Nick Bostrom is a Swedish philosopher at the University of Oxford. ...
Most of the water on Mars exists as ice, but there are also small q...
"If extraterrestrial life exists, the closest life forms beyond Ear...
Here is a list of reported UFO sightings going back all the way to ...
SETI Institute ("search for extraterrestrial intelligence”) is a no...
The Fermi paradox, named after physicist Enrico Fermi (1901–1954) s...
The Great filter comes from "The Great Filter - Are We Almost Past ...
Here is an excerpt from Carl Sagan's "The Abundance of Life-Bearing...
There is a forthcoming paper by Snyder-Beattie and Sandberg, resear...
A von Neumann probe is a spacecraft capable of replicating itself. ...
The Drake Equation, first presented by Frank Drake in 1961 identifi...
Read the vulnerable world hypothesis for more from Nick Bolstrom ab...
yikes.
W
HEREARETHEY?
WHYIHOPETHESEARCHFOREXTRATERRESTRIALLIFE
FINDSNOTHING
NickBostrom
FutureofHumanityInstitute
OxfordUniversity
www.nickbostrom.com
[PublishedintheMITTechnologyReview,May/Juneissue(2008):pp.7277]
WhenwaterwasdiscoveredonMars,peoplegotveryexcited.Wherethereiswater,
theremaybelife.Scientistsareplanningnewmissionstostudytheplanetupclose.
NASA’snextMarsroverisscheduledtoarrivein2010.Inthedecadefollowing,aMars
SampleReturnmissionmightbelaunched,whichwoulduseroboticsystemstocollect
samplesofMartianrocks,soils,andatmosphere,andreturnthemtoEarth.Wecould
thenanalyzethesampletoseeifitcontainsanytracesoflife,whetherextinctorstill
active.Suchadiscoverywouldbeoftremendousscientificsignificance.Whatcouldbe
morefascinatingthandiscoveringlifethathadevolvedentirelyindependentlyoflife
hereonEarth?Manypeoplewouldalsofindithearteningtolearnthatwearenot
entirelyaloneinthisvastcoldcosmos.
ButIhopethatourMarsprobeswilldiscovernothing.Itwouldbegoodnewsifwefind
Marstobecompletelysterile.Deadrocksandlifelesssandswouldliftmyspirit.
Conversely,ifwediscoveredtracesofsomesimpleextinctlifeform—somebacteria,
somealgae—itwouldbebadnews.Ifwefoundfossilsofsomethingmoreadvanced,
perhapssomethinglookingliketheremnantsofatrilobiteoreventheskeletonofa
smallmammal,itwouldbeverybadnews.Themorecomplexthelifewefound,the
moredepressingthenewsofitsexistencewouldbe.Scientificallyinteresting,certainly,
butabadomenforthefutureofthehumanrace.
HowdoIarriveatthisconclusion?Ibeginbyreflectingonawellknownfact.UFO
spotters,Raeliancultists,andselfcertifiedalienabducteesnotwithstanding,humans
have,todate,seennosignofanyextraterrestrialintelligentcivilization.Wehavenot
receivedanyvisitorsfromspace,norhaveourradiotelescopesdetectedanysignals
transmittedbyanyextraterrestrialcivilization.TheSearchforExtraTerrestrial
IntelligentLife(SETI)hasbeengoingfornearlyfiftyyears,employingincreasingly
powerfultelescopesanddataminingtechniques,andhassofarconsistently
1
corroboratedthenullhypothesis.Asbestwehavebeenabletodetermine,thenightsky
isemptyandsilent—thequestion“Wherearethey?”thusbeingatleastaspertinent
todayasitwaswhenEnricoFermifirstposeditduringalunchdiscussionwithsomeof
hisphysicistcolleaguesbackin1950.
Hereisanotherfact:Thereareontheorderof100billionstarsinourgalaxyalone,and
theobservableuniversecontainsontheorderof100billiongalaxies.Inthelastcoupleof
decades,wehavelearntthatmanyofthesestarshaveplanetscirclingaroundthem.By
now,severalhundredexoplanetswehavediscovered.Mostofthesearegigantic,but
thisisduetoaselectioneffect:Itisverydifficulttodetectsmallerexoplanetswith
currentobservationmethods.(Inmostcases,theplanetscannotbedirectlyobserved.
Theirexistenceisinferredfromtheirgravitationalinfluenceontheirparentsun,which
wobblesslightlywhenpulledtowardsalargeorbitingplanet;oralternativelybyaslight
fluctuationintheirsun’sperceivedluminositywhichoccurswhenitispartiallyeclipsed
bytheexoplanet.)Wehaveeveryreasontobelievethattheobservableuniversecontains
vastnumbersofsolarsystems,includingmanythathaveplanetsthatareEarthlikeat
leastinthesenseofhavingamassandtemperaturesimilartothoseofourownorb.We
alsoknowthatmanyofthesesolarsystemsaremucholderthanours.
Fromthesetwofactsitfollowsthatthereexistsa“GreatFilter”.
1
TheGreatFiltercanbe
thoughtofasaprobabilitybarrier.Itconsistsofexistoneofmorehighlyimprobable
evolutionarytransitionsorstepswhoseoccurrenceisrequiredinorderforanEarthlike
planettoproduceanintelligentcivilizationofatypethatwouldbevisibletouswithour
currentobservationtechnology.Youstartwithbillionsandbillionsofpotential
germinationpointsforlife,andyouendupwithasumtotalofzeroextraterrestrial
civilizationsthatwecanobserve.TheGreatFiltermustthereforebepowerfulenough—
whichistosay,thecriticalstepsmustbeimprobableenough—thatevenwithmany
billionsrollsofthedice,oneendsupwithnothing:noaliens,nospacecraft,nosignals,at
leastnonethatwecandetectinourneckofthewoods.
