A Short History of Nearly Everything-第31章

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they　die。　carbon…14　has　ahalf…life—that　is；　the　time　it　takes　for　half　of　any　sample　to　disappear1—of　about　5；600　years；so　by　working　out　how　much　a　given　sample　of　carbon　had　decayed；　libby　could　get　a　goodfix　on　the　age　of　an　object—though　only　up　to　a　point。　after　eight　half…lives；　only　1/256　of　theoriginal　radioactive　carbon　remains；　which　is　too　little　to　make　a　reliable　measurement；　soradiocarbon　dating　works　only　for　objects　up　to　forty　thousand　or　so　years　old。

curiously；　just　as　the　technique　was　being　widespread；　certain　flaws　within　it　becameapparent。　to　begin　with；　it　was　discovered　that　one　of　the　basic　ponents　of　libby’sformula；　known　as　the　decay　constant；　was　off　by　about　3　percent。　by　this　time；　however；thousands　of　measurements　had　been　taken　throughout　the　world。　rather　than　restate　everyone；　scientists　decided　to　keep　the　inaccurate　constant。　“thus；”　tim　flannery　notes；　“everyraw　radiocarbon　date　you　read　today　is　given　as　too　young　by　around　3　percent。”　theproblems　didn’t　quite　stop　there。　it　was　also　quickly　discovered　that　carbon…14　samples　can　beeasily　contaminated　with　carbon　from　other　sources—a　tiny　scrap　of　vegetable　matter；　forinstance；　that　has　been　collected　with　the　sample　and　not　noticed。　for　younger　samples—those　under　twenty　thousand　years　or　so—slight　contamination　does　not　always　matter　somuch；　but　for　older　samples　it　can　be　a　serious　problem　because　so　few　remaining　atoms　arebeing　counted。　in　the　first　instance；　to　borrow　from　flannery；　it　is　like　miscounting　by　a　dollarwhen　counting　to　a　thousand；　in　the　second　it　is　more　like　miscounting　by　a　dollar　when　youhave　only　two　dollars　to　count。

libby’s　method　was　also　based　on　the　assumption　that　the　amount　of　carbon…14　in　theatmosphere；　and　the　rate　at　which　it　has　been　absorbed　by　living　things；　has　been　consistentthroughout　history。　in　fact　it　hasn’t　been。　we　now　know　that　the　volume　of　atmosphericcarbon…14　varies　depending　on　how　well　or　not　earth’s　magnetism　is　deflecting　cosmic　rays；and　that　that　can　vary　significantly　over　time。　this　means　that　some　carbon…14　dates　are　more1if　you　have　ever　wondered　how　the　atoms　determine　which　50　percent　will　die　and　which　50　percent　willsurvive　for　the　next　session；　the　answer　is　that　the　half…life　is　really　just　a　statistical　convenience…a　kind　ofactuarial　table　for　elemental　things。　imagine　you　had　a　sample　of　material　with　a　half…life　of　30　seconds。　it　isntthat　every　atom　in　the　sample　will　exist　for　exactly　30　seconds　or　60　seconds　or　90　seconds　or　some　other　tidilyordained　period。　each　atom　will　in　fact　survive　for　an　entirely　random　length　of　time　that　has　nothing　to　do　withmultiples　of　30；　it　might　last　until　two　seconds　from　now　or　it　might　oscillate　away　for　years　or　decades　orcenturies　to　e。　no　one　can　say。　but　what　we　can　say　is　that　for　the　sample　as　a　whole　the　rate　ofdisappearance　will　be　such　that　half　the　atoms　will　disappear　every　30　seconds。　its　an　average　rate；　in　otherwords；　and　you　can　apply　it　to　any　large　sampling。　someone　once　worked　out；　for　instance；　that　dimes　have　ahalf…life　of　about　30　years。

dubious　than　others。　this　is　particularly　so　with　dates　just　around　the　time　that　people　firstcame　to　the　americas；　which　is　one　of　the　reasons　the　matter　is　so　perennially　in　dispute。

finally；　and　perhaps　a　little　unexpectedly；　readings　can　be　thrown　out　by　seeminglyunrelated　external　factors—such　as　the　diets　of　those　whose　bones　are　being　tested。　onerecent　case　involved　the　long…running　debate　over　whether　syphilis　originated　in　the　newworld　or　the　old。　archeologists　in　hull；　in　the　north　of　england；　found　that　monks　in　amonastery　graveyard　had　suffered　from　syphilis；　but　the　initial　conclusion　that　the　monks　haddone　so　before　columbus’s　voyage　was　cast　into　doubt　by　the　realization　that　they　had　eaten　alot　of　fish；　which　could　make　their　bones　appear　to　be　older　than　in　fact　they　were。　the　monksmay　well　have　had　syphilis；　but　how　it　got　to　them；　and　when；　remain　tantalizinglyunresolved。

because　of　the　accumulated　shortings　of　carbon…14；　scientists　devised　other　methods　ofdating　ancient　materials；　among　them　thermoluminesence；　which　measures　electrons　trappedin　clays；　and　electron　spin　resonance；　which　involves　bombarding　a　sample　withelectromagnetic　waves　and　measuring　the　vibrations　of　the　electrons。　but　even　the　best　ofthese　could　not　date　anything　older　than　about　200；000　years；　and　they　couldn’t　date　inorganicmaterials　like　rocks　at　all；　which　is　of　course　what　you　need　if　you　wish　to　determine　the　ageof　your　planet。

