下载后可任意编辑新概念第四册课文翻译及学习笔记Lesson43~45新概念第四册课文翻译及学习笔记Lesson43【课文】Firstlistenandthenanswerthefollowingquestion.听录音，然后回答以下问题。Whatdoesthe‘uniquelyrationalway’forustocommunicatewithotherintelligentbeingsinspacedependon?Wemustconcludefromtheworkofthosewho下载后可任意编辑havestudiedtheoriginoflife,thatgivenaplanetonlyapproximatelylikeourown,lifeisalmostcertaintostart.Ofalltheplanetsinoursolarsystem,wewarenowprettycertaintheEarthistheonlyoneonwhichlifecansurvive.Marsistoodryandpoorinoxygen,Venusfartoohot,andsoisMercury,andtheouterplanetshavetemperaturesnearabsolutezeroandhydrogen-dominatedatmospheres.Butothersuns,startastheastronomerscallthem,areboundtohaveplanetslikeourown,andasisthenumberofstarsintheuniverseissovast,thispossibilitybecomesvirtualcertainty.ThereareonehundredthousandmillionstartsinourownMilkyWayalone,andthentherearethreethousandmillionotherMilkyWays,orgalaxies,intheuniverse.Sothenumberofthestarsthatweknowexistisnowestimatedatabout300millionmillionmillion.Althoughperhapsonly1percentofthelifethathasstartedsomewherewilldevelopintohighlycomplexandintelligentpatterns,sovastisthe下载后可任意编辑numberofplanets,thatintelligentlifeisboundtobeanaturalpartoftheuniverse.Ifthenwearesocertainthatotherintelligentlifeexistsintheuniverse,whyhavewehadnovisitorsfromouterspaceyet?Firstofall,theymayhavecometothisplanetofoursthousandsormillionsofyearsago,andfoundourthenprevailingprimitivestatecompletelyuninterestingtotheirownadvancedknowledge.ProfessorRonaldBracewell,aleadingAmericanradioastronomer,arguedinNaturethatsuchasuperiorcivilization,onavisittoourownsolarsystem,mayhaveleftanautomaticmessengerbehindtoawaitthepossibleawakeningofanadvancedcivilization.Suchamessenger,receivingourradioandtelevisionsignals,mightwellre-transmitthembacktoitshome-planet,althoughwhatimpressionanyothercivilizationwouldthusgetfromusisbestleftunsaid.Butherewecomeupagainstthemostdifficult下载后可任意编辑ofallobstaclestocontactwithpeopleonotherplanets--theastronomicaldistanceswhichseparateus.Asareasonableguess,theymight,onanaverage,be100lightyearsaway.(Alightyearisthedistancewhichlighttravelsat186,000milespersecondinoneyear,namely6millionmillionmiles.)Radiowavesalsotravelatthespeedoflight,andassumingsuchanautomaticmessengerpickedupourfirstbroadcastsofthe1920’s,themessagetoitshomeplanetisbarelyhalfwaythere.Similarly,ourownpresentprimitivechemicalrockets,thoughgoodenoughtoorbitmen,havenochanceoftransportingustothenearestotherstar,fourlightyearsaway,letalonedistancesoftensorhundredsoflightyears.Fortunately,thereisa‘uniquelyrationalway’forustocommunicatewithotherintelligentbeings,asWalterSullivanhasputitinhisexcellentbook,WeArenotAlone.Thisdependsonthepreciseradiofrequencyofthe21-cmwavelength,or1420megacyclespersecond.Itis下载后可任意编辑thenaturalfrequencyofemissionofthehydrogenatomsinspaceandwasdiscoveredbyusin1951;itmustbeknowntoanykindofradioastronomerintheuniverse.Oncetheexistenceofthiswave-lengthhadbeendiscovered,itwasnotlongbeforeitsuseastheuniquelyrecognizablebroadcastingfrequencyforinterstellarcommunicationwassuggested.Withoutsomethingofthiskind,searchingforintelligencesonotherplanetswouldbeliketryingtomeetafriendinLondonwithoutapre-arrangedrendezvousandabsurdlywanderingthestreetsinthehopeofachanceencounter.ANTHONYMICHAELISAreThereStrangersinSpa...
