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1、Whatifaglassofwaterwas,allofasudden,literallyhalfempty?—VittorioIacovellaThepessimistisprobablymorerightabouthowitturnsoutthantheoptimist.Whenpeoplesay“glasshalfempty”,theyusuallymeansomethinglikeaglasscontainingequalpartswaterandair:Traditionally,theoptimistseestheglassashalffull
2、whilethepessimistseesitashalfempty.Thishasspawnedazillionjokevariants—e.g.,theengineerseesaglassthat’stwiceasbigasitneedstobe,thesurrealistseesagiraffeeatinganecktie,etc.Butwhatiftheemptyhalfoftheglasswereactuallyempty—avacuum?(Evenavacuumarguablyisn’ttrulyempty,butthat’saquestion
3、forquantumsemantics.)Thevacuumwoulddefinitelynotlastlong.Butexactlywhathappensdependsonakeyquestionthatnobodyusuallybotherstoask:Whichhalfisempty?Forourscenario,we’llimaginethreedifferenthalf-emptyglasses,andfollowwhathappenstothemmicrosecondbymicrosecond.Inthemiddleisthetradition
4、alair/waterglass.Ontherightisaglasslikethetraditionalone,excepttheairisreplacedbyavacuum.Theglassontheleftishalffullofwaterandhalfempty—butit’sthebottomhalfthat’sempty.We’llimaginethevacuumsappearattimet=0.Forthefirsthandfulofmicroseconds,nothinghappens.Onthistimescale,eventheairm
5、oleculesarenearlystationary.Forthemostpart,airmoleculesjigglearoundatspeedsofafewhundredmeterspersecond.Butatanygiventime,somehappentobemovingfasterthanothers.Thefastestfewaremovingatover1000meterspersecond.Thesearethefirsttodriftintothevacuumintheglassontheright.Thevacuumonthelef
6、tissurroundedbybarriers,soairmoleculescan’teasilygetin.Thewater,beingaliquid,doesn’texpandtofillthevacuuminthesamewayairdoes.However,inthevacuumoftheglasses,itdoesstarttoboil,slowlysheddingwatervaporintotheemptyspace.Whilethewateronthesurfaceinbothglassesstartstoboilaway,intheglas
7、sontheright,theairrushinginstopsitbeforeitreallygetsgoing.Theglassontheleftcontinuestofillwithaveryfaintmistofwatervapor.Afterafewhundredmicroseconds,theairrushingintotheglassontherightfillsthevacuumcompletelyandramsintothesurfaceofthewater,sendingapressurewavethroughtheliquid.The
8、sidesoftheglassbulgeslightly,butt
