A Short History of Nearly Everything- 第40部分

according　to　timothy　ferris；　writing　in　the　new　yorker；　such　near　misses　probably　happentwo　or　three　times　a　week　and　go　unnoticed。

an　object　a　hundred　yards　across　couldn’t　be　picked　up　by　any　earth…based　telescope　untilit　was　within　just　a　few　days　of　us；　and　that　is　only　if　a　telescope　happened　to　be　trained　on　it；which　is　unlikely　because　even　now　the　number　of　people　searching　for　such　objects　ismodest。　the　arresting　analogy　that　is　always　made　is　that　the　number　of　people　in　the　worldwho　are　actively　searching　for　asteroids　is　fewer　than　the　staff　of　a　typical　mcdonald’srestaurant。　（it　is　actually　somewhat　higher　now。　but　not　much。）while　gene　shoemaker　was　trying　to　get　people　galvanized　about　the　potential　dangers　ofthe　inner　solar　system；　another　development—wholly　unrelated　on　the　face　of　it—was　quietlyunfolding　in　italy　with　the　work　of　a　young　geologist　from　the　lamont　doherty　laboratory　atcolumbia　university。　in　the　early　1970s；　walter　alvarez　was　doing　fieldwork　in　a　elydefile　known　as　the　bottaccione　gorge；　near　the　umbrian　hill　town　of　gubbio；　when　he　grewcurious　about　a　thin　band　of　reddish　clay　that　divided　two　ancient　layers　of　limestone—onefrom　the　cretaceous　period；　the　other　from　the　tertiary。　this　is　a　point　known　to　geology　asthe　kt　boundary；1and　it　marks　the　time；　sixty…five　million　years　ago；　when　the　dinosaurs　androughly　half　the　world’s　other　species　of　animals　abruptly　vanish　from　the　fossil　record。

alvarez　wondered　what　it　was　about　a　thin　lamina　of　clay；　barely　a　quarter　of　an　inch　thick；that　could　account　for　such　a　dramatic　moment　in　earth’s　history。

at　the　time　the　conventional　wisdom　about　the　dinosaur　extinction　was　the　same　as　it　hadbeen　in　charles　lyell’s　day　a　century　earlier—namely　that　the　dinosaurs　had　died　out　overmillions　of　years。　but　the　thinness　of　the　clay　layer　clearly　suggested　that　in　umbria；　if1it　is　kt　rather　than　ct　because　c　had　already　been　appropriated　for　cambrian。　depending　on　which　sourceyou　credit；　the　k　es　either　from　the　greek　kreta　or　german　kreide。　both　conveniently　mean　“chalk；”　whichis　also　what　cretaceous　means。

nowhere　else；　something　rather　more　abrupt　had　happened。　unfortunately　in　the　1970s　notests　existed　for　determining　how　long　such　a　deposit　might　have　taken　to　accumulate。

in　the　normal　course　of　things；　alvarez　almost　certainly　would　have　had　to　leave　theproblem　at　that；　but　luckily　he　had　an　impeccable　connection　to　someone　outside　hisdiscipline　who　could　help—his　father；　luis。　luis　alvarez　was　an　eminent　nuclear　physicist；he　had　won　the　nobel　prize　for　physics　the　previous　decade。　he　had　always　been　mildlyscornful　of　his　son’s　attachment　to　rocks；　but　this　problem　intrigued　him。　it　occurred　to　himthat　the　answer　might　lie　in　dust　from　space。

every　year　the　earth　accumulates　some　thirty　thousand　metric　tons　of　“cosmicspherules”—space　dust　in　plainer　language—which　would　be　quite　a　lot　if　you　swept　it　intoone　pile；　but　is　infinitesimal　when　spread　across　the　globe。　scattered　through　this　thin　dustingare　exotic　elements　not　normally　much　found　on　earth。　among　these　is　the　element　iridium；which　is　a　thousand　times　more　abundant　in　space　than　in　the　earth’s　crust　（because；　it　isthought；　most　of　the　iridium　on　earth　sank　to　the　core　when　the　planet　was　young）。

alvarez　knew　that　a　colleague　of　his　at　the　lawrence　berkeley　laboratory　in　california；frank　asaro；　had　developed　a　technique　for　measuring　very　precisely　the　chemicalposition　of　clays　using　a　process　called　neutron　activation　analysis。　this　involvedbombarding　samples　with　neutrons　in　a　small　nuclear　reactor　and　carefully　counting　thegamma　rays　that　were　emitted；　it　was　extremely　finicky　work。　previously　asaro　had　used　thetechnique　to　analyze　pieces　of　pottery；　but　alvarez　reasoned　that　if　they　measured　the　amountof　one　of　the　exotic　elements　in　his　son’s　soil　samples　and　pared　that　with　its　annual　rateof　deposition；　they　would　know　how　long　it　had　taken　the　samples　to　form。　on　an　octoberafternoon　in　1977；　luis　and　walter　alvarez　dropped　in　on　asaro　and　asked　him　if　he　wouldrun　the　necessary　tests　for　them。

it　was　really　quite　a　presumptuous　request。　they　were　asking　asaro　to　devote　months　tomaking　the　most　painstaking　measurements　of　geological　samples　merely　to　confirm　whatseemed　entirely　self…evident　to　begin　with—that　the　thin　layer　of　clay　had　been　formed　asquickly　as　its　thinness　suggested。　certainly　no　one　expected　his　survey　to　yield　any　dramaticbreakthroughs。

“well；　they　were　very　charming；　very　persuasive；”　asaro　recalled　in　an　interview　in　2002。

