Copyright©2018bySpacetimePublicationsLimitedForewordcopyright©2018byEddieRedmayneIntroductioncopyright©2018byKipThorneAfterwordcopyright©2018byLucyHawking

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Contents

CoverTitlePage

Copyright

ANotefromthePublisher

Foreword:EddieRedmayneAnIntroduction:KipThorne

WhyWeMustAsktheBigQuestionsChapter1:IsThereaGod?

Chapter2:HowDidItAllBegin?

Chapter3:IsThereOtherIntelligentLifeintheUniverse?

Chapter4:CanWePredicttheFuture?

Chapter5:WhatIsInsideaBlackHole?

Chapter6:IsTimeTravelPossible?

Chapter7:WillWeSurviveonEarth?

Chapter8:ShouldWeColoniseSpace?

Chapter9:WillArtificialIntelligenceOutsmartUs?

Chapter10:HowDoWeShapetheFuture?

Afterword:LucyHawking

Acknowledgements

ByStephenHawkingAbouttheAuthor

ANotefromthePublisher

Stephen Hawking was regularly asked for his thoughts on the “bigquestions” of the day by scientists, tech entrepreneurs, senior businessfigures,political leaders and thegeneralpublic.Stephenmaintainedanenormous personal archive of his responses, which took the form ofspeeches,interviewsandessays.Thisbookdrawsfromthispersonalarchiveandwasindevelopmentat

the time of his death. It has been completed in collaboration with hisacademiccolleagues,hisfamilyandtheStephenHawkingEstate.A percentage of the royalties will go to the Motor Neurone Disease

AssociationandtheStephenHawkingFoundation.

ForewordEddieRedmayne

ThefirsttimeImetStephenHawking,Iwasstruckbyhisextraordinarypowerandhisvulnerability.Thedeterminedlookinhiseyescoupledwiththeimmobilebodywasfamiliartomefrommyresearch—Ihadrecentlybeen engaged to play the role of Stephen inTheTheory of Everythingand had spent severalmonths studying hiswork and the nature of hisdisability,attempting tounderstandhow tousemybody toexpress thepassageofmotorneuronediseaseovertime.And yet when I finally met Stephen, the icon, this scientist of

phenomenal talent, whose main communication was through acomputerised voice along with a pair of exceptionally expressiveeyebrows, I was floored. I tend to get nervous in silences and talk toomuchwhereas Stephen absolutely understood the power of silence, thepoweroffeelinglikeyouarebeingscrutinised.Flustered,Ichosetotalktohimabouthowourbirthdayswereonlydaysapart,puttingus in thesame zodiacal sign. After a few minutes Stephen replied, “I’m anastronomer.Notanastrologer.”HealsoinsistedthatIcallhimStephenandstopreferringtohimasProfessor.Ihadbeentold…The opportunity to portray Stephenwas an extraordinary one. I was

drawntotherolebecauseofthedualityofStephen’sexternaltriumphinhisscientificworkandtheinternalbattleagainstmotorneuronediseasestarting in his early twenties.His was a unique, complex, rich story ofhuman endeavour, family life, huge academic achievement and sheerdefiance in the face of all obstacles. While we wanted to portray theinspiration, we also wanted to show the grit and courage involved inStephen’slife,displayedbothbyhimandbythosewhocaredforhim.ButitwasequallyimportanttoportraythatsideofStephenwhichwas

pure showman. In my trailer I ended up having three images that Ireferred to.OnewasEinsteinwith his tongue out, because there’s thatsimilar playful wit with Hawking. Another was the Joker in a pack ofcardswho’sapuppeteer,becauseIfeelStephenalwayshadpeopleinthepalmofhishand.And the thirdwasJamesDean.And thatwaswhat Igainedfromseeinghim—theglintandthehumour.Thegreatestpressureinplayingalivingpersonisthatyouwillhaveto

account for your performance to the person you have portrayed. InStephen’s case, the accountingwas also tohis family,whohadbeen sogeneroustomeduringmypreparationforthefilm.BeforeStephenwentintothescreening,hesaidtome,“IwilltellyouwhatIthink.Good.Orotherwise.”Irepliedthat if itwas“otherwise”perhapshecould justsay“otherwise” and spare me the excoriating details. Generously, Stephensaidhehadenjoyedthefilm.Hewasmovedby it,but famouslyhealsostated that he thought there should have beenmore physics and fewerfeelings.Thisisimpossibletoarguewith.Since The Theory of Everything, I have stayed in contact with the

Hawkingfamily.IwastouchedtobeaskedtogiveareadingatStephen’sfuneral.Itwasanincrediblysadbutbrilliantday, fullof loveandjoyfulmemoriesandreflectionson thismostcourageousofmen,whohad ledthe world in his science and in his quest to have disabled peoplerecognisedandgivenproperopportunitiestothrive.Wehave lost a truly beautifulmind, an astonishing scientist and the

funniestmanIhaveeverhadthepleasuretomeet.ButashisfamilysaidatthetimeofStephen’sdeath,hisworkandlegacywillliveonandsoitiswithsadnessbutalsogreatpleasurethatIintroduceyoutothiscollectionofStephen’swritingsondiverseandfascinatingtopics.Ihopeyouenjoyhiswritingsand,toquoteBarackObama,IhopeStephenishavingfunupthereamongthestars.

LoveEddie

AnIntroductionProfessorKipS.Thorne

I first met Stephen Hawking in July 1965, in London, England, at aConference on General Relativity and Gravitation. Stephen was in themidst of his PhD studies at the University of Cambridge; I had justcompleted mine at Princeton University. Rumours swirled around theconference halls that Stephen had devised a compelling argument thatour universemust have been born at some finite time in the past. Itcannotbeinfinitelyold.So,alongwithsome100people, I squeezed intoa roomdesigned for

forty,tohearStephenspeak.Hewalkedwithacaneandhisspeechwasabit slurred, but otherwise he showed only modest signs of the motorneuronediseasewithwhichhehadbeendiagnosedjusttwoyearsearlier.Hismindwasclearlyunaffected.HislucidreasoningreliedonEinstein’sgeneral relativity equations, and on astronomers’ observations that ouruniverseisexpanding,andonafewsimpleassumptionsthatseemedverylikelytobetrue,anditmadeuseofsomenewmathematicaltechniquesthat Roger Penrose had recently devised. Combining all these in waysthatwere clever, powerful and compelling, Stephendeducedhis result:ouruniversemusthavebeguninsomesortofsingularstate,roughlytenbillionyearsago.(Overthenextdecade,StephenandRoger,combiningforces, would go on to prove, ever more convincingly, this singularbeginningoftime,andalsoproveevermoreconvincinglythatthecoreofeveryblackholeisinhabitedbyasingularitywheretimeends.)I emerged fromStephen’s 1965 lecture tremendously impressed.Not

just by his argument and conclusion, but more importantly by hisinsightfulness and creativity. So I sought him out and spent an hourtalking privately with him. That was the beginning of a lifelong

friendship,afriendshipbasednotjustoncommonscienceinterests,butonaremarkablemutualsympathy,anuncannyabilitytounderstandeachotherashumanbeings.Soonwewerespendingmoretimetalkingaboutour lives, our loves, and even death than about science, though oursciencewasstillmuchofthegluethatboundustogether.In September 1973 I took Stephen and his wife Jane to Moscow,

Russia.DespitetheragingColdWar,IhadbeenspendingamonthorsoinMoscow every other year since 1968, collaborating on researchwithmembersofagroupledbyYakovBorisovichZel’dovich.Zel’dovichwasasuperb astrophysicist, and also a father of the Soviet hydrogen bomb.Because of his nuclear secrets, he was forbidden to travel to WesternEurope or America. He craved discussions with Stephen; he could notcometoStephen;sowewenttohim.InMoscow,StephenwowedZel’dovichandhundredsofotherscientists

with his insights, and in return Stephen learned a thing or two fromZel’dovich.MostmemorablewasanafternoonthatStephenandIspentwith Zel’dovich and his PhD student Alexei Starobinsky in Stephen’sroom in the Rossiya Hotel. Zel’dovich explained in intuitive ways aremarkable discovery they had made, and Starobinsky explained itmathematically.Tomake a black hole spin requires energy.We already knew that. A

blackhole,theyexplained,canuseitsspinenergytocreateparticles,andtheparticleswill flyawaycarrying thespinenergywith them.Thiswasnew and surprising—but not terribly surprising. When an object hasenergy of motion, nature usually finds a way to extract it. We alreadyknewotherwaysofextractingablackhole’sspinenergy;thiswasjustanew,thoughunexpectedway.Now,thegreatvalueofconversationslikethis isthattheycantrigger

newdirectionsof thought.And so itwaswithStephen.Hemulledoverthe Zel’dovich/Starobinsky discovery for several months, looking at itfirstfromonedirectionandthenfromanother,untilonedayittriggereda truly radical insight in Stephen’s mind: after a black hole stopsspinning,theholecanstillemitparticles.Itcanradiate—anditradiatesasthoughtheblackholewashot,liketheSun,thoughnotveryhot,justmildlywarm.Theheavierthehole,theloweritstemperature.Aholethatweighs as much as the Sun has a temperature of 0.00000006 Kelvin,

