Extraterrestrialcivilizations

(地外文明）ExtraterrestriallifeContentsExtraterrestriallifeislifethatdoesnotoriginatefromEarth.Itisalsocalledalienlife,or,ifitisasentientandrelativelycomplexindividual,an"extraterrestrial"or"alien“.外星生命指存在于地球以外的生命體。這個概念囊括了簡單的細菌到具有高度智慧的外星人。研究和測試關于外星生命猜想的學科被稱作地外生物學或天體生物學。Possiblebasis(存在與否）ScientificsearchBackgroundCulturalimpactCONTENTS1952年美國新澤西州所拍攝的UFO人類普遍對外星人的刻板印象BackgroundPossiblebasisAlienlife,suchasmicroorganisms（微生物）,hasbeenhypothesizedtoexistintheSolarSystemandthroughouttheuniverse.Thishypothesisreliesonthevastsize（廣博的面積）andconsistentphysicallaws（一致的物理定律）oftheobservableuniverse.Accordingtothisargument,madebyscientistssuchasCarlSagan(卡爾·薩根:太空科學家)andStephenHawking(史蒂芬·霍金),itwouldbeimprobableforlifenottoexistsomewhereotherthanEarth.ThisargumentisembodiedintheCopernicanprinciple,whichstatesthatEarthdoesnotoccupyauniquepositionintheUniverse,andthemediocrityprinciple,whichsuggeststhatthereisnothingspecialaboutlifeonEarth.PossiblebasisPossiblebasisPossiblebasisAlllifeonEarthisbasedupon26chemicalelements.However,about95%ofthislifeisbuiltupononlysixoftheseelements:carbon,hydrogen,nitrogen,oxygen,phosphorusandsulfur,abbreviatedCHNOPS（碳、氫、氧、磷及硫）.ThesesixelementsformthebasicbuildingblocksofvirtuallyalllifeonEarth,whereasmostoftheremainingelementsarefoundonlyintraceamounts（微量）.PossiblebasisPossiblebasis認為地球之外存在生命有著很簡單的事實的支持：僅僅在銀河系中就有2000億至4000億顆恒星，而銀河系只是宇宙中超過1000億星系中的一員。據估計至少有十分之一的類似太陽的恒星具有行星系統。Possiblebasis科學家在搜索外星生命時，多以液態水和有機物的存在作為外星生命存在之前提條件。水為地球生命體內各種化學反應提供了場所。有人批評這樣以地球生物為藍本的先入為主的觀點阻礙了外星生命的探索。卡爾·薩根在1973年提出碳沙文主義，認為這些以人類為中心的思想限制了我們對于地外生命可能性的想像。例如在宜居帶之外，有可能通過地熱等方式維持地底的生物圈；也有生物能夠在高砷低磷的環境下存活，這說明生物組成“必備”的六大基本元素：碳、氫、氧、氮、磷、硫，可能不是必需的。LifeonEarthrequires

water

asits

solvent

inwhichbiochemicalreactionstakeplace.Sufficientquantitiesofcarbonandtheotherelementsalongwithwater,mayenabletheformationoflivingorganismsonotherplanetswithachemicalmake-upandtemperaturerangesimilartothatofEarth.PossiblebasisPossiblebasisScientificsearchScientificsearchScientificsearchScientificsearchDirectsearch直接探索Indirectsearch間接探索Extrasolarplanets可能存在的外星生命ScientistsaredirectlysearchingforbiosignatureswithintheSolarSystem,carryingoutstudiesonthesurfaceofMarsandexaminingmeteorsthathavefallentoEarth.Atthemoment,noconcreteplanexistsforexplorationofEuropaforlife.In2008,ajointmissionbyNASAandtheEuropeanSpaceAgencywasannouncedthatwouldhaveincludedstudiesofEuropa.However,in2011NASAwasforcedtodeprioritizethemissionduetoalackoffunding,anditispossiblethattheESAwilltakeonthemissionbyitself.Directsearch2003年6月10日和7月7日，美國國家航空航天局分別發射了勇氣號和機遇號火星探測器，其目標是探測火星上水存在的可能性、分析火星巖石和土壤的成分、評估火星是否適合生命生存等。這兩個雙胞胎探測器均在2004年1月在火星成功著陸。在2011年11月26日發射的好奇號火星探測器也在2012年8月6日成功登陸火星，用于評估火星上是否存在過適合微生物生存的環境TerrestrialPlanetFinderTheNASAKeplerforthesearchforextrasolarplanets.IndirectsearchScientistshaveproposedradiated

heat

fromaplanetasawaytofindadvancedcivilizations.

Large-scalepowergenerationreleaseslargeamountsofheat,whichcanbedetectedusinganadvancedtelescope,suchastheproposedColossus

interferometric

telescope.

Thefactthattheheatsignatureswouldbecomingfrompowergeneration,especiallyinurbanareas,where

heatislands

arecreated,wouldalsoconfirmthattheobservedcivilizationisadvanced.Largerpowerconsumptioncorrelateswithmore-advancedsocieties.Detectingthermalsignaturesallowstheabilitytodetectadvancedcivilizationsindirectly,asopposedtotryingtomakedirectcontactthroughmessageslikethosecreatedby

ActiveSETI.SETI對外星智能的探索(searchforextraterrestrialintelligence).科學家認為生命可能曾經存在過或者依然存在的地方包括金星、火星、木星的衛星、土星的衛星。除了太陽系的行星和衛星之外，科學家在太陽系外也找到了數十顆位于宜居帶太陽系以外的行星。科學家保守估計每兩顆恒星中就有一顆擁有行星，每200顆恒星中就有一顆位于宜居地帶。ExtrasolarplanetsAstronomerssearchforextrasolarplanetsthatmaybeconducivetolife,narrowingthesearchtoterrestrialplanetswithinthehabitablezone.Currentradiodetectionmethodshavebeeninadequateforsuchasearch,becausetheresolutionaffordedbyrecenttechnologyisinadequateforadetailedstudyofextrasolarplanetaryobjects.CultureimpactThefailuresofaroftheSETIprogramtodetectanintelligentradiosignalafterdecadesofeffort,hasatleastpartiallydimmedtheprevailingoptimismofthebeginningofthespaceage.Notwithstanding,beliefinextraterrestrialbeingscontinuestobevoicedinpseudoscience,conspiracytheories,andinpopularfolklore,notably"Area51"andlegends.Ithasbecomeapopculturetropegivenless-than-serioustreatmentinpopularentertainmentwithe.g.theALFTVseries(1986–1990),TheX-Files(1993–2002),etc.內華達375號州道旁的路牌，由于靠近51區而被稱命名為“外星人公路”。火星偵察軌道器的高分辨率成像科學設備拍攝的高分辨率“火星人臉”影像，右下角為海盜號拍攝的影像。Theoreticalphysicist

StephenHawking

in2010warnedthathumansshouldnottrytocontactalienlifeforms.HewarnedthataliensmightpillageEarthforresources."Ifaliensvisitus,theoutcomewouldbemuchaswhen

Columbus

landedin

America,whichdidn'tturnoutwellfo