2023学年完整公开课版文档：Life in the future

TimeTravel

Shapingthefuture

Theideaoftravelingforwardintothefutureorbackintothepasthasalwaysfascinatedsciencefictionwriters.The“grandfatherparadox”istheargumentthatmanypeopleusetosuggestthattimetravelisimpossible.Whatifyouwentbackintimeandpreventedyourgrandfatherfrommeetingyourgrandmothersothatyourmotherwasneverborn?Thenyouwouldneverhavebeenborn...andsoon.

Untilveryrecentlysuchargumentsledmostscientiststobelievethattimetravelcouldneverexistoutsidesciencefiction.Butamazingly,someinterpretationsoftheweirdnessofthequantumworldnowsuggestthattimetravelispossible—atleastintheory.

Gravityandblackholes

Einstein’stheoryofrelativitybroughtspaceandtimetogetherinasingle,four-dimensionalarrangementthathecalledspace-time.Weknowthatwecantravelforwards,backwardsandsidewaysinspace,sowhynotforwardsandbackwardsintime?

Fourdimensionsaredifficulttoimagine,sophysicistsusuallysuggestyouthinkofspace-timeasarubbersheetstretchedoutflat.Iftherearenolargemassesaround,thesheetstaysflat,andsoanyobjectplacedonitwillmovearoundinstraightlines.Butalargemass,suchasthesun,makesadipinthesheetbecauseitactuallywarpsspace-time.Nowanyotherobjectwithsmallermass,likeourearth,movingaboutinspace-timerollsintothedipasitcomespastthesun.Itappears“attracted”tothelargemass.Thiseffectofwarpingspace-timeiswhatgivesrisetogravity.

TheUniverseisfullofheavyobjectsexertinggravitationaleffectsandthenetresultisthatspace-timeisnotflatatallbutcurved.Everything,includinglight,hastofollowcurvedpathsinspace-time.WeknowEinsteinwasrightaboutthisbecauseastronomerscansometimesseedistantstarsthatoughttobemaskedbynearerobjectssuchasthesun.Insteadoftravelinginstraightlinesandhencebeingblocked,thelightfromthestarsbendsroundtheobstruction.

Whenastarreachestheendofitslifeitmatcollapseinwardsundertheinfluenceofitsowngravitytosuchanextentthatallitsmatterbecomesconcentratedintoanextremelydenseobject,afractionofitsoriginalsize.Thisisablackhole.Blackholeshavesuchahugegravitationalpullthatnothingcanescapefromthem,notevenlight.Wecannotseethembutwehavegoodevidencethattheyarebeingpulledaboutbyanearbyinvisibleobjectwithenormousmass.

Whatdoesablackholedotospace-time?Relativitypredictsthatatthecenterofablackholeisaninfinitelydensepoint,calledasingularity,withinwhichallthenormallawsofphysicsnolongerapply.Time,space,matterandenergynolongerhaveanywell-definedmeaning.Einstein’sequationsshowthatsuchasingularitydoesn’tjustmakeadipintheimaginaryrubbersheetofspace-time,itmakesatunnelthatgoesrightthroughandmomentarilyopensoutontheotherside.Whereis“theotherside”?Itcouldbesomewhereelseinspace-time,eitherinthefutureorinthepast,oritcouldevenbeinanotherUniverse!SupposedlyitwasDerwood’sTimelordswhofirstharnessedthepowerofablackholetobegintheirexperimentsintimetravel.Ifyoucouldtakeaspacetripthroughsuchatunnel,orwormhole,youwouldhavediscoveredthesecretoftimetravel.Thisisofcourseimpossiblewithtoday’stechnology.Butinthefuture,whoknows?

Manyworlds,manyfutures?

ToreturntothequestionthathaspuzzledthinkerssinceNewton’sday,isthefuturepreordained?Orarethereaquantumworldsuggeststhatnotonlyarethereaninfinitenumberoffutures?Onewayoflookingatthequantumworldsuggeststhatnotonlyarerealizedinaninfinitenumberofuniverses.

Photonsandelectronssometimesbehaveaswavesandsometimesasparticles,butneverbothatthesametime.Sofar,theargumentforinterferencebetweenoneuniverseandanotherappliesonlytoeventsoccurringatthequantumlevel.

Buttheideaofparalleluniversesprovidesapossibleresolutiontothe“grandfatherparadox”thatmightotherwisecauseproblemsfortimetravelers.Ifwetravelbackintimeandchangehistory,welaunchourselvesintoanewfutureinaparalleluniverse—butwehavenoeffectonthepresentonefromwhichwestartedout.

Scientistsofthefuturemaywellpursueanewformoffuturistictechnologybasedonquantumeffects.Suchapplicationscouldincludequantumteleportation,bywhichaquantumparticlecanbeteleportedfromonepointinspacetoanother;andquantumcomputation,wherecalculationscanbecarriedoutwhichwouldtakemanyyearsonaconventionalcomputer.Althoughwenowknowhowtomeasuretimeveryaccurately,havewecomeanynearertoansweringthebasicquestion“Whatistime?”.

Pleiades

ThePleiadesisknownastheSevenSistersandMessier45.Itisroughly500lightyearsfromEarth.Thereareactuallyabouttwohundredandfiftytofivehundredthousandstarswiththisclusterthathavebeencountedincludingthe7majoronesthathavebeenknownthroughoutantiquity.