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=== Extraterrestrial Civilizations ===

=== Extraterrestrial Civilizations ===

Do one or more extraterrestrial civilizations exist? Until recently, speculation about extraterrestrial races was confined either to [[science fiction]] or to innumerable anecdotal reports of "[[UFO|unidentified flying objects]]." These latter reports were once the subject of an investigation, known as [[Project Blue Book]], by the [[United States Air Force]].<ref>Anonymous, "[http://www.ufoevidence.org/topics/ProjectBlueBook.htm Project Blue Book]", ''[http://www.ufoevidence.org/ UFO Evidence.org]'', retrieved April 16, 2007</ref><ref>Anonymous, ''[http://www.bluebookarchive.org/ The Project Blue Book Archive]'', retrieved April 16, 2007</ref><ref>Fund for Unidentified Flying Object Research (FUFOR), ''[http://www.nicap.org/index.htm Official Site of the National Investigative Committee for Aerial Phenomena]'', Francis L. Ridge, editor and Webmaster, December 15, 1997 to present. Retrieved April 16, 2007.</ref><ref>Presumably the [[Union of Soviet Socialist Republics|Soviet]] Army Air Forces had their own version of Blue Book as well.</ref> But years after Blue Book wound down, a large cadre of scientists began to entertain seriously the notion that extraterrestrial civilizations might exist.

Do one or more extraterrestrial civilizations exist? Until recently, speculation about extraterrestrial races was confined either to [[science fiction]] or to innumerable anecdotal reports of "[[UFO|unidentified flying objects]]." These latter reports were once the subject of an investigation, known as Project Blue Book, by the [[United States Air Force]].<ref>Anonymous, "[http://www.ufoevidence.org/topics/ProjectBlueBook.htm Project Blue Book]", ''[http://www.ufoevidence.org/ UFO Evidence.org]'', retrieved April 16, 2007</ref><ref>Anonymous, ''[http://www.bluebookarchive.org/ The Project Blue Book Archive]'', retrieved April 16, 2007</ref><ref>Fund for Unidentified Flying Object Research (FUFOR), ''[http://www.nicap.org/index.htm Official Site of the National Investigative Committee for Aerial Phenomena]'', Francis L. Ridge, editor and Webmaster, December 15, 1997 to present. Retrieved April 16, 2007.</ref><ref>Presumably the [[Union of Soviet Socialist Republics|Soviet]] Army Air Forces had their own version of Blue Book as well.</ref> But years after Blue Book wound down, a large cadre of scientists began to entertain seriously the notion that extraterrestrial civilizations might exist.

SETI ('''S'''earch for '''E'''xtra'''t'''errestrial '''I'''ntelligence)<ref>[http://www.seti.org/site/pp.asp?c=ktJ2J9MMIsE&b=178025 Home of the SETI Institute]</ref> is the name given to several government and privately-funded projects that over the years have searched for evidence of extraterrestrial civilizations.  This is done by listening via [[radiotelescope]] on a defined frequency band for any transmission having any semblance of order. The frequency band is one that, they believe, is the likeliest region in the [[electromagnetic spectrum]] for anyone to be sending a signal intended to cross interstellar, or even intergalactic, space.

[[Search for Extra-Terrestrial Intelligence|SETI]] ('''S'''earch for '''E'''xtra'''t'''errestrial '''I'''ntelligence)<ref>[http://www.seti.org/site/pp.asp?c=ktJ2J9MMIsE&b=178025 Home of the SETI Institute]</ref> is the name given to several government and privately-funded projects that over the years have searched for evidence of extraterrestrial civilizations.  This is done by listening via radio [[telescope]] on a defined frequency band for any transmission having any semblance of order. The frequency band is one that, they believe, is the likeliest region in the [[electromagnetic spectrum]] for anyone to be sending a signal intended to cross interstellar, or even intergalactic, space.

No definitive evidence exists for any form of extraterrestrial civilization. While the [[Project Blue Book]] investigators found a number of cases that they had to classify as "unknown," they were able to classify the bulk of the anecdotes as anything from outright hoaxes to misinterpretations of common weather features to misinterpretation of the sightings of ordinary aircraft.<ref>The Air Force might also have acted in a deceptive manner in certain isolated cases in which what the witnesses took for [[UFO]]s were actually secret experimental prototypes. Perhaps some of these prototypes exist today as the [[B-2]], [[F-117]], and other [[Stealth aircraft]].</ref> Neither has any [[Astronomy|astronomer]] or [[astronaut]] reported a definite sighting of, much less contact with, any carrier-like vessel that would have been capable of launching any of the alleged two-seater or similar small-crewed craft that various "witnesses" report having encountered. Nor has any [[physics|physicist]] yet suggested a method by which such a craft could have visited our [[solar system]], given the fixed speed limit set by the [[Special Theory of Relativity]]. Finally, the SETI project, for all their searching for a signal (even to recruiting civilians to participate in a "distributed computing" project to process the noise they have received from their radiotelescopes), have never isolated anything like a definite signal.

No definitive evidence exists for any form of extraterrestrial civilization. While the Project Blue Book investigators found a number of cases that they had to classify as "unknown," they were able to classify the bulk of the anecdotes as outright hoaxes, misinterpretations of common weather features, or misinterpretations of sightings of ordinary aircraft.<ref>The Air Force might also have acted in a deceptive manner in certain isolated cases in which what the witnesses took for [[UFO]]s were actually secret experimental prototypes. Perhaps some of these prototypes exist today as the [[B-2]], [[F-117]], and other [[Stealth aircraft]].</ref> Neither has any [[Astronomy|astronomer]] or [[astronaut]] reported a definite sighting of a vessel that could have launched any of the craft that witnesses report having encountered. And the SETI projects, despite years of searching for a signal (at one point recruiting civilians to participate in a "distributed computing" project to process the noise they have received from their radiotelescopes), have never isolated anything like a definite signal.

====The Drake Equation====

====The Drake Equation====