Extraterrestrial life

Extraterrestrial life is life that may exist and originate outside the planet Earth, the only place in the universe known to support life. Its existence is currently hypothetical; there is yet no evidence of extraterrestrial life that has been widely accepted by the scientific community.

Most scientists hold that if extraterrestrial life exists, its evolution would have occurred independently in different places in the universe. An alternative hypothesis, held by a minority, is panspermia, which suggests that life in the universe could have stemmed from a smaller number of points of origin, and then spread across the universe, from habitable planet to habitable planet. These two hypotheses are not mutually exclusive.

Speculative forms of extraterrestrial life range from humanoid and monstrous beings seen in works of science fiction to life at the much smaller scale of bacteria.

Extraterrestrial life forms, especially intelligent ones, are often referred to in popular culture as aliens or ETs. The putative study and theorisation of ET life is known as astrobiology or xenobiology.

Possible basis of extraterrestrial life

Biochemistry

All life on Earth is based on the building block element carbon with water as the solvent in which bio-chemical reactions take place.

• Water is useful because it has a neutral pH, and due to its continued dissociation between hydroxide and hydronium ions.

2H₂O → H₃O⁺ + OH^-

As a result, it can dissolve both positive metallic ions and negative non metallic ions with equal ability. Furthermore, the fact that organic molecules can be either hydrophobic (repelled by water) or hydrophilic (soluble in water) creates the ability of organic compounds to orient themselves to form water enclosing membranes. The fact that solid water (ice) is less dense than liquid water also means that ice floats, thereby preventing the Earth's oceans from slowly freezing solid. Additionally, the Van der Waals forces between water molecules gives it an ability to store energy with evaporation, which upon condensation is released. This helps moderate climate, cooling the tropics and warming the poles, helping to maintain a thermodynamic stability needed for life.

• Carbon is fundamental to conventional terrestrial life for its immense flexibility in creating covalent chemical bonds with a variety of non-metallic elements, principally nitrogen, oxygen and hydrogen. Carbon dioxide and water together enable the storage of solar energy in sugars, such as glucose. The oxidisation of glucose releases biochemical energy needed to fuel all other biochemical reactions

6CO₂ + 6H₂O + sun energy → C₆H₁₂O₆ + 6O₂

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + biochemical energy

The ability to form organic acids (-COOH) and amine bases (NH₂-) gives it the possibility of neutralisation dehydrating reactions to build long polymer peptides and catalytic proteins from monomer amino acids, and with phosphates to build not only DNA, the information storing molecule of inheritance, but also adenosine triphosphate (ATP) the principle energy "currency" of cellular life.

Given their relative abundance and usefulness in sustaining life it has long been assumed that life forms elsewhere in the universe will also utilize these basic components. However, other elements and solvents might be capable of providing a basis for life. Silicon is usually considered the most likely alternative to carbon, though this remains improbable. Silicon life forms are proposed to have a crystalline morphology, and are theorized to be able to exist in high temperatures, such as planets closer to the sun. Life forms based in ammonia rather than water are also considered, though this solution appears less optimal than water.

Indeed, technically life is little more than any self-replicating reaction, which could arise in a great many conditions and with various ingredients, though carbon-oxygen within the liquid temperature range of water seems most conducive. Suggestions have even been made that self-replicating reactions of some sort could occur within the plasma of a star, though it would be highly unconventional, since plasma is essentially the fourth state of matter, where electrons are not bound in their orbits around atomic nuclei.

Theoretical evolution and morphology

Along with the biochemical basis of extraterrestrial life, there remains a broader consideration of evolution and morphology. What might an alien look like? Science fiction has long shown a bias towards humanoid or (often in the case of villains) reptilian forms. The classical alien is light green or grey skinned, with a large head, and the typical four limb and two to five digit structure—i.e., it is fundamentally humanoid with a large brain to indicate great intelligence. Other subjects from animal mythos such as felines and insects have also featured strongly in fictional representations of aliens.

In considering the subject more seriously, a useful division has been suggested between universal and parochial (narrowly restricted) characteristics. Universals are features which have evolved independently more than once on Earth (and thus presumably are not difficult to develop) and are so intrinsically useful that species will inevitably tend towards them. These include flight, sight, photosynthesis and limbs, all of which have evolved several times here on Earth with differing materialization. There are a huge variety of eyes, for example, many of which have radically different working schematics as well as different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochials, by contrast, are essentially arbitrary evolutionary forms which often serve little utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Parochials include the five digits of mammals, the genitalia and sexual mechanics of animals, as well as the curious and often fatal conjunction of the feeding and breathing passages found within many animals.

A consideration of which features are ultimately parochial challenges many taken for granted notions about morphological necessity. Skeletons, in some form, are likely to be replicated elsewhere, yet the vertebrate spine—while a profound development on Earth—is just as likely to be unique. Similarly, it is reasonable to expect some type of egg laying amongst off-Earth creatures but the mammary glands which set apart mammals may be a singular case.

The assumption of radical diversity amongst putative extraterrestrials is by no means settled. While many exobiologists do stress that the enormously heterogeneous nature of Earth life foregrounds even greater variety in space, others point out that convergent evolution dictates substantial similarities between Earth and off-Earth life. These two schools of thought are called "divergionism" and "convergionism", respectively.

Beliefs in extraterrestrial life

Ancient and Early Modern ideas

Belief in extraterrestrial life may have been present in ancient Egypt, Babylon, and Sumer, although in these societies, cosmology was fundamentally supernatural and the notion of aliens is difficult to distinguish from that of gods, demons, and such. The first important Western thinkers to argue systematically for a universe full of other planets and, therefore, possible extraterrestrial life were the ancient Greek writers Thales and his student Anaximander in the 7th and 6th centuries B.C.E. The atomists of Greece took up the idea, arguing that an infinite universe ought to have an infinity of populated worlds. Ancient Greek cosmology worked against the idea of extraterrestrial life in one critical respect, however: the geocentric universe, championed by Aristotle and codified by Ptolemy, privileged the Earth and Earth-life (Aristotle denied there could be a plurality of worlds) and seemingly rendered extraterrestrial life impossible.

Giordano Bruno, De l'Infinito, Univirso e Mondi, 1584

Authors of ancient Jewish sources also considered extraterrestrial life. The Talmud states that there are at least 18,000 other worlds, but provides little elaboration on the nature of the worlds and on whether they are physical or spiritual. Based on this, however, the medieval exposition "Sefer HaB'rit" posits that extraterrestrial creatures exist but that they have no free will (and are thus equivalent to animal life). It adds that human beings should not expect creatures from another world to resemble earthly life, any more than sea creatures resemble land animals.