0.06 millionths of a degree above absolute zero. The formula forcalculatingthistemperature isnowengravedonStephen’sheadstoneinWestminsterAbbeyinLondon,wherehisashesresidebetweenthoseofIsaacNewtonandCharlesDarwin.This “Hawking temperature” of a black hole and its “Hawking

radiation” (as they came to be called) were truly radical—perhaps themost radical theoretical physics discovery in the second half of thetwentieth century. They opened our eyes to profound connectionsbetweengeneralrelativity(blackholes),thermodynamics(thephysicsofheat)andquantumphysics(thecreationofparticleswherebeforetherewerenone).Forexample,theyledStephentoprovethatablackholehasentropy,whichmeans that somewhere inside or around the black holethereisenormousrandomness.Hededucedthattheamountofentropy(thelogarithmofthehole’samountofrandomness)isproportionaltothehole’ssurfacearea.HisformulafortheentropyisengravedonStephen’smemorial stone at Gonville and Caius College in Cambridge, where heworked.Forthepastforty-fiveyears,Stephenandhundredsofotherphysicists

have struggled to understand the precise nature of a black hole’srandomness.Itisaquestionthatkeepsongeneratingnewinsightsaboutthemarriageofquantumtheorywithgeneralrelativity—thatis,abouttheill-understoodlawsofquantumgravity.Inautumn1974StephenbroughthisPhDstudentsandhisfamily(his

wife Jane and their two children Robert and Lucy) to Pasadena,Californiaforayear,sothatheandhisstudentscouldparticipateintheintellectual life ofmy university, Caltech, andmerge, temporarily, withmyown researchgroup. Itwasaglorious year, at thepinnacleofwhatcametobecalled“thegoldenageofblackholeresearch.”Duringthatyear,Stephenandhisstudentsandsomeofminestruggled

to understand black holes more deeply, as did I to some degree. ButStephen’s presence, and his leadership in our joint group’s black holeresearch, gave me freedom to pursue a new direction that I had beencontemplatingforsomeyears:gravitationalwaves.Thereareonly two typesofwaves that can travelacross theuniverse

bringing us information about things far away: electromagnetic waves(which include light, X-rays, gamma rays, microwaves, radio waves…);

andgravitationalwaves.Electromagnetic waves consist of oscillating electric and magnetic

forcesthattravelatlightspeed.Whentheyimpingeonchargedparticles,suchas theelectrons ina radioorTVantenna, theyshake theparticlesback and forth, depositing in the particles the information the wavescarry.ThatinformationcanthenbeamplifiedandfedintoaloudspeakerorontoaTVscreenforhumanstocomprehend.Gravitational waves, according to Einstein, consist of an oscillatory

spacewarp: anoscillating stretch and squeezeof space. In 1972Rainer(Rai)WeissattheMassachusettsInstituteofTechnologyhadinventedagravitational-wavedetector, inwhichmirrorshanging inside the cornerandendsofanL-shapedvacuumpipearepushedapartalongonelegoftheLbythestretchofspace,andpushedtogetheralongtheotherlegbythe squeeze of space. Rai proposed using laser beams to measure theoscillating pattern of this stretch and squeeze. The laser light couldextract a gravitationalwave’s information, and the signal could thenbeamplifiedandfedintoacomputerforhumancomprehension.The study of the universe with electromagnetic telescopes

(electromagnetic astronomy) was initiated by Galileo, when he built asmallopticaltelescope,pointeditatJupiteranddiscoveredJupiter’sfourlargest moons. During the 400 years since then, electromagneticastronomy has completely revolutionised our understanding of theuniverse.In1972mystudentsandIbeganthinkingaboutwhatwemight learn

about the universe using gravitational waves: we began developing avisionforgravitational-waveastronomy.Becausegravitationalwavesarea form of spacewarp, they are producedmost strongly by objects thatthemselvesaremadewhollyorpartiallyfromwarpedspace–time—whichmeans,especially,byblackholes.Gravitationalwaves,weconcluded,arethe ideal tool for exploring and testing Stephen’s insights about blackholes.More generally, it seemed to us, gravitational waves are so radically

differentfromelectromagneticwavesthattheywerealmostguaranteedtocreate their own, new revolution in our understanding of the universe,perhaps comparable to the enormous electromagnetic revolution thatfollowedGalileo—iftheseelusivewavescouldbedetectedandmonitored.

But thatwasabig if:weestimatedthat thegravitationalwavesbathingtheEarthare soweak thatmirrorsat theendsofRaiWeiss’sL-shapeddevicewouldbemovedbackandforthrelativetoeachotherbynomorethan1/100ththediameterofaproton(whichmeans1/10,000,000thofthesizeofanatom),evenifthemirrorseparationwasseveralkilometres.Thechallengeofmeasuringsuchtinymotionswasenormous.So during that glorious year,with Stephen’s andmy research groups

mergedatCaltech,Ispentmuchofmytimeexploringtheprospects forgravitational-wavesuccess.Stephenwashelpful in thisas, severalyearsearlier, he and his student Gary Gibbons had designed a gravitational-wavedetectoroftheirown(whichtheyneverbuilt).Shortly after Stephen’s return to Cambridge,my exploration reached

fruitionwithanall-night, intensediscussionbetweenRaiWeissandmein Rai’s hotel room in Washington DC. I became convinced that theprospectsforsuccessweregreatenoughthatIshoulddevotemostofmyowncareer, andmy future students’ research, tohelpingRai andotherexperimentersachieveourgravitational-wavevision.Andtherest,astheysay,ishistory.On September 14, 2015, the LIGO gravitational-wave detectors (built

bya1,000-personprojectthatRaiandIandRonaldDreverco-founded,and Barry Barish organised, assembled and led) registered their firstgravitational waves. By comparing the wave patterns with predictionsfrom computer simulations, our team concluded that the waves wereproducedwhentwoheavyblackholes,1.3billionlightyearsfromEarth,collided. This was the beginning of gravitational-wave astronomy. Ourteam had achieved, for gravitational waves, what Galileo achieved forelectromagneticwaves.I am confident that, over the coming several decades, the next

generation of gravitational-wave astronomers will use these waves notonly to testStephen’s lawsofblackholephysics,but also todetect andmonitorgravitationalwavesfromthesingularbirthofouruniverse,andtherebytestStephen’sandothers’ideasabouthowouruniversecametobe.During our glorious year of 1974–5, while I was dithering over

gravitationalwaves,andStephenwasleadingourmergedgroupinblackholeresearch,Stephenhimselfhadaninsightevenmoreradicalthanhis

discovery of Hawking radiation. He gave a compelling, almost airtightproof that, when a black hole forms and then subsequently evaporatesawaycompletelybyemittingradiation,theinformationthatwentintotheblackholecannotcomebackout.Informationisinevitablylost.This is radical because the laws of quantum physics insist

unequivocally that informationcanneverget totally lost.So, ifStephenwas right, blackholes violate amost fundamental quantummechanicallaw.How could this be? The black hole’s evaporation is governed by the

combined laws of quantum mechanics and general relativity—the ill-understoodlawsofquantumgravity;andso,Stephenreasoned,thefierymarriage of relativity and quantum physics must lead to informationdestruction.The great majority of theoretical physicists find this conclusion

abhorrent. They are highly sceptical. And so, for forty-four years theyhave struggled with this so-called information-loss paradox. It is astrugglewellworth the effort and anguish that have gone into it, sincethis paradox is a powerful key for understanding the quantum gravitylaws. Stephen himself, in 2003, found a way that information mightescape during the hole’s evaporation, but that did not quell theorists’struggles. Stephen did not prove that the information escapes, so thestrugglecontinues.InmyeulogyforStephen,attheintermentofhisashesatWestminster

Abbey, Imemorialised thatstrugglewith thesewords: “Newtongaveusanswers. Hawking gave us questions. And Hawking’s questionsthemselves keep on giving, generating breakthroughs decades later.Whenultimatelywemaster thequantumgravity laws,andcomprehendfully the birth of our universe, it may largely be by standing on theshouldersofHawking.”

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Just as our glorious 1974–5 year was only the beginning for mygravitational-wavequest, so italsowas just thebeginning forStephen’squesttounderstandindetailthelawsofquantumgravityandwhatthose

laws say about the true nature of a black hole’s information andrandomness, and also about the true nature of our universe’s singularbirth,andthetruenatureofthesingularitiesinsideblackholes—thetruenatureofthebirthanddeathoftime.Thesearebigquestions.Verybig.I have shied away from big questions. I don’t have enough skills,

wisdom or self-confidence to tackle them. Stephen, by contrast, wasalwaysattractedtobigquestions,whethertheyweredeeplyrootedinhisscience or not. He did have the necessary skills, wisdom and self-confidence.Thisbookisacompilationofhisanswerstothebigquestions,answers

onwhichhewasstillworkingatthetimeofhisdeath.Stephen’s answers to six of the questions are deeply rooted in his

science.(IsthereaGod?Howdiditallbegin?Canwepredictthefuture?Whatisinsideablackhole?Istimetravelpossible?Howdoweshapethefuture?).Hereyouwill findhimdiscussing indepth the issues that I’vedescribedbrieflyinthisIntroduction,andalsomuch,muchmore.Hisanswerstotheotherfourbigquestionscannotpossiblyberooted

solidlyinhisscience.(WillwesurviveonEarth?Isthereotherintelligentlifeintheuniverse?Shouldwecolonisespace?Willartificialintelligenceoutsmart us?) Nevertheless, his answers display deep wisdom andcreativity,asweshouldexpect.IhopeyoufindhisanswersasstimulatingandinsightfulasdoI.Enjoy!

KipS.ThorneJuly2018

WHYWEMUSTASKTHEBIGQUESTIONS

Peoplehavealwayswantedanswerstothebigquestions.Wheredidwecomefrom?Howdidtheuniversebegin?Whatisthemeaninganddesignbehinditall?Isthereanyoneoutthere?Thecreationaccountsofthepastnowseemlessrelevantandcredible.Theyhavebeenreplacedbyavarietyofwhat canonlybecalled superstitions, ranging fromNewAge toStarTrek.Butrealsciencecanbefarstrangerthansciencefiction,andmuchmoresatisfying.Iama scientist.Anda scientistwithadeep fascinationwithphysics,

cosmology,theuniverseandthefutureofhumanity.Iwasbroughtupbymy parents to have an unwavering curiosity and, like my father, toresearch and try to answer themany questions that science asks us. Ihave spent my life travelling across the universe, inside my mind.Through theoretical physics, Ihave sought to answer someof the greatquestions.Atonepoint, I thought Iwouldsee theendofphysicsasweknowit,butnowIthinkthewonderofdiscoverywillcontinuelongafterIamgone.Weareclosetosomeoftheseanswers,butwearenotthereyet.Theproblemis,mostpeoplebelievethatrealscienceistoodifficultand

complicatedforthemtounderstand.ButIdon’tthinkthisisthecase.Todo research on the fundamental laws that govern the universe wouldrequire a commitment of time that most people don’t have; the worldwould soon grind to a halt ifwe all tried to do theoretical physics. Butmost people can understand and appreciate the basic ideas if they arepresented in a clearwaywithout equations, which I believe is possibleandwhichissomethingIhaveenjoyedtryingtodothroughoutmylife.Ithasbeenaglorioustimetobealiveanddoingresearchintheoretical

physics.Ourpictureoftheuniversehaschangedagreatdeal inthe lastfiftyyears,andI’mhappyifIhavemadeacontribution.Oneofthegreatrevelations of the space age has been the perspective it has givenhumanity on ourselves. When we see the Earth from space, we seeourselvesasawhole.Weseetheunity,andnotthedivisions.Itissuchasimpleimagewithacompellingmessage;oneplanet,onehumanrace.Iwanttoaddmyvoicetothosewhodemandimmediateactiononthe

keychallengesforourglobalcommunity.Ihopethatgoingforward,evenwhen I am no longer here, people with power can show creativity,courageandleadership.Letthemrisetothechallengeofthesustainabledevelopmentgoals,andact,notoutof self-interest,butoutof commoninterest.Iamveryawareofthepreciousnessoftime.Seizethemoment.Actnow.

