Difference between revisions of "Extraterrestrial life" - New World Encyclopedia

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[[Image:Glieseupdated.jpg|thumb|235px|right|Artist's impression of Gilese 581 c, the first extrasolar planet discovered in its star's habitable zone]]
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'''Extraterrestrial life''' is the term used to define any form of [[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 no evidence of extraterrestrial life that has been widely accepted by the [[scientific community]]. The putative study and theorization of ET life is known as [[astrobiology]] or ''exobiology'', and the term "exopolitics" may be used to denote the study of political relations between humanity and extraterrestrial [[civilization]]s. Speculative forms of extraterrestrial life range from [[humanoid]] and monstrous beings to life at the much smaller scale of [[bacterium|bacteria]].
  
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Extraterrestrial life forms, especially intelligent ones, are often referred to in popular culture as "aliens" or "ETs." The popular belief in extraterrestrial life has been fueled since the mid-twentieth century by numerous reports of [[Unidentified Flying Object (UFO)|UFO]]s (Unidentified Flying Objects) and claims of [[abduction]]. On the other hand, Christian skeptics consider them to be merely apparitions of [[angel]]s or [[demon]]s from the [[spirit world]].
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Scientists have approached the effort to discover extraterrestrial life by looking to the [[star]]s—exploring the closest planets and moons for evidence of life; listening for [[radio]] or [[electromagnetic]] transmissions from life beyond Earth; and even sending a message into space. While evidence has not yet been discovered of any life currently existing outside of earth, many scientists remain hopeful and continue the search. Given that the existence of life beyond earth has ramifications of the greatest significance for humankind, there are serious questions and preparations that need to be made on earth before this endeavor succeeds.
  
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'''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]].
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==Historical belief in extraterrestrial life==
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[[Astronomy]] played a significant role in ancient societies, the arrangement of heavenly bodies inspiring cultural beliefs and [[mythology]]. Beliefs in creatures and lifeforms that populated the sky  became almost commonplace in the ancient world. The [[atomism|atomists]] of [[ancient Greece|Greece]] took up the idea of an infinite universe, with an infinity of populated worlds. The [[Judaism|Jewish]] [[Talmud]] states that there are at least 18,000 other worlds. The [[Babylon|Babylonians]], [[Ancient Egypt|Egyptians]], [[Sumer|Sumerians]], [[China|Chinese]], and [[Aztec Civilization|Aztec]]s all held somewhat similar ideas of multiple worlds that existed amongst the [[star]]s. However, these beliefs were often [[spirit]]ually focused, as the sky and stars were often the domain of [[deity|deities]] and supernatural powers. The notion of alien biological lifeforms as they are thought of today, did not take shape for centuries.
  
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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 [[Planetary habitability|habitable planet]] to habitable planet. These two hypotheses are not [[mutually exclusive]].  
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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.  
  
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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 [[bacterium|bacteria]].  
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The best known early-modern proponent of such ideas was [[Giordano Bruno]], who argued in the sixteenth century for an infinite universe in which every star is surrounded by its own [[solar system]]; he was eventually burned at the stake for [[heresy|heretical]] ideas.<ref>Al Van Helden, [http://galileo.rice.edu/chr/bruno.html "Giordano Bruno (1548-1600)"] (1995). Retrieved February 27, 2017.</ref> Dominican monk [[Tommaso Campanella]] wrote about a Solarian alien race in his ''[[Civitas Solis]].''
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The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. [[William Herschel]], the discoverer of [[Uranus]], was one of many eighteenth and nineteenth century astronomers convinced that our [[solar system]], and perhaps others, could very well be populated by alien life. Other luminaries of the period who championed "cosmic pluralism" included [[Immanuel Kant]] and [[Benjamin Franklin]]. At the height of the [[Age of Enlightenment|Enlightenment]] even the [[Sun]] and [[Moon]] were considered candidates for hosting aliens.
  
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Extraterrestrial life forms, especially intelligent ones, are often referred to in popular culture as [[Extraterrestrial life in popular culture|aliens]] or ETs. The putative study and theorisation of ET life is known as [[astrobiology]] or xenobiology.
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==Extraterrestrials and the modern era==
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This enthusiasm towards the possibility of alien life continued into the twentieth century. As long as humankind was unable to travel into space, extraterrestrial life seemed as probable to scientists and the general public alike. However, a split in the belief in ET life forms occurred in the beginning of the twentieth century. With advancement in the areas of [[physics]], [[astronomy]], and evolutionary [[biology]], scientists started to understand how complicated and fragile life on earth was, how unlikely it was that life even developed somewhere hospitable to it, let alone elsewhere in a largely dangerous and inhospitable universe.  
  
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==Historical Belief in Extraterrestrial Life==
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Furthermore, the early explorations of space by probes showed no signs of life on the barren [[planet]] of [[Mars]] and exposed [[Venus]] to be a planet of such dramatic climate as to be not conducive to supporting any known forms of life. The major scientific argument against extraterrestrial life was formed: Earth was unique in its [[evolution]], randomly composed of the exact conditions to support the fragile, and somewhat random itself, process that led to life. Most scientists believed that for such conditions to happen twice, even in a universe as big as this one, is unlikely.
  
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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.
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==Popular belief in extraterrestrials==
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[[Image:Giordano_Bruno.jpg|thumb|[[Giordano Bruno]], ''De l'Infinito, Univirso e Mondi, 1584'']]
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===UFOs and alien abductions===
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[[Image:Alienigena.jpg|thumb|right|180px|Artistic depiction of a grey alien]]
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Yet, as science was dismissing the notion of extraterrestrial life, it was becoming more popular in public imagination. The connection between extraterrestrials, the sudden frequent reports of unidentifiable, un-Earthly aircraft known now as [[Unidentified Flying Object (UFO)|UFO]]s, and the anthropomorphizing of extraterrestrials into the common identity of aliens, as they are known today, happened in the time period between 1920 and 1950. The possibility of intelligent life forms became an alluring mystery to the public. Early [[science fiction]] of the period used aliens frequently in its stories. Even those who claimed not to believe in aliens shared a widespread fear of malevolent creatures from space, possibly best demonstrated  by [[Orson Welles]]' infamous radio broadcast of ''The War of the Worlds'', in which hundreds of people in the [[New York]] and [[New Jersey]] area were led to believe an invasion by aliens was taking place.  
  
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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.<ref>{{cite web| url=http://ohr.edu/ask_db/ask_main.php/318/Q1/| publisher=ohr.edu| title=Star Struck, a letter to a Rabbi}}</ref><ref>{{cite web| url=http://www.torah.org/features/secondlook/extraterrestrial.html| publisher=torah.org| title=Extraterrestrial life| first=Rabbi Aryeh| last=Kaplan}}</ref>
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Belief in aliens has become one of the most interesting and complicated aspects of [[pop culture]] and the [[new age movement]] in the later half of the twentieth century. Not only was it commonly believed that UFOs were in fact alien space craft, but reports of contact between humans and aliens have been prevalent since the 1950s. The development of the highly controversial [[alien abduction]]s is perhaps the most bizarre phenomenon in the UFO culture. The incident in [[New Hampshire]] in 1961, involving [[Betty Hill and Barney Hill|Betty and Barney Hill]] was the first reported [[abduction]] of humans by extraterrestrials for the purpose of [[medicine|medical]] experiments and testing. The tenacity of the couple, their desire to keep the incident a secret, and the recognition of their [[psychologist]] that something traumatic had happened to them all lent a sense of credibility to their story when it was revealed some years later. Since then, thousands of people around the world have made similar claims.  
  
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[[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.
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"Contactees" (persons who claim to be in regular contact with extraterrestrials) have typically reported that they were given messages or profound wisdom by aliens, and that they were compelled to share these messages. As a cultural phenomenon, contactees perhaps had their greatest notoriety from the late 1940s to the late 1950s, but individuals have continued to make similar claims. Some shared their messages with small groups of followers, and many issued newsletters or spoke at UFO conventions.  
  