Hindu beliefs of endlessly repeated cycles of life have lead to descriptions of multiple worlds in existence and their mutual contacts ( Sanskrit word Sampark means 'contact' as in Mahasamparka = the great contact). However the relevance of such descriptions have to be evaluated in the context of understanding of geography and science at those times.

Within Islam, the statement of the Qur'an "All praise belongs to God, Lord of all the worlds" indicates multiple universal bodies and maybe even multiple universes that may indicate extraterrestrial and even extradimensional life. Surat Al-Jinn also mentioned a statement from a Jinn regarding the current status and ability of his group in the heavens.

When Christianity spread throughout the West, the Ptolemaic system became widely accepted, and although the Church never issued any formal pronouncement on the question of alien life at least tacitly the idea was aberrant. In 1277 the Bishop of Paris, Étienne Tempier, did overturn Aristotle on one point: God could have created more than one world (given His omnipotence) yet we know by revelation He only made one. Taking a further step and arguing that aliens actually existed remained rare. Notably, Cardinal Nicholas of Cusa speculated about aliens on the moon and sun.

This situation changed, however, with the dramatic shift in thinking initiated by the invention of the telescope and the Copernican assault on geocentric cosmology. Once it became clear that the Earth was merely one planet amongst countless bodies in the universe the extraterrestrial idea moved towards the scientific mainstream. God's omnipotence, it could be argued, not only allowed for other worlds and other life, on some level it necessitated them. The best known early-modern proponent of such ideas was Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own solar system; he was eventually burned at the stake for heretical ideas. In the early 17th century the Czech astronomer Anton Maria Schyrleus of Rheita mused that "if Jupiter has…inhabitants…they must be larger and more beautiful than the inhabitants of the Earth, in proportion to the [characteristics] of the two spheres." Dominican monk Tommaso Campanella wrote about a Solarian alien race in his Civitas Solis.

Such comparisons also appeared in poetry of the era. In "The Creation: a Philosophical Poem in Seven Books" (1712) Sir Richard Blackmore observed: "We may pronounce each orb sustains a race / Of living things adapted to the place". The didactic poet Henry More took up the classical theme of the Greek Democritus in "Democritus Platonissans, or an Essay Upon the Infinity of Worlds" (1647). With the new relative viewpoint that the Copernican revolution had wrought, he suggested "our world's sunne / Becomes a starre elsewhere." Fontanelle's "Conversations on the Plurality of Worlds" (translated into English in 1686) offered similar excursions on the possibility of extraterrestrial life, expanding rather than denying the creative sphere of a Maker.

The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. William Herschel, the discoverer of Uranus, was one of many 18th-19th century astronomers convinced that our Solar System, and perhaps others, would be well populated by alien life. Other luminaries of the period who championed "cosmic pluralism" included Immanuel Kant and Benjamin Franklin. At the height of the Enlightenment even the Sun and Moon were considered candidates for hosting aliens.

Extraterrestrials and the Modern era

This enthusiasm towards the possibility of alien life continued well into the 20th century. Indeed, the roughly three centuries from the Scientific Revolution through the beginning of the modern era of solar system probes were essentially the zenith for belief in extraterrestrials in the West: many astronomers and other secular thinkers, at least some religious thinkers, and much of the general public were largely satisfied that aliens were a reality. This trend was finally tempered as actual probes visited potential alien abodes in the solar system. The moon was decisively ruled out as a possibility, while Venus and Mars, long the two main candidates for extraterrestrials, showed no obvious evidence of current life. The other large moons of our system which have been visited appear similarly lifeless, though the interesting geothermic forces observed (Io's volcanism, Europa's ocean, Titan's thick atmosphere) have underscored how broad the range of potentially habitable environments may be. Although the hypothesis of a deliberate cosmic silence of advanced extraterrestrials should also be considered, the failure of the SETI program to detect anything resembling an intelligent radio signal after four decades of effort has partially dimmed the optimism that prevailed at the beginning of the space age. Emboldened critics view the search for extraterrestrials as unscientific, despite the fact the SETI program is not the result of a continuous, dedicated search but instead utilizes what resources and manpower it can, when it can.

Thus, the three decades preceding the turn of the second millennium saw a crossroads reached in beliefs in alien life. The prospect of ubiquitous, intelligent, space-faring civilizations in our solar system appears increasingly dubious to many scientists. Still, in the words of SETI's Frank Drake, "All we know for sure is that the sky is not littered with powerful microwave transmitters." Drake has also noted that it is entirely possible advanced technology results in communication being carried out in some way other than conventional radio transmission. At the same time, the data returned by space probes and giant strides in detection methods have allowed science to begin delineating habitability criteria on other worlds and to confirm that, at least, other planets are plentiful though aliens remain a question mark.

At present, some enthusiasts in the topic believe that extraterrestrial beings regularly visit or have visited the Earth. Some think that unidentified flying objects observed in the skies are in fact sightings of the spacecraft of intelligent extraterrestrials, and even claim to have met such beings. Crop circle patterns have also been attributed to the actions of extraterrestrials, although many were later found to be hoaxes. While at least one recent scientific paper published in a respected, peer-reviewed journal has urged a re-evaluation of the UFO phenomenon, as of this time mainstream scientific opinion holds that such claims are unsupportable by the evidence currently available and unlikely to be true.

The possible existence of primitive (microbial) life outside of Earth is much less controversial to mainstream scientists although at present no direct evidence of such life has been found. Indirect evidence has been offered for the current existence of primitive life on the planet Mars; however, the conclusions that should be drawn from such evidence remain in debate.

Scientific search for extraterrestrial life

The scientific search for extraterrestrial life is being carried out in two different ways, directly and indirectly.

Direct search

The Arecibo message is a digital message sent to globular star cluster M13, and is a well-known symbol of human attempts to contact extraterrestrials.

Scientists are directly searching for evidence of unicellular life within the solar system, carrying out studies on the surface of Mars and examining meteors that have fallen to Earth. A mission is also proposed to Europa, one of Jupiter's moons with a liquid water layer under its surface, which might contain life.

There is some limited evidence that microbial life might possibly exist or have existed on Mars. An experiment on the Viking Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes. However, the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. Recently, Circadian rhythms have been allegedly discovered in Viking data. The interpretation is controversial. Independently in 1996 structures resembling bacteria were reportedly discovered in a meteorite, ALH84001, thought to be formed of rock ejected from Mars. This report is also controversial and scientific debate continues. (See Viking biological experiments.)

