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Vigorous scientific research on the subject is fairly limited, and has focused on both the "micro" and "macro" aspects of the problem:

Vigorous scientific research on the subject is fairly limited, and has focused on both the "micro" and "macro" aspects of the problem:

# The genesis, nature, survival, and movement of primitive life forms, including the hazard that such microbes might pose to [[astronaut]]s or to the earth itself.

# The genesis, nature, survival, and movement of primitive life forms, including the hazard that spaceborne microbes might pose to [[astronaut]]s or to the earth itself.

# The possibility of prior or future detection, contact, trade, or war with an extraterrestrial [[civilization]].

# The possibility of prior or future detection, contact, trade, or war with an extraterrestrial [[civilization]].

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.

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====

====Theoretical approaches====

In 1961, astronomer Frank Drake developed a method for estimating of the number of extraterrestrial civilizations in the galaxy.  The Drake Equation simply begins with the number of stars, then multiplies that number by a series of probabilities (probability that a star has planets, that a planet can support life, etc.) to obtain the quantity desired (number of populated planets, number of civilizations whose existence is detectable, etc.).

In 1961, astronomer Frank Drake developed a method for estimating of the number of extraterrestrial civilizations in the galaxy.  The Drake Equation simply begins with the number of stars, then multiplies that number by a series of probabilities (probability that a star has planets, that a planet can support life, etc.) to obtain the quantity desired (number of populated planets, number of civilizations whose existence is detectable, etc.).

In principle, the Drake Equation can provide an upper and lower bound for the number of extraterrestrial civilizations by setting upper and lower bounds for each probability factor, which can then be improved upon as better information becomes available.  Because the number of stars in so huge, even the lower bound is often calculated to be a respectable number.  However, a serious problem is that some of the probabilities are completely unknown, or even unknowable.  For example, the probability of life emerging on a planet is a meaningless concept if life is created directly by God and not by some naturalistic process.

In principle, the Drake Equation can provide an upper and lower bound for the number of extraterrestrial civilizations by setting upper and lower bounds for each probability factor, which can then be improved upon as better information becomes available.  Because the number of stars in so huge, even the lower bound is often calculated to be a respectable number.  However, a serious problem is that some of the probabilities are completely unknown, or even unknowable.  For example, the probability of life emerging on a planet is a meaningless concept if life is created directly by God and not by some naturalistic process. If any of the probabilities appearing in the equation is zero, then the number of extraterrestrial civilizations of course goes to zero.

 

If any of the probabilities appearing in the equation is zero, then the number of extraterrestrial civilizations of course goes to zero. This is a serious problem if one relies only on naturalistic explanations of the genesis of life.  One of the most well-accepted scientific laws, the [[law of biogenesis]], says that life only comes from life, which means that it cannot come from non-life.  Furthermore, scientific observations are that evolution itself is impossible, as evolution requires ''increasing'' genetic [[information]], whereas observations show that living things ''lose'' genetic information.<ref>Wieland, Carl, [http://www.creationontheweb.com/content/view/431/ The evolution train’s a-comin’], ''Creation'' 24(2):16–19, March 2002.</ref>

== Extraterrestrial Life in Science Fiction ==

== Extraterrestrial Life in Science Fiction ==