A Short History of Nearly Everything-第35章

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east　forthe　moment。

the　difficulty　in　making　final　determinations　is　that　there　are　often　acres　of　room　forinterpretation。　imagine　standing　in　a　field　at　night　and　trying　to　decide　how　far　away　twodistant　electric　lights　are。　using　fairly　straightforward　tools　of　astronomy　you　can　easilyenough　determine　that　the　bulbs　are　of　equal　brightness　and　that　one　is；　say；　50　percent　moredistant　than　the　other。　but　what　you　can’t　be　certain　of　is　whether　the　nearer　light　is；　let　ussay；　a　58…watt　bulb　that　is　122　feet　away　or　a　61…watt　light　that　is　119　feet；　8　inches　away。　ontop　of　that　you　must　make　allowances　for　distortions　caused　by　variations　in　the　earth’satmosphere；　by　intergalactic　dust；　contaminating　light　from　foreground　stars；　and　many　otherfactors。　the　upshot　is　that　your　putations　are　necessarily　based　on　a　series　of　nestedassumptions；　any　of　which　could　be　a　source　of　contention。　there　is　also　the　problem　thataccess　to　telescopes　is　always　at　a　premium　and　historically　measuring　red　shifts　has　beennotably　costly　in　telescope　time。　it　could　take　all　night　to　get　a　single　exposure。　inconsequence；　astronomers　have　sometimes　been　pelled　（or　willing）　to　base　conclusionson　notably　scanty　evidence。　in　cosmology；　as　the　journalist　geoffrey　carr　has　suggested；　wehave　“a　mountain　of　theory　built　on　a　molehill　of　evidence。”　or　as　martin　rees　has　put　it：

“our　present　satisfaction　＇with　our　state　of　understanding＇　may　reflect　the　paucity　of　the　datarather　than　the　excellence　of　the　theory。”

this　uncertainty　applies；　incidentally；　to　relatively　nearby　things　as　much　as　to　the　distantedges　of　the　universe。　as　donald　goldsmith　notes；　when　astronomers　say　that　the　galaxy　m87is　60　million　light…years　away；　what　they　really　mean　（“but　do　not　often　stress　to　the　generalpublic”）　is　that　it　is　somewhere　between　40　million　and　90　million　light…years　away—not2you　are　of　course　entitled　to　wonder　what　is　meant　exactly　by　〃a　constant　of　50〃　or　〃a　constant　of　100。〃　theanswer　lies　in　astronomical　units　of　measure。　except　conversationally；　astronomers　dont　use　light…years。　theyuse　a　distance　called　the　parsec　（a　contraction　of　parallax　and　second）；　based　on　a　universal　measure　called　thestellar　parallax　and　equivalent　to　3。26　light…years。　really　big　measures；　like　the　size　of　a　universe；　are　measuredin　megaparsecs：　a　million　parsecs。　the　constant　is　expressed　in　terms　of　kilometers　per　second　per　megaparsec。

thus　when　astronomers　refer　to　a　hubble　constant　of　50；　what　they　really　mean　is　〃50　kilometers　per　second　permegaparsec。〃　for　most　of　us　that　is　of　course　an　utterly　meaningless　measure；　but　then　with　astronomicalmeasures　most　distances　are　so　huge　as　to　be　utterly　meaningless。

quite　the　same　thing。　for　the　universe　at　large；　matters　are　naturally　magnified。　bearing　allthat　in　mind；　the　best　bets　these　days　for　the　age　of　the　universe　seem　to　be　fixed　on　a　range　ofabout　12　billion　to　13。5　billion　years；　but　we　remain　a　long　way　from　unanimity。

one　interesting　recently　suggested　theory　is　that　the　universe　is　not　nearly　as　big　as　wethought；　that　when　we　peer　into　the　distance　some　of　the　galaxies　we　see　may　simply　bereflections；　ghost　images　created　by　rebounded　light。

the　fact　is；　there　is　a　great　deal；　even　at　quite　a　fundamental　level；　that　we　don’t　know—notleast　what　the　universe　is　made　of。　when　scientists　calculate　the　amount　of　matter　needed　tohold　things　together；　they　always　e　up　desperately　short。　it　appears　that　at　least　90　percentof　the　universe；　and　perhaps　as　much　as　99　percent；　is　posed　of　fritz　zwicky’s　“darkmatter”—stuff　that　is　by　its　nature　invisible　to　us。　it　is　slightly　galling　to　think　that　we　live　ina　universe　that；　for　the　most　part；　we　can’t　even　see；　but　there　you　are。　at　least　the　names　forthe　two　main　possible　culprits　are　entertaining：　they　are　said　to　be　either　wimps　（for　weaklyinteracting　massive　particles；　which　is　to　say　specks　of　invisible　matter　left　over　from　the　bigbang）　or　machos　（for　massive　pact　halo　objects—really　just　another　name　for　blackholes；　brown　dwarfs；　and　other　very　dim　stars）。

particle　physicists　have　tended　to　favor　the　particle　explanation　of　wimps；　astrophysiciststhe　stellar　explanation　of　machos。　for　a　time　machos　had　the　upper　hand；　but　not　nearlyenough　of　them　were　found；　so　sentiment　swung　back　toward　wimps　but　with　the　problemthat　no　wimp　has　ever　been　found。　because　they　are　weakly　interacting；　they　are　（assumingthey　even　exist）　very　hard　to　detect。　cosmic　rays　would　cause　too　much　interference。　soscientists　must　go　deep　underground。　one　kilometer　underground　cosmic　bombardmentswould　be　one　millionth　what　they　would　be　on　the　surface。　but　even　when　all　these　are　addedin；　“two…thirds　of　the　universe　is　still　missing　from　the　balance　sheet；”　as　one　mentatorhas　put　it。　for　the　moment　we　might　very　well　call　them　dunnos　（for　dark　unknownnonreflective　nondetectable　objects　somewhere）。