In February 2005, NASA scientists reported that they had found strong evidence of present life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claims on methane signatures found in Mars' atmosphere that resemble the methane production of some forms of primitive life on Earth, as well as their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon denied the scientists' claims, and Stoker herself backed off from her initial assertions. However, only a few days after Stoker and Lemke made their claims, scientists from the European Space Agency reported that their own measurements of methane on Mars suggested an organic origin.

Though such findings are still very much in debate, support among scientists for the belief in the existence of life on Mars seems to be growing. In an informal survey conducted at the conference in which the European Space Agency presented its findings, 75 percent of the scientists in attendance reported to believe that life once existed on Mars; 25 percent reported a belief that life currently exists there.

Recently, scientists have discovered mysterious red cells in India unlike any cells on Earth. The cells appear to be replicating without the presence of DNA, raising questions about whether they are a new kind of life altogether, or whether they can even be classified as life at all, without genetic material. It is believed that these red cells may have been injected into Earth's atmosphere by a comet or meteor from elsewhere in the universe; some believe this may be evidence of panspermia's occurrence. Little is known about these cells, however, and the issue is controversial. Some scientists believe the cells may actually be native to Earth.

Indirect search

It is theorised that any technological society in space will be transmitting information. Projects such as SETI are conducting an astronomical search for radio activity that would confirm the presence of intelligent life. A related suggestion is that aliens might broadcast pulsed and continuous laser signals in the optical as well as infrared spectrum; laser signals have the advantage of not "smearing" in the interstellar medium and may prove more conducive to communication between the stars.

Extrasolar planets

Terrestrial Planet Finder - Infrared interferometer concept of finding Earth-like extrasolar planets

Astronomers also search for extrasolar planets that would be conducive to life, especially those like OGLE-2005-BLG-390Lb which have been found to have Earth-like qualities. Current radiodetection methods have been inadequate for such a search, as the resolution afforded by recent technology is inadequate for detailed study of extrasolar planetary objects. Future telescopes should be able to image planets around nearby stars, which may reveal the presence of life (either directly or through spectrography which would reveal key information such as the presence of free oxygen in a planet's atmosphere). Darwin is an ESA mission designed to find Earth-like planets, and analyse their atmosphere. It has been argued that one of the best candidates for the discovery of life-supporting planets may be Alpha Centauri, the closest star system to Earth, given that two of the three stars in the system are broadly sun-like.

Extraterrestrial life in the Solar System

Many bodies in the Solar System have been suggested as being likely to contain conventional organic life. The most commonly suggested ones are listed below; of these, three of the five are moons, and are thought to have large bodies of underground liquid, where life may have evolved in a similar fashion to deep sea vents.

• Venus - Carbonyl sulfide was recently discovered in Venus' atmosphere, which is suggestive of life.
• Mars - Liquid water is widely thought to have existed on Mars in the past and there may still be liquid water beneath the surface. Recently, methane was found in the atmosphere of Mars. Main article: Life on Mars
• Titan - Saturn's largest moon - The only known moon with a significant atmosphere was recently visited by the Huygens probe. Latest discoveries indicate that there is no global or widespread ocean, but small and/or seasonal liquid hydrocarbon lakes are almost surely present on surface.
• Europa - Jupiter's fourth largest moon - Likely to have a salt ocean under a thick ice crust. If the moon is host to life, many expect to find it living in habitats similar to the hydrothermal vents of Earth. Moreover, astrobiologists are hopeful that we could find aerobic life, living on oxygen brought down into the ocean through the churning of the moon's icy surface.
• Enceladus - Saturn's sixth largest moon - Geologically active, with liquid water and geysers known to exist beneath its south pole.

Numerous other bodies have been suggested as potential hosts for microbial life. For example, atmospheric life has been hypothesised on Venus and the gas giants. Fred Hoyle has proposed that life might exist on comets, as some Earth microbes managed to survive on a lunar probe for many years. However, it is considered highly unlikely that complex multicellular organisms of the conventional chemistry of terrestrial life (animals, plants) could exist under these living conditions.

Extraterrestrials in Popular Culture

In popular culture and conspiracy theories, life forms, especially intelligent life forms, that are of extraterrestrial origin, i.e. not coming from the Earth, are referred to collectively as aliens or sometimes visitors.

This usage is clearly anthropocentric: when humans in fictional accounts accomplish interstellar travel and land on a planet elsewhere in the universe, the local inhabitants of these other planets are usually still referred to as "alien," even though they are the native life form and the humans are the intruders. In general they are seen as unfriendly life forms. This may be seen as a reversion to the classic meaning of "alien" (see Foreigner ) as referring to "other," in contrast to "us" in the context of the writer's frame of reference.

File:Grey alien close1.jpg

A grey alien close-up

Aliens in movies

There are several reasons for this humanoid depiction in movies. It makes it easier for an alien in a movie scene to simply be a disguised human actor. Aliens in movies, in order to catch our attention, must trigger instantaneous emotional reaction; this requires a design based on recognizable human facial features and expressions. It is easier to relate to an alien with features we recognize such as arms and legs, two eyes, a nose and a mouth, as well as behavior we recognize such as baring its teeth in anger or widening its eyes in shock or surprise.

However, if real extraterrestrial life exists, few scientists expect to find humanoid characteristics, believing that this would be too great a coincidence given an entirely different evolutionary scale. On the other hand, some of humanity's most defining characteristics are also extremely advantageous, such as bipedalism, opposable thumbs, dual forward facing eyes. Therefore, it is possible that alien life similar to humankind exists.

Prime examples of how aliens are viewed are found in the movies Alien, Predator, E.T. the Extra-Terrestrial, Close Encounters of the Third Kind, The X-Files:Fight The Future, War of the Worlds, Independence Day, Signs and Cocoon.

Aliens in poetry

There is a long history of writing about imagined meetings between aliens and humans, and poetry is no exception. Many serious poets, including former Poet Laureates Stanley Kunitz and Robert Hayden, have written celebrated poems on the topic of life beyond our world. The best of these poems complicate the expectations of the reader, such as Kunitz's poem "The Abduction" which subverts the popular notion of alien abduction by describing the event surreally and without the typical cast of characters. Other poems take on the topic as a way to offer an alternate view of humanity, or even a cultural critique. In Robert Hayden's poem "American Journal," an extraterrestrial describes American behavior to his superiors, and similarly, "The White Fires of Venus" by Denis Johnson, relates the observations of the inhabitants of Venus about humanity.