“and　it　seemed　an　interesting　challenge；　so　i　agreed　to　try。　unfortunately；　i　had　a　lot　of　otherwork　on；　so　it　was　eight　months　before　i　could　get　to　it。”　he　consulted　his　notes　from　theperiod。　“on　june　21；　1978；　at　1：45　p。m。；　we　put　a　sample　in　the　detector。　it　ran　for　224minutes　and　we　could　see　we　were　getting　interesting　results；　so　we　stopped　it　and　had　alook。”

the　results　were　so　unexpected；　in　fact；　that　the　three　scientists　at　first　thought　they　had　tobe　wrong。　the　amount　of　iridium　in　the　alvarez　sample　was　more　than　three　hundred　timesnormal　levels—far　beyond　anything　they　might　have　predicted。　over　the　following　monthsasaro　and　his　colleague　helen　michel　worked　up　to　thirty　hours　at　a　stretch　（“once　youstarted　you　couldn’t　stop；”　asaro　explained）　analyzing　samples；　always　with　the　same　results。

tests　on　other　samples—from　denmark；　spain；　france；　new　zealand；　antarctica—showedthat　the　iridium　deposit　was　worldwide　and　greatly　elevated　everywhere；　sometimes　by　asmuch　as　five　hundred　times　normal　levels。　clearly　something　big　and　abrupt；　and　probablycataclysmic；　had　produced　this　arresting　spike。

after　much　thought；　the　alvarezes　concluded　that　the　most　plausible　explanation—plausible　to　them；　at　any　rate—was　that　the　earth　had　been　struck　by　an　asteroid　or　et。

the　idea　that　the　earth　might　be　subjected　to　devastating　impacts　from　time　to　time　was　notquite　as　new　as　it　is　now　sometimes　presented。　as　far　back　as　1942；　a　northwesternuniversity　astrophysicist　named　ralph　b。　baldwin　had　suggested　such　a　possibility　in　anarticle　in　popular　astronomy　magazine。　（he　published　the　article　there　because　no　academicpublisher　was　prepared　to　run　it。）　and　at　least　two　well…known　scientists；　the　astronomerernst　？pik　and　the　chemist　and　nobel　laureate　harold　urey；　had　also　voiced　support　for　thenotion　at　various　times。　even　among　paleontologists　it　was　not　unknown。　in　1956　a　professorat　oregon　state　university；　m。　w。　de　laubenfels；　writing　in　the　journal　of　paleontology；　hadactually　anticipated　the　alvarez　theory　by　suggesting　that　the　dinosaurs　may　have　been　dealt　adeath　blow　by　an　impact　from　space；　and　in　1970　the　president　of　the　americanpaleontological　society；　dewey　j。　mclaren；　proposed　at　the　group’s　annual　conference　thepossibility　that　an　extraterrestrial　impact　may　have　been　the　cause　of　an　earlier　event　knownas　the　frasnian　extinction。

as　if　to　underline　just　how　un…novel　the　idea　had　bee　by　this　time；　in　1979　ahollywood　studio　actually　produced　a　movie　called　meteor　（“it’s　five　miles　wide　。　。　。　it’sing　at　30；000　m。p。h。—and　there’s　no　place　to　hide！”）　starring　henry　fonda；　nataliewood；　karl　malden；　and　a　very　large　rock。

so　when；　in　the　first　week　of　1980；　at　a　meeting　of　the　american　association　for　theadvancement　of　science；　the　alvarezes　announced　their　belief　that　the　dinosaur　extinctionhad　not　taken　place　over　millions　of　years　as　part　of　some　slow　inexorable　process；　butsuddenly　in　a　single　explosive　event；　it　shouldn’t　have　e　as　a　shock。

but　it　did。　it　was　received　everywhere；　but　particularly　in　the　paleontological　munity；as　an　outrageous　heresy。

“well；　you　have　to　remember；”　asaro　recalls；　“that　we　were　amateurs　in　this　field。　walterwas　a　geologist　specializing　in　paleomagnetism；　luis　was　a　physicist　and　i　was　a　nuclearchemist。　and　now　here　we　were　telling　paleontologists　that　we　had　solved　a　problem　that　hadeluded　them　for　over　a　century。　it’s　not　terribly　surprising　that　they　didn’t　embrace　itimmediately。”　as　luis　alvarez　joked：　“we　were　caught　practicing　geology　without　alicense。”

but　there　was　also　something　much　deeper　and　more　fundamentally　abhorrent　in　the　impacttheory。　the　belief　that　terrestrial　processes　were　gradual　had　been　elemental　in　natural　historysince　the　time　of　lyell。　by　the　1980s；　catastrophism　had　been　out　of　fashion　for　so　long　that　ithad　bee　literally　unthinkable。　for　most　geologists　the　idea　of　a　devastating　impact　was；　aseugene　shoemaker　noted；　“against　their　scientific　religion。”

nor　did　it　help　that　luis　alvarez　was　openly　contemptuous　of　paleontologists　and　theircontributions　to　scientific　knowledge。　“they’re　really　not　very　good　scientists。　they’re　morelike　stamp　collectors；”　he　wrote　in　the　new　york　times　in　an　article　that　stings　yet。

opponents　of　the　alvarez　theory　produced　any　number　of　alternative　explanations　for　theiridium　deposits—for　instance；　that　they　were　generated　by　prolonged　volcanic　eruptions　inindia　called　the　deccan　traps—and　above　all　insisted　that　there　was　no　proof　that　thedinosaurs　disappeared　abruptly　from　the　fossil　record　at　the　iridium　boundary。　one　of　themost　vigorous　o