Now,animportantquestionforusis,justwheremightthisGreatFilterbelocated?
Therearetwobasicpossibilities:Itmightbebehindus,somewhereinourdistantpast.
Oritmightbeaheadofus,somewhereinthemillenniaordecadestocome.Letus
ponderthesepossibilitiesinturn.
Considerfirstthepossibilitythatthefilterisinourpast.Thiswouldmeanthatthereis
someextremelyimprobablestepinthesequenceofeventswherebyanEarthlikeplanet
givesrisetoanintelligentlifeformcomparableinitstechnologicalsophisticationtoour
1
IborrowthistermfromRobinHanson’s“TheGreatFilter—AreWe AlmostPastIt?”
(http://hanson.gmu.edu/greatfilter.html
),apaperwhichpresentsanargumentsimilartotheone
expoundedhere.
2
contemporaryhumancivilization.Somepeopleseemtotakeitforgrantedthatevolution
ofintelligentlifeonthisplanetwasstraightforward—lengthy,yes,complex,sure,yet
ultimatelyinevitableornearlyso.CarlSaganappearstohaveheldthisview;heonce
wrotethat“theoriginoflifemustbeahighlyprobablecircumstance;assoonas
conditionspermit,upitpops!”
2
Butthisviewmightwellbecompletelymistaken.There
isatanyratehardlyanyevidencetosupportit.Evolutionarybiology,atthemoment,
doesnotenableustocalculatefromfirstprincipleshowprobableorimprobablethe
evolutionofintelligentlifeonEarthwas.Moreover,ifwelookbackatthehistoryoflife
onthisplanet,wecanidentifyanumberofevolutionarytransitionseachoneofwhichis
aplausiblecandidateGreatFilter.
Forexample,perhapsitisvery,veryimprobablethatevensimpleselfreplicatorsshould
emergeonanygivenEarthlikeplanet.Attemptstocreatelifeinthelaboratoryby
mixingwaterandgasesbelievedtohaveexistedintheearlyatmosphereonEarthhave
failedtogetmuchbeyondthesynthesisofafewsimpleaminoacids.Noinstanceof
abiogenesishaseverbeenobserved.
Theoldestconfirmedmicrofossilsdatefromapproximately3,500millionyearsago,and
thereistentativeevidencethatlifemighthaveexistedafewhundredmillionyearsprior
tothatdate,butnoevidenceoflifebefore3,800millionyearsago.Lifemightwellhave
arisenconsiderablyearlierthanthatwithoutleavinganytraces.Thereareveryfew
preservedrockformationsthisoldandsuchashavesurvivedhaveundergonemajor
remoldingovertheeons.Nevertheless,thereisaperiodlastingseveralhundredsof
millionsofyearsbetweentheformationofEarthandthefirstknownlife.Theevidence
isthusconsistentwiththehypothesisthattheemergenceofliferequiredanextremely
improbablesetofcoincidences,andthatittookhundredsofmillionsofyearsoftrial
anderror,ofmoleculesandsurfacestructuresrandomlyinteracting,beforesomething
capableofselfreplicationhappenedtoappearbyastrokeofastronomicalluck.For
aughtweknow,thisfirstcriticalstepcouldbeaGreatFilter.
Sincewecannotrerunthehistoryoflifemultipletimestoobtainrigorousstatistics,itis
difficultdetermineconclusivelythe“difficulty”ofanygivenevolutionarydevelopment.
Thereare,however,somecriteriathatwecanusetoidentifyevolutionarytransitionsthat
areatleastgoodcandidatesforbeingaGreatFilter,i.e.thatarebothextremely
improbableandpracticallynecessaryfortheeventualemergenceofintelligent
technologicalcivilization.Onecriterionisthatthetransitionshouldhaveoccurredonly
once.Flight,sight,photosynthesis,andlimbshaveallevolvedseveraltimeshereon
Earth,andarethusruledout.Anotherindicationthatanevolutionarystepwasvery
improbableisthatittookaverylongtimeforittooccurevenafteritsprerequisiteswere

2
Sagan,C.(1995).“Theabundanceoflifebearingplanets.”BioastronomyNews7(4):
1–4.
3
inplace.Alongdelaysuggeststhatavastlymanyrandomrecombinationshadtobe
triedbeforeonewasfoundthatworked.Perhapsseveralimprobablemutationshadto
occurallatonceinordertoleapfromonelocalfitnesspeaktoanother:themutations
mightindividuallybedeleteriousandonlyfitnessenhancingwhentheyoccurtogether.
(TheevolutionofHomosapiensfromoneofourrecenthominidancestors,suchasHomo
erectus,happenedratherquicklyongeologicaltimescales,sothisstepwouldbea
relativelyweakcandidateforaGreatFilter.)
Theoriginalemergenceoflifeappearstomeetthesetwocriteria.Asfarasweknow,it
mighthaveoccurredonlyonceanditmighthavetakenhundredsofmillionsofyearsfor
ittohappenevenaftertheplanethadcooleddownsufficientlytoenableawiderangeof
organicmoleculestobestable.Laterevolutionaryhistoryoffersadditionalcandidates
fortheGreatFilter.Forexample,ittooksome1.8billionyearsforprokaryotes(themost
basictypeofsinglecellorganism)toevolveintoeukaryotes(amorecomplexkindofcell
withamembraneenclosednucleus).1.8billionyearsisalongtime,andasfaraswe
knoweukaryotesevolvedonlyonce,makingthistransitionanexcellentpossibleGreat
Filter.Otherstrongcandidatesincludetheriseofmulticellularorganismsandsexual
reproduction.