Historical ideas

The fictionalization of extraterrestrial life occurred before the 20th century. The didactic poet Henry More took up the classical theme of Cosmic pluralism of the Greek Democritus in "Democritus Platonissans, or an Essay Upon the Infinity of Worlds" (1647). With the new relative viewpoint that understood "our world's sunne / Becomes a starre elsewhere", More made the speculative leap to extrasolar planets,

the frigid spheres that 'bout them fare;

Which of themselves quite dead and barren are,

But by the wakening warmth of kindly dayes,

And the sweet dewie nights, in due course raise

Long hidden shapes and life, to their great Maker's praise.

The possibility of extraterrestrial life was a commonplace of educated discourse in the 17th century, though in Paradise Lost (1667) Milton cautiously employed the conditional when the angel suggests to Adam the possibility of life on the Moon:

Her spots thou seest

As clouds, and clouds may rain, and rain produce

Fruits in her softened soil, for some to eat

Allotted there; and other Suns, perhaps,

With their attendant Moons, thou wilt descry,

Communicating male and female light,

Which two great sexes animate the World,

Stored in each Orb perhaps with some that live.

Fontanelle's "Conversations on the Plurality of Worlds" with its similar excursions on the possibility of extraterrestrial life, expanding rather than denying the creative sphere of a Maker, was translated into English in 1686. In "The Excursion" (1728) David Mallet exclaimed, "Ten thousand worlds blaze forth; each with his train/Of peopled worlds."

Artistic depiction of a Grey Alien

SETI

SETI (pronounced ['sɛti]) is the acronym for Search for Extra-Terrestrial Intelligence; organized efforts by humans to detect intelligent aliens. A number of efforts with "SETI" in the project name have been organized, including projects funded by the United States Government. The generic approach of SETI projects is to survey the sky to detect the existence of transmissions from a civilization on a distant planet - an approach widely endorsed by the scientific community as hard science.

There are great challenges in searching across the sky to detect a first transmission that can be characterised as intelligent, since its direction, spectrum and method of communication are all unknown beforehand. SETI projects necessarily make assumptions to narrow the search, and thus no exhaustive search has so far been conducted.

Overview

Visiting another civilization on a distant world is presently beyond human capabilities (see Project Orion and Project Daedalus for some hypothetical explorations of the concept). However, it is currently technologically feasible to develop a communications system which uses a powerful transmitter and a sensitive receiver to search the sky for extraterrestrial worlds whose citizens have a similar inclination as terrestrials.

Assumptions

SETI is not generally viewed by scientists as a trivial task. Our galaxy, the Milky Way, is 100,000 light years across and contains approximately a hundred billion stars. Searching the entire sky for some far-away and faint signal is an exhausting exercise. A number of assumptions are needed for SETI to be feasible.

A basic assumption of SETI is that of "Mediocrity": the idea that humanity is not exotic in the cosmos but in a sense "typical" or "medium" when compared with other intelligent species.^{[citation needed]} This would mean that humanity has sufficient similarities with other intelligent beings that communications would be mutually desirable and understandable. If this basic assumption of Mediocrity is correct, and other intelligent species are present in any number in the galaxy at our technological level or above, then communications between the two worlds should be inevitable.

Another assumption is that the vast majority of known life-forms in our galaxy are based on carbon chemistries, as all life-forms are on Earth. While it is possible that life could be based around elements other than carbon, carbon is well known for the unusually wide variety of molecules that can be formed around it.

The presence of liquid water is also a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life.

Another assumption is to focus on Sun-like stars. Very big stars have relatively short lifetimes, meaning that intelligent life would likely not have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally locked to the star, with one side of the planet perpetually baked and the other perpetually frozen. (However, some speculate that a thick cloud cover may mitigate these differences. [1])

About 10% of the stars in the Milky Way galaxy are Sun-like, and there are about a thousand such stars within 100 light-years of the Sun. These stars would be useful primary targets for interstellar listening.

However, we know of only one planet where life exists, our own. There is no way to know if any of the simplifying assumptions are correct, and so as a second priority the entire sky must be searched.

Searching the Electromagnetic Spectrum

In order to find an electromagnetic transmission from an alien civilization we also have to search through most of the useful radio spectrum, as there is no way to know what frequencies aliens might be using. Trying to transmit a powerful signal over a wide range of wavelengths is impractical, and so it is likely that such a signal would be transmitted on a relatively narrow band. This means that a wide range of frequencies must be searched at every spatial coordinate of the sky.

There is also the problem of knowing what to listen for, as we have no idea how a signal sent by aliens might be modulated, and how the data transmitted by it might be encoded. Narrow-bandwidth signals that are stronger than background noise and constant in intensity are obviously interesting, and if they have a regular and complex pulse pattern are likely to be artificial.

However, while studies have been performed on how to send a signal that could be easily deciphered, there is no way to know if the assumptions of those studies are valid, and deciphering the information from an alien signal could be very difficult.

There is yet another problem in listening for interstellar radio signals. Cosmic and receiver noise sources impose a threshold to power of signals that we can detect. For us to detect an alien civilization 100 light years away that is broadcasting "omnidirectionally", that is, in all directions, the aliens would have to be using a transmitter power equivalent to several thousand times the entire current power-generating capacity of the entire Earth.

It is much more effective in terms of communication to generate a narrow-beam signal whose "effective radiated power" is very high along the path of the beam, but negligible everywhere else. This places the transmitter power within reasonable ranges, the problem being now of having the good luck to coincide with the path of the beam, with the possibility approaching to zero as distance increases.

Such a beam might be very hard to detect, not only because it is very narrow, but because it could be blocked by interstellar dust clouds or garbled by "multipath effects", the same phenomenon that causes "ghosted" TV images. Such ghosts occur when TV transmissions are bounced off a mountain or other large object, while also arriving at our TV antenna by a shorter, direct route, with the TV picking up two signals separated by a delay.

Similarly, interstellar narrow-beam communications could be bent or "refracted" by interstellar clouds to produce multipath effects that could obscure the signal. If interstellar signals are being transmitted on narrow beams, there is nothing we can do at this end to deal with this problem other than to be alert.

Modern SETI efforts began with a paper written by physicists Giuseppe Cocconi and Philip Morrison and published in the science press in 1959. Cocconi and Morrison suggested that the microwave frequencies between 1 and 10 gigahertz would be best suited for interstellar communications.

Below 1 gigahertz, "synchrotron radiation" emitted from electrons moving in galactic magnetic fields tends to drown out other radio sources. Above 10 gigahertz, radio noise from water and oxygen atoms in our atmosphere tends to also become a source of interference. Even if alien worlds have substantially different atmospheres, quantum noise effects make it difficult to build a receiver that can pick up signals above 100 gigahertz.