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Ihavewrittenaboutmylifebeforebutsomeofmyearlyexperiencesareworth repeating as I think about my lifelong fascination with the bigquestions.Iwasbornexactly300yearsafterthedeathofGalileo,andIwouldlike

tothinkthatthiscoincidencehashadabearingonhowmyscientificlifehas turned out. However, I estimate that about 200,000 other babieswere also born that day; I don’t knowwhether any of themwere laterinterestedinastronomy.Igrewupinatall,narrowVictorianhouseinHighgate,London,which

myparentshadboughtverycheaplyduringtheSecondWorldWarwheneveryone thought London was going to be bombed flat. In fact, a V2rocketlandedafewhousesawayfromours.Iwasawaywithmymotherandsisteratthetime,andfortunatelymyfatherwasnothurt.Foryearsafterwards,therewasalargebombsitedowntheroadinwhichIusedtoplaywithmyfriendHoward.Weinvestigatedtheresultsoftheexplosionwiththesamecuriositythatdrovememywholelife.In 1950, my father’s place of work moved to the northern edge of

London,tothenewlyconstructedNationalInstituteforMedicalResearchin Mill Hill, so my family relocated to the cathedral city of St Albans

nearby. Iwas sent to theHighSchool forGirls,whichdespite itsnametookboysup to the ageof ten.Later Iwent toStAlbansSchool. Iwasnevermorethanabouthalfwayuptheclass—itwasaverybrightclass—butmy classmates gaveme thenicknameEinstein, sopresumably theysawsignsofsomethingbetter.WhenIwastwelve,oneofmyfriendsbetanotherfriendabagofsweetsthatIwouldnevercometoanything.Ihad sixor sevenclose friends inStAlbans, and I rememberhaving

longdiscussionsandargumentsabouteverything,fromradio-controlledmodelstoreligion.Oneofthebigquestionswediscussedwastheoriginoftheuniverse,andwhetheritrequiredaGodtocreateitandsetitgoing.Ihadheardthat light fromdistantgalaxieswasshifted towards theredendofthespectrumandthiswassupposedtoindicatethattheuniversewasexpanding.ButIwassuretheremustbesomeotherreasonfor thered shift. Maybe light got tired and more red on its way to us? Anessentially unchanging and everlasting universe seemed somuchmorenatural. (It was only years later, after the discovery of the cosmicmicrowave background about two years into my PhD research, that IrealisedIhadbeenwrong.)Iwasalwaysveryinterestedinhowthingsoperated,andIusedtotake

themaparttoseehowtheyworked,butIwasnotsogoodatputtingthemback together again. My practical abilities never matched up to mytheoretical qualities. My father encouraged my interest in science andwasverykeenthatIshouldgotoOxfordorCambridge.HehimselfhadgonetoUniversityCollege,Oxford,sohethoughtIshouldapplythere.Atthattime,UniversityCollegehadnofellowinmathematics,soIhadlittleoptionbuttotry forascholarship innaturalscience.Isurprisedmyselfbybeingsuccessful.TheprevailingattitudeatOxfordatthattimewasveryanti-work.You

weresupposedtobebrilliantwithouteffort,ortoacceptyourlimitationsandgetafourth-classdegree.Itookthisasaninvitationtodoverylittle.I’mnotproudofthis,I’mjustdescribingmyattitudeatthetime,sharedby most of my fellow students. One result of my illness has been tochangeallthat.Whenyouarefacedwiththepossibilityofanearlydeath,itmakes you realise that there are lots of things youwant todobeforeyourlifeisover.Because of my lack of work, I had planned to get through the final

exam by avoiding questions that required any factual knowledge andfocus instead on problems in theoretical physics. But I didn’t sleep thenightbeforetheexamandsoIdidn’tdoverywell.Iwasontheborderlinebetweenafirst-andsecond-classdegree,andIhadtobeinterviewedbythe examiners to determine which I should get. In the interview theyaskedmeaboutmyfutureplans.IrepliedthatIwantedtodoresearch.Iftheygavemea first, Iwouldgo toCambridge. If I onlygota second, IwouldstayinOxford.Theygavemeafirst.In the long vacation following my final exam, the college offered a

numberofsmalltravelgrants.IthoughtmychancesofgettingonewouldbegreaterthefurtherIproposedtogo,soIsaidIwantedtogotoIran.Inthe summer of 1962 I set out, taking a train to Istanbul, then on toErzuerumineasternTurkey,thentoTabriz,Tehran,Isfahan,ShirazandPersepolis, thecapitalof theancientPersiankings.Onmywayhome, Iandmytravellingcompanion,RichardChiin,werecaught intheBouin-Zahra earthquake, amassive 7.1 Richter quake that killed over 12,000people. I must have been near the epicentre, but I was unaware of itbecauseIwasill,andinabusthatwasbouncingaroundontheIranianroadsthatwerethenveryuneven.WespentthenextseveraldaysinTabriz,whileIrecoveredfromsevere

dysenteryandfromabrokenribsustainedonthebuswhenIwasthrownagainst the seat in front, still not knowing of the disaster because wedidn’tspeakFarsi. ItwasnotuntilwereachedIstanbul thatwe learnedwhat had happened. I sent a postcard to my parents, who had beenanxiously waiting for ten days, because the last they had heard I wasleavingTehran for thedisaster regionon thedayof thequake.Despitetheearthquake,IhavemanyfondmemoriesofmytimeinIran.Intensecuriosityabouttheworldcanputoneinharm’sway,butformethiswasprobablytheonlytimeinmylifethatthiswastrue.I was twenty in October 1962, when I arrived in Cambridge at the

departmentofappliedmathematicsandtheoreticalphysics.IhadappliedtoworkwithFredHoyle,themostfamousBritishastronomerofthetime.I say astronomer, because cosmology then was hardly recognised as alegitimate field.However,Hoylehadenoughstudentsalready,so tomygreat disappointment Iwas assigned toDennis Sciama, ofwhom I hadnot heard. But it was just as well I hadn’t been a student of Hoyle,becauseIwouldhavebeendrawnintodefendinghissteady-statetheory,

ataskwhichwouldhavebeenharderthannegotiatingBrexit.Ibeganmyworkbyreadingoldtextbooksongeneralrelativity—asever,drawntothebiggestquestions.AssomeofyoumayhaveseenfromthefilminwhichEddieRedmayne

playsaparticularlyhandsomeversionofme,inmythirdyearatOxfordInoticedthatIseemedtobegettingclumsier.Ifelloveronceortwiceandcouldn’t understand why, and I noticed that I could no longer row ascullingboatproperly.Itbecameclearsomethingwasnotquiteright,andIwassomewhatdisgruntledtobetoldbyadoctorat thetimeto layoffthebeer.ThewinterafterIarrivedinCambridgewasverycold.Iwashomefor

theChristmasbreakwhenmymotherpersuadedmetogoskatingonthelake inStAlbans,eventhoughIknewIwasnotupto it. I felloverandhadgreatdifficultygettingupagain.Mymotherrealisedsomethingwaswrongandtookmetothedoctor.IspentweeksinStBartholomew’sHospital inLondonandhadmany

tests.In1962,thetestsweresomewhatmoreprimitivethantheyarenow.Amusclesamplewastakenfrommyarm,Ihadelectrodesstuckintomeand radio-opaque fluid was injected into my spine, which the doctorswatchedgoingupanddownonX-rays,asthebedwastilted.Theyneveractually toldmewhatwaswrong,but I guessedenough toknow itwaspretty bad, so I didn’t want to ask. I had gathered from the doctors’conversationsthatit,whatever“it”was,wouldonlygetworse,andtherewas nothing they could do except giveme vitamins. In fact, the doctorwhoperformed the testswashedhis hands ofme and I never sawhimagain.AtsomepointImusthavelearnedthatthediagnosiswasamyotrophic

lateral sclerosis (ALS), a type of motor neurone disease, in which thenervecellsofthebrainandspinalcordatrophyandthenscarorharden.Ialso learned that people with this disease gradually lose the ability tocontroltheirmovements,tospeak,toeatandeventuallytobreathe.My illness seemed to progress rapidly. Understandably, I became

depressedandcouldn’tseethepointofcontinuingtoresearchmyPhD,becauseIdidn’tknowifIwouldlivelongenoughtofinishit.ButthentheprogressionsloweddownandIhadarenewedenthusiasmformywork.Aftermyexpectationshadbeenreducedtozero,everynewdaybecamea

bonus,andIbegantoappreciateeverythingIdidhave.Whilethere’slife,thereishope.And,ofcourse,therewasalsoayoungwomancalledJane,whomIhad

metataparty.Shewasverydeterminedthattogetherwecouldfightmycondition. Her confidence gave me hope. Getting engaged lifted myspirits,andIrealised,ifweweregoingtogetmarried,IhadtogetajobandfinishmyPhD.Andasalways,thosebigquestionsweredrivingme.IbegantoworkhardandIenjoyedit.To support myself during my studies, I applied for a research

fellowship at Gonvillle and Cauis College. To my great surprise, I waselectedandhavebeenafellowofCaiuseversince.Thefellowshipwasaturning point in my life. It meant that I could continue my researchdespitemyincreasingdisability.ItalsomeantthatJaneandIcouldgetmarried, which we did in July 1965. Our first child, Robert, was bornafterwehadbeenmarriedabouttwoyears.Oursecondchild,Lucy,wasbornaboutthreeyearslater.Ourthirdchild,Timothy,wouldbebornin1979.As a father, Iwould try to instill the importanceof askingquestions,

always. My son Tim once told a story in an interview about asking aquestionwhichIthinkatthetimeheworriedwasabitsilly.Hewantedtoknowiftherewerelotsoftinyuniversesdottedaround.Itoldhimnevertobeafraidtocomeupwithanideaorahypothesisnomatterhowdaft(hiswordsnotmine)itmightseem.