SoonepossibilityisthattheGreatFilterisbehindus.Thiswouldexplaintheabsenceof
observablealiens.Why?Becauseiftheriseofintelligentlifeonanyoneplanetis
sufficientlyimprobable,thenitfollowsthatwearemostlikelytheonlysuchcivilization
inourgalaxyorevenintheentireobservableuniverse.(Theobservableuniverse
containsapproximately10
22
stars.Theuniversemightwellextendinfinitelyfarbeyond
partthatisobservablebyus,andmaycontaininfinitelymanystars.Ifso,thenitis
virtuallycertainthatthereexistsaninfinitenumberofintelligentextraterrestrialspecies,
nomatterhowimprobabletheirevolutiononanygivenplanet.However,cosmological
theoryimpliesthat,duetotheexpansionoftheuniverse,anylifeoutsidetheobservable
universeisandwillforeverremaincausallydisconnectedfromus:itcannevervisitus,
communicatewithus,orbeseenbyusorourdescendants.)
TheotherpossibilityisthattheGreatFilterisafterus,inourfuture.Thiswouldmean
thatthereissomegreatimprobabilitythatpreventsalmostalltechnologicalcivilizations
atourcurrenthumanstageofdevelopmentfromprogressingtothepointwherethey
engageinlargescalespacecolonizationandmaketheirpresenceknowntoother
technologicalcivilizations.Forexample,itmightbethatanysufficientlytechnologically
advancedcivilizationdiscoverssometechnology—perhapssomeverypowerful
weaponstechnology—thatcausesitsextinction.
Iwillreturntothisscenarioshortly,butfirstIshallsayafewwordsaboutanother
theoreticalpossibility:thattheextraterrestrialsareoutthere,inabundancebuthidden
fromourview.Ithinkthisisunlikely,becauseifextraterrestrialsdoexistinany
4
numbers,it’sreasonabletothinkatleastonespecieswouldhavealreadyexpanded
throughoutthegalaxy,orbeyond.Yetwehavemetnoone.
Variousschemeshavebeenproposedforhowanintelligentspeciesmightcolonize
space.Theymightsendout“manned”spaceships,whichwouldestablishcoloniesand
“terraform”newplanets,beginningwithworldsintheirownsolarsystembefore
movingontomoredistantdestinations.Butmuchmorelikely,inmyview,wouldbe
colonizationbymeansofsocalled“vonNeumannprobes”,namedaftertheHungarian
bornprodigyJohnvonNeumann,whoincludedamonghismanymathematicaland
scientificachievementsthedevelopmentoftheconceptofauniversalconstructor.Avon
Neumannprobewouldbeanunmannedselfreplicatingspacecraft,controlledby
artificialintelligence,capableofinterstellartravel.Aprobewouldlandonaplanet(ora
moonorasteroid),whereitwouldminerawmaterialstocreatemultiplereplicasofitself,
perhapsusingadvancedformsofnanotechnology.Thesereplicaswouldthenbe
launchedinvariousdirections,thussettinginmotionamultiplyingcolonizationwave.
3

Ourgalaxyisabout100,000lightyearsacross.Ifa probewerecapableoftravellingat
onetenthofthespeedoflight,everyplanetinthegalaxycouldthusbecolonizedwithin
acoupleofmillionyears(allowingsometimeforthebootstrappingprocessthatneedsto
takeplacebetweenaprobe’slandingonaresourcesite,settingupthenecessary
infrastructure,andproducingdaughterprobes).Iftravelspeedwerelimitedto1%of
lightspeed,colonizationmighttaketwentymillionyearsinstead.Theexactnumbersdo
notmattermuchbecausetheyareatanyrateveryshortcomparedtotheastronomical
timescalesinvolvedintheevolutionofintelligentlifefromscratch(billionsofyears).
IfbuildingavonNeumannprobeseemslikeaverydifficultthingtodo—well,surelyit
is,butwearenottalkingaboutaproposalforsomethingthatNASAortheEuropean
SpaceAgencyshouldgettoworkontoday.Rather,weareconsideringwhatwouldbe
accomplishwithsomefutureveryadvancedtechnology.Weourselvesmightbuild
Neumannprobesindecades,centuries,ormillennia—intervalsthataremereblips
comparedtothelifespanofaplanet.Consideringthatspacetravelwassciencefictiona
merehalfcenturyago,weshould,Ithink,beextremelyreluctanttoproclaimsomething
forevertechnologicallyinfeasibleunlessitconflictswithsomehardphysicalconstraint.
Ourearlyspaceprobesarealreadyoutthere:Voyager1,forexample,isnowbeyondour
solarsystem.
Evenifanadvancedtechnologicalcivilizationcouldspreadthroughoutthegalaxyina
relativelyshortperiodoftime(andthereafterspreadtoneighboringgalaxies),onemight
stillwonderwhetheritwouldopttodoso.Perhapsitwouldratherchoosetostayat
homeandliveinharmonywithnature.However,thereareanumberofconsiderations
thatmakethisalessplausibleexplanationofthegreatsilence.First,weobservethatlife

3
ThisscenariowasdevelopedbyFrankTiplerin1981.