The low end of this "microwave window" is particularly attractive for communications, because it is in general easier to generate and receive signals at lower frequency. The lower frequencies are also desirable because of the "Doppler shifting" of a narrow-band signal due to planetary motions.

Doppler shifting is a change in the frequency of a signal due to the motion of the source of that signal. If the source is approaching, the signal will be shifted up in frequency, while if the source is moving away, the signal will be shifted down in frequency. The rotation of a planet and its orbit around a star causes a Doppler shift in the frequency of any signal generated from that planet, and over the course of a day the signal can drift in frequency far out of its intended bandwidth. The problem gets worse with higher frequencies, and so lower frequencies are preferred.

Cocconi and Morrison suggested that the frequency of 1.420 gigahertz was particularly interesting. This is the frequency emitted by neutral hydrogen. Radio astronomers often search the sky on this frequency to map the great hydrogen clouds in our galaxy. Transmitting a communications signal near this "marker" frequency would improve the chances of its detection by accident.

The frequencies between 1.420 and 1.640 gigahertz have been considered particularly interesting by SETI researchers, and have been given the nickname the "Water Hole".

Radio SETI experiments

Early work

In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named "Project Ozma", after the Queen of Oz in L. Frank Baum's fantasy books. Drake used a 25-meter-diameter radio telescope at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency. A 400 kilohertz band was scanned around the marker frequency, using a single-channel receiver with a bandwidth of 100 hertz. The information was stored on tape for off-line analysis. Nothing of great interest was found.

The first SETI conference took place at Green Bank in 1961. The Soviets took a strong interest in SETI during the 1960s and performed a number of searches with omnidirectional antennas in the hope of picking up powerful radio signals beginning in 1964. TV-Host/American astronomer Carl Sagan and Soviet astronomer Iosif Shklovskii together wrote the pioneering book in the field, Intelligent Life in the Universe which was published in 1966 .

In the March 1955 issue of Scientific American, Dr. John Kraus, Professor Emeritus and McDougal Professor of Electrical Engineering and Astronomy at the Ohio State University, described a concept to scan the cosmos for natural radio signals using a flat-plane radio telescope equipped with a parabolic reflector. Within one year, his concept was approved for construction by the Ohio State University. With the aid of $71,000 in total grants by the National Science Foundation, construction of the first Kraus-style radio telescope began on a 20-acre plot in Delaware, Ohio. The 360-feet wide, 500-feet long, and 70-feet high telescope was powered up in 1963. This Ohio State University radio telescope was called Big Ear. Later, it began the world's first continuous SETI program, called the Ohio State University SETI program.

In 1971, the U.S. National Aeronautics and Space Administration (NASA) funded a SETI study that involved Drake, Bernard Oliver of Hewlett-Packard Corporation, and others. The report that resulted proposed the construction of an Earth-based radio telescope array with 1,500 dishes, known as "Project Cyclops". The price tag for the Cyclops array was $10 billion USD, and, not surprisingly, Cyclops was not built.

The "Wow!" Signal

The OSU SETI program gained fame on August 15, 1977 when Jerry Ehman, a project volunteer, witnessed a startlingly strong signal received by the telescope. He quickly circled the indication on a printout and scribbled the phrase “Wow!” in the margin.

This signal, dubbed the Wow! signal, is considered by some to be the most likely candidate from an artificial, extraterrestrial source ever discovered, but it has not been detected again in several additional searches.

Arecibo message

In 1974, a largely symbolic attempt was made to send a message to other worlds. To celebrate a substantial upgrading of the 305 meter Arecibo Radio Telescope in Puerto Rico, a coded message of 1,679 bits was transmitted towards the Globular Cluster M13, about 25,100 light years away.

The pattern of 0s and 1s contained in the message defines a 23 × 73 two dimensional grid which when plotted reveals some data about our location in the Solar System, a stylized figure of a human being, chemical formulae and an outline of the radio telescope itself. The 23 by 73 grid was chosen because both 23 and 73 are prime numbers, which makes it easier to decode the message. The reasons for this are:

• an attempt to factorize the length of the message would show that it can't contain a grid with more than two dimensions (since there are just two factors);
• assuming a two dimensional grid, there are only two possible resultant images, with dimensions 23x73 or 73x23.

Given the limitations of the speed of light for message transmission, no reply would be possible before the year 52,174 (approximately) and hence has been dismissed by some as a publicity stunt. A controversy arose because the transmission raised the serious question of whether a small group should be allowed to speak for Earth.

SERENDIP

In 1979 the University of California, Berkeley launched a SETI project named "Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations (SERENDIP)" .

SETI@home

SETI@home is a highly successful distributed computing project that was launched by U.C. Berkeley in May 1999, and is heavily sponsored by The Planetary Society. Any individual can become involved with SETI research by downloading and running the SETI@home software package, which then runs signal analysis on a "work unit" of data recorded from the central 2.5 MHz wide band of the SERENDIP IV instrument. The results are then automatically reported back to UC Berkeley. Over 5 million computer users in more than 200 countries have signed up for SETI@home and have collectively contributed over 19 billion hours of computer processing time.

The cost of this prodigious processing time is not to be overlooked: over $1,000,000,000 of electricity has been used in the effort, making SETI@home amongst the most costly failed scientitific experiments undertaken. This hidden cost is almost invariably overlooked in understanding the value and risk of SETI as an enterprise. ^{[citation needed]}

Sentinel, META, and BETA

In 1980, Carl Sagan, Bruce Murray, and Louis Friedman founded the U.S. Planetary Society, partly as a vehicle for SETI studies.

In the early 1980s, Harvard University physicist Paul Horowitz took the next step and proposed the design of a spectrum analyzer specifically intended to search for SETI transmissions. Traditional desktop spectrum analyzers were of little use for this job, as they sampled frequencies using banks of analog filters and so were restricted in the number of channels they could acquire. However, modern integrated-circuit digital signal processing (DSP) technology could be used to build autocorrelation receivers to check far more channels. This work led in 1981 to a portable spectrum analyzer named "Suitcase SETI" that had a capacity of 131,000 narrowband channels. After field tests that lasted into 1982, Suitcase SETI was put into use in 1983 with the 26-meter Harvard/Smithsonian radio telescope at Harvard, Massachusetts. This project was named "Sentinel", and continued into 1985.