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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.
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===Belief in extraterrestrials as a religious phenomenon===
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The contactee movement has been regarded by academics as a variety of religious belief, analogous to the [[new religious movements|new religions]], and has thus been studied by sociologists of religion. The classic 1956 study, ''When Prophecy Fails'' by [[Leon Festinger]], which included information about, and analysis of, contactee groups and was foundational in the development of the theory of [[cognitive dissonance]].<ref>Leon Festinger, Henry W. Riecken, and Stanley Schachter, ''When Prophecy Fails: A Social and Psychological Study of a Modern Group That Predicted the Destruction of the World'' (Harper-Torchbooks, 1956, ISBN 0061311324).</ref> Thus, astronomer [[J. Allen Hynek]] described contactees as asserting
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<blockquote>…the visitation to the earth of generally benign beings whose ostensible purpose is to communicate (generally to a relatively few selected and favored persons—almost invariably without witnesses) messages of "cosmic importance." These chosen recipients generally have repeated contact experiences, involving additional messages. The transmission of such messages to willing and uncritical true believers frequently, in turn, leads to the formation of a flying saucer cult, with the "communicator" or "contactee" the willing and obvious cult leader. Although relatively few in number, such flying saucer advocates have by their irrational acts strongly influenced public opinion.<ref>J. Allen Hynek, ''The UFO Experience: A Scientific Inquiry'' (Marlow & Company, 1998, ISBN 156924782X).</ref></blockquote>
  
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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<ref>{{cite web| url=http://www.crisismagazine.com/november2002/feature7.htm| publisher=crisismagazine.com| title=Christianity and the Search for Extraterrestrial Life| first=Benjamin D.| last=Wiker}}</ref> 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]].  
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These beliefs are fueled by the [[mass media|media]], in which aliens are commonplace elements of [[science fiction]] in films and television shows, as well as books and documentaries. Conspiracy theorists have claimed that world governments have clandestine relationships with aliens or are otherwise covering up alien existence. Commonly, these aliens are seen as small, with grayish skin, large head, and black, lidless eyes. They are sometimes referred to as "greys." Other depictions perceive aliens as [[reptile|reptilian]], [[insect]]-like, or even resembling people of [[Nordic]] descent.
  
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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."<ref>{{cite web| url=http://cosmovisions.com/Rheita.htm| publisher=cosmovisions.com| title=Rheita.htm}}</ref> Dominican monk [[Tommaso Campanella]] wrote about a Solarian alien race in his ''[[Civitas Solis]]''.
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Aliens are depicted with conflicting [[motivation]]s: Some believe aliens exist to help humankind, while other times aliens are seen as hostile and using humankind for their own advancement. Either way, the notion that there are other life forms that possess similar intelligence and cognitive abilities as human beings seems to strike a chord in people. Perhaps it is an overactive imagination projecting onto the infinity of the cosmos, of which science still knows little, or perhaps the fear of the ultimate unknown, that which is "alien" to humankind, that helps fuel the belief.
  
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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." [[Bernard le Bovier de Fontenelle|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.  
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===Alternative spiritual explanations===
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On the other hand, some Christians theorize that purported aliens are in fact [[angel]]s or [[demon]]s&mdash;spiritual creatures rather than material beings from outer space. They point out that angels and demons can appear in any form, including "aliens" if that is what matches [[human being]]s' expectations. The mixed motivations of the so-called aliens also fits with the behavior of spiritual beings, who may either seek to help or create problems for the people they visit. Numerous traditions teach that the [[Earth]] is unique among the planets as the place of free choice and responsibility.
  
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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 [[Age of Enlightenment|Enlightenment]] even the [[Sun]] and [[Moon]] were considered candidates for hosting aliens.
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===Exopolitics===
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A development at the end of the twentieth century in the field of extraterrestrial life is "exopolitics," or the [[politics|political]] implications of extraterrestrial contact, a term that came into use by Alfred Webre in his groundbreaking work as a futurist at the Stanford Research Institute. His view is that humanity lives on an isolated planet in the midst of a populated, evolving, and highly organized universal society.
  
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==Extraterrestrials and the Modern era==
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==Scientific approaches==
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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 (moon)|Io]]'s volcanism,  [[Europa (moon)|Europa]]'s  ocean, [[Titan (moon)|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,<ref>http://controlled-hominization.com</ref> 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.<ref>{{cite web| url=http://www.crichton-official.com/speeches/speeches_quote04.html| publisher=crichton-official.com| title=Aliens Cause Global Warming| first=Michael| last=Crichton| date=January 17, 2003}}</ref>
 
  
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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 [[Planetary habitability|habitability criteria]] on other worlds and to confirm that, at least, [[extrasolar planet|other planets]] are plentiful though aliens remain a question mark.
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The stigma attached to the question of alien lifeforms from the more outlandish pop cultural ideas left many scientists in the latter half of the twentieth century unwilling to pursue the topic, either because they were afraid of ridicule within their discipline or they firmly believed that such endeavors were pointless. However, there have always been, and continue to be, serious scientists willing to examine these questions.  
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[[Image:Alienigena.jpg|thumb|180px|Artistic depiction of a Grey Alien]]
 
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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,<ref>{{cite web| url=http://www.ufoskeptic.org/JBIS.pdf| format=PDF| publisher=ufodkrptic.org| title=Inflation-Theory Implications for Extraterrestrial Visitation| last=Deardorff| coauthors= et al.| year=2005}}</ref> as of this time mainstream scientific opinion holds that such claims are unsupportable by the evidence currently available and unlikely to be true.  
 
  
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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.
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[[Astrobiology]] as a field of study has gained legitimacy and acceptance in the scientific community. Its main objectives include the search for extraterrestrial life, but in so doing seek to develop a better understanding of life on Earth.
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In popular culture and [[conspiracy theory|conspiracy theories]], life forms, especially intelligent life forms, that are of [[extraterrestrial life|extraterrestrial origin]], i.e. not coming from the [[Earth]], are referred to  collectively as '''aliens''' or sometimes '''visitors'''.  
 
  
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This usage is clearly [[anthropocentrism|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]].
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===Basis of life===
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A difficult term to define, [[life]] can be considered the characteristic state of living organisms and individual [[Cell (biology)|cells]], or that quality or property that distinguishes living organisms from dead organisms and inanimate objects. Although universal consensus on a definition is lacking, [[biology|biological]] properties common to the known organisms found on [[Earth]] ([[plant]]s, [[animal]]s, [[fungus|fungi]], [[protist]]s, [[archaea]], and [[bacteria]]) are that they are [[carbon]]-and-[[water]]-based, are cellular with complex organization, use [[energy]] and undergo [[metabolism]], possess a capacity to grow, maintain [[homeostasis]], respond to stimuli, reproduce, and have various adaptations to the environment. Beyond these biological manifestations of matter, some [[philosophy|philosophical]] and [[religion|religious]] perspectives argue that living organisms possess an inner aspect or character that confers the quality of life.<ref>S. H. Lee, ''Unification Thought'' (New York: Unification Thought Institute, 1981, ISBN 0960648003).</ref>
  
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[[Image:grey_alien_close1.jpg|thumb|180px|right|A [[grey alien]] close-up]]
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===Speculation===
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Without concrete facts to analyze or direct phenomena to observe, most scientists studying extraterrestrial life have had to do so by speculation. Most speculation is, however, based on what is accepted knowledge of [[astronomy]] and [[biology]], as well as careful scientific postulating. For example, scientists must first try to understand if the [[evolution]] of extraterrestrial life would have occurred independently in different places in the [[universe]], or if, as a minority of scientists contend, life in the universe could have stemmed from a smaller number of points of origin, and then spread across the universe, from [[Planetary habitability|habitable planet]] to habitable planet, known as [[panspermia]].
  
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===Aliens in movies===
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Another concern is the evolutionary state of extraterrestrial life. There seems to be a greater possibility in the mind of many scientists that simple life forms, such as basic [[protein]]s and [[microbe]]s, are more probable than more radically evolved organisms. One reason for this is that these simpler lifeforms are easier to develop and maintain than a more complicated life form, such as the [[human being]], and therefore can possibly exist in a wider variety of environments. The other reasoning for simpler life forms does not exclude the notion of complex lifeforms, but rather that humankind will never find any, because by the time a probe or spacecraft could reach other planets, the [[star]]s supporting the alien species would have died out and destroyed them. In this scenario, the most likely organisms humanity would discover are simple proteins in its own [[solar system]].
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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.  
 