5
hereonEarthmanifestsaverystrongtendencytospreadwhereveritcan.Onourplanet,
lifehasspreadtoeverynookandcrannythatcansustainit:East,West,North,and
South;land,water,andair;desert,tropic,andarcticice;undergroundrocks,
hydrothermalvents,andradioactivewastedumps;thereareevenlivingbeingsinside
thebodiesofotherlivingbeings.Thisempiricalfindingisofcourseentirelyconsonant
withwhatonewouldexpectonthebasisofelementaryevolutionarytheory.Second,if
weconsiderourownspeciesinparticular,wealsofindthatithasspreadtoeverypartof
theplanet,andweevenhaveevenestablishedapresenceinspace,atvastexpense,with
theinternationalspacestation.Third,thereisanobviousreasonforanadvanced
civilizationthathasthetechnologytogointospacerelativelycheaplytodoso:namely,
that’swheremostoftheresourcesare.Land,minerals,energy,negentropy,matter:all
abundantoutthereyetlimitedonanyonehomeplanet.Theseresourcescouldbeused
tosupportagrowingpopulationandtoconstructgianttemplesorsupercomputersor
whateverstructuresacivilizationvalues.Fourth,evenifsomeadvancedcivilization
werenonexpansionarytobeginwith,itmightchangeitsmindafterahundredyearsor
fiftythousandyears—adelaytooshorttomatter.Fifth,evenifsomeadvanced
civilizationchosetoremainnonexpansionistforever,itwouldstillnotmakeany
differenceiftherewereatleastoneothercivilizationouttherethatatsomepointopted
tolaunchacolonizationprocess:thatexpansionarycivilizationwouldthenbetheone
whoseprobes,colonies,ordescendantswouldfillthegalaxy.Ittakesbutonematchto
startafire;onlyoneexpansionistcivilizationtolaunchthecolonizationoftheuniverse.
Forallthesereasonsitseemsunlikelythatthegalaxyisteemingwithintelligentlifeand
thatthereasonwehaven’tseenanyofthemisthattheyallconfinethemselvestotheir
homeplanets.Now,itispossibletoconcoctscenariosinwhichtheuniverseisswarming
withadvancedcivilizationseveryoneofwhichchoosestokeepitselfwellhiddenfrom
ourview.Maybethereisasecretsocietyofadvancedcivilizationsthatknowaboutus
buthavedecidednottocontactusuntilwe’rematureenoughtobeadmittedintotheir
club.Perhapsthey’reobservingus,likeanimalsinazoo.Idon’tseehowwecan
conclusivelyruleoutthispossibility.ButIwillsetitasidefortheremainderofthisessay
inordertoconcentratewhattomeappearstobemoreplausibleanswerstoFermi’s
question.
AdisconcertinghypothesisisthattheGreatFilterconsistsinsomedestructivetendency
commontovirtuallyallsufficientlyadvancedtechnologicalcivilizations.Throughout
history,greatcivilizationsonEarthhaveimploded—theRomanEmpire,theMayan
civilizationthatonceflourishedinCentralAmerica,andmanyothers.However,the
kindofsocietalcollapsethatmerelydelaystheeventualemergenceofaspacecolonizing
civilizationbyafewhundredorafewthousandyearswouldnothelpexplainwhyno
suchcivilizationhasvisitedusfromanotherplanet.Athousandyearsmayseemalong
timetoanindividual,butinthiscontextit’sasneeze.Thereareplanetsthatarebillions
ofyearsolderthanEarth.Anyintelligentspeciesonthoseplanetswouldhavehad
6
ampletimetorecoverfromrepeatedsocialorecologicalcollapses.Eveniftheyfaileda
thousandtimesbeforetheysucceeded,theycouldstillhavearrivedherehundredsof
millionsofyearsago.
ToconstituteaneffectiveGreatFilter,wehypothesizeaterminalglobalcataclysm:an
existentialcatastrophe.Anexistentialriskisonewhereanadverseoutcomewould
annihilateEarthoriginatingintelligentlifeorpermanentlyanddrasticallycurtailits
potentialforfuturedevelopment.Wecanidentifyanumberofpotentialexistential
risks:nuclearwarfoughtwithstockpilesmuchgreaterthanthosethatexisttoday
(mayberesultingfromfuturearmsraces);ageneticallyengineeredsuperbug;
environmentaldisaster;asteroidimpact;warsorterroristsactcommittedwithpowerful
futureweapons,perhapsbasedonadvancedformsofnanotechnology;superintelligent
generalartificialintelligencewithdestructivegoals;highenergyphysicsexperiments;a
permanentglobalBraveNewWorldliketotalitarianregimeprotectedfromrevolution
bynewsurveillanceandmindcontroltechnologies.Thesearejustsomeoftheexistential
risksthathavebeendiscussedintheliterature,andconsideringthatmanyofthesehave
beenconceptualizedonlyinrecentdecades,itisplausibletoassumethatthereare
furtherexistentialrisksthatwehavenotyetthoughtof.
