A Short History of Nearly Everything-第28章

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however；　was　that　an　atom　was　more　like　a　currant　bun　or　aplum　pudding：　a　dense；　solid　object　that　carried　a　positive　charge　but　that　was　studded　withnegatively　charged　electrons；　like　the　currants　in　a　currant　bun。

in　1910；　rutherford　（assisted　by　his　student　hans　geiger；　who　would　later　invent　theradiation　detector　that　bears　his　name）　fired　ionized　helium　atoms；　or　alpha　particles；　at　asheet　of　gold　foil。

2to　rutherford’s　astonishment；　some　of　the　particles　bounced　back。　it　wasas　if；　he　said；　he　had　fired　a　fifteen…inch　shell　at　a　sheet　of　paper　and　it　rebounded　into　his　lap。

this　was　just　not　supposed　to　happen。　after　considerable　reflection　he　realized　there　could　beonly　one　possible　explanation：　the　particles　that　bounced　back　were　striking　something　smalland　dense　at　the　heart　of　the　atom；　while　the　other　particles　sailed　through　unimpeded。　anatom；　rutherford　realized；　was　mostly　empty　space；　with　a　very　dense　nucleus　at　the　center。

this　was　a　most　gratifying　discovery；　but　it　presented　one　immediate　problem。　by　all　the　lawsof　conventional　physics；　atoms　shouldn’t　therefore　exist。

let　us　pause　for　a　moment　and　consider　the　structure　of　the　atom　as　we　know　it　now。　everyatom　is　made　from　three　kinds　of　elementary　particles：　protons；　which　have　a　positiveelectrical　charge；　electrons；　which　have　a　negative　electrical　charge；　and　neutrons；　which　haveno　charge。　protons　and　neutrons　are　packed　into　the　nucleus；　while　electrons　spin　aroundoutside。　the　number　of　protons　is　what　gives　an　atom　its　chemical　identity。　an　atom　with　oneproton　is　an　atom　of　hydrogen；　one　with　two　protons　is　helium；　with　three　protons　is　lithium；and　so　on　up　the　scale。　each　time　you　add　a　proton　you　get　a　new　element。　（because　thenumber　of　protons　in　an　atom　is　always　balanced　by　an　equal　number　of　electrons；　you　willsometimes　see　it　written　that　it　is　the　number　of　electrons　that　defines　an　element；　it　es　tothe　same　thing。　the　way　it　was　explained　to　me　is　that　protons　give　an　atom　its　identity；electrons　its　personality。）neutrons　don’t　influence　an　atom’s　identity；　but　they　do　add　to　its　mass。　the　number　ofneutrons　is　generally　about　the　same　as　the　number　of　protons；　but　they　can　vary　up　and　downslightly。　add　a　neutron　or　two　and　you　get　an　isotope。　the　terms　you　hear　in　reference　todating　techniques　in　archeology　refer　to　isotopes—carbon…14；　for　instance；　which　is　an　atomof　carbon　with　six　protons　and　eight　neutrons　（the　fourteen　being　the　sum　of　the　two）。

neutrons　and　protons　occupy　the　atom’s　nucleus。　the　nucleus　of　an　atom　is　tiny—only　onemillionth　of　a　billionth　of　the　full　volume　of　the　atom—but　fantastically　dense；　since　itcontains　virtually　all　the　atom’s　mass。　as　cropper　has　put　it；　if　an　atom　were　expanded　to　thesize　of　a　cathedral；　the　nucleus　would　be　only　about　the　size　of　a　fly—but　a　fly　manythousands　of　times　heavier　than　the　cathedral。　it　was　this　spaciousness—this　resounding；unexpected　roominess—that　had　rutherford　scratching　his　head　in　1910。

it　is　still　a　fairly　astounding　notion　to　consider　that　atoms　are　mostly　empty　space；　and　thatthe　solidity　we　experience　all　around　us　is　an　illusion。　when　two　objects　e　together　in　the2geiger　would　also　later　bee　a　loyal　nazi；　unhesitatingly　betraying　jewish　colleagues；　including　many　whohad　helped　him。

real　world—billiard　balls　are　most　often　used　for　illustration—they　don’t　actually　strike　eachother。　“rather；”　as　timothy　ferris　explains；　“the　negatively　charged　fields　of　the　two　ballsrepel　each　other　。　。　。　were　it　not　for　their　electrical　charges　they　could；　like　galaxies；　pass　rightthrough　each　other　unscathed。”　when　you　sit　in　a　chair；　you　are　not　actually　sitting　there；　butlevitating　above　it　at　a　height　of　one　angstrom　（a　hundred　millionth　of　a　centimeter）；　yourelectrons　and　its　electrons　implacably　opposed　to　any　closer　intimacy。

the　picture　that　nearly　everybody　has　in　mind　of　an　atom　is　of　an　electron　or　two　flyingaround　a　nucleus；　like　planets　orbiting　a　sun。　this　image　was　created　in　1904；　based　on　littlemore　than　clever　guesswork；　by　a　japanese　physicist　named　hantaro　nagaoka。　it　ispletely　wrong；　but　durable　just　the　same。　as　isaac　asimov　liked　to　note；　it　inspiredgenerations　of　science　fiction　writers　to　create　stories　of　worlds　within　worlds；　in　which　atomsbee　tiny　inhabited　solar　systems　or　our　solar　system　turns　out　to　be　merely　a　mote　in　somemuch　larger　scheme。　even　now　cern；　the　european　organization　for　nuclear　research；　usesnagaoka’s　image　as　a　logo　on　its　website。　in　fact；　as　physicists　were　soon　to　realize；　electronsare　not　like　orbiting　planets　at　all；　but　more　like　the　blades　of　a　spinning　fan；　managing　to　fillevery　bit　of　space　in　their　orbits　simultaneously　（but　with　the　crucial　difference　that　the　bladesof　a　fan　only　seem　to　be　everywhere　at　once；　electrons　are　）。