  
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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.
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The assumption of radical diversity amongst putative extraterrestrials is by no means settled. While many exobiologists 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.<ref>David Darling, [http://www.daviddarling.info/encyclopedia/E/etlifevar.html "Variety of extraterrestrial life,"] The Encyclopedia of Astrobiology, Astronomy and Spaceflight. Retrieved February 27, 2017.</ref>
  
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Prime examples of how aliens are viewed are found in the movies ''[[Alien (film)|Alien]]'', ''[[Predator (film)|Predator]]'', ''[[E.T. the Extra-Terrestrial]]'', ''[[Close Encounters of the Third Kind]]'', ''[[The X-Files:Fight The Future]], [[War of the Worlds (2005 film)|War of the Worlds]]'', ''[[Independence Day (film)|Independence Day]]'', ''[[Signs (film)|Signs]]'' and ''[[Cocoon (film) |Cocoon]]''.
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===The search for extraterrestrials===
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Since the last quarter of the twentieth century, scientists have been actively searching the universe for other lifeforms with all the different types of [[technology]] available. Yet, even if all of [[NASA]] and the [[European Space Agency]]'s budgets were directed towards this one goal, the [[universe]] is still too large and the technological means inadequate to exhaustively search those areas in space that could possibly harbor life. Still, there are undaunted scientists who continue to search for possible life, whether it be [[bacteria]] or advanced [[civilization]]s. There are two different methods scientists use in this search: the direct and indirect method.
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====The direct method====
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Scientists are directly searching for evidence of unicellular life within the [[solar system]], carrying out studies on the surface of [[Mars]], [[Europa]] (one of [[Jupiter]]'s [[natural satellite]]s), and [[Titan]] (a moon of [[Saturn]]). The possibility of life on Mars has always been a matter of speculation, but became an even more heated issue in 1996, when structures resembling [[bacterium|bacteria]] were reportedly discovered in a [[meteorite]], ALH84001, thought to be formed of rock ejected from Mars. Such potentialities have helped to keep interest in alien life open to a number of prominent scientists. NASA's Mars Lander probes have surveyed the landscape of the red planet, analyzing the geological and chemical makeup of the environment, in order to determine, among other things, if the planet could have at some point supported life. There is circumstantial evidence that Mars was at one time supportive of life; the frozen [[water]] at the Martian poles most likely once flowed over the surface of the planet, and the levels of [[methane]] in the atmosphere, a [[gas]] that occurs in [[Earth's atmosphere]] as the result of biological processes.  
  
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==Possible basis of extraterrestrial life==
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Studies of Saturn's moon Titan have discovered high levels of methane and [[nitrogen]] in the atmosphere, along with other organic compounds that point to the possibility of life forms. Its extremely low temperatures (-289 &deg;F) and hostile weather more than likely rule out current life, at least on the surface. Underground liquid reservoirs, coupled with recent discoveries on Earth of small life forms surviving in incredibly harsh environments, suggested that perhaps there are, or were, micro-sized organisms on Titan.<ref>Ker Than, [http://www.space.com/scienceastronomy/050913_titan_life.html "Scientists Reconsider Habitability of Saturn's Moon,"] SPACE.com (September 13, 2005). Retrieved February 27, 2017.</ref>
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=== Biochemistry ===
 
  
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[[Image:Terrestrial Planet Finder PIA04499.jpg|thumb|250px|[[Terrestrial Planet Finder]] &ndash; Infrared interferometer concept of finding Earth-like [[extrasolar planet]]s]]
  
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All life on Earth is based on the building block element [[carbon]] with [[water]] as the [[solvent]] in which bio-chemical reactions take place.
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Astronomers are also searching for [[extrasolar planet]]s that would be conducive to life, especially those like OGLE-2005-BLG-390Lb, which have been found to have Earth-like qualities. Radiodetection methods have been inadequate for such a search, as the resolution has been 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 atomic [[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 analyze 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.
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*Water is useful because it has a neutral [[pH]], and due to its continued dissociation between [[hydroxide]] and [[hydronium]] [[ionic bond|ions]].
 
−
 
 
−
::2H<sub>2</sub>O → H<sub>3</sub>O<sup>+</sup> + OH<sup>-</sup>
 
−
 
 
−
:As a result, it can dissolve both positive [[cations|metallic ions]] and negative [[anions|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 force]]s between water molecules gives it an ability to store [[latent heat|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 bond|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
 
−
 
 
−
::6CO<sub>2</sub> + 6H<sub>2</sub>O + sun energy → C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + 6O<sub>2</sub>
 
−
::C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + 6O<sub>2</sub> → 6CO<sub>2</sub> + 6H<sub>2</sub>O + biochemical energy
 
−
 
 
−
:The ability to form [[organic acid]]s (-COOH) and [[amine]] [[bases]] (NH<sub>2</sub>-) gives it the possibility of [[neutralisation]] dehydrating reactions to build long [[polymer]] [[peptides]] and [[catalyst|catalytic]] [[proteins]] from [[monomer]] [[amino acids]], and with [[phosphate]]s 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.<ref>{{cite web| url=http://www.daviddarling.info/encyclopedia/A/ammonialife.html| publisher=daviddarling.info| title=Ammonia based life}}</ref>
 
−
 
 
−
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 [[Phase (matter)|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 [[comparative anatomy|morphology]]. What might an alien look like? Science fiction has long shown a bias towards humanoid or (often in the case of villains) [[reptiles|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 [[cats|felines]] and [[insects]] have also featured strongly in fictional representations of aliens.
 
−
 
 
−
In considering the subject more seriously, a useful division has [[Evolving the Alien|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 [[eye]]s, 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]] [[vertebral column|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.<ref>{{cite web| url=http://www.daviddarling.info/encyclopedia/E/etlifevar.html| publisher=daviddarling.info| title=Variety of extraterrestrial life}}</ref>
 
−
 
 
−
 
 
−
 
 
−
 
 
−
 
 
−
 
 
−
 
 
−
==Scientific search for extraterrestrial life==
 
−
The scientific search for extraterrestrial life is being carried out in two different ways, directly and indirectly.
 
−
 
 
−
===Direct search===
 
−
[[Image:arecibo_message.png|160px|right|thumb|The [[Arecibo message]] is a digital message sent to [[Great Globular Cluster in Hercules|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 (moon)|Europa]], one of [[Jupiter (planet)|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 program|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 [[bacterium|bacteria]] were reportedly discovered in a meteorite, [[ALH84001]], thought to be formed of [[martian meteorite|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]].<ref>{{cite news| url=http://www.space.com/scienceastronomy/mars_life_050216.html| title= Exclusive: NASA Researchers Claim Evidence of Present Life on Mars| last= Berger| first= Brian| date=2005}}</ref> 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.<ref>{{cite news| url=http://www.spacetoday.net/Summary/2804| title= NASA denies Mars life reports| publisher=spacetoday.net| date=2005}}</ref> 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.<ref name="Michelson 2005">{{cite news| url=http://story.news.yahoo.com/news?tmpl=story&u=/nm/space_mars_dc| publisher=Yahoo News| title=European Scientists Believe in Life on Mars| last=Michelson| date= 2005}}</ref>
 
−
 
 
−
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.<ref name="Michelson 2005"/>
 
−
 
 
−
Recently, scientists have discovered mysterious red cells in India unlike any cells on Earth.<ref>Astrophysics and Space Science 302 #1-6 (2006) 175–187
 
−
The Red Rain Phenomenon of Kerala and its Possible Extraterrestrial Origin
 
−
Godfrey Louis and A. Santhosh Kumar</ref>  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.<ref>{{cite news
 
−
|url=http://www.cnn.com/2006/TECH/science/06/02/red.rain/index.html
 
−
|publisher=CNN.com
 
−
|date=2006-06-02
 
−
|title= Mysterious red cells might be aliens
 
−
|accessdate=2006-08-09}}
 
−
</ref>
 
  
 
===Indirect search===
 
===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;<ref>{{cite web| url=http://www.coseti.org/| publisher=The Columbus Optical SETI Observatory| title=The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum}}</ref> laser signals have the advantage of not "smearing" in the interstellar medium and may prove more conducive to communication between the stars.
+
Some scientists theorize that any technological [[society]] in space would be transmitting information, and that instruments on Earth could possibly pick up those transmissions. In 1960, [[Cornell University]] astronomer [[Frank Drake]] performed the first modern experiment, named "[[Project Ozma]]" after the [[Princess Ozma|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. Although nothing of great interest was discovered, the notion spawned other searches that continue to this day. Such searches had been categorized under the acronym '''SETI''' (Search for Extra-Terrestrial Intelligence). Using radio telescopes to scan for cosmic transmissions is an approach widely endorsed by the scientific community as hard science.
  