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The big question in cosmology in the early 1960swas did the universehaveabeginning?Manyscientistswereinstinctivelyopposedtotheidea,becausetheyfeltthatapointofcreationwouldbeaplacewheresciencebrokedown.OnewouldhavetoappealtoreligionandthehandofGodtodetermine how the universe would start off. This was clearly afundamentalquestion,anditwasjustwhatIneededtocompletemyPhDthesis.RogerPenrosehad shown that once adying starhad contracted to a

certain radius, there would inevitably be a singularity, that is a point

wherespaceandtimecametoanend.Surely,Ithought,wealreadyknewthatnothingcouldpreventamassivecoldstarfromcollapsingunderitsowngravityuntilitreachedasingularityofinfinitedensity.Irealisedthatsimilarargumentscouldbeappliedto theexpansionof theuniverse. Inthiscase,Icouldprovethereweresingularitieswherespace–timehadabeginning.A eureka moment came in 1970, a few days after the birth of my

daughter,Lucy.Whilegettingintobedoneevening,whichmydisabilitymadeaslowprocess,IrealisedthatIcouldapplytoblackholesthecasualstructure theory I had developed for singularity theorems. If generalrelativityiscorrectandtheenergydensityispositive,thesurfaceareaoftheeventhorizon—theboundaryofablackhole—hasthepropertythatitalways increases when additional matter or radiation falls into it.Moreover, if two black holes collide and merge to form a single blackhole, the area of the event horizon around the resulting black hole isgreater than the sum of the areas of the event horizons around theoriginalblackholes.Thiswasagoldenage,inwhichwesolvedmostofthemajorproblems

inblackholetheoryevenbeforetherewasanyobservationalevidenceforblack holes. In fact, we were so successful with the classical generaltheory of relativity that I was at a bit of a loose end in 1973 after thepublicationwithGeorgeEllis of ourbookTheLargeScaleStructureofSpace–Time. My work with Penrose had shown that general relativitybrokedownatsingularities,sotheobviousnextstepwouldbetocombinegeneral relativity—the theory of the very large—with quantum theory—the theory of the very small. In particular, I wondered, can one haveatomsinwhichthenucleusisatinyprimordialblackhole,formedintheearly universe? My investigations revealed a deep and previouslyunsuspected relationship between gravity and thermodynamics, thescience of heat, and resolved a paradox that had been argued over forthirty years without much progress: how could the radiation left overfromashrinkingblackholecarryalloftheinformationaboutwhatmadethe black hole? I discovered that information is not lost, but it is notreturnedinausefulway—likeburninganencyclopediabutretainingthesmokeandashes.Toanswerthis,Istudiedhowquantumfieldsorparticleswouldscatter

offablackhole. Iwasexpectingthatpartofan incidentwavewouldbe

absorbed,andtheremainderscattered.ButtomygreatsurpriseIfoundthereseemedtobeemissionfromtheblackholeitself.Atfirst,Ithoughtthismustbeamistakeinmycalculation.Butwhatpersuadedmethatitwasrealwasthattheemissionwasexactlywhatwasrequiredtoidentifytheareaofthehorizonwiththeentropyofablackhole.Thisentropy,ameasureofthedisorderofasystem,issummedupinthissimpleformula

whichexpressestheentropyintermsoftheareaofthehorizon,andthethreefundamentalconstantsofnature,c,thespeedoflight,G,Newton’sconstant of gravitation, and ħ, Planck’s constant. The emission of thisthermal radiation from the black hole is now calledHawking radiationandI’mproudtohavediscoveredit.In1974,IwaselectedafellowoftheRoyalSociety.Thiselectioncame

asasurprisetomembersofmydepartmentbecauseIwasyoungandonlyalowlyresearchassistant.ButwithinthreeyearsIhadbeenpromotedtoprofessor. My work on black holes had given me hope that we woulddiscovera theoryof everything, and thatquest forananswerdrovemeon.In the same year, my friend Kip Thorne invited me and my young

family and a number of others working in general relativity to theCaliforniaInstituteofTechnology(Caltech).Forthepreviousfouryears,Ihad been using a manual wheelchair as well as a blue electric three-wheeled car, which went at a slow cycling speed, and in which Isometimes illegally carriedpassengers.Whenwewent toCalifornia,westayedinaCaltech-ownedcolonial-stylehousenearcampusandthereIwasabletoenjoyfull-timeuseofanelectricwheelchairforthefirsttime.It gaveme a considerable degree of independence, especially as in theUnitedStatesbuildingsandsidewalksaremuchmoreaccessible for thedisabledthantheyareinBritain.When we returned from Caltech in 1975, I initially felt rather low.

EverythingseemedsoparochialandrestrictedinBritaincomparedtothecan-doattitude inAmerica.At thetime, the landscapewas litteredwithdead trees killed by Dutch elm disease and the country was beset by

strikes.However,mymood liftedas I sawsuccess inmyworkandwaselected, in 1979, to the Lucasian Professorship of Mathematics, a postonceheldbySirIsaacNewtonandPaulDirac.During the 1970s, Ihadbeenworkingmainlyonblackholes,butmy

interest in cosmology was renewed by the suggestions that the earlyuniverse had gone through a period of rapid inflationary expansion inwhich its size grew at an ever-increasing rate, like theway prices haveincreasedsincetheUK’sBrexitvote.IalsospenttimeworkingwithJimHartle, formulating a theory of the universe’s birth that we called “noboundary.”By the early 1980s, my health continued to worsen, and I endured

prolongedchokingfitsbecausemylarynxwasweakeningandwaslettingfood intomy lungs as I ate. In 1985, I caught pneumonia on a trip toCERN,theEuropeanOrganisationforNuclearResearch,inSwitzerland.Thiswas a life-alteringmoment. Iwas rushed to theLucerneCantonalHospital andputon toa ventilator.Thedoctors suggested toJane thatthingshadprogressedtothestagewherenothingcouldbedoneandthattheyturnoffmyventilatortoendmylife.ButJanerefusedandhadmeflownbacktoAddenbrooke’sHospitalinCambridgebyairambulance.As youmay imagine thiswas a verydifficult time, but thankfully the

doctors atAddenbrooke’s tried hard to getme back to how I had beenbefore the visit to Switzerland. However, because my larynx was stillallowing food and saliva into my lungs, they had to perform atracheostomy.Asmostofyouwillknow,atracheostomytakesawaytheabilitytospeak.Yourvoiceisveryimportant.Ifitisslurred,asminewas,people can think you arementally deficient and treat you accordingly.Before the tracheostomymy speech was so indistinct that only peoplewho knewmewell could understandme.My childrenwere among thefewwhocoulddoso.Forawhileafterthetracheostomy,theonlywayIcouldcommunicatewastospelloutwords,letterbyletter,byraisingmyeyebrowswhensomeonepointedtotherightletteronaspellingcard.LuckilyacomputerexpertinCalifornianamedWaltWoltoszheardof

my difficulties. He sentme a computer program he had written calledEqualizer.Thisallowedmetoselectwholewordsfromaseriesofmenuson the computer screen onmy wheelchair by pressing a switch in myhand.Overtheyearssincethen,thesystemhasdeveloped.TodayIusea

programcalledAcat,developedbyIntel,whichIcontrolbyasmallsensorinmy glasses viamy cheekmovements. It has amobile phone, whichgives me access to the internet. I can claim to be the most connectedperson in theworld. I have kept the original speech synthesiser I had,however, partly because I haven’t heard one with better phrasing, andpartly because by now I identify with this voice, despite its Americanaccent.I first had the idea of writing a popular book about the universe in

1982,aroundthetimeofmyno-boundarywork.IthoughtImightmakeamodestamounttohelpsupportmychildrenatschoolandmeettherisingcostsofmycare,butthemainreasonwasthatIwantedtoexplainhowfar I felt we had come in our understanding of the universe: how wemightbenearfindingacompletetheorythatwoulddescribetheuniverseandeverythinginit.Notonlyisitimportanttoaskquestionsandfindtheanswers, as a scientist I felt obligated to communicate with the worldwhatwewerelearning.Appropriatelyenough,ABriefHistoryofTimewasfirstpublishedon

April Fool’s Day in 1988. Indeed, the book was originallymeant to becalledFromtheBigBangtoBlackHoles:AShortHistoryofTime.Thetitlewasshortenedandchangedto“brief,”andtherestishistory.I never expected A Brief History of Time to do as well as it has.

Undoubtedly, the human-interest story of how I havemanaged to be atheoreticalphysicistandabestsellingauthordespitemydisabilitieshashelped.Noteveryonemayhavefinisheditorunderstoodeverythingtheyread, but they at least grappled with one of the big questions of ourexistenceandgottheideathatweliveinauniversegovernedbyrationallawsthat,throughscience,wecandiscoverandunderstand.Tomycolleagues,I’mjustanotherphysicist,buttothewiderpublicI

became possibly the best-known scientist in the world. This is partlybecausescientists,apartfromEinstein,arenotwidelyknownrockstars,and partly because I fit the stereotype of a disabled genius. I can’tdisguise myself with a wig and dark glasses—the wheelchair gives meaway. Being well known and easily recognisable has its pluses andminuses, but the minuses are more than outweighed by the pluses.People seem genuinely pleased to see me. I even had my biggest-everaudiencewhenIopenedtheParalympicGamesinLondonin2012.