needless　to　say；　very　little　of　this　was　understood　in　1910　or　for　many　years　afterward。

rutherford’s　finding　presented　some　large　and　immediate　problems；　not　least　that　no　electronshould　be　able　to　orbit　a　nucleus　without　crashing。　conventional　electrodynamic　theorydemanded　that　a　flying　electron　should　very　quickly　run　out　of　energy—in　only　an　instant　orso—and　spiral　into　the　nucleus；　with　disastrous　consequences　for　both。　there　was　also　theproblem　of　how　protons　with　their　positive　charges　could　bundle　together　inside　the　nucleuswithout　blowing　themselves　and　the　rest　of　the　atom　apart。　clearly　whatever　was　going　ondown　there　in　the　world　of　the　very　small　was　not　governed　by　the　laws　that　applied　in　themacro　world　where　our　expectations　reside。

as　physicists　began　to　delve　into　this　subatomic　realm；　they　realized　that　it　wasn’t　merelydifferent　from　anything　we　knew；　but　different　from　anything　ever　imagined。　“becauseatomic　behavior　is　so　unlike　ordinary　experience；”　richard　feynman　once　observed；　“it　isvery　difficult　to　get　used　to　and　it　appears　peculiar　and　mysterious　to　everyone；　both　to　thenovice　and　to　the　experienced　physicist。”　when　feynman　made　that　ment；　physicists　hadhad　half　a　century　to　adjust　to　the　strangeness　of　atomic　behavior。　so　think　how　it　must　havefelt　to　rutherford　and　his　colleagues　in　the　early　1910s　when　it　was　all　brand　new。

one　of　the　people　working　with　rutherford　was　a　mild　and　affable　young　dane　namedniels　bohr。　in　1913；　while　puzzling　over　the　structure　of　the　atom；　bohr　had　an　idea　soexciting　that　he　postponed　his　honeymoon　to　write　what　became　a　landmark　paper。　becausephysicists　couldn’t　see　anything　so　small　as　an　atom；　they　had　to　try　to　work　out　its　structurefrom　how　it　behaved　when　they　did　things　to　it；　as　rutherford　had　done　by　firing　alphaparticles　at　foil。　sometimes；　not　surprisingly；　the　results　of　these　experiments　were　puzzling。

one　puzzle　that　had　been　around　for　a　long　time　had　to　do　with　spectrum　readings　of　thewavelengths　of　hydrogen。　these　produced　patterns　showing　that　hydrogen　atoms　emittedenergy　at　certain　wavelengths　but　not　others。　it　was　rather　as　if　someone　under　surveillancekept　turning　up　at　particular　locations　but　was　never　observed　traveling　between　them。　no　onecould　understand　why　this　should　be。

it　was　while　puzzling　over　this　problem　that　bohr　was　struck　by　a　solution　and　dashed　offhis　famous　paper。　called　“on　the　constitutions　of　atoms　and　molecules；”　the　paper　explainedhow　electrons　could　keep　from　falling　into　the　nucleus　by　suggesting　that　they　could　occupyonly　certain　well…defined　orbits。　according　to　the　new　theory；　an　electron　moving　betweenorbits　would　disappear　from　one　and　reappear　instantaneously　in　another　without　visiting　thespace　between。　this　idea—the　famous　“quantum　leap”—is　of　course　utterly　strange；　but　itwas　too　good　not　to　be　true。　it　not　only　kept　electrons　from　spiraling　catastrophically　into　thenucleus；　it　also　explained　hydrogen’s　bewildering　wavelengths。　the　electrons　only　appearedin　certain　orbits　because　they　only　existed　in　certain　orbits。　it　was　a　dazzling　insight；　and　itwon　bohr　the　1922　nobel　prize　in　physics；　the　year　after　einstein　received　his。

meanwhile　the　tireless　rutherford；　now　back　at　cambridge　as　j。　j。　thomson’s　successor　ashead　of　the　cavendish　laboratory；　came　up　with　a　model　that　explained　why　the　nuclei　didn’tblow　up。　he　saw　that　they　must　be　offset　by　some　type　of　neutralizing　particles；　which　hecalled　neutrons。　the　idea　was　simple　and　appealing；　but　not　easy　to　prove。　rutherford’sassociate；　james　chadwick；　devoted　eleven　intensive　years　to　hunting　for　neutrons　beforefinally　succeeding　in　1932。　he；　too；　was　awarded　with　a　nobel　prize　in　physics；　in　1935。　asboorse　and　his　colleagues　point　out　in　their　history　of　the　subject；　the　delay　in　discovery　wasprobably　a　very　good　thing　as　mastery　of　the　neutron　was　essential　to　the　development　of　theatomic　bomb。　（because　neutrons　have　no　charge；　they　aren’t　repelled　by　the　electrical　fields　atthe　heart　of　an　atom　and　thus　could　be　fired　like　tiny　torpedoes　into　an　atomic　nucleus；　settingoff　the　destructive　process　known　as　fission。）　had　the　neutron　been　isolated　in　the　1920s；　theynote；　it　is　“very　likely　the　atomic　bomb　would　have　been　developed　first　in　europe；undoubtedly　by　the　germans。”

as　it　was；　the　europeans　had　their　hands　full　trying　to　understand　the　strange　behavior