Thestudyofexistentialrisksisanextremelyimportantalbeitratherneglectedfieldof
inquiry.Butherewemustlimitourselvestomakingjustonepoint.Inorderforsome
existentialrisktoconstituteaplausibleGreatFilter,itisnotsufficientthatwejudgeitto
haveasignificantsubjectiveprobabilityofdestroyinghumanity.Rather,itmustbeofa
kindthatcouldwithsomeplausibilitybepostulatedtodestroyvirtuallyallsufficiently
advancedcivilizations.Forinstance,stochasticnaturaldisasterssuchasasteroidhitsand
supervolcaniceruptionsareunlikelyGreatFiltercandidates,becauseevenifthey
destroyedasignificantnumberofcivilizationswewouldexpectsomecivilizationstoget
luckyandescapedisaster;andsomeofthesecivilizationscouldthengoontocolonize
theuniverse.Perhapsthemostlikelytypeofexistentialrisksthatcouldconstitutea
GreatFilterarethosethatarisefromtechnologicaldiscovery.Itisnotfarfetchedto
supposethattheremightbesomepossibletechnologywhichissuchthat(a)virtuallyall
sufficientlyadvancedcivilizationseventuallydiscoveritand(b)itsdiscoveryleads
almostuniversallytoexistentialdisaster.
SowhereistheGreatFilter?Behindus,ornotbehindus?
IftheGreatFilterisaheadofus,wehavestilltoconfrontit.Ifitistruethatalmostall
intelligentspeciesgoextinctbeforetheymasterthetechnologyforspacecolonization,
thenwemustexpectthatourownspecies,too,willgoextinctbeforereaching
technologicalmaturity,sincewehavenoreasontothinkthatwewillbeanyluckierthan
mostotherspeciesatourstageofdevelopment.IftheGreatFilterisaheadofus,we
mustrelinquishallhopeofevercolonizingthegalaxy;andwemustfearthatour
7
adventurewillendsoon,oratanyrate,prematurely.Therefore,webetterhopethatthe
GreatFilterisbehindus.
WhathasallthisgottodowithfindinglifeonMars?Considertheimplicationsof
discoveringthatlifehadevolvedindependentlyonMars(orsomeotherplanetinour
solarsystem).Thatdiscoverywouldsuggestthattheemergenceoflifeisnotavery
improbableevent.Ifithappenedindependentlytwicehereinourownbackyard,it
mustsurelyhavehappenedmillionstimesacrossthegalaxy.Thiswouldmeanthatthe
GreatFilterislesslikelytooccurintheearlylifeofplanetsandisthereforemorelikely
stilltocome.
IfwediscoveredsomeverysimplelifeformsonMarsinitssoilorundertheiceatthe
polarcaps,itwouldshowthattheGreatFiltermustexistsomewhereafterthatperiodin
evolution.Thiswouldbedisturbing,butwemightstillhopethattheGreatFilterwas
locatedinourpast.Ifwediscoveredamoreadvancedlifeform,suchassomekindof
multicellularorganism,thatwouldeliminateamuchlargerstretchofpotentiallocations
wheretheGreatFiltercouldbe.Theeffectwouldbetoshifttheprobabilitymore
stronglytothehypothesisthattheGreatFilterisaheadofus,notbehindus.Andifwe
discoveredthefossilsofsomeverycomplexlifeform,suchasofsomevertebratelike
creature,wewouldhavetoconcludethattheprobabilityisverygreatthatthebulkofthe
GreatFilterisaheadofus.Suchadiscoverywouldbeacrushingblow.Itwouldbeby
fartheworstnewseverprintedonanewspapercover.
Yetmostpeoplereadingtheaboutthediscoverywouldbethrilled.Theywouldnot
understandtheimplications.IftheGreatFilterisnotbehindus,itisaheadofus.
SothisiswhyI’mhopingthatourspaceprobeswilldiscoverdeadrocksandlifeless
sandsonMars,onJupiter’smoonEuropa,andeverywhereelseourastronomerslook.It
wouldkeepalivethehopeforagreatfutureforhumanity.
Now,itmightbethoughtanamazingcoincidenceifEarthweretheonlyplanetinthe
galaxyonwhichintelligentlifeevolved.Ifithappenedhere—theoneplanetwehave
studiedclosely—surelyonewouldexpectittohavehappenedonalotofotherplanetsin
thegalaxyalso,whichwehavenotyethadthechancetoexamine?Thisobjection,
however,restsonafallacy:Itoverlookswhatisknownasan“observationselection
effect.”Whetherintelligentlifeiscommonorrare,everyobserverisguaranteedtofind
themselvesoriginatingfromaplacewhereintelligentlifedid,indeed,arise.Sinceonly
thesuccessesgiverisetoobserverswhocanwonderabouttheirexistence,itwouldbea
mistaketoregardourplanetasarandomlyselectedsamplefromallplanets.(Itwould
beclosertothemarktoregardourplanetasthoughitwerearandomsamplefromthe
subsetofplanetsthatdidengenderintelligentlife:thisbeingacrudeformulationofone
8
ofthesaneelementsextractablefromthemotleyoreofideasreferredtoasthe“anthropic
principle”.)
Sincethispointconfusesmany,itisworthexpoundingitslightly.Considertwodifferent
hypotheses.Onesaysthattheevolutionofintelligentlifeisfairlyeasyandhappensona
significantfractionofallsuitableplanets.Theotherhypothesissaysthattheevolutionof
intelligentlifeisextremelydifficultandhappensperhapsonlyononeoutofamillion
billionsplanets.Toevaluatetheirplausibilityinlightofyourevidence,youmustask
yourself,“WhatdothesehypothesespredictthatIshouldobserve?”Ifyouthinkabout
it,itisclearthatbothhypothesespredictthatyoushouldobservethatyourcivilization
originatedinplaceswhereintelligentlifeevolved.Allobserverswillobserveprecisely
that,whethertheevolutionofintelligentlifehappenedonalargeorasmallfractionofall
planets.Anobservationselectioneffectguaranteesthatwhateverplanetwecall“ours”
wasasuccessstory.Andaslongasthetotalnumberofplanetsintheuniverseislarge
enoughtocompensateforthelowprobabilityofanygivenoneofthemgivingriseto
intelligentlife,itisnotasurprisethatafewsuccessstoriesexist.