−
===Extrasolar planets===
+
There are great challenges in searching across the sky to detect a first transmission that can be characterized 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. 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.<ref>Jennifer Kahn, [http://discovermagazine.com/2002/apr/featnotes "Notes From Another Universe,"] ''Discover'' (April 2002). Retrieved February 27, 2017.</ref> This would mean that humankind has sufficient similarities with other intelligent beings that [[communication]] 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 humanity's technological level or above, then communication between the two worlds should be inevitable.  
−
[[Image:Terrestrial Planet Finder PIA04499.jpg|thumb|250px|[[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 [[Atomic absorption spectroscopy|spectrography]] which would reveal key information such as the presence of free [[oxygen]] in a planet's atmosphere). [[Darwin (ESA)|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 ==
+
Another assumption is to focus on [[Sun]]-like [[star]]s. 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 [[tidal locking|tidally locked]] to the star, with one side of the planet perpetually baked and the other perpetually frozen.
−
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.
+
About 10 percent 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, comprising the useful primary targets for interstellar listening. Finding an electromagnetic transmission from an alien civilization involves searching through most of the useful radio [[electromagnetic spectrum|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 [[Celestial coordinate system|spatial coordinate]] of the sky.  
−
* [[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 (moon)|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.<ref>{{cite web| url=http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1577|  title=Land of Lakes? Full-Res: PIA06240| date=June 28, 2005}}</ref><ref>{{cite web| url=http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2214| title=Lakes on Titan, Full-Res: PIA08630| date=July 24, 2006}}</ref>
 
−
* [[Europa (moon)|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 vent]]s 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 (moon)|Enceladus]] - Saturn's sixth largest moon - Geologically active, with liquid water and [[geysers]] known to exist beneath its [[south pole]].<ref>{{cite news |title=Watery atmosphere discovered on Saturn's moon| date=2005-03-17| publisher=[[New Scientist]]| url=http://www.newscientist.com/article.ns?id=dn7159}}</ref>
 
  
−
Numerous other bodies have been suggested as potential hosts for [[bacteria|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.
+
There is also the problem of knowing what to listen for, as scientists 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.
 +
[[Image:arecibo_message.png|160px|right|thumb|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]]
 +
However, while studies have been performed on how to send a signal that could be easily [[decipher]]ed, 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.
  
−
==SETI==
+
Many different SETI projects have come and gone since the mid-twentieth century. The massive radio dish in Arecibo, [[Puerto Rico]], once was distinguished in its SETI program, until the U.S. government cut its funding. Since then, most of the SETI work being done has been in the private sector, such as the [[Allen Telescope Array]] in [[California]], the [[Phoenix Project]] in [[Australia]], and [[Harvard University]]'s Optical SETI project.
  
 +
===Arecibo message===
  
−
'''SETI''' ([[International Phonetic Alphabet for English|pronounced]] {{IPA|['sɛti]}}) is the acronym for '''Search for Extra-Terrestrial Intelligence'''; organized efforts by humans to detect [[Extraterrestrial life|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 [[interstellar communication|transmissions]] from a civilization on a distant planet - an approach widely endorsed by the [[scientific community]] as [[hard science]].
+
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 [[bit]]s was transmitted towards the [[globular cluster]] M13, about 25,100 light years away.
  
−
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.
+
The message contains a pattern of 0s and 1s, defining 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]], [[chemistry|chemical]] formulae, and an outline of the [[radio telescope]] itself. The 23 by 73 grid was chosen because both 23 and 73 are [[prime number]]s, which makes it easier to decode the message.  
  
−
===Overview===
+
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 many as a [[publicity stunt]]. It also generated controversy over the serious question of whether a small group should be allowed to speak for Earth.
−
Visiting another [[civilization]] on a [[Extrasolar planets|distant world]] is presently beyond human capabilities (see [[Project Orion (nuclear propulsion)|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 life|extraterrestrial worlds]] whose citizens have a similar inclination as terrestrials.
 
  
−
==== Assumptions ====
+
==Notes==
−
SETI is not generally viewed by scientists as a trivial task. Our [[galaxy]], the [[Milky Way]], is 100,000 [[light year]]s 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.
+
<references/>
−
 
 
−
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 [[chemical element|elements]] [[Alternative biochemistry|other than carbon]], carbon is well known for the unusually wide variety of [[molecule]]s 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 [[star]]s.  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 [[tidal locking|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. [http://www.universetoday.com/am/publish/seti_sets_site_on_mdwarfs.html])
 
−
 
 
−
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 [[Extraterrestrial life|alien]] civilization we also have to search through most of the useful radio [[electromagnetic spectrum|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 [[Celestial coordinate system|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 [[decipher]]ed, 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.
 
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−
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.
 
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−
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 [[television|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.
 
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−
Similarly, interstellar narrow-beam communications could be bent or "refracted" by [[interstellar cloud]]s 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.
 
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−
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.
 
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−
Below 1 gigahertz, "[[synchrotron radiation]]" emitted from [[electron]]s moving in galactic [[magnetic field]]s 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.
 
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−
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 shift]]ing" of a narrow-band signal due to planetary motions.
 
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−
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.
 
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−
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 [[Princess Ozma|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.
 
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−
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 antenna]]s in the hope of picking up powerful radio signals beginning in 1964. TV-Host/American astronomer [[Carl Sagan]] and Soviet astronomer [[Iosif Samuilovich Shklovskii|Iosif Shklovskii]] together wrote the pioneering book in the field, ''Intelligent Life in the Universe'' which was published in 1966 <ref name="sagan_shklovskii">{{cite book | first = Carl | last = Sagan | coauthors = Iosif Scklovskii | title = Intelligent Life in the Universe | year = 1966}}</ref>.
 
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−
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.
 
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−
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.
 
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−
====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 [[bit]]s 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 &times; 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 number]]s, 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.
 
−
 
 
−
 
 
−
 
 
−
==== 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 [[Planetary Society|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. <ref name="SETI@home">{{cite web | url = http://seticlassic.ssl.berkeley.edu/totals.html | title = SETI@home Classic - Current Total Statistics | accessdate = 2006-06-12}}</ref> <ref name="BOINC">{{cite web | url = http://www.boincstats.com/ | title = BOINCstats | accessdate = 2006-06-12}}</ref>
 
−
 
 
−
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 C. Murray|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 number#Signal analysis|complex]] [[fast Fourier transform]] in two seconds, and 21 general-purpose [[IBM PC compatible|PC]]s 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 [[Congress of the United States|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, Santa Clara County, California|Mountain View, California]], resurrected the work under the name of Project "Phoenix," backed by private sources of funding.  [[Project Phoenix (SETI)|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 Observatory|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. <ref name="Allen">{{cite web | url = http://www.seti.org/site/pp.asp?c=ktJ2J9MMIsE&b=179146 | title = Allen Telescope Array General Overview | publisher = [[SETI Institute]] | accessdate = 2006-06-12}}</ref>
 
−
 
 
−
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 [[laser]]s for interstellar communications at optical wavelengths.
 
