does not cite any references or sources. ... Astounding Stories was a seminal science fiction magazine founded in 1930. ... Peter D. Ward is a paleontologist and professor of Biology and of Earth and Space Sciences at the University of Washington. ... This article is about the search for extra-terrestrial intelligence. ... Fred Saberhagens Berserker series is a space opera in which robotic self_replicating machines intend to destroy all organic life. ... The Privileged Planet: How Our Place in the Cosmos is Designed for Discovery is a book by Guillermo Gonzalez and Jay Richards which attempts to use scientific evidence to show that the Earth isnt a result of natural processes, and that it was optimized for life and learning. ... The current Star Trek franchise logo Star Trek is an American science fiction entertainment series and media franchise. ...

Suggested reading

• Ben Zuckerman and Michael H. Hart, Extraterrestrials: Where Are They? ISBN 0521448034 Amazon
• 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. 
• Michaud, Michael (2006). Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials. Copernicus Books. ISBN 978-0387-28598-6. 
• Evolving the Alien: The Science of Extraterrestrial Life

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

Eric Schulman is an American astronomer and science humorist. ... Mercury is a science magazine that features articles and columns about astronomy for a general audience. ... John Smart is a developmental systems theorist whose interests include accelerating change, computational autonomy, evolutionary development, and the technological singularity. ... Robin Hanson is an associate professor of economics at George Mason University. ... Robert A. Freitas Jr. ...