If—asIhopeisthecase—wearetheonlyintelligentspeciesthathaseverevolvedinour
galaxy,andperhapsintheentireobservableuniverse,itdoesnotfollowthatoursurvival
isnotindanger.NothingintheabovereasoningprecludestheGreatFilterfrombeing
locatedbothbehindusandaheadofus.Itmightbothbeextremelyimprobablethat
intelligentlifeshouldariseonanygivenplanet,andveryimprobablethatintelligentlife,
onceevolved,shouldsucceedinbecomingadvancedenoughtocolonizespace.
ButwewouldhavesomegroundsforhopethatallormostoftheGreatFilterisinour
pastifMarsisindeedfoundtobebarren.Inthatcase,wemayhaveasignificantchance
ofonedaygrowingintosomethingalmostunimaginablygreaterthanwearetoday.
Inthisscenario,theentirehistoryofhumankindtodateisamereinstantcomparedto
theeonsofhistorythatliestillbeforeus.Allthetriumphsandtribulationsofthe
millionsofpeoplesthehavewalkedtheEarthsincetheancientcivilizationof
Mesopotamiawouldbelikemerebirthpangsinthedeliveryofakindoflifethathasn’t
reallybegunyet.Forsurelyitwouldbetheheightofnaivetétothinkthatwiththe
transformativetechnologiesalreadyinsight—genetics,nanotechnologyandsoon—and
withthousandsofmillenniastillaheadofustoperfectandapplythesetechnologiesand
othersthatwehaven’tyetconceivedof,humannatureandthehumanconditionwill
remainunchangedforallfuture.Instead,ifwesurviveandprosper,wewillpresumably
developintosomekindofposthumanexistence.
SothisiswhyIconcludethatthesilenceofthenightskyisgolden,andwhy,inthe
searchforextraterrestriallife,nonewsisgoodnews.Itpromisesapotentiallygreat
futureforhumanity.
9
10
NoneofthismeansthatweoughttocancelourplanstohaveacloserlookatMars.Ifthe
redplaneteverharboredlife,wemightaswellfindoutaboutit.Itmightbebadnews,
butitwouldtellussomethingaboutourplaceintheuniverse,ourfuturetechnological
prospects,theexistentialrisksconfrontingus,thepossibilitiesforhumantransformation:
issuesofconsiderableimportance.
Itisimpossibletoknowinadvancewhatinsightsmightbegleanedbyapplyingthekind
ofcarefulandsystematicstudytosuchbigquestionsthatweapplyeverydaytosmaller
andlessconsequentialtechnologicalandscientificproblems.Theremaybesurprising
argumentsandideasouttheremerelywaitingtobediscovered.Someofthesemight
eventurnouttohavepracticalramificationsofsuchimportanceastochangeourwhole
schemeofpriorities.PerhapsthegreatestbenefitfromtheSETIprogramwillresultifit
promptsthinkingabouttheselargermatters.
Theoretically,smartambitiousscholarscouldstartthinkingwithoutwaitingforsuch
prompts.Expensiveinstruments,however,haveawayoflendingscientificstatusand
respectabilitytoafieldofinquiry.Academicsarekeentoputasmuchdistanceas
possiblebetweenthemselvesandthekooksandcranksthatflocktothesebigquestions.
Iflargetelescopes,NASAsatellites,andcomplicatedmathematicaldataanalysisare
involved,itbecomesharderforoutsideobserverstomistaketheworkfortheramblings
ofUFOnutsandothercrackpots.Theremaybenosignalsfromspace,yetthosewith
theirantennastunedtomoreanthropomorphicwavelengthsaresuretopickupabuzzof
socialsignalinginpeople’sattitudestowardsthe searchforextraterrestrialbeings.Such
socialbackgroundnoisemightinfactbeoneofthemainobstaclestointellectualprogress
onmanybigpicturetopics.
NickBostromistheDirectoroftheFutureofHumanityInstituteattheUniversityofOxford.
Hishomepage,withmanyofhispapers,isathttp://www.nickbostrom.com
.