−
The idea was first suggested in a paper published in the British journal ''[[Nature (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|Harmonic analysis (Fourier analysis)]], emitting light in narrow pulses results in a broad spectrum of emission{{fact}}, 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 "[[piggyback]]ing" 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 [http://www.rfreitas.com/Astro/InterstellarProbesJBIS1980.htm] [http://www.rfreitas.com/Astro/ProbeMyths1983.htm] [http://www.rfreitas.com/Astro/TheCaseForInterstellarProbes1983.htm] 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]]
 
−
[http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v431/n7004/index.html],
 
−
[[Christopher Rose (professor)|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
 
−
[http://www.nature.com/nature/journal/v431/n7004/full/nature02884.html]
 
−
[http://www.winlab.rutgers.edu/~crose/papers/nature.pdf]
 
−
[http://www.winlab.rutgers.edu/~crose/cgi-bin/cosmic23.html].
 
−
Thus, a solarcentric Search for Extraterrestrial Artifacts (SETA) [http://www.rfreitas.com/Astro/SETAJBISNov1983.htm] 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 [http://www.rfreitas.com/Astro/SearchIcarus1980.htm] 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 [http://www.rfreitas.com/Astro/SearchIcarus1980.htm] 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 [http://www.rfreitas.com/Astro/SearchIcarus1983.htm] 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 [http://www.rfreitas.com/Astro/TritiumSearch1986.htm] 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<sup>-21</sup> W/m<sup>2</sup>/channel and 0.7-2 x 10<sup>-24</sup> W/m<sup>2</sup>/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:
 
−
<blockquote>
 
−
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.
 
−
</blockquote>
 
−
 
 
−
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 [http://www.iaanet.org/publicat/acta.html Acta Astronautica] (and subsequently in [[Allen Tough]]'s book ''[http://www.ieti.org/tough/books/succeeds.htm 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 [http://www.pbs.org/wgbh/nova/gamma/ 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 [[packet]]s.[http://www.valdostamuseum.org/hamsmith/GRB.html#CETI ]
 
−
 
 
−
 
 
−
 
 
−
===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 [[Karl Popper|Popperian]] criteria for [[falsifiability]] [http://web.archive.org/web/*/http://www.theness.com/articles/seti-nejs0101.html]. [[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." [http://www.crichton-official.com/speeches/speeches_quote04.html].
 
−
 
 
−
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]].
 
  
 
== References ==
 
== References ==
−
<references/>
+
* Baird, John C. ''The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings.'' Hanover, NH: University Press of New England, 1987. ISBN 0874514061
−
 
+
* Cohen, Jack and Ian Stewart. ''Evolving the Alien: The Science of Extraterrestrial Life.'' Ebury Press, 2002. ISBN 0091879272
−
== Further reading ==
+
* Crowe, Michael J. ''The Extraterrestrial Life Debate, 1750-1900.'' Mineola, NY: Dover Publications.
−
* {{cite book | author = Exers, Ronald, D. Cullers, J. Billingham, L. Scheffer (editors) | title = SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence | year = 2003 | publisher = SETI Press | id = ISBN 0-9666335-3-9}}
+
* Exers, Ronald, D. Cullers, J. Billingham, L. Scheffer (eds.). ''SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence.'' SETI Press, 2003. ISBN 0966633539
−
* {{cite book | last = McConnell | first = Brian | coauthors = Chuck Toporek | title = Beyond Contact: A Guide to SETI and Communicating with Alien Civilizations | year = 2001 | publisher = O'Reilly | id = ISBN 0-596-00037-5}}
+
* Festinger, Leon, Henry W. Riecken, and Stanley Schachter. ''When Prophecy Fails: A Social and Psychological Study of a Modern Group That Predicted the Destruction of the World''. Harper-Torchbooks, 1956. ISBN 0061311324
−
 
+
* Goldsmith, Donald. ''The Hunt for Life on Mars.'' New York: Dutton Books, 1997. ISBN 0525943366
−
 
+
* Grinspoon, David. ''The Lonely Planets: The Natural Philosophy of Alien Life.'' New York: HarperCollins, 2003. ISBN 0060185406
−
 
+
* Hynek, J. Allen. ''The UFO Experience: A Scientific Inquiry''. Marlow & Company, 1998. ISBN 156924782X
−
*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.
+
* Lee, Sang Hun. ''Unification Thought''. Rose of Sharon Pr, 1981. ISBN 0960648003
−
 
+
* Lemnick, Michael T. ''Other Worlds: The Search for Life in the Universe.'' New York: Touchstone Books, 1998.
−
* [[Carl Sagan|Sagan, Carl]]. 1996. ''The Demon-Haunted World: Science as a Candle in the Dark: chapter 4: "Aliens"
+
* McConnell, Brian and Chuck Toporek. ''Beyond Contact: A Guide to SETI and Communicating with Alien Civilizations.'' O'Reilly, 2001. ISBN 0596000375
−
 
+
* Pickover, Cliff. ''The Science of Aliens.'' New York: Basic Books, 2003. ISBN 0465073158
−
 
+
* Roth, Christopher F. "Ufology as Anthropology: Race, Extraterrestrials, and the Occult" in Debbora Battagilia, ''E.T. Culture: Anthropology in Outerspaces.'' Durham, NC: Duke University Press, 2005.  
−
 
+
* Sagan, Carl, and I.S. Shklovskii. ''Intelligent Life in the Universe.'' New York: Random House, 1966.
−
 
+
* Sagan, Carl. ''Communication with Extraterrestrial Intelligence.'' Cambridge, MA: MIT Press, 1973.
−
 
+
* Sagan, Carl. ''The Demon-Haunted World: Science as a Candle in the Dark'', 1996. chap. 4: "Aliens."
−
* {{cite book
+
* Webre, Alfred. ''Exopolitics: Politics, Government, and Law in the Universe.'' Universebooks, 2008 (original 2005). ISBN 0973766301
−
| author = [[David Grinspoon (scientist)|David Grinspoon]]
 
−
| year =2003
 
−
| title=[[Lonely Planets|Lonely Planets: The Natural Philosophy of Alien Life]]
 
−
| publisher = HarperCollins
 
−
| id = ISBN 0-06-018540-6
 
−
}}
 
−
* {{cite book
 
−
| author = [[Jack Cohen (scientist)|Jack Cohen]] and [[Ian Stewart (mathematician)|Ian Stewart]]
 
−
| year =2002
 
−
| title=[[Evolving the Alien|Evolving the Alien: The Science of Extraterrestrial Life]]
 
−
| publisher = Ebury Press
 
−
| id = ISBN 0-09-187927-2
 
−
}}
 
−
* {{cite book
 
−
| author = [[John C. Baird]]
 
−
| year = 1987
 
−
| title = [[The Inner Limits of Outer Space]]: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings
 
−
| location = Hanover
 
−
| publisher = University Press of New England
 
−
| id = ISBN 0-87451-406-1
 
−
}}
 
−
* {{cite book
 
−
| author = [[Donald Goldsmith]]
 
−
| year = 1997
 
−
| title = [[The Hunt for Life on Mars]]
 
−
| location = New York
 
−
| publisher = A Dutton Book
 
−
| id = ISBN 0-525-94336-6
 
−
}}
 
−
* {{cite book
 
−
| author = [[Michael T. Lemnick]]
 
−
| year = 1998
 
−
| title = Other Worlds: The Search for Life in the Universe
 
−
| location = New York
 
−
| publisher = A Touchstone Book
 
−
}}
 
−
* {{cite book
 
−
| author = [[Cliff Pickover]]
 
−
| year = 2003
 
−
| title = [[The Science of Aliens]]
 
−
| location = New York
 
−
| publisher = Basic Books
 
−
| id = 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.
 
−
* {{cite book
 
−
| last = Crowe | first = Michael J.
 