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I have led an extraordinary life on this planet, while at the same timetravellingacrosstheuniversebyusingmymindandthelawsofphysics.Ihavebeentothefurthestreachesofourgalaxy,travelledintoablackholeand gone back to the beginning of time. On Earth, I have experiencedhighsandlows,turbulenceandpeace,successandsuffering.Ihavebeenrichandpoor,Ihavebeenable-bodiedanddisabled.Ihavebeenpraisedand criticised, but never ignored. I have been enormously privileged,throughmywork,inbeingabletocontributetoourunderstandingoftheuniverse.ButitwouldbeanemptyuniverseindeedifitwerenotforthepeopleIlove,andwholoveme.Withoutthem,thewonderofitallwouldbelostonme.Andattheendofallthis,thefactthatwehumans,whoareourselves

mere collections of fundamental particles of nature, have been able tocometoanunderstandingofthelawsgoverningus,andouruniverse,isagreattriumph.Iwanttosharemyexcitementaboutthesebigquestionsandmyenthusiasmaboutthisquest.Oneday, Ihopewewillknow theanswers toall thesequestions.But

thereareotherchallenges,otherbigquestionsontheplanetwhichmustbe answered, and these will also need a new generation who areinterestedandengaged,andhaveanunderstandingofscience.Howwillwe feed an ever-growing population? Provide clean water, generaterenewableenergy,preventandcurediseaseandslowdownglobalclimatechange? Ihope that scienceand technologywillprovide theanswers tothese questions, but it will take people, human beings with knowledgeandunderstanding, to implement these solutions.Letus fight foreverywoman and every man to have the opportunity to live healthy, securelives, fullofopportunityandlove.Weareall timetravellers, journeyingtogether into the future.But letuswork together tomake that futureaplacewewanttovisit.Be brave, be curious, be determined, overcome the odds. It can be

done.

Whatwasyourdreamwhenyouwereachild,anddiditcometrue?

Iwantedtobeagreatscientist.However,Iwasn’taverygoodstudentwhenIwasatschool,andwasrarelymorethan

halfwayupmyclass.Myworkwasuntidy,andmyhandwritingnotverygood.ButIhadgoodfriendsatschool.Andwetalkedabouteverythingand,specifically,theoriginoftheuniverse.Thisiswheremydreambegan,andIamvery

fortunatethatithascometrue.

1

ISTHEREAGOD?

Scienceisincreasinglyansweringquestionsthatusedtobetheprovinceofreligion.Religionwasanearlyattempttoanswerthequestionsweallask:whyarewehere,wheredidwecomefrom?Longago,theanswerwasalmost always the same: godsmade everything. Theworldwas a scaryplace, so even people as tough as the Vikings believed in supernaturalbeings to make sense of natural phenomena like lightning, storms oreclipses.Nowadays,scienceprovidesbetterandmoreconsistentanswers,butpeoplewillalwaysclingtoreligion,becauseitgivescomfort,andtheydonottrustorunderstandscience.Afewyearsago,TheTimesnewspaperranaheadlineonthefrontpage

which said “Hawking: God did Not Create Universe.” The article wasillustrated. God was shown in a drawing by Michelangelo, lookingthunderous.Theyprintedaphotoofme,lookingsmug.Theymadeitlooklikeaduelbetweenus.But Idon’thaveagrudgeagainstGod. IdonotwanttogivetheimpressionthatmyworkisaboutprovingordisprovingtheexistenceofGod.Mywork isabout findinga rational framework tounderstandtheuniversearoundus.Forcenturies, itwasbelievedthatdisabledpeople likemewereliving

underacursethatwasinflictedbyGod.Well,Isupposeit’spossiblethatI’veupsetsomeoneupthere,butIprefertothinkthateverythingcanbeexplained anotherway, by the laws of nature. If youbelieve in science,likeIdo,youbelievethattherearecertainlawsthatarealwaysobeyed.Ifyou like, you can say the laws are thework ofGod, but that ismore adefinition of God than a proof of his existence. In about 300 BCE, aphilosopher called Aristarchus was fascinated by eclipses, especially

eclipses of the Moon. He was brave enough to question whether theyreallywerecausedbygods.Aristarchuswasatruescientificpioneer.Hestudiedtheheavenscarefullyandreachedaboldconclusion:herealisedtheeclipsewasreallytheshadowoftheEarthpassingovertheMoon,andnotadivineevent.Liberatedby thisdiscovery,hewasable toworkoutwhatwasreallygoingonabovehishead,anddrawdiagramsthatshowedthetruerelationshipoftheSun,theEarthandtheMoon.Fromtherehereached evenmore remarkable conclusions.Hededuced that theEarthwasnot thecentreof theuniverse,aseveryonehad thought,but that itinsteadorbitstheSun.Infact,understandingthisarrangementexplainsalleclipses.WhentheMooncastsitsshadowontheEarth,that’sasolareclipse.AndwhentheEarthshadestheMoon,that’salunareclipse.ButAristarchustookitevenfurther.Hesuggestedthatstarswerenotchinksinthefloorofheaven,ashiscontemporariesbelieved,butthatstarswereother suns, like ours, only a very long way away. What a stunningrealisation it must have been. The universe is a machine governed byprinciplesorlaws—lawsthatcanbeunderstoodbythehumanmind.I believe that the discovery of these laws has been humankind’s

greatestachievement,forit’stheselawsofnature—aswenowcallthem—thatwilltelluswhetherweneedagodtoexplaintheuniverseatall.Thelawsofnatureareadescriptionofhowthingsactuallyworkinthepast,presentandfuture.Intennis,theballalwaysgoesexactlywheretheysayit will. And there are many other laws at work here too. They governeverythingthatisgoingon,fromhowtheenergyoftheshotisproducedin the players’ muscles to the speed at which the grass grows beneaththeirfeet.Butwhat’sreallyimportantisthatthesephysicallaws,aswellasbeingunchangeable,areuniversal.Theyapplynotjusttotheflightofaball, but to themotionof aplanet, andeverything else in theuniverse.Unlikelawsmadebyhumans,thelawsofnaturecannotbebroken—that’swhy they are so powerful and, when seen from a religious standpoint,controversialtoo.Ifyouaccept,asIdo,thatthelawsofnaturearefixed,thenitdoesn’t

take long to ask: what role is there for God? This is a big part of thecontradictionbetweenscienceandreligion,andalthoughmyviewshavemadeheadlines,itisactuallyanancientconflict.OnecoulddefineGodasthe embodiment of the laws of nature.However, this is notwhatmostpeoplewouldthinkofasGod.Theymeanahuman-likebeing,withwhom

onecanhaveapersonal relationship.Whenyou lookat thevast sizeoftheuniverse,andhowinsignificantandaccidentalhumanlifeisinit,thatseemsmostimplausible.Iusetheword“God”inanimpersonalsense,likeEinsteindid,forthe

laws of nature, so knowing the mind of God is knowing the laws ofnature.MypredictionisthatwewillknowthemindofGodbytheendofthiscentury.Theoneremainingareathatreligioncannowlayclaimtoistheorigin

oftheuniverse,butevenherescienceismakingprogressandshouldsoonprovideadefinitiveanswertohowtheuniversebegan.Ipublishedabookthat asked ifGod created theuniverse, and that caused something of astir.Peoplegotupsetthatascientistshouldhaveanythingtosayonthematterofreligion.Ihavenodesiretotellanyonewhattobelieve,butformeaskingifGodexistsisavalidquestionforscience.Afterall,itishardtothinkofamoreimportant,orfundamental,mysterythanwhat,orwho,createdandcontrolstheuniverse.I think the universe was spontaneously created out of nothing,

according to the laws of science. The basic assumption of science isscientificdeterminism.Thelawsofsciencedeterminetheevolutionoftheuniverse, given its state at one time.These lawsmay, ormaynot,havebeendecreedbyGod,buthecannotintervenetobreakthelaws,ortheywouldnotbelaws.ThatleavesGodwiththefreedomtochoosetheinitialstateof theuniverse,butevenhere itseemstheremaybe laws.SoGodwouldhavenofreedomatall.Despitethecomplexityandvarietyoftheuniverse,itturnsoutthatto

makeoneyouneedjustthreeingredients.Let’simaginethatwecouldlisttheminsomekindofcosmiccookbook.Sowhatarethethreeingredientsweneedtocookupauniverse?Thefirst ismatter—stuff thathasmass.Matterisallaroundus,inthegroundbeneathourfeetandoutinspace.Dust,rock, ice, liquids.Vastcloudsofgas,massivespiralsofstars,eachcontainingbillionsofsuns,stretchingawayforincredibledistances.The second thing you need is energy. Even if you’ve never thought

aboutit,weallknowwhatenergyis.Somethingweencountereveryday.LookupattheSunandyoucanfeelitonyourface:energyproducedbyastar ninety-three million miles away. Energy permeates the universe,drivingtheprocessesthatkeepitadynamic,endlesslychangingplace.

So we havematter and we have energy. The third thing we need tobuildauniverse isspace.Lotsofspace.Youcancall theuniversemanythings—awesome, beautiful, violent—but one thing you can’t call it iscramped.Wherever we look we see space, more space and even morespace.Stretchinginalldirections.It’senoughtomakeyourheadspin.Sowherecouldallthismatter,energyandspacecomefrom?Wehadnoideauntilthetwentiethcentury.The answer came from the insights of one man, probably the most

remarkable scientistwhohas ever lived.HisnamewasAlbertEinstein.Sadly I never got tomeet him, since Iwas only thirteenwhenhedied.Einstein realised something quite extraordinary: that two of the mainingredientsneeded tomakeauniverse—massandenergy—arebasicallythe same thing, two sides of the same coin if you like. His famousequationE=mc2simplymeansthatmasscanbethoughtofasakindofenergy, andvice versa.So insteadof three ingredients,we cannowsaythat the universe has just two: energy and space. Sowhere did all thisenergy and space come from? The answer was found after decades ofworkbyscientists:spaceandenergywerespontaneously inventedinaneventwenowcalltheBigBang.AtthemomentoftheBigBang,anentireuniversecameintoexistence,

andwith it space. It all inflated, just like a balloonbeingblownup. Sowhere did all this energy and space come from? How does an entireuniversefullofenergy,theawesomevastnessofspaceandeverythinginit,simplyappearoutofnothing?For some, this iswhereGodcomesback into thepicture. ItwasGod

who created the energy and space. The Big Bang was the moment ofcreation.Butscience tellsadifferentstory.At theriskofgettingmyselfinto trouble, I think we can understand much more the naturalphenomena that terrified the Vikings. We can even go beyond thebeautifulsymmetryofenergyandmatterdiscoveredbyEinstein.Wecanuse the laws of nature to address the very origins of the universe, anddiscoveriftheexistenceofGodistheonlywaytoexplainit.AsIwasgrowingupinEnglandaftertheSecondWorldWar,itwasa

timeofausterity.Weweretoldthatyounevergetsomethingfornothing.Butnow,afteralifetimeofwork,Ithinkthatactuallyyoucangetawholeuniverseforfree.