the　problems　of　dating　rocks　were　such　that　at　one　point　almost　everyone　in　the　world　hadgiven　up　on　them。　had　it　not　been　for　a　determined　english　professor　named　arthur　holmes；the　quest　might　well　have　fallen　into　abeyance　altogether。

holmes　was　heroic　as　much　for　the　obstacles　he　overcame　as　for　the　results　he　achieved。

by　the　1920s；　when　holmes　was　in　the　prime　of　his　career；　geology　had　slipped　out　offashion—physics　was　the　new　excitement　of　the　age—and　had　bee　severely　underfunded；particularly　in　britain；　its　spiritual　birthplace。　at　durham　university；　holmes　was　for　manyyears　the　entire　geology　department。　often　he　had　to　borrow　or　patch　together　equipment　inorder　to　pursue　his　radiometric　dating　of　rocks。　at　one　point；　his　calculations　were　effectivelyheld　up　for　a　year　while　he　waited　for　the　university　to　provide　him　with　a　simple　addingmachine。　occasionally；　he　had　to　drop　out　of　academic　life　altogether　to　earn　enough　tosupport　his　family—for　a　time　he　ran　a　curio　shop　in　newcastle　upon　tyne—and　sometimeshe　could　not　even　afford　the　￡5　annual　membership　fee　for　the　geological　society。

the　technique　holmes　used　in　his　work　was　theoretically　straightforward　and　arose　directlyfrom　the　process；　first　observed　by　ernest　rutherford　in　1904；　in　which　some　atoms　decayfrom　one　element　into　another　at　a　rate　predictable　enough　that　you　can　use　them　as　clocks。　ifyou　know　how　long　it　takes　for　potassium…40　to　bee　argon…40；　and　you　measure　theamounts　of　each　in　a　sample；　you　can　work　out　how　old　a　material　is。　holmes’s　contributionwas　to　measure　the　decay　rate　of　uranium　into　lead　to　calculate　the　age　of　rocks；　and　thus—hehoped—of　the　earth。

but　there　were　many　technical　difficulties　to　overe。　holmes　also　needed—or　at　leastwould　very　much　have　appreciated—sophisticated　gadgetry　of　a　sort　that　could　make　veryfine　measurements　from　tiny　samples；　and　as　we　have　seen　it　was　all　he　could　do　to　get　asimple　adding　machine。　so　it　was　quite　an　achievement　when　in　1946　he　was　able　toannounce　with　some　confidence　that　the　earth　was　at　least　three　billion　years　old　and　possiblyrather　more。　unfortunately；　he　now　met　yet　another　formidable　impediment　to　acceptance：　theconservativeness　of　his　fellow　scientists。　although　happy　to　praise　his　methodology；　manymaintained　that　he　had　found　not　the　age　of　the　earth　but　merely　the　age　of　the　materials　fromwhich　the　earth　had　been　formed。

it　was　just　at　this　time　that　harrison　brown　of　the　university　of　chicago　developed　a　newmethod　for　counting　lead　isotopes　in　igneous　rocks　（which　is　to　say　those　that　were　createdthrough　heating；　as　opposed　to　the　laying　down　of　sediments）。　realizing　that　the　work　wouldbe　exceedingly　tedious；　he　assigned　it　to　young　clair　patterson　as　his　dissertation　project。

famously　he　promised　patterson　that　determining　the　age　of　the　earth　with　his　new　methodwould　be　“duck　soup。”　in　fact；　it　would　take　years。

patterson　began　work　on　the　project　in　1948。　pared　with　thomas　midgley’s　colorfulcontributions　to　the　march　of　progress；　patterson’s　discovery　of　the　age　of　the　earth　feelsmore　than　a　touch　anticlimactic。　for　seven　years；　first　at　the　university　of　chicago　and　then　atthe　california　institute　of　technology　（where　he　moved　in　1952）；　he　worked　in　a　sterile　lab；making　very　precise　measurements　of　the　lead/uranium　ratios　in　carefully　selected　samples　ofold　rock。

the　problem　with　measuring　the　age　of　the　earth　was　that　you　needed　rocks　that　wereextremely　ancient；　containing　lead…　and　uranium…bearing　crystals　that　were　about　as　old　as　theplanet　itself—anything　much　younger　would　obviously　give　you　misleadingly　youthfuldates—but　really　ancient　rocks　are　only　rarely　found　on　earth。　in　the　late　1940s　no　onealtogether　understood　why　this　should　be。　indeed；　and　rather　extraordinarily；　we　would　bewell　into　the　space　age　before　anyone　could　plausibly　account　for　where　all　the　earth’s　oldrocks　went。　（the　answer　was　plate　tectonics；　which　we　shall　of　course　get　to。）　patterson；meantime；　was　left　to　try　to　make　sense　of　things　with　very　limited　materials。　eventually；　andingeniously；　it　occurred　to　him　that　he　could　circumvent　the　rock　shortage　by　using　rocksfrom　beyond　earth。　he　turned　to　meteorites。

the　assumption　he　made—rather　a　large　one；　but　correct　as　it　turned　out—was　that　manymeteorites　are　essentially　leftover　building　materials　from　the　early　days　of　the　solar　system；and　thus　have　managed　to　preserve　a　more　or　less　pristine　interior　chemistry。　measure　the　ageof　these　wandering　rocks　and　you　would　have　the　age　also　（near　enough）　of　the　earth。

as　always；　however；　nothing　was　quite　as　straightforward　as　such　a　breezy　descriptionmakes　it　sound。　m