−
| title = The Extraterrestrial Life Debate, 1750—1900
 
−
| publisher = Dover Publications
 
−
}}
 
−
* {{cite book
 
−
| author = [[Carl Sagan|Sagan, Carl]] and [[Iosif Samuilovich Shklovskii|I.S. Shklovskii]]
 
−
| title = [[Intelligent Life in the Universe]]
 
−
| publisher = Random House
 
−
| year = 1966
 
−
}}
 
−
* {{cite book
 
−
| last = Sagan | first = Carl
 
−
| title = [[Communication with Extraterrestrial Intelligence]]
 
−
| publisher = MIT Press
 
−
| year = 1973
 
−
}}
 
  
 
==External links==
 
==External links==
−
*[http://www.vega.org.uk/video/programme/9 'Is There Anybody Out There?'] Freeview video by the Vega Science Trust and the BBC/OU.
+
All links retrieved March 23, 2024.
−
*[http://www.actualaliens.com Actual Aliens]: Extraterrestrial News and Current Events
 
−
*[http://www.pbs.org/lifebeyondearth/index.html ''PBS: Life Beyond Earth'' a film by Timothy Ferris]
 
−
*[http://www.pbs.org/exploringspace/ ''PBS: Exploring Space - The Quest for Life'' by Scott Pearson]
 
−
*[http://www.ufoskeptic.org ''ufoskeptic.org'' by Bernard Haisch]
 
−
*[http://www.rfreitas.com/Astro/Xenopsychology.htm ''Xenopsychology'' by Robert A. Freitas Jr.]
 
−
*[http://news.nationalgeographic.com/news/2005/05/0520_050520_tv_aliens.html "What Aliens Might Look Like"] from [[National Geographic]]
 
−
*[http://www.sylviaengdahl.com/space/poetry1.htm Sylvia Engdahl , "Early Space Poetry" Part I]: Didactic and other poetry concerning other inhabited worlds, well-known and obscure poets, 17th-18th centuries. [http://www.sylviaengdahl.com/space/poetry2.htm "Part II":] 19th century
 
−
*[http://news.bbc.co.uk/1/hi/sci/tech/4728228.stm Top stars picked in alien search]
 
−
 
 
−
 
 
−
* [http://ufologie.net/htm/caponi.htm ufologie.net - The Filiberto Caponi close encounter of the 3rd kind, 1993]
 
−
* [http://www.ufocasebook.com/caponi.html ufocasebook.com - Filiberto Caponi Close Encounter 1993]
 
−
* [http://ufologie.net/htm/ilkleymoor.htm The Ilkley Moor encounter of the 3rd kind, 1987]
 
  
−
{{cleanup-date|2006 September}}<!-- too many links surely? —>
 
 
* [http://www.seti.org/ SETI Institute]
 
* [http://www.seti.org/ SETI Institute]
−
* [http://setiathome.ssl.berkeley.edu/ SETI@home], the [[shared computing]] project
+
*[http://www.pbs.org/exploringspace/ ''PBS: Exploring Space - The Quest for Life'' by Scott Pearson]
−
* [http://www.planetary.org/ The Planetary Society]
+
*[http://www.pbs.org/lifebeyondearth/index.html ''PBS: Life Beyond Earth'' a film by Timothy Ferris]
−
* [http://www.setileague.org/ The SETI League]
+
* [http://SkyandTelescope.com/resources/seti/ ''SETI: Searching for Life''] – ''Sky & Telescope''
−
:* [http://www.setileague.org/iaaseti/index.html IAA SETI Permanent Study Group]
+
*[http://news.nationalgeographic.com/news/2005/05/0520_050520_tv_aliens.html "Flying Whales, Other Aliens Theorized by Scientists"] from ''National Geographic Magazine''
−
:* [http://www.setileague.org/editor/clarke.htm Where Is Everybody?] : an [[essay]] by [[Arthur C. Clarke]] on SETI
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*[http://news.bbc.co.uk/1/hi/sci/tech/4728228.stm “Top stars picked in alien search”] &ndash; BBC
−
* [http://www.iaaweb.org/ International Academy of Astronautics]
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*[http://www.rfreitas.com/Astro/Xenopsychology.htm ''Xenopsychology''] by Robert A. Freitas Jr.
−
* [http://www.boulder.swri.edu/CSEPR/projects/readings/race-need_for_operating_guidelines.pdf ''SETI Principles''] as defined from the [[Department of Space Studies]] of the [[Southwest Research Institute]] (PDF File)
+
* [http://www.anthropic-principle.com/preprints/milan-seti.pdf “Galactic Gradients, Postbiological Evolution, and the Apparent Failure of SETI”]
−
* [http://seti.harvard.edu/seti/ Harvard University SETI page]
 
−
* [http://www.coseti.org The Columbus Optical SETI Observatory, Dr. Stuart A, Kingsley]
 
−
* [http://www.spacedaily.com/news/seti-04e.html ''First Contact Within 20 Years: Shostak''] &ndash; From SpaceDaily.com, 22 July 2004 (based on calculations to be published in [[Acta Astronautica]])
 
−
* [http://SkyandTelescope.com/resources/seti/ ''SETI: Searching for Life''] &ndash; Article series in [[Sky & Telescope]] magazine
 
−
* Cirkovic, Milan M., and Bradbury, R. J., 200n, "[http://www.anthropic-principle.com/preprints/milan-seti.pdf Galactic Gradients, Postbiological Evolution, and the Apparent Failure of SETI.]"
 
−
* [http://history.nasa.gov/garber.pdf ''Searching for Good Science: The Cancellation of NASA's SETI Program''] ([[Portable Document Format|PDF]]) &ndash; 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.
 
−
* [http://www.openseti.org Open SETI Initiative] &ndash; Gerry Zeitlin's site concerned with reforming SETI's approach
 
−
* [http://groups.myspace.com/galacticdrifterseti Galactic Drifter SETI] &ndash; Example of an economical SETI contact strategy in terms of look direction and timing constraints
 
−
* [http://history.nasa.gov/SP-419/sp419.htm ''The Search For Extraterrestrial Intelligence'' (NASA SP-419, 1977)]
 
−
* [http://www.winlab.rutgers.edu/~crose/cgi-bin/cosmic23.html  ET Might Write, Not Radiate]
 
−
* [http://www.satsig.net/seticalc.htm  SETI radio link-budget range calculator]
 
−
* [http://www.ccnmag.com/story.php?id=146  IT & SETI: The Role of Computer Technology in the Search for Extraterrestrial Intelligence]
 
−
* [http://www.markelowitz.com/exobiology.htm Mark Elowitz's Web site on Exobiology and SETI]
 
−
* [http://www.bigear.org/ Big Ear Memorial Website] - discovered the "Wow!" signal and has entry in Guinness Book of Records
 
−
* [http://www.naapo.org/ NAAPO], North American AstroPhysical Observatory (formerly Big Ear)
 
−
* [http://spsr.utsi.edu/ SPSR] The Society for Planetary SETI Research
 
−
* [http://www.setifaq.org/ SETI@home FAQ for newsgroups alt.sci.seti and sci.astro.seti]
 
−
* [http://www.spacedaily.com/reports/Has_Seti_Been_Barking_Up_The_Wrong_Tree_Mostly_999.html Has SETI Been Barking Up the Wrong Tree (Mostly?]
 
−
* {{cite news | title=Top stars picked in alien search | publisher=BBC News | date=2006-02-19 | url=http://news.bbc.co.uk/1/hi/sci/tech/4728228.stm | accessdate=2006-08-13 }}
 
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{{Credits|Extraterrestrial_life|79457760|Extraterrestrial_life_in_popular_culture|78634268|SETI|79467501|}}

Latest revision as of 00:00, 25 March 2024


Artist's impression of Gilese 581 c, the first extrasolar planet discovered in its star's habitable zone

Extraterrestrial life is the term used to define any form of 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 no evidence of extraterrestrial life that has been widely accepted by the scientific community. The putative study and theorization of ET life is known as astrobiology or exobiology, and the term "exopolitics" may be used to denote the study of political relations between humanity and extraterrestrial civilizations. Speculative forms of extraterrestrial life range from humanoid and monstrous beings 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 popular belief in extraterrestrial life has been fueled since the mid-twentieth century by numerous reports of UFOs (Unidentified Flying Objects) and claims of abduction. On the other hand, Christian skeptics consider them to be merely apparitions of angels or demons from the spirit world.