The greatmystery at the heart of the Big Bang is to explain how anentire, fantastically enormous universe of space and energy canmaterialise out of nothing. The secret lies in one of the strangest factsabout our cosmos. The laws of physics demand the existence ofsomethingcalled“negativeenergy.”Tohelpyougetyourheadaroundthisweirdbutcrucialconcept,letme

drawonasimpleanalogy.Imagineamanwantstobuildahillonaflatpiece of land. The hillwill represent the universe. Tomake this hill hedigsaholeinthegroundandusesthatsoiltodighishill.Butofcoursehe’snot justmakingahill—he’salsomakingahole, ineffectanegativeversionofthehill.Thestuffthatwasintheholehasnowbecomethehill,so it all perfectly balances out. This is the principle behind whathappenedatthebeginningoftheuniverse.When theBigBangproducedamassiveamountofpositiveenergy, it

simultaneously produced the same amount of negative energy. In thisway,thepositiveandthenegativeadduptozero,always.It’sanotherlawofnature.Sowhereisallthisnegativeenergytoday?It’sinthethirdingredientin

ourcosmiccookbook:it’sinspace.Thismaysoundodd,butaccordingtothe lawsofnatureconcerninggravityandmotion—laws thatareamongthe oldest in science—space itself is a vast store of negative energy.Enoughtoensurethateverythingaddsuptozero.I’lladmitthat,unlessmathematicsisyourthing,thisishardtograsp,

but it’s true.The endlesswebof billionsuponbillionsof galaxies, eachpulling on each other by the force of gravity, acts like a giant storagedevice.Theuniverseislikeanenormousbatterystoringnegativeenergy.Thepositivesideofthings—themassandenergyweseetoday—islikethehill. The corresponding hole, or negative side of things, is spreadthroughoutspace.Sowhatdoes thismean inourquest to findout if there is aGod? It

meansthatiftheuniverseaddsuptonothing,thenyoudon’tneedaGodtocreateit.Theuniverseistheultimatefreelunch.Sinceweknowthatthepositiveandthenegativeadduptozero,allwe

need to do now is to work out what—or dare I say who—triggered thewhole process in the first place. What could cause the spontaneousappearanceofauniverse?Atfirst,itseemsabafflingproblem—afterall,

inourdaily lives thingsdon’t justmaterialiseoutof theblue.Youcan’tjustclickyourfingersandsummonupacupofcoffeewhenyoufeellikeone.Youhave tomake itoutofother stuff likecoffeebeans,waterandperhaps some milk and sugar. But travel down into this coffee cup—throughthemilkparticles,downtotheatomiclevelandrightdowntothesub-atomiclevel,andyouenteraworldwhereconjuringsomethingoutofnothing is possible. At least, for a short while. That’s because, at thisscale,particlessuchasprotonsbehaveaccordingtothelawsofnaturewecall quantum mechanics. And they really can appear at random, stickaroundforawhileandthenvanishagain,toreappearsomewhereelse.Sinceweknowtheuniverseitselfwasonceverysmall—perhapssmaller

than a proton—this means something quite remarkable. It means theuniverse itself, in all its mind-boggling vastness and complexity, couldsimplyhavepopped into existencewithout violating the known laws ofnature.Fromthatmomenton,vastamountsofenergywerereleasedasspaceitselfexpanded—aplacetostoreallthenegativeenergyneededtobalancethebooks.Butofcoursethecriticalquestionisraisedagain:didGod create the quantum laws that allowed theBigBang to occur? In anutshell,doweneedaGodtosetitupsothattheBigBangcouldbang?Ihavenodesiretooffendanyoneof faith,butI thinksciencehasamorecompellingexplanationthanadivinecreator.Oureverydayexperiencemakesusthinkthateverythingthathappens

mustbecausedbysomethingthatoccurredearlierintime,soit’snaturalfor us to think that something—maybe God—must have caused theuniverse to come into existence. But when we’re talking about theuniverseasawhole, that isn’tnecessarilyso.Letmeexplain. Imagineariver, flowing down a mountainside. What caused the river? Well,perhapstherainthatfellearlierinthemountains.Butthen,whatcausedtherain?AgoodanswerwouldbetheSun,thatshonedownontheoceanandliftedwatervapourupintotheskyandmadeclouds.Okay,sowhatcausedtheSuntoshine?Well,ifwelookinsideweseetheprocessknownas fusion, inwhichhydrogen atoms join to formhelium, releasing vastquantities of energy in the process. So far so good. Where does thehydrogen come from?Answer: theBigBang.But here’s the crucial bit.The laws of nature itself tell us that not only could the universe havepopped into existence without any assistance, like a proton, and haverequired nothing in terms of energy, but also that it is possible that

nothingcausedtheBigBang.Nothing.TheexplanationliesbackwiththetheoriesofEinstein,andhisinsights

intohowspaceandtime intheuniverseare fundamentally intertwined.Something very wonderful happened to time at the instant of the BigBang.Timeitselfbegan.Tounderstandthismind-boggling idea,considerablackholefloating

inspace.Atypicalblackholeisastarsomassivethatithascollapsedinonitself.It’ssomassivethatnotevenlightcanescapeitsgravity,whichiswhy it’s almost perfectly black. It’s gravitational pull is so powerful, itwarps and distorts not only light but also time. To see how, imagine aclock is being sucked into it. As the clock gets closer and closer to theblackhole, itbeginstogetslowerandslower.Timeitselfbeginstoslowdown.Nowimaginetheclockasitenterstheblackhole—well,assumingof course that it could withstand the extreme gravitational forces—itwouldactuallystop.Itstopsnotbecauseitisbroken,butbecauseinsidetheblackholetimeitselfdoesn’texist.Andthat’sexactlywhathappenedatthestartoftheuniverse.In the lasthundredyears,wehavemadespectacularadvances inour

understandingoftheuniverse.Wenowknowthelawsthatgovernwhathappens in all but the most extreme conditions, like the origin of theuniverse,orblackholes.Theroleplayedbytimeatthebeginningoftheuniverse is, I believe, the final key to removing the need for a granddesignerandrevealinghowtheuniversecreateditself.As we travel back in time towards themoment of the Big Bang, the

universegetssmallerandsmallerandsmaller,untilitfinallycomestoapointwhere thewholeuniverse is a space so small that it is in effect asingleinfinitesimallysmall, infinitesimallydenseblackhole.Andjustaswithmodern-dayblackholes,floatingaroundinspace,thelawsofnaturedictate something quite extraordinary. They tell us that here too timeitselfmust come to a stop.You can’t get to a timebefore theBigBangbecause therewas no time before the Big Bang.We have finally foundsomething that doesn’t have a cause, because there was no time for acause to exist in. For me this means that there is no possibility of acreator,becausethereisnotimeforacreatortohaveexistedin.Peoplewantanswers to thebigquestions, likewhywearehere.They

don’texpecttheanswerstobeeasy,sotheyarepreparedtostruggleabit.

Whenpeople askme if aGod created theuniverse, I tell them that thequestionitselfmakesnosense.Timedidn’texistbeforetheBigBangsothere is no time for God to make the universe in. It’s like asking fordirections to the edge of the Earth—the Earth is a sphere that doesn’thaveanedge,solookingforitisafutileexercise.DoIhavefaith?Weareeachfreetobelievewhatwewant,andit’smy

viewthatthesimplestexplanationisthatthereisnoGod.Noonecreatedthe universe and no one directs our fate. This leadsme to a profoundrealisation:thereisprobablynoheavenandafterlifeeither.Ithinkbeliefinanafterlifeisjustwishfulthinking.Thereisnoreliableevidenceforit,anditfliesinthefaceofeverythingweknowinscience.Ithinkthatwhenwediewereturntodust.Butthere’sasenseinwhichweliveon,inourinfluence,andinourgenesthatwepassontoourchildren.Wehavethisonelifetoappreciatethegranddesignoftheuniverse,andforthatIamextremelygrateful.

HowdoesGod’sexistencefitintoyourunderstandingofthebeginningandtheendoftheuniverse?AndifGodwastoexistandyouhadthechancetomeethim,what

wouldyouaskhim?

Thequestionis,“IsthewaytheuniversebeganchosenbyGodforreasonswecan’tunderstand,orwasitdeterminedbyalawofscience?”Ibelievethesecond.Ifyoulike,youcancallthelawsofscience“God,”butitwouldn’tbeapersonalGodthatyouwouldmeetandputquestionsto.Although,ifthereweresuchaGod,IwouldliketoaskhoweverdidhethinkofanythingascomplicatedasM-theoryineleven

dimensions.

2

HOWDIDITALLBEGIN?