Even 131,000 channels weren't enough to search the sky in detail at a fast rate, so Suitcase SETI was followed in 1985 by Project "META", for "Megachannel Extra-Terrestrial Assay". The META spectrum analyzer had a capacity of 8.4 million channels and a channel resolution of 0.05 hertz. An important feature of META was its use of frequency doppler shift to distinguish between signals of terrestrial and extraterrestrial origin. The project was led by Horowitz with the help of the Planetary Society, and was partly funded by movie maker Steven Spielberg. A second such effort, META II, was begun in Argentina in 1990 to search the southern sky. META II is still in operation, after an equipment upgrade in 1996. The next year, in 1986, UC Berkeley initiated their second SETI effort, SERENDIP II, and has continued with two more SERENDIP efforts to the present day.

The follow-on to META was named "BETA", for "Billion-channel ExtraTerrestrial Assay", and it commenced observation on October 30, 1995. The heart of BETA's processing capability consisted of 63 dedicated FFT engines, each capable of performing a 2^22-point complex fast Fourier transform in two seconds, and 21 general-purpose PCs equipped with custom digital signal processing boards. This allowed BETA to receive 250 million simultaneous channels with a resolution of 0.5 hertz per channel. It scanned through the microwave spectrum from 1.400 to 1.720 gigahertz in eight hops, with two seconds of observation per hop. An important capability of the BETA search was rapid and automatic reobservation of candidate signals, achieved by observing the sky with two adjacent beams, one slightly to the east and the other slightly to the west. A successful candidate signal would first transit the east beam, and then the west beam and do so with a speed consistent with the earth's sidereal rotation rate. A third receiver observed the horizon to veto signals of obvious terrestrial origin. On March 23, 1999 the 26-meter radio telescope on which Sentinel, META and BETA were based was blown over by strong winds and seriously damaged. This forced the BETA project to cease operation.

MOP and Project Phoenix

In 1992, the U.S. government finally funded an operational SETI program, in the form of the NASA "Microwave Observing Program (MOP)". MOP was planned as a long-term effort, performing a "Targeted Search" of 800 specific nearby stars, along with a general "Sky Survey" to scan the sky. MOP was to be performed by radio dishes associated with the NASA Deep Space Network, as well as a 43-meter dish at Green Bank and the big Arecibo dish. The signals were to be analyzed by spectrum analyzers, each with a capacity of 15 million channels. These spectrum analyzers could be ganged to obtain greater capacity. Those used in the Targeted Search had a bandwidth of 1 hertz per channel, while those used in the Sky Survey had a bandwidth of 30 hertz per channel.

MOP drew the attention of the U.S. Congress, where the program was strongly ridiculed, and was canceled a year after its start. SETI advocates did not give up, and in 1995 the nonprofit "SETI Institute" of Mountain View, California, resurrected the work under the name of Project "Phoenix", backed by private sources of funding. Project Phoenix, under the direction of Dr. Jill Tarter, previously Project Scientist for the NASA project, is a continuation of the Targeted Search program, studying roughly 1,000 nearby Sunlike stars. Seth Shostak also worked on Project Phoenix. From 1995 through March 2004, Phoenix conducted observing campaigns at the 64-meter Parkes radio telescope in Australia, the 140 Foot Telescope of the National Radio Astronomy Observatory in West Virginia, USA, and the Arecibo Observatory in Puerto Rico. The project observed the equivalent of 800 stars over the available channels in the frequency range from 1200 to 3000 MHz. The search was sensitive enough to pick up transmitters with power output equivalent to airport radars to a distance of about 200 light years.

Allen Telescope Array

The SETI Institute is now collaborating with the Radio Astronomy Laboratory at UC Berkeley to develop a specialized radio telescope array for SETI studies, something like a mini-Cyclops array. The new array concept is named the "Allen Telescope Array" (ATA) (formerly, One Hectare Telescope [1HT]) after the project's benefactor Paul Allen. Its sensitivity will be equivalent to a single large dish more than 100 meters on a side. The array is being constructed at the Hat Creek Observatory in rural northern California.

The full array is planned to consist of 350 or more Gregorian radio dishes, each 6.1 meters (20 feet) in diameter. These dishes are the largest producable with commercially available satellite television dish technology. The ATA was planned for a 2007 completion date, at a very modest cost of $25 million USD, but the completion date will surely slip. The SETI Institute provides money for building the ATA while UC Berkeley designs the telescope and provides operational funding. Berkeley astronomers will use the ATA to pursue other deep space radio observations. The ATA is intended to support a large number of simultaneous observations through a technique known as "multibeaming", in which DSP technology is used to sort out signals from the multiple dishes. The DSP system planned for the ATA is extremely ambitious.

The ATA schedule has slipped, not surprising for an ambitious project on a limited budget. The individual antennas work, can be fabricated, and meet specifications. As of summer 2006, roughly 10 of the antennas are complete and 42 are under final construction. Although not yet capable of significant radio astronomy or SETI observations, the ATA has become a testbed for array technology, as needed for the Square Kilometre Array, the US Navy, and DARPA. Completion of the full 350 element array will depend on funding and the technical results from the 42 element sub-array.

Optical SETI experiments

While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. The idea was first suggested in a paper published in the British journal Nature in 1961, and in 1983 Charles Townes, one of the inventors of the laser, published a detailed study of the idea in the US journal Proceedings of the National Academy of Sciences.

Most SETI researchers agreed with the idea. The 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central sun of a remote star system would be too difficult. Now some SETI advocates, such as Frank Drake, have suggested that such a judgement was too conservative.

There are two problems with optical SETI, one of which is easy to deal with, the second of which is troublesome. The first problem is that lasers are highly "monochromatic", that is, they emit light only on one frequency, making it troublesome to figure out what frequency to look for. However, according to Harmonic analysis (Fourier analysis), emitting light in narrow pulses results in a broad spectrum of emission^{[citation needed]}, with the frequencies becoming higher as the pulse width becomes narrower, and an interstellar communications system could use pulsed lasers.

The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. This means that a laser beam could be easily blocked by clouds of interstellar dust, and more to the point, we could pick it up only if we happened to cross its line of fire. As it is unlikely an alien civilization would focus an interstellar laser communications beam on Earth deliberately, we would have to cross such a beam by accident.

However, as discussed earlier, the power requirements for omnidirectional interstellar radio broadcasts are tremendous, and narrow-beam radio communications are technically more plausible. As SETI researchers have adjusted to the idea that interstellar radio communications may be over narrow beams, the idea of hunting for interstellar laser beams has become no more troublesome.

In the 1980s, two Soviet researchers conducted a short optical SETI search, but turned up nothing. During much of the 1990s, the optical SETI cause was kept alive through searches by Stuart Kingsley, a British dedicated amateur living in the US state of Ohio.

Now the SETI old-timers have warmed to the concept of optical SETI. Paul Horowitz of Harvard and researchers with the SETI institute have conducted simple optical SETI searches using a telescope and a photon pulse detection system, and are considering further searches. Horowitz says: "Everyone's been mesmerized by radio, but we've done that experiment a lot and we're a little tired of it."

Optical SETI enthusiasts have conducted paper studies of the effectiveness of using contemporary high-energy lasers and a ten-meter focus mirror as an interstellar beacon. The analysis shows that an infrared pulse from a laser, focussed into a narrow beam by a such a mirror, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire. The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see.

Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a rate, say, of once a second. This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors. This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting to contact us.

Several optical SETI experiments are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter (61 inch) optical telescope. This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" on that effort.

Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue. The Harvard-Smithsonian group is now working with Princeton to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes will be "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise.

The Harvard-Smithsonian group is now building a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope. The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.

The University of California, Berkeley, home of SERENDIP and SETI@home, is also conducting optical SETI searches. One is being directed by Geoffrey Marcy, the well-known extrasolar planet hunter, and involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal.

The other Berkeley optical SETI effort is more like that being pursued by the Harvard-Smithsonian group and is being directed by Dan Werthimer of Berkeley, who built the laser detector for the Harvard-Smithsonian group. The Berkeley survey uses a 76-centimeter (30-inch) automated telescope and an older laser detector built by Wertheimer.

Probe SETI and SETA Experiments

The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960 (see Bracewell probe), and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments [2] [3] [4] for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals. Subsequently, in a September 2004 paper featured on the cover of Nature Magazine [5], Christopher Rose and Gregory Wright showed that inscribing a message in matter and transporting it to the destination is vastly more energy-efficient than communication using electromagnetic waves if the message can tolerate delivery delay beyond light transit time [6] [7] [8]. Thus, a solarcentric Search for Extraterrestrial Artifacts (SETA) [9] would seem to be favored over the more traditional radio or optical searches.

Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth libration orbits [10] might therefore constitute the most universally convenient parking places for automated extraterrestrial spacecraft exploring arbitrary stellar systems. A viable long-term SETI program may be founded upon a search for these objects.

In 1979 Freitas and Valdes [11] conducted a photographic search of the vicinity of the Earth-Moon triangular libration points L4 and L5, and of the solar-synchronized positions in the associated halo orbits, seeking possible orbiting extraterrestrial interstellar probes, but found nothing to a detection limit of about 14th magnitude. The authors conducted a second more comprehensive photographic search for probes in 1982 [12] that examined the five Earth-Moon Lagrangian positions and included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near L1/L2, Earth-Moon L3, and also L2 in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17-19th magnitude near L3/L4/L5, 10-18th magnitude for L1/L2, and 14-16th magnitude for Sun-Earth L2.

In June 1983, Valdes and Freitas [13] used the 26-m radiotelescope at Hat Creek Radio Observatory to search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars including all visible stars within a 20 light-year radius. The tritium frequency was deemed highly attractive for SETI work because (1) the isotope is cosmically rare, (2) the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5-14 x 10^-21 W/m²/channel and 0.7-2 x 10^-24 W/m²/channel, respectively, but no detections were made.

Where are they?

Italian physicist Enrico Fermi suggested in the 1950s that if technologically advanced civilizations are common in the universe, then they should be detectable in one way or another. (Perhaps apocryphally, Fermi is said to have asked "Where are they?")

The Fermi paradox can be stated more completely as follows:

The size and age of the universe incline us to believe that many technologically advanced civilizations must exist. However, this belief seems logically inconsistent with our lack of observational evidence to support it. Either the initial assumption is incorrect and technologically advanced intelligent life is much rarer than we believe, our current observations are incomplete and we simply have not detected them yet, or our search methodologies are flawed and we are not searching for the correct indicators.

The fact that radio-based SETI searches have not come up with anything very interesting so far is not cause to rule out the existence of contactable alien intelligence. As the previous sections of this document show, trying to find another civilization in space is a difficult proposition, and we have searched only a small fraction of the entire "parameter space" of targets, frequencies, power levels, and so on.

However, it is important to emphasize that our SETI hunts have been based on assumptions on communications frequencies and technologies that may be irrelevant to alien societies. It is possible that intelligent species abandon radio when new technologies are discovered, making the length of time a world is transmitting on conventional radio extremely short. Thus, the lack of positive results doesn't imply that alien civilizations don't exist. It only tells us that if they do, our most optimistic assumptions for getting in touch with them have proven unrealistic.

There is another issue that provides another possible explanation as to why we don't see evidence of a large number of alien societies. That issue is time. Our Sun is not a first-generation star. All first-generation stars are either very small and dim, or have exploded, or have burned out. This first generation synthesized the heavy elements needed to create planets and lifeforms. Later generations of stars, including our Sun, have been born and have died or will die in their turn. Our galaxy is more than 10 billion years old. Intelligent life and technological societies may have arisen and died out many times during this ten billion years. Assuming that an intelligent species survives for ten million years, that means that only 0.1% of all societies that have arisen during the history of our galaxy are in existence now.

Science writer Timothy Ferris has suggested that since galactic societies could be only transitory, then if there is in fact an interstellar communications network, it consists mostly of automated systems that store the cumulative knowledge of vanished civilizations and communicate that knowledge through the galaxy. Ferris calls this the "Interstellar Internet", with the various automated systems acting as network "servers".

Ferris suspects that if such an Interstellar Internet exists, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network could maintain some broadcast nodes in hopes of making contact with new civilizations. The Interstellar Internet may be out there, waiting for us to figure out how to link up with it.

Another theory which has been proposed to explain the apparent lack of interstellar communication is the suggestion that the galaxy may contain predatory (or otherwise aggressive) species. Those species smart enough to maintain radio silence are those that survive such predation. Another suggestion, made by astrophysicist Ray Norris in 1999 in Acta Astronautica (and subsequently in Allen Tough's book When SETI Succeeds: The Impact of High-Information Contact - ISBN 0-9677252-2-4) was that gamma-ray burst events are sufficiently frequent to sterilize vast swaths of galactic real-estate. This idea was subsequently popularised by physicist Arnon Dar, and described in the PBS Nova show 'Death Star'. On the other hand, Robin Corbet suggests that gamma-ray bursts may be useful to synchronize interstellar communication, and Tony Smith speculates that some gamma-ray bursts may actually be ultra wideband communication packets.[14]

Public information

In 2005, the International Academy of Astronautics established the SETI: Post-Detection Science and Technology Taskgroup (Chairman, Professor Paul Davies) "to act as a Standing Committee to be available to be called on at any time to advise and consult on questions stemming from the discovery of a putative signal of extraterrestrial intelligent (ETI) origin." [15] It will be using in part the Rio Scale [16] to evaluate the importance of public release of the information.