Discussion

yikes. Nick Bostrom is a Swedish philosopher at the University of Oxford. He is known for his work on existential risk, the anthropic principle, human enhancement ethics, superintelligence risks, the reversal test, and consequentialism. He authored more than 200 publications including New York Times bestseller Superintelligence: Paths, Dangers, Strategies. You can learn more about Nick on his personal home page: nickbostrom.com  ![Imgur](https://i.imgur.com/3Yk7r3r.png) Here is a list of reported UFO sightings going back all the way to Ancient Egypt: https://en.wikipedia.org/wiki/List_of_reported_UFO_sightings ![Imgur](https://i.imgur.com/56YtVMR.png) Here is an excerpt from Carl Sagan's "The Abundance of Life-Bearing Planets": "We live in an age of remarkable exploration and discovery. Fully half of the nearby Sun-like stars have circumstellar disks of gas and dust like the solar nebula out of which our planets formed 4.6 billion years ago. By a most unexpected technique -- radio timing residuals -- we have discovered two Earth-like planets around the pulsar B1257+12. An apparent Jovian planet has been astrometrically detected around the star 51 Pegasi. A range of new Earth-based and space-borne techniques--including astrometry, spectrophotometry, radial velocity measurements, adaptive optics and interferometry-- all seem to be on the verge of being able to detect Jovian- type planets, if they exist, around the nearest stars. At least one proposal (The FRESIP[Frequency of Earth Sized Inner Planets] Project, a spaceborne spectrophotometric system) holds the promise of detecting terrestrial planets more readily than Jovian ones. If there is not a sudden cutoff in support, we are likely entering a golden age in the study of the planets of other stars in the Milky Way galaxy. Once you have found another planet of Earth-like mass, however, it of course does not follow that it is an Earth- like world. Consider Venus. But there are means by which, even from the vantage point of Earth, we can investigate this question. We can look for the spectral signature of enough water to be consistent with oceans. We can look for oxygen and ozone in the planet's atmosphere. We can seek molecules like methane, in such wild thermodynamic disequilibrium with the oxygen that it can only be produced by life. (In fact, all of these tests for life were successfully performed by the Galileo spacecraft in its close approaches to Earth in 1990 and 1992 as it wended its way to Jupiter [Sagan et al., 1993].) The best current estimates of the number and spacing of Earth-mass planets in newly forming planetary systems (as George Wetherill reported at the first international conference on circumstellar habitable zones [Doyle, 1995]) combined with the best current estimates of the long-term stability of oceans on a variety of planets (as James Kasting reported at that same meeting [Doyle, 1995]) suggest one to two blue worlds around every Sun-like star. Stars much more massive than the Sun are comparatively rare and age quickly. Stars comparatively less massive than the Sun are expected to have Earth-like planets, but the planets that are warm enough for life are probably tidally locked so that one side always faces the local sun. However, winds may redistribute heat from one hemisphere to another on such worlds, and there has been very little work on their potential habitability. Nevertheless, the bulk of the current evidence suggests a vast number of planets distributed through the Milky Way with abundant liquid water stable over lifetimes of billions of years. Some will be suitable for life--our kind of carbon and water life--for billions of years less than Earth, some for billions of years more. And, of course, the Milky Way is one of an enormous number, perhaps a hundred billion, other galaxies." Source: http://www2.hawaii.edu/~pine/sagan.html The Drake Equation, first presented by Frank Drake in 1961 identifies specific factors thought to play a role in the development of advanced civilizations. Although there is no unique solution to this equation, it is a generally accepted tool used by the scientific community to examine these factors and get at an estimation of the number of technological civilizations that might exist among the stars. ![Imgur](https://i.imgur.com/OhEQ8NC.jpg) Source: https://www.seti.org/drake-equation-index A von Neumann probe is a spacecraft capable of replicating itself. The concept is named after the scientist John von Neumann (December 28, 1903 – February 8, 1957), who studied the concept of self-replicating machines that he called "Universal Assemblers" and which are now referred as "von Neumann machines". von Neumann never applied his work to the idea of spacecrafts, but theoreticians since then have done so. If a self-replicating probe finds evidence of primitive life (or a primitive, low-level culture) it might be programmed to lie dormant, silently observe, attempt to make contact (this variant is known as a Bracewell probe), or even interfere with or guide the evolution of life in some way. Nick Bostrom frequently discusses the idea that future powerful superintelligences will create efficient cost-effective space travel and interstellar Von Neumann probes. Source: https://en.wikipedia.org/wiki/Self-replicating_spacecraft It's worth noting that more recent work by Sandberg, Drexler, and Ord entitled "dissolving the Fermi Paradox - https://arxiv.org/abs/1806.02404 shows that we should be surprised to find life, and shows that typical Fermi estimates are incorrect in using point estimates instead of distributions. By correcting this, it becomes clear that life is very unlikely. There is a forthcoming paper by Snyder-Beattie and Sandberg, researchers in Bostrom's group at Oxford, show how these transition steps can be used to update reasonable prior distributions. This builds on the paper mentioned in an earlier note by Sandberg, Drexler, and Ord entitled "dissolving the Fermi Paradox" - https://arxiv.org/abs/1806.02404 This provides further strong evidence that the great filter lies in the past. David-> really good point, we'll be annotating that paper soon, their approach to the Fermi Paradox using distributions is more mathematically sound. Thanks for sharing! The Great filter comes from "The Great Filter - Are We Almost Past It?", published on Sept. 15, 1998 by Robin Hanson. Here is how it begins: "Humanity seems to have a bright future, i.e., a non-trivial chance of expanding to fill the universe with lasting life. But the fact that space near us seems dead now tells us that any given piece of dead matter faces an astronomically low chance of begating such a future. There thus exists a great filter between death and expanding lasting life, and humanity faces the ominous question: how far along this filter are we? Combining standard stories of biologists, astronomers, physicists, and social scientists would lead us to expect a much smaller filter than we observe. Thus one of these stories must be wrong. To find out who is wrong, and to inform our choices, we should study and reconsider all these areas. For example, we should seek evidence of extraterrestrials, such as via signals, fossils, or astronomy. But contrary to common expectations, evidence of extraterrestrials is likely bad (though valuable) news. The easier it was for life to evolve to our stage, the bleaker our future