Hamletsaid,“Icouldbeboundedinanutshell,andcountmyselfakingof infinite space.” I thinkwhathemeantwas thatalthoughwehumansare very limited physically, particularly inmy own case, ourminds arefreetoexplorethewholeuniverse,andtoboldlygowhereevenStarTrekfears to tread. Is theuniverseactually infinite,or justvery large?Did ithaveabeginning?Will it last foreveror justa longtime?Howcanourfiniteminds comprehendan infiniteuniverse? Isn’t it pretentiousofuseventomaketheattempt?AttheriskofincurringthefateofPrometheus,whostolefirefromthe

ancient gods for human use, I believe we can, and should, try tounderstandtheuniverse.Prometheus’punishmentwasbeingchainedtoa rock for eternity, although happily he was eventually liberated byHercules.Wehavealreadymaderemarkableprogress inunderstandingthecosmos.Wedon’tyethaveacompletepicture.Iliketothinkwemaynotbefaroff.According to theBoshongopeople of centralAfrica, in the beginning

there was only darkness, water and the great god Bumba. One dayBumba, inpainfromstomachache,vomiteduptheSun.TheSundriedupsomeofthewater, leavingland.Still inpain,BumbavomiteduptheMoon, thestarsand thensomeanimals—the leopard, thecrocodile, theturtleand,finally,man.Thesecreationmyths,likemanyothers,trytoanswerthequestionswe

all ask. Why are we here? Where did we come from? The answergenerally given was that humans were of comparatively recent originbecauseitmusthavebeenobviousthatthehumanracewasimprovingits

knowledge and technology. So it can’t havebeenaround that longor itwould have progressed even more. For example, according to BishopUssher,theBookofGenesisplacedthebeginningoftimeonOctober22,4004 BCE at 6 p.m.On the other hand, the physical surroundings, likemountainsandrivers,changevery little inahumanlifetime.Theywerethereforethoughttobeaconstantbackground,andeithertohaveexistedforeverasanemptylandscape,ortohavebeencreatedatthesametimeasthehumans.Noteveryone,however,washappywiththeideathattheuniversehad

a beginning. For example, Aristotle, the most famous of the Greekphilosophers,believedthattheuniversehadexistedforever.Somethingeternalismoreperfectthansomethingcreated.Hesuggestedthereasonweseeprogresswasthatfloods,orothernaturaldisasters,hadrepeatedlysetcivilisationbacktothebeginning.Themotivationforbelievinginaneternaluniversewas thedesire toavoid invokingdivine intervention tocreatetheuniverseandsetitgoing.Conversely,thosewhobelievedthattheuniversehadabeginninguseditasanargumentfortheexistenceofGodasthefirstcause,orprimemover,oftheuniverse.If one believed that the universe had a beginning, the obvious

questions were, “What happened before the beginning?What was Goddoingbeforehemadetheworld?WashepreparingHellforpeoplewhoaskedsuchquestions?”Theproblemofwhetherornottheuniversehadabeginning was a great concern to the German philosopher ImmanuelKant.Hefelttherewerelogicalcontradictions,orantimonies,eitherway.Iftheuniversehadabeginning,whydiditwaitaninfinitetimebeforeitbegan?Hecalledthatthethesis.Ontheotherhand, if theuniversehadexisted for ever, why did it take an infinite time to reach the presentstage?He called that the antithesis. Both the thesis and the antithesisdepended on Kant’s assumption, along with almost everyone else, thattimewas absolute. That is to say, it went from the infinite past to theinfinite future independently of any universe that might or might notexist.Thisisstillthepictureinthemindofmanyscientiststoday.However,

in1915Einsteinintroducedhisrevolutionarygeneraltheoryofrelativity.In this, space and time were no longer absolute, no longer a fixedbackgroundtoevents.Instead,theyweredynamicalquantitiesthatwereshapedbythematterandenergyintheuniverse.Theyweredefinedonly

within the universe, so it made no sense to talk of a time before theuniverse began. It would be like asking for a point south of the SouthPole.Itisnotdefined.Although Einstein’s theory unified time and space, it didn’t tell us

much about space itself. Something that seems obvious about space isthat itgoesonandonandon.Wedon’texpecttheuniversetoendinabrickwall,althoughthere’snologicalreasonwhyitcouldn’t.ButmoderninstrumentsliketheHubblespacetelescopeallowustoprobedeepintospace.Whatweseeisbillionsandbillionsofgalaxies,ofvariousshapesandsizes.Therearegiantellipticalgalaxies,andspiralgalaxies likeourown.Each galaxy containsbillions andbillions of stars,manyofwhichwillhaveplanetsroundthem.Ourowngalaxyblocksourviewincertaindirections, but apart from that the galaxies are distributed roughlyuniformly throughout space,with some local concentrations and voids.The density of galaxies appears to drop off at very large distances, butthatseemstobebecausetheyaresofarawayandfaintthatwecan’tmakethemout.Asfaraswecantell,theuniversegoesoninspaceforeverandismuchthesamenomatterhowfaritgoeson.Althoughtheuniverseseemstobemuchthesameateachpositionin

space, it is definitely changing in time. This was not realised until theearlyyearsofthelastcentury.Uptothen,itwasthoughttheuniversewasessentiallyconstantintime.Itmighthaveexistedforaninfinitetime,butthatseemedtoleadtoabsurdconclusions.Ifstarshadbeenradiatingforaninfinitetime,theywouldhaveheateduptheuniverseuntilitreachedtheirowntemperature.Evenatnight,thewholeskywouldbeasbrightastheSun,becauseeverylineofsightwouldhaveendedeitheronastaroronacloudofdustthathadbeenheatedupuntilitwasashotasthestars.Sotheobservationthatwehaveallmade,thattheskyatnightisdark,isveryimportant.Itimpliesthattheuniversecannothaveexistedforever,inthestateweseetoday.Somethingmusthavehappenedinthepasttomakethestarsturnonafinitetimeago.Thenthelightfromverydistantstarswouldn’thavehadtimetoreachusyet.Thiswouldexplainwhytheskyatnightisn’tglowingineverydirection.Ifthestarshadjustbeensittingthereforever,whydidtheysuddenly

lightupafewbillionyearsago?Whatwastheclockthattoldthemitwastimetoshine?Thispuzzledthosephilosophers,likeImmanuelKant,whobelievedthattheuniversehadexistedforever.Butformostpeopleitwas

consistentwiththeideathattheuniversehadbeencreated,muchasitisnow,onlyafewthousandyearsago,justasBishopUssherhadconcluded.However,discrepanciesinthisideabegantoappear,withobservationsbythe hundred-inch telescope onMountWilson in the 1920s. First of all,EdwinHubblediscoveredthatmanyfaintpatchesoflight,callednebulae,wereinfactothergalaxies,vastcollectionsofstarslikeourSun,butatagreat distance. In order for them to appear so small and faint, thedistances had to be so great that light from them would have takenmillions or even billions of years to reach us. This indicated that thebeginningof theuniverse couldn’thavebeen just a few thousandyearsago.ButthesecondthingHubblediscoveredwasevenmoreremarkable.By

ananalysisofthelightfromothergalaxies,Hubblewasabletomeasurewhethertheyweremovingtowardsusoraway.Tohisgreatsurprise,hefoundtheywerenearlyallmovingaway.Moreover,thefurthertheywerefromus,thefastertheyweremovingaway.Inotherwords,theuniverseisexpanding.Galaxiesaremovingawayfromeachother.The discovery of the expansion of the universe was one of the great

intellectual revolutions of the twentieth century. It came as a totalsurprise, and it completely changed the discussion of the origin of theuniverse. If the galaxies aremoving apart, theymust have been closertogetherinthepast.Fromthepresentrateofexpansion,wecanestimatethat they must have been very close together indeed, about 10 to 15billionyears ago.So it looks as though theuniversemighthave startedthen,witheverythingbeingatthesamepointinspace.But many scientists were unhappy with the universe having a

beginning, because it seemed to imply that physics broke down. Onewouldhavetoinvokeanoutsideagency,whichforconvenienceonecancallGod,todeterminehowtheuniversebegan.Theythereforeadvancedtheories in which the universe was expanding at the present time, butdidn’t have a beginning. One of these was the steady-state theory,proposedbyHermannBondi,ThomasGoldandFredHoylein1948.In thesteady-state theory,asgalaxiesmovedapart, the ideawas that

newgalaxieswouldformfrommatterthatwassupposedtobecontinuallybeing created throughout space. The universe would have existed forever,andwouldhavelookedthesameatalltimes.Thislastpropertyhad

the great virtue of being a definite prediction that could be tested byobservation.TheCambridgeradioastronomygroup,underMartinRyle,did a survey of weak sources of radio waves in the early 1960s. Theseweredistributed fairlyuniformlyacross the sky, indicating thatmostofthesourceslayoutsideourgalaxy.Theweakersourceswouldbefurtheraway,onaverage.Thesteady-statetheorypredictedarelationshipbetweenthenumberof

sources and their strength. But the observations showed more faintsources than predicted, indicating that the density of the sources washigher in the past. This was contrary to the basic assumption of thesteady-state theory, that everything was constant in time. For this andotherreasons,thesteady-statetheorywasabandoned.Another attempt to avoid the universe having a beginning was the

suggestion that therewas a previous contracting phase, but because ofrotationandlocalirregularitiesthematterwouldnotallfalltothesamepoint. Instead,differentpartsof thematterwouldmisseachother,andthe universe would expand again with the density always remainingfinite. Two Russians, Evgeny Lifshitz and Isaak Khalatnikov, actuallyclaimed to have proved that a general contraction without exactsymmetry would always lead to a bounce, with the density remainingfinite. This result was very convenient for Marxist–Leninist dialecticalmaterialism,becauseitavoidedawkwardquestionsaboutthecreationoftheuniverse.ItthereforebecameanarticleoffaithforSovietscientists.IbeganmyresearchincosmologyjustaboutthetimethatLifshitzand