Scientists have approached the effort to discover extraterrestrial life by looking to the stars—exploring the closest planets and moons for evidence of life; listening for radio or electromagnetic transmissions from life beyond Earth; and even sending a message into space. While evidence has not yet been discovered of any life currently existing outside of earth, many scientists remain hopeful and continue the search. Given that the existence of life beyond earth has ramifications of the greatest significance for humankind, there are serious questions and preparations that need to be made on earth before this endeavor succeeds.

Historical belief in extraterrestrial life

Astronomy played a significant role in ancient societies, the arrangement of heavenly bodies inspiring cultural beliefs and mythology. Beliefs in creatures and lifeforms that populated the sky became almost commonplace in the ancient world. The atomists of Greece took up the idea of an infinite universe, with an infinity of populated worlds. The Jewish Talmud states that there are at least 18,000 other worlds. The Babylonians, Egyptians, Sumerians, Chinese, and Aztecs all held somewhat similar ideas of multiple worlds that existed amongst the stars. However, these beliefs were often spiritually focused, as the sky and stars were often the domain of deities and supernatural powers. The notion of alien biological lifeforms as they are thought of today, did not take shape for centuries.

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.

The best known early-modern proponent of such ideas was Giordano Bruno, who argued in the sixteenth 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.[1] Dominican monk Tommaso Campanella wrote about a Solarian alien race in his Civitas Solis. The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. William Herschel, the discoverer of Uranus, was one of many eighteenth and nineteenth century astronomers convinced that our solar system, and perhaps others, could very well be 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 into the twentieth century. As long as humankind was unable to travel into space, extraterrestrial life seemed as probable to scientists and the general public alike. However, a split in the belief in ET life forms occurred in the beginning of the twentieth century. With advancement in the areas of physics, astronomy, and evolutionary biology, scientists started to understand how complicated and fragile life on earth was, how unlikely it was that life even developed somewhere hospitable to it, let alone elsewhere in a largely dangerous and inhospitable universe.

Furthermore, the early explorations of space by probes showed no signs of life on the barren planet of Mars and exposed Venus to be a planet of such dramatic climate as to be not conducive to supporting any known forms of life. The major scientific argument against extraterrestrial life was formed: Earth was unique in its evolution, randomly composed of the exact conditions to support the fragile, and somewhat random itself, process that led to life. Most scientists believed that for such conditions to happen twice, even in a universe as big as this one, is unlikely.

Popular belief in extraterrestrials

UFOs and alien abductions

Artistic depiction of a grey alien

Yet, as science was dismissing the notion of extraterrestrial life, it was becoming more popular in public imagination. The connection between extraterrestrials, the sudden frequent reports of unidentifiable, un-Earthly aircraft known now as UFOs, and the anthropomorphizing of extraterrestrials into the common identity of aliens, as they are known today, happened in the time period between 1920 and 1950. The possibility of intelligent life forms became an alluring mystery to the public. Early science fiction of the period used aliens frequently in its stories. Even those who claimed not to believe in aliens shared a widespread fear of malevolent creatures from space, possibly best demonstrated by Orson Welles' infamous radio broadcast of The War of the Worlds, in which hundreds of people in the New York and New Jersey area were led to believe an invasion by aliens was taking place.

Belief in aliens has become one of the most interesting and complicated aspects of pop culture and the new age movement in the later half of the twentieth century. Not only was it commonly believed that UFOs were in fact alien space craft, but reports of contact between humans and aliens have been prevalent since the 1950s. The development of the highly controversial alien abductions is perhaps the most bizarre phenomenon in the UFO culture. The incident in New Hampshire in 1961, involving Betty and Barney Hill was the first reported abduction of humans by extraterrestrials for the purpose of medical experiments and testing. The tenacity of the couple, their desire to keep the incident a secret, and the recognition of their psychologist that something traumatic had happened to them all lent a sense of credibility to their story when it was revealed some years later. Since then, thousands of people around the world have made similar claims.

"Contactees" (persons who claim to be in regular contact with extraterrestrials) have typically reported that they were given messages or profound wisdom by aliens, and that they were compelled to share these messages. As a cultural phenomenon, contactees perhaps had their greatest notoriety from the late 1940s to the late 1950s, but individuals have continued to make similar claims. Some shared their messages with small groups of followers, and many issued newsletters or spoke at UFO conventions.

Belief in extraterrestrials as a religious phenomenon

The contactee movement has been regarded by academics as a variety of religious belief, analogous to the new religions, and has thus been studied by sociologists of religion. The classic 1956 study, When Prophecy Fails by Leon Festinger, which included information about, and analysis of, contactee groups and was foundational in the development of the theory of cognitive dissonance.[2] Thus, astronomer J. Allen Hynek described contactees as asserting

…the visitation to the earth of generally benign beings whose ostensible purpose is to communicate (generally to a relatively few selected and favored persons—almost invariably without witnesses) messages of "cosmic importance." These chosen recipients generally have repeated contact experiences, involving additional messages. The transmission of such messages to willing and uncritical true believers frequently, in turn, leads to the formation of a flying saucer cult, with the "communicator" or "contactee" the willing and obvious cult leader. Although relatively few in number, such flying saucer advocates have by their irrational acts strongly influenced public opinion.[3]

These beliefs are fueled by the media, in which aliens are commonplace elements of science fiction in films and television shows, as well as books and documentaries. Conspiracy theorists have claimed that world governments have clandestine relationships with aliens or are otherwise covering up alien existence. Commonly, these aliens are seen as small, with grayish skin, large head, and black, lidless eyes. They are sometimes referred to as "greys." Other depictions perceive aliens as reptilian, insect-like, or even resembling people of Nordic descent.

Aliens are depicted with conflicting motivations: Some believe aliens exist to help humankind, while other times aliens are seen as hostile and using humankind for their own advancement. Either way, the notion that there are other life forms that possess similar intelligence and cognitive abilities as human beings seems to strike a chord in people. Perhaps it is an overactive imagination projecting onto the infinity of the cosmos, of which science still knows little, or perhaps the fear of the ultimate unknown, that which is "alien" to humankind, that helps fuel the belief.

Alternative spiritual explanations

On the other hand, some Christians theorize that purported aliens are in fact angels or demons—spiritual creatures rather than material beings from outer space. They point out that angels and demons can appear in any form, including "aliens" if that is what matches human beings' expectations. The mixed motivations of the so-called aliens also fits with the behavior of spiritual beings, who may either seek to help or create problems for the people they visit. Numerous traditions teach that the Earth is unique among the planets as the place of free choice and responsibility.

Exopolitics

A development at the end of the twentieth century in the field of extraterrestrial life is "exopolitics," or the political implications of extraterrestrial contact, a term that came into use by Alfred Webre in his groundbreaking work as a futurist at the Stanford Research Institute. His view is that humanity lives on an isolated planet in the midst of a populated, evolving, and highly organized universal society.

Scientific approaches

The stigma attached to the question of alien lifeforms from the more outlandish pop cultural ideas left many scientists in the latter half of the twentieth century unwilling to pursue the topic, either because they were afraid of ridicule within their discipline or they firmly believed that such endeavors were pointless. However, there have always been, and continue to be, serious scientists willing to examine these questions.

Astrobiology as a field of study has gained legitimacy and acceptance in the scientific community. Its main objectives include the search for extraterrestrial life, but in so doing seek to develop a better understanding of life on Earth.

Basis of life

A difficult term to define, life can be considered the characteristic state of living organisms and individual cells, or that quality or property that distinguishes living organisms from dead organisms and inanimate objects. Although universal consensus on a definition is lacking, biological properties common to the known organisms found on Earth (plants, animals, fungi, protists, archaea, and bacteria) are that they are carbon-and-water-based, are cellular with complex organization, use energy and undergo metabolism, possess a capacity to grow, maintain homeostasis, respond to stimuli, reproduce, and have various adaptations to the environment. Beyond these biological manifestations of matter, some philosophical and religious perspectives argue that living organisms possess an inner aspect or character that confers the quality of life.[4]

Speculation

Without concrete facts to analyze or direct phenomena to observe, most scientists studying extraterrestrial life have had to do so by speculation. Most speculation is, however, based on what is accepted knowledge of astronomy and biology, as well as careful scientific postulating. For example, scientists must first try to understand if the evolution of extraterrestrial life would have occurred independently in different places in the universe, or if, as a minority of scientists contend, 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, known as panspermia.

Another concern is the evolutionary state of extraterrestrial life. There seems to be a greater possibility in the mind of many scientists that simple life forms, such as basic proteins and microbes, are more probable than more radically evolved organisms. One reason for this is that these simpler lifeforms are easier to develop and maintain than a more complicated life form, such as the human being, and therefore can possibly exist in a wider variety of environments. The other reasoning for simpler life forms does not exclude the notion of complex lifeforms, but rather that humankind will never find any, because by the time a probe or spacecraft could reach other planets, the stars supporting the alien species would have died out and destroyed them. In this scenario, the most likely organisms humanity would discover are simple proteins in its own solar system.

The assumption of radical diversity amongst putative extraterrestrials is by no means settled. While many exobiologists 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.[5]

The search for extraterrestrials

Since the last quarter of the twentieth century, scientists have been actively searching the universe for other lifeforms with all the different types of technology available. Yet, even if all of NASA and the European Space Agency's budgets were directed towards this one goal, the universe is still too large and the technological means inadequate to exhaustively search those areas in space that could possibly harbor life. Still, there are undaunted scientists who continue to search for possible life, whether it be bacteria or advanced civilizations. There are two different methods scientists use in this search: the direct and indirect method.

The direct method

Scientists are directly searching for evidence of unicellular life within the solar system, carrying out studies on the surface of Mars, Europa (one of Jupiter's natural satellites), and Titan (a moon of Saturn). The possibility of life on Mars has always been a matter of speculation, but became an even more heated issue in 1996, when structures resembling bacteria were reportedly discovered in a meteorite, ALH84001, thought to be formed of rock ejected from Mars. Such potentialities have helped to keep interest in alien life open to a number of prominent scientists. NASA's Mars Lander probes have surveyed the landscape of the red planet, analyzing the geological and chemical makeup of the environment, in order to determine, among other things, if the planet could have at some point supported life. There is circumstantial evidence that Mars was at one time supportive of life; the frozen water at the Martian poles most likely once flowed over the surface of the planet, and the levels of methane in the atmosphere, a gas that occurs in Earth's atmosphere as the result of biological processes.

Studies of Saturn's moon Titan have discovered high levels of methane and nitrogen in the atmosphere, along with other organic compounds that point to the possibility of life forms. Its extremely low temperatures (-289 °F) and hostile weather more than likely rule out current life, at least on the surface. Underground liquid reservoirs, coupled with recent discoveries on Earth of small life forms surviving in incredibly harsh environments, suggested that perhaps there are, or were, micro-sized organisms on Titan.[6]

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

Astronomers are also searching 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. Radiodetection methods have been inadequate for such a search, as the resolution has been 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 atomic 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 analyze 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.

Indirect search

Some scientists theorize that any technological society in space would be transmitting information, and that instruments on Earth could possibly pick up those transmissions. In 1960, Cornell University astronomer Frank Drake performed the first modern 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. Although nothing of great interest was discovered, the notion spawned other searches that continue to this day. Such searches had been categorized under the acronym SETI (Search for Extra-Terrestrial Intelligence). Using radio telescopes to scan for cosmic transmissions is 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 characterized 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. 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.[7] This would mean that humankind has sufficient similarities with other intelligent beings that communication 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 humanity's technological level or above, then communication between the two worlds should be inevitable.

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.

About 10 percent 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, comprising the useful primary targets for interstellar listening. Finding an electromagnetic transmission from an alien civilization involves searching 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 scientists 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.

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

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.

Many different SETI projects have come and gone since the mid-twentieth century. The massive radio dish in Arecibo, Puerto Rico, once was distinguished in its SETI program, until the U.S. government cut its funding. Since then, most of the SETI work being done has been in the private sector, such as the Allen Telescope Array in California, the Phoenix Project in Australia, and Harvard University's Optical SETI project.

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 message contains a pattern of 0s and 1s, defining 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.

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 many as a publicity stunt. It also generated controversy over the serious question of whether a small group should be allowed to speak for Earth.

Notes

  1. ↑ Al Van Helden, "Giordano Bruno (1548-1600)" (1995). Retrieved February 27, 2017.
  2. ↑ Leon Festinger, Henry W. Riecken, and Stanley Schachter, When Prophecy Fails: A Social and Psychological Study of a Modern Group That Predicted the Destruction of the World (Harper-Torchbooks, 1956, ISBN 0061311324).
  3. ↑ J. Allen Hynek, The UFO Experience: A Scientific Inquiry (Marlow & Company, 1998, ISBN 156924782X).
  4. ↑ S. H. Lee, Unification Thought (New York: Unification Thought Institute, 1981, ISBN 0960648003).
  5. ↑ David Darling, "Variety of extraterrestrial life," The Encyclopedia of Astrobiology, Astronomy and Spaceflight. Retrieved February 27, 2017.
  6. ↑ Ker Than, "Scientists Reconsider Habitability of Saturn's Moon," SPACE.com (September 13, 2005). Retrieved February 27, 2017.
  7. ↑ Jennifer Kahn, "Notes From Another Universe," Discover (April 2002). Retrieved February 27, 2017.

References
ISBN links support NWE through referral fees

  • Baird, John C. The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover, NH: University Press of New England, 1987. ISBN 0874514061
  • Cohen, Jack and Ian Stewart. Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press, 2002. ISBN 0091879272
  • Crowe, Michael J. The Extraterrestrial Life Debate, 1750-1900. Mineola, NY: Dover Publications.
  • Exers, Ronald, D. Cullers, J. Billingham, L. Scheffer (eds.). SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence. SETI Press, 2003. ISBN 0966633539
  • Festinger, Leon, Henry W. Riecken, and Stanley Schachter. When Prophecy Fails: A Social and Psychological Study of a Modern Group That Predicted the Destruction of the World. Harper-Torchbooks, 1956. ISBN 0061311324
  • Goldsmith, Donald. The Hunt for Life on Mars. New York: Dutton Books, 1997. ISBN 0525943366
  • Grinspoon, David. The Lonely Planets: The Natural Philosophy of Alien Life. New York: HarperCollins, 2003. ISBN 0060185406
  • Hynek, J. Allen. The UFO Experience: A Scientific Inquiry. Marlow & Company, 1998. ISBN 156924782X
  • Lee, Sang Hun. Unification Thought. Rose of Sharon Pr, 1981. ISBN 0960648003
  • Lemnick, Michael T. Other Worlds: The Search for Life in the Universe. New York: Touchstone Books, 1998.
  • McConnell, Brian and Chuck Toporek. Beyond Contact: A Guide to SETI and Communicating with Alien Civilizations. O'Reilly, 2001. ISBN 0596000375
  • Pickover, Cliff. The Science of Aliens. New York: Basic Books, 2003. ISBN 0465073158
  • Roth, Christopher F. "Ufology as Anthropology: Race, Extraterrestrials, and the Occult" in Debbora Battagilia, E.T. Culture: Anthropology in Outerspaces. Durham, NC: Duke University Press, 2005.
  • Sagan, Carl, and I.S. Shklovskii. Intelligent Life in the Universe. New York: Random House, 1966.
  • Sagan, Carl. Communication with Extraterrestrial Intelligence. Cambridge, MA: MIT Press, 1973.
  • Sagan, Carl. The Demon-Haunted World: Science as a Candle in the Dark, 1996. chap. 4: "Aliens."
  • Webre, Alfred. Exopolitics: Politics, Government, and Law in the Universe. Universebooks, 2008 (original 2005). ISBN 0973766301

External links

All links retrieved March 23, 2024.

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