recent　evidence　suggests　that　not　only　are　the　galaxies　of　the　universe　racing　away　fromus；　but　that　they　are　doing　so　at　a　rate　that　is　accelerating。　this　is　counter　to　all　expectations。　itappears　that　the　universe　may　not　only　be　filled　with　dark　matter；　but　with　dark　energy。

scientists　sometimes　also　call　it　vacuum　energy　or；　more　exotically；　quintessence。　whatever　itis；　it　seems　to　be　driving　an　expansion　that　no　one　can　altogether　account　for。　the　theory　isthat　empty　space　isn’t　so　empty　at　all—that　there　are　particles　of　matter　and　antimatterpopping　into　existence　and　popping　out　again—and　that　these　are　pushing　the　universeoutward　at　an　accelerating　rate。　improbably　enough；　the　one　thing　that　resolves　all　this　iseinstein’s　cosmological　constant—the　little　piece　of　math　he　dropped　into　the　general　theoryof　relativity　to　stop　the　universe’s　presumed　expansion；　and　called　“the　biggest　blunder　of　mylife。”　it　now　appears　that　he　may　have　gotten　things　right　after　all。

the　upshot　of　all　this　is　that　we　live　in　a　universe　whose　age　we　can’t　quite　pute；surrounded　by　stars　whose　distances　we　don’t　altogether　know；　filled　with　matter　we　can’tidentify；　operating　in　conformance　with　physical　laws　whose　properties　we　don’t　trulyunderstand。

and　on　that　rather　unsettling　note；　let’s　return　to　planet　earth　and　consider　something　thatwe　do　understand—though　by　now　you　perhaps　won’t　be　surprised　to　hear　that　we　don’tunderstand　it　pletely　and　what　we　do　understand　we　haven’t　understood　for　long。

m。



12THE　EARTH　MOVES

　生小说＿网　
in　one　of　his　last　professional　acts　before　his　death　in　1955；　albert　einstein　wrote　a　shortbut　glowing　foreword　to　a　book　by　a　geologist　named　charles　hapgood　entitled　earth’sshifting　crust：　a　key　to　some　basic　problems　of　earth　science。　hapgood’s　book　was　asteady　demolition　of　the　idea　that　continents　were　in　motion。　in　a　tone　that　all　but　invited　thereader　to　join　him　in　a　tolerant　chuckle；　hapgood　observed　that　a　few　gullible　souls　hadnoticed　“an　apparent　correspondence　in　shape　between　certain　continents。”　it　would　appear；he　went　on；　“that　south　america　might　be　fitted　together　with　africa；　and　so　on。　。　。　。　it　is　evenclaimed　that　rock　formations　on　opposite　sides　of　the　atlantic　match。”

mr。　hapgood　briskly　dismissed　any　such　notions；　noting　that　the　geologists　k。　e。　casterand　j。　c。　mendes　had　done　extensive　fieldwork　on　both　sides　of　the　atlantic　and　hadestablished　beyond　question　that　no　such　similarities　existed。　goodness　knows　what　outcropsmessrs。　caster　and　mendes　had　looked　at；　beacuse　in　fact　many　of　the　rock　formations　onboth　sides　of　the　atlanticare　the　same—not　just　very　similar　but　the　same。

this　was　not　an　idea　that　flew　with　mr。　hapgood；　or　many　other　geologists　of　his　day。　thetheory　hapgood　alluded　to　was　one　first　propounded　in　1908　by　an　amateur　americangeologist　named　frank　bursley　taylor。　taylor　came　from　a　wealthy　family　and　had　both　themeans　and　freedom　from　academic　constraints　to　pursue　unconventional　lines　of　inquiry。　hewas　one　of　those　struck　by　the　similarity　in　shape　between　the　facing　coastlines　of　africa　andsouth　america；　and　from　this　observation　he　developed　the　idea　that　the　continents　had　onceslid　around。　he　suggested—presciently　as　it　turned　out—that　the　crunching　together　ofcontinents　could　have　thrust　up　the　world’s　mountain　chains。　he　failed；　however；　to　producemuch　in　the　way　of　evidence；　and　the　theory　was　considered　too　crackpot　to　merit　seriousattention。

in　germany；　however；　taylor’s　idea　was　picked　up；　and　effectively　appropriated；　by　atheorist　named　alfred　wegener；　a　meteorologist　at　the　university　of　marburg。　wegenerinvestigated　the　many　plant　and　fossil　anomalies　that　did　not　fit　fortably　into　the　standardmodel　of　earth　history　and　realized　that　very　little　of　it　made　sense　if　conventionallyinterpreted。　animal　fossils　repeatedly　turned　up　on　opposite　sides　of　oceans　that　were　clearlytoo　wide　to　swim。　how；　he　wondered；　did　marsupials　travel　from　south　america　to　australia？

how　did　identical　snails　turn　up　in　scandinavia　and　new　england？　and　how；　e　to　that；did　one　account　for　coal　seams　and　other　semi…tropical　remnants　in　frigid　spots　likespitsbergen；　four　hundred　miles　north　of　norway；　if　they　had　not　somehow　migrated　therefrom　warmer　climes？

wegener　d