Criticism of SETI

SETI has occasionally been the target of criticism by those who suggest that it is a form of pseudoscience. In particular, critics allege that no observed phenomena suggest the existence of extraterrestrial intelligence, and furthermore that the assertion of the existence of extraterrestrial intelligence has no good Popperian criteria for falsifiability [17]. Science fiction writer Michael Crichton, in a 2003 lecture at Caltech, stated that "The Drake equation cannot be tested and therefore SETI is not science. SETI is unquestionably a religion." [18].

In response, SETI advocates note, among other things, that the existence of intelligent life on Earth is a plausible reason to expect it elsewhere, and that individual SETI projects have clearly defined "stop" conditions. Concerning the latter argument, the justification for SETI projects doesn't necessarily require an acceptance of the Drake equation. The search for extra-terrestrial intelligence is not an assertion that extra-terrestrial intelligence exists, and conflating the two can be seen as a straw man argument. There is an effort to distinguish the SETI projects from UFOlogy, the study of UFOs considered to be pseudoscience by many.

In 1983 Stanislaw Lem, disappointed with the lack of results of SETI in spite of the huge effort, has noted that he considers the Universe silent, coining the moniker silencium universi.

Further reading

• Exers, Ronald, D. Cullers, J. Billingham, L. Scheffer (editors) (2003). SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence. SETI Press. ISBN 0-9666335-3-9. 
• McConnell, Brian and Chuck Toporek (2001). Beyond Contact: A Guide to SETI and Communicating with Alien Civilizations. O'Reilly. ISBN 0-596-00037-5. 

• Roth, Christopher F., "Ufology as Anthropology: Race, Extraterrestrials, and the Occult." In E.T. Culture: Anthropology in Outerspaces, ed. by Debbora Battaglia. Durham, N.C.: Duke University Press, 2005.

• Sagan, Carl. 1996. The Demon-Haunted World: Science as a Candle in the Dark: chapter 4: "Aliens"

• David Grinspoon (2003). Lonely Planets: The Natural Philosophy of Alien Life. HarperCollins. ISBN 0-06-018540-6. 
• Jack Cohen and Ian Stewart (2002). Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN 0-09-187927-2. 
• John C. Baird (1987). The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover: University Press of New England. ISBN 0-87451-406-1. 
• Donald Goldsmith (1997). The Hunt for Life on Mars. New York: A Dutton Book. ISBN 0-525-94336-6. 
• Michael T. Lemnick (1998). Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book. 
• Cliff Pickover (2003). The Science of Aliens. New York: Basic Books. ISBN 0-465-07315-8. 
• Roth, Christopher F. (2005) "Ufology as Anthropology: Race, Extraterrestrials, and the Occult." In E.T. Culture: Anthropology in Outerspaces, ed. by Debbora Battaglia. Durham, N.C.: Duke University Press.
• Crowe, Michael J.. The Extraterrestrial Life Debate, 1750—1900. Dover Publications. 
• Sagan, Carl and I.S. Shklovskii (1966). Intelligent Life in the Universe. Random House. 
• Sagan, Carl (1973). Communication with Extraterrestrial Intelligence. MIT Press. 

External links

• 'Is There Anybody Out There?' Freeview video by the Vega Science Trust and the BBC/OU.
• Actual Aliens: Extraterrestrial News and Current Events
• PBS: Life Beyond Earth a film by Timothy Ferris
• PBS: Exploring Space - The Quest for Life by Scott Pearson
• ufoskeptic.org by Bernard Haisch
• Xenopsychology by Robert A. Freitas Jr.
• "What Aliens Might Look Like" from National Geographic
• Sylvia Engdahl , "Early Space Poetry" Part I: Didactic and other poetry concerning other inhabited worlds, well-known and obscure poets, 17th-18th centuries. "Part II": 19th century
• Top stars picked in alien search

• ufologie.net - The Filiberto Caponi close encounter of the 3rd kind, 1993
• ufocasebook.com - Filiberto Caponi Close Encounter 1993
• The Ilkley Moor encounter of the 3rd kind, 1987

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• SETI Institute
• SETI@home, the shared computing project
• The Planetary Society
• The SETI League

• IAA SETI Permanent Study Group
• Where Is Everybody? : an essay by Arthur C. Clarke on SETI

• International Academy of Astronautics
• SETI Principles as defined from the Department of Space Studies of the Southwest Research Institute (PDF File)
• Harvard University SETI page
• The Columbus Optical SETI Observatory, Dr. Stuart A, Kingsley
• First Contact Within 20 Years: Shostak – From SpaceDaily.com, 22 July 2004 (based on calculations to be published in Acta Astronautica)
• SETI: Searching for Life – Article series in Sky & Telescope magazine
• Cirkovic, Milan M., and Bradbury, R. J., 200n, "Galactic Gradients, Postbiological Evolution, and the Apparent Failure of SETI."
• Searching for Good Science: The Cancellation of NASA's SETI Program (PDF) – By Stephen J. Garber, NASA History Office. Journal of the British Interplanetary Society, Vol. 52, pp. 3-12, 1999. Provides more details on the elimination of SETI funding by the US Congress in 1993.
• Open SETI Initiative – Gerry Zeitlin's site concerned with reforming SETI's approach
• Galactic Drifter SETI – Example of an economical SETI contact strategy in terms of look direction and timing constraints
• The Search For Extraterrestrial Intelligence (NASA SP-419, 1977)
• ET Might Write, Not Radiate
• SETI radio link-budget range calculator
• IT & SETI: The Role of Computer Technology in the Search for Extraterrestrial Intelligence
• Mark Elowitz's Web site on Exobiology and SETI
• Big Ear Memorial Website - discovered the "Wow!" signal and has entry in Guinness Book of Records
• NAAPO, North American AstroPhysical Observatory (formerly Big Ear)
• SPSR The Society for Planetary SETI Research
• SETI@home FAQ for newsgroups alt.sci.seti and sci.astro.seti
• Has SETI Been Barking Up the Wrong Tree (Mostly?
• "Top stars picked in alien search", BBC News, 2006-02-19. Retrieved 2006-08-13.

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