chances probably are." Source: http://mason.gmu.edu/~rhanson/greatfilter.html "If extraterrestrial life exists, the closest life forms beyond Earth may very well be on Mars. Our search for life on Mars has turned up compelling findings. With the Mariner missions in the 1960s, we learned that Mars has a tenuous atmosphere dominated by carbon dioxide, which would be good for plants and anaerobic microbes, though not for us. Measurements made during the Viking missions in the 1970s hinted at the presence of primitive Martian life forms. Two decades later, scientists studying a meteorite from Mars found evidence that some think suggests the presence of ancient life on the planet. More recently, our robotic explorers have uncovered evidence of an abundance of frozen water, and in 2015, NASA's Curiosity rover found more methane gas in the Martian air than many scientists think Mars should have — unless microbes are responsible for producing some of that methane. The evidence for ancient and present life on Mars is controversial and inconclusive at best, but there is no proof that Mars is sterile, either. Both scenarios would carry enormous implications. It is possible microscopic Martians exist already. Although the planet looks barren and sterile on the surface, it could be home to thriving colonies of subsurface microbes. And if there are subsurface microbes, we might be related. All of life on Earth could conceivably have descended from Martian microbes that caught a ride to Earth on an ancient meteorite and then slowly evolved into our ancestors. This knowledge would tell us that life can survive long journeys in space, perhaps even long journeys from one star system to another. Alternatively, life on Mars might have no connection to life on Earth. If Martian life is not based on DNA or RNA, genesis happened twice, on two nearby planets orbiting the same star. That knowledge would tell us that life is likely very common in the universe. If Mars turns out to be uninhabited altogether, that knowledge, too, would be profound. We might be alone in the universe, or far more isolated than many experts currently believe. This ambiguity presents us with a moral quandary. If we start dropping habitats, greenhouses, food, fuel and people onto the planet's surface, we will contaminate Mars. We would inject bacteria and viruses into the Martian ecosystem and provide them with places where they could survive and even thrive. Martian life, if it exists, is not likely to outlive an onslaught of terrestrial germs." David Weintraub is a professor of astronomy at Vanderbilt University and the author of "Life on Mars: What to Know Before We Go." Source:https://www.latimes.com/opinion/op-ed/la-oe-weintraub-life-on-mars-insight-lander-20180429-story.html Read the vulnerable world hypothesis for more from Nick Bolstrom about these existential dangers and their relative risks: https://nickbostrom.com/papers/vulnerable.pdf "Scientific and technological progress might change people’s capabilities or incentives in ways that would destabilize civilization. For example, advances in DIY biohacking tools might make it easy for anybody with basic training in biology to kill millions; novel military technologies could trigger arms races in which whoever strikes first has a decisive advantage; or some economically advantageous process may be invented that produces disastrous negative global externalities that are hard to regulate. This paper introduces the concept of a vulnerable world: roughly, one in which there is some level of technological development at which civilization almost certainly gets devastated by default, i.e. unless it has exited the “semi-anarchic default condition”. Several counterfactual historical and speculative future vulnerabilities are analyzed and arranged into a typology. A general ability to stabilize a vulnerable world would require greatly amplified capacities for preventive policing and global governance. The vulnerable world hypothesis thus offers a new perspective from which to evaluate the risk-benefit balance of developments towards ubiquitous surveillance or a unipolar world order." Most of the water on Mars exists as ice, but there are also small quantities of vapor in its atmosphere and occasionally low-volume liquid brines in shallow Martian soil. In December of 2013, NASA reported that the Curiosity Rover studying Aeolis Palus, Gale Crrater found an ancient freshwater lake that might have once hospitable environment for microbial life. There have also been over 60 meteorites that have been found that came from Mars, many containing evidence that they were exposed to water on Mars. Nasa’s next Mars rover will look for signs of life on an ancient crater lake. This was just announced today (November 19th 2018): https://www.washingtonpost.com/science/2018/11/19/nasas-next-mars-rover-will-look-signs-life-an-ancient-crater-lake/?utm_term=.dfbc753afedf For more about water/life on Mars: https://en.wikipedia.org/wiki/Water_on_Mars The Fermi paradox, named after physicist Enrico Fermi (1901–1954) states the conflict between the argument that scale and probability seem to favor intelligent life being common in the universe, and total lack of evidence of intelligent life having ever arisen anywhere other than on the Earth. Here is a description of the mentioned lunch: “In 1950, while working at Los Alamos National Laboratory, Fermi had a casual conversation while walking to lunch with colleagues Emil Konopinski, Edward Teller and Herbert York. The men discussed a recent spate of UFO reports and an Alan Dunn cartoon facetiously blaming the disappearance of municipal trashcans on marauding aliens. The conversation shifted to other subjects, until during lunch Fermi suddenly exclaimed, "Where are they?". Teller remembers, "The result of his question was general laughter because of the strange fact that in spite of Fermi's question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life." Herbert York recalls that Fermi followed up on his comment with a series of calculations on the probability of Earth-like planets, the probability of life, the likely rise and duration of high technology, etc., and concluded that we ought to have been visited long ago and many times over.” 
Source: https://en.wikipedia.org/wiki/Fermi_paradox SETI Institute ("search for extraterrestrial intelligence”) is a not-for-profit research organization incorporated in 1984 with headquarters in Mountain View, California. Their mission is to explore, understand and explain the origin and nature of life in the universe, and apply the knowledge gained to inspire and guide present and future generations. On 13 February 2015, scientists (including David Grinspoon, Seth Shostak, and David Brin) at an annual meeting of the American Association for the Advancement of Science, discussed Active SETI and whether transmitting a message to possible intelligent extraterrestrials in the cosmos was a good idea. That same week, a statement was released, signed by many in the SETI community, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent". Here is an oped written by Seth Shostak and published in the New York Times titled: “Should We Keep a Low Profile in Space?” https://www.nytimes.com/2015/03/28/opinion/sunday/messaging-the-stars.html ![Imgur](https://i.imgur.com/1dGo7VK.jpg) For more on SETI: https://en.wikipedia.org/wiki/SETI_Institute