Khalatnikov published their conclusion that the universe didn’t have abeginning.Irealisedthatthiswasaveryimportantquestion,butIwasn’tconvincedbytheargumentsthatLifshitzandKhalatnikovhadused.Weareusedtotheideathateventsarecausedbyearlierevents,which

in turn are caused by still earlier events. There is a chain of causality,stretching back into the past. But suppose this chain has a beginning,supposetherewasafirstevent.Whatcausedit?Thiswasnotaquestionthatmanyscientistswanted toaddress.They tried toavoid it,eitherbyclaimingliketheRussiansandthesteady-statetheoriststhattheuniversedidn’thaveabeginningorbymaintainingthattheoriginoftheuniversedid not lie within the realm of science but belonged tometaphysics orreligion. Inmy opinion, this is not a position any true scientist should

take.Ifthelawsofsciencearesuspendedatthebeginningoftheuniverse,mightnottheyalsofailatothertimes?Alawisnotalawifitonlyholdssometimes. I believe thatwe should try tounderstand thebeginningoftheuniverseonthebasisofscience.Itmaybeataskbeyondourpowers,butatleastweshouldmaketheattempt.RogerPenroseandImanagedtoprovegeometricaltheoremstoshow

thattheuniversemusthavehadabeginningifEinstein’sgeneraltheoryofrelativitywascorrect,andcertainreasonableconditionsweresatisfied.Itisdifficulttoarguewithamathematicaltheorem,sointheendLifshitzand Khalatnikov conceded that the universe should have a beginning.Although the idea of a beginning to the universe might not be verywelcometocommunistideas,ideologywasneverallowedtostandinthewayof science inphysics.Physicswasneeded for thebomb,and itwasimportantthatitworked.However,Sovietideologydidpreventprogressinbiologybydenyingthetruthofgenetics.Although the theoremsRoger Penrose and I proved showed that the

universemusthavehadabeginning,theydidnotgivemuchinformationabout the nature of that beginning. They indicated that the universebeganinaBigBang,apointwherethewholeuniverseandeverythinginitwerescrunchedupintoasinglepointofinfinitedensity,aspace–timesingularity.AtthispointEinstein’sgeneraltheoryofrelativitywouldhavebroken down. Thus one cannot use it to predict in what manner theuniverse began. One is left with the origin of the universe apparentlybeingbeyondthescopeofscience.Observationalevidencetoconfirmtheideathattheuniversehadavery

dense beginning came in October 1965, a few months after my firstsingularityresult,withthediscoveryofafaintbackgroundofmicrowavesthroughout space. These microwaves are the same as those in yourmicrowave oven, but very much less powerful. They would heat yourpizza only to minus 270.4 degrees centigrade (minus 518.72 degreesFahrenheit),notmuchgoodfordefrostingthepizza,letalonecookingit.You can actually observe thesemicrowaves yourself. Those of youwhoremember analogue televisions have almost certainly observed thesemicrowaves.Ifyoueversetyourtelevisiontoanemptychannel,afewpercentofthesnowyousawonthescreenwascausedbythisbackgroundofmicrowaves.Theonlyreasonableinterpretationofthebackgroundisthatit is radiation left over from an early very hot and dense state. As the

universe expanded, the radiation would have cooled until it is just thefaintremnantweobservetoday.ThattheuniversebeganwithasingularitywasnotanideathatIora

numberofotherpeoplewerehappywith.ThereasonEinstein’sgeneralrelativity breaks down near the Big Bang is that it is what is called aclassical theory.That is, it implicitlyassumedwhatseemsobvious fromcommonsense,thateachparticlehadawell-definedpositionandawell-defined speed. In such a so-called classical theory, if one knows thepositionsandspeedsofalltheparticlesintheuniverseatonetime,onecancalculatewhattheywouldbeatanyothertime,inthepastorfuture.However, in the early twentieth century scientists discovered that theycouldn’tcalculateexactlywhatwouldhappenoververyshortdistances.Itwasn’tjustthattheyneededbettertheories.Thereseemstobeacertainlevel of randomness or uncertainty in nature that cannot be removedhowever good our theories. It can be summed up in the UncertaintyPrinciple that was proposed in 1927 by the German scientist WernerHeisenberg. One cannot accurately predict both the position and thespeedofaparticle.Themoreaccuratelythepositionispredicted,thelessaccuratelyyouwillbeabletopredictthespeed,andviceversa.Einsteinobjectedstronglytotheideathattheuniverseisgovernedby

chance.His feelingsweresummedup inhisdictum“Goddoesnotplaydice.”ButalltheevidenceisthatGodisquiteagambler.Theuniverseislikeagiantcasinowithdicebeingrolled,orwheelsbeingspun,oneveryoccasion.Acasinoownerriskslosingmoneyeachtimedicearethrownorthe roulette wheel is spun. But over a large number of bets the oddsaverageout,andthecasinoownermakessuretheyaverageoutinhisorher favour. That’swhy casino owners are so rich. The only chance youhaveofwinningagainstthemistostakeallyourmoneyonafewrollsofthediceorspinsofthewheel.Itisthesamewiththeuniverse.Whentheuniverseisbig,therearea

very large number of rolls of the dice, and the results average out tosomethingonecanpredict.Butwhentheuniverseisverysmall,neartheBig Bang, there are only a small number of rolls of the dice, and theUncertaintyPrincipleisveryimportant.Inordertounderstandtheoriginof the universe, one therefore has to incorporate the UncertaintyPrinciple into Einstein’s general theory of relativity. This has been thegreatchallengeintheoreticalphysicsforatleastthelastthirtyyears.We

haven’tsolvedityet,butwehavemadealotofprogress.Nowsupposewetrytopredictthefuture.Becauseweonlyknowsome

combinationofpositionandspeedofaparticle,wecannotmakeprecisepredictions about the future positions and speeds of particles.We canonly assign a probability to particular combinations of positions andspeeds. Thus there is a certain probability to a particular future of theuniverse.Butnowsupposewetrytounderstandthepastinthesameway.Giventhenatureoftheobservationswecanmakenow,allwecandois

assign a probability to a particular history of the universe. Thus theuniverse must have many possible histories, each with its ownprobability.There isahistoryof theuniverse inwhichEnglandwintheWorldCupagain,thoughmaybetheprobabilityislow.Thisideathattheuniverse hasmultiple historiesmay sound like science fiction, but it isnowacceptedassciencefact.ItisduetoRichardFeynman,whoworkedat the eminently respectable California Institute of Technology andplayedthebongodrumsinastripjointuptheroad.Feynman’sapproachtounderstandinghowthingsworksistoassigntoeachpossiblehistoryaparticular probability, and then use this idea to make predictions. Itworksspectacularlywelltopredictthefuture.Sowepresumeitworkstoretrodictthepasttoo.Scientists are now working to combine Einstein’s general theory of

relativity and Feynman’s idea of multiple histories into a completeunifiedtheorythatwilldescribeeverythingthathappensintheuniverse.This unified theory will enable us to calculate how the universe willevolve,ifweknowitsstateatonetime.Buttheunifiedtheorywillnotinitselftellushowtheuniversebegan,orwhatitsinitialstatewas.Forthat,we need something extra. We require what are known as boundaryconditions, things that tell us what happens at the frontiers of theuniverse,theedgesofspaceandtime.Butifthefrontieroftheuniversewasjustatanormalpointofspaceandtimewecouldgopastitandclaimthe territory beyond as part of the universe. On the other hand, if theboundaryoftheuniversewasatajaggededgewherespaceortimewerescrunchedup, and thedensitywas infinite, itwouldbe verydifficult todefinemeaningfulboundaryconditions.Soitisnotclearwhatboundaryconditionsareneeded.Itseemsthere isno logicalbasis forpickingonesetofboundaryconditionsoveranother.

However, Jim Hartle of the University of California, Santa Barbara,and I realised therewas a third possibility.Maybe the universe has noboundary in space and time. At first sight, this seems to be in directcontradictiontothegeometricaltheoremsthatImentionedearlier.Theseshowedthattheuniversemusthavehadabeginning,aboundaryintime.However, in order to make Feynman’s techniques mathematically welldefined,themathematiciansdevelopedaconceptcalledimaginarytime.It isn’t anything to do with the real time that we experience. It is amathematicaltricktomakethecalculationsworkanditreplacestherealtimeweexperience.Our ideawas to say that therewasnoboundary inimaginarytime.Thatdidawaywithtryingtoinventboundaryconditions.Wecalledthistheno-boundaryproposal.Iftheboundaryconditionoftheuniverseisthatithasnoboundaryin

imaginary time, it won’t have just a single history. There are manyhistoriesinimaginarytimeandeachofthemwilldetermineahistoryinreal time.Thuswehaveasuperabundanceofhistories for theuniverse.Whatpicksout theparticularhistory,or setofhistories thatwe live in,fromthesetofallpossiblehistoriesoftheuniverse?One point that we can quickly notice is that many of these possible

histories of the universe won’t go through the sequence of forminggalaxiesandstars,somethingthatwasessentialtoourowndevelopment.Itmaybethatintelligentbeingscanevolvewithoutgalaxiesandstars,butitseemsunlikely.Thustheveryfactthatweexistasbeingsthatcanaskthequestion “Why is theuniverse theway it is?” is a restrictionon thehistorywelivein.Itimpliesitisoneoftheminorityofhistoriesthathavegalaxies and stars. This is an example of what is called the AnthropicPrinciple.TheAnthropicPrinciplesaysthattheuniversehastobemoreorlessasweseeit,becauseifitweredifferenttherewouldn’tbeanyoneheretoobserveit.ManyscientistsdisliketheAnthropicPrinciple,becauseitseemslittle

morethanhandwaving,andnottohavemuchpredictivepower.ButtheAnthropicPrinciplecanbegivenapreciseformulation,anditseemstobeessentialwhendealingwiththeoriginoftheuniverse.M-theory,whichisour best candidate for a complete unified theory, allows a very largenumberofpossiblehistoriesfortheuniverse.Mostofthesehistoriesarequite unsuitable for the development of intelligent life. Either they areempty,ortooshortlasting,ortoohighlycurved,orwronginsomeother

way.Yet,according toRichardFeynman’smultiple-histories idea, theseuninhabitedhistoriesmighthavequiteahighprobability.We really don’t care how many histories there may be that don’t

contain intelligent beings. We are interested only in the subset ofhistoriesinwhichintelligentlifedevelops.Thisintelligentlifeneednotbeanything like humans. Little greenmenwould do aswell. In fact, theymightdoratherbetter.Thehumanracedoesnothaveaverygoodrecordofintelligentbehaviour.As an example of the power of the Anthropic Principle, consider the

numberofdirectionsinspace.Itisamatterofcommonexperiencethatwe live in three-dimensionalspace.That is tosay,wecanrepresent theposition of a point in space by three numbers. For example, latitude,longitude and height above sea level. But why is space three-dimensional? Why isn’t it two, or four, or some other number ofdimensions, like in science fiction? In fact, in M-theory space has tendimensions(aswellasthetheoryhavingonedimensionoftime),butitisthoughtthatsevenofthetenspatialdirectionsarecurledupverysmall,leavingthreedirectionsthatarelargeandnearlyflat.Itislikeadrinkingstraw.Thesurfaceofastrawistwo-dimensional.However,onedirectioniscurledupintoasmallcircle,sothatfromadistancethestrawlookslikeaone-dimensionalline.Why don’t we live in a history in which eight of the dimensions are

curled up small, leaving only two dimensions that we notice? A two-dimensionalanimalwouldhaveahardjobdigestingfood.Ifithