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cky　or　so　prudent；　perish。

Since；　in　the　long　run；　every　planetary　society　will　be　endangered　by　impacts　from　space；　every　surviving　civilization　is　obliged　to　bee　spacefaring—…not　because　of　exploratory　or　romantic　zeal；　but　for　the　most　practical　reason　imaginable：　staying　alive。　And　once　you're　out　there　in　space　for　centuries　and　millennia；　moving　little　worlds　around　and　engineering　planets；　your　species　has　been　pried　loose　from　its　cradle。　If　they　exist；　many　other　civilizations　will　eventually　venture　far　from　home。*

*　Might　a　planetary　civilization　which　has　survived　its　adolescence　wish　to　encourage　others　struggling　with　their　emerging　technologies？　Perhaps　they　would　make　special　efforts　to　broadcast　news　of　their　existence；　the　triumphant　announcement　that　it's　possible　to　avoid　self…annihilation。　Or　would　they　at　first　be　very　cautious？　Having　avoided　catastrophes　of　their　own　making；　perhaps　they　would　fear　giving　away　knowledge　of　their　existence；　lest　some　other；　unknown；　aggrandizing　civilization　out　there　in　the　dark　is　looking　for　Lebensraum　or　slavering　to　put　down　the　potential　petition。　That　might　be　a　reason　for　us　to　explore　neighboring　star　systems；　but　discreetly。

Maybe　they　would　be　silent　for　another　reason：　because　broadcasting　the　existence　of　an　advanced　civilization　might　encourage　emerging　civilizations　to　do　less　than　their　best　efforts　to　safeguard　their　future—　hoping　instead　that　someone　will　e　out　of　the　dark　and　save　them　from　themselves。



A　MEANS　HAS　BEEN　OFFERED　of　estimating　how　precarious　our　circumstances　are—remarkably；　without　in　any　way　addressing　the　nature　of　the　hazards。　J。　Richard　Gott　III　is　an　astrophysicist　at　Princeton　University。　He　asks　us　to　adopt　a　generalized　Copernican　principle；　something　I've　described　elsewhere　as　the　Principle　of　Mediocrity。　Chances　are　that　we　do　not　live　in　a　truly　extraordinary　time。　Hardly　anyone　ever　did。　The　probability　is　high　that　we're　born；　live　out　our　days；　and　die　somewhere　in　the　broad　middle　range　of　the　lifetime　of　our　species　（or　civilization；　or　nation）。　Almost　certainly；　Gott　says；　we　do　not　live　in　first　or　last　times。　So　if　your　species　is　very　young；　it　follows　that　it's　unlikely　to　last　long—because　if　it　were　to　last　long；　you　（and　the　rest　of　us　alive　today）　would　be　extraordinary　in　living；　proportionally　speaking；　so　near　the　beginning。

What　then　is　the　projected　longevity　of　our　species？　Gott　concludes；　at　the　97。5　percent　confidence　level；　that　there　will　be　humans　for　no　more　than　8　million　years。　That's　his　upper　limit；　about　the　same　as　the　average　lifetime　of　many　mammalian　species。　In　that　case；　our　technology　neither　harms　nor　helps。　But　Gott's　lower　limit；　with　the　same　claimed　reliability；　is　only　12　years。　He　will　not　give　you　40…to…1　odds　that　humans　will　still　be　around　by　the　time　babies　now　alive　bee　teenagers。　In　everyday　life　we　try　very　hard　not　to　take　risks　so　large；　not　to　board　airplanes；　say；　with　1　chance　in　40　of　crashing。　We　will　agree　to　surgery　in　which　95　percent　of　patients　survive　only　if　our　disease　has　a　greater　than　5　percent　chance　of　killing　us。　Mere　40…to…1　odds　on　our　species　surviving　another　12　years　Would　be；　if　valid；　a　cause　for　supreme　concern。　If　Gott　is　right；　not　only　may　we　never　be　out　among　the　stars；　there's　a　fair　chance　we　may　not　be　around　long　enough　even　to　make　the　first　footfall　on　another　planet。

To　me；　this　argument　has　a　strange；　vaporish　quality。　Knowing　nothing　about　our　species　except　how　old　it　is；　we　make　numerical　estimates；　claimed　to　be　highly　reliable；　about　its　future　prospects。　How？　We　go　with　the　winners。　Those　who　have　been　around　are　likely　to　stay　around。　Newers　tend　to　disappear。　The　only　assumption　is　the　quite　plausible　one　that　there　is　nothing　special　about　the　moment　at　which　we　inquire　into　the　matter。　So　why　is　the　argument　unsatisfying？　Is　it　just　that　we　are　appalled　by　its　implications？

Something　like　the　Principle　of　Mediocrity　must　have　very　broad　applicability。　But　we　are　not　so　ignorant　as　to　imagine　that　everything　is　mediocre。　There　is　something　special　about　our　time—not　just　the　temporal　chauvinism　that　those　who　reside　in　any　epoch　doubtless　feel；　but　something；　as　outlined　above；　clearly　unique　and　strictly　relevant　to　our　species'　future　chances：　This　is　the　first　time　that　（a）　our　exponentiating　technology　has　reached　the　precipice　of　self…destruction；　but　also　the　first　time　that　（b）　we　can　postpone　or　avoid　destruction　by　going　somewhere　else；　somewhere　off　the　Earth。

These　two　clusters　of　capabilities；　（a）　and　（b）；　make　our　time　extraordinary　in　directly　contradictory　ways—which　both　（a）　strengthen　and　（b）　weaken　Gott's　argument。　I　don't　know　how　to　predict　whether　the　new　destructive　technologies　will　hasten；　more　than　the　new　spaceflight　technologies　will　delay；　human　extinction。　But　since　never　before　have　we　contrived　the　means　of　annihilating　ourselves；　and　never　before　have　w　e　developed　the　technology　for　settling　other　worlds；　I　think　a　pelling　case　can　be　made　that　our　time　is　extraordinary　precisely　in　the　context　of　Gott's　argument。　If　this　is　true；　it　significantly　increases　the　margin　of　error　in　such　estimates　of　future　longevity。　The　worst　is　worse；　and　the　best　better：　Our　short…term　prospects　are　even　bleaker　and—if　we　can　survive　the　short…term—our　long…term　chances　even　brighter　than　Gott　calculates。

But　the　former　is　no　more　cause　for　despair　than　the　latter　is　for　placency。　Nothing　forces　us　to　be　passive　observers；　clucking　in　dismay　as　our　destiny　inexorably　works　itself　out。　If　we　cannot　quite　seize　fate　by　the　neck；　perhaps　we　can　misdirect　it；　or　mollify　it；　or　escape　it。

Of　course　we　must　keep　our　planet　habitable—not　on　a　leisurely　timescale　of　centuries　or　millennia；　but　urgently；　on　a　timescale　of　decades　or　even　years。　This　will　involve　changes　in　government；　in　industry；　in　ethics；　in　economics；　and　in　religion。　We've　never　done　such　a　thing　before；　certainly　not　on　a　global　scale。　It　may　be　too　difficult　for　us。　Dangerous　technologies　may　be　too　widespread。　Corruption　may　be　too　pervasive。　Too　many　leaders　may　be　focused　on　the　short　term　rather　than　the　long。　There　may　be　too　many　quarreling　ethnic　groups；　nation…states；　and　ideologies　for　the　right　kind　of　global　change　to　be　instituted。　We　may　be　too　foolish　to　perceive　even　what　the　real　dangers　are；　or　that　much　of　what　we　hear　about　them　is　determined　by　those　with　a　vested　interest　in　minimizing　fundamental　change。

However；　we　humans　also　have　a　history　of　making　long…lasting　social　change　that　nearly　everyone　thought　impossible。　Since　our　earliest　days；　we've　worked　not　just　for　our　own　advantage　but　for　our　children　and　our　grandchildren。　My　grandparents　and　parents　did　so　for　me。　We　have　often；　despite　our　diversity；　despite　endemic　hatreds；　pulled　together　to　face　a　mon　enemy。　We　seem；　these　days；　much　more　willing　to　recognize　the　dangers　before　us　than　we　were　even　a　decade　ago。　The　newly　recognized　dangers　threaten　all　of　us　equally。　No　one　can　say　how　it　will　turn　out　down　here。



THE　MOON　WAS　WHERE　the　tree　of　immortality　grew　in　ancient　Chinese　myth。　The　tree　of　longevity　if　not　of　immortality；　it　seems；　indeed　grows　on　other　worlds。　If　we　were　up　there　among　the　planets；　if　there　were　self…sufficient　human　munities　oil　many　worlds；　our　species　would　be　insulated　from　catastrophe。　The　depletion　of　the　ultraviolet…absorbing　shield　on　one　world　would；　if　anything；　be　a　warning　to　take　special　care　of　the　shield　on　another。　A　cataclysmic　impact　on　one　world　would　likely　leave　all　the　others　untouched。　The　more　of　us　beyond　the　Earth；　the　greater　the　diversity　of　worlds　we　inhabit；　the　more　varied　the　planetary　engineering；　the　greater　the　range　of　societal　standards　and　values—then　the　safer　the　human　species　will　be。

If　you　grow　up　living　underground　in　a　world　with　a　hundredth　of　an　Earth　gravity　and　black　skies　through　the　portals；　you　have　a　very　different　set　of　perceptions；　interests；　prejudices；　and　predispositions　than　someone　who　lives　on　the　surface　of　the　home　planet。　Likewise　if　you　live　on　the　surface　of　Mars　in　the　throes　of　terraforming；　or　Venus；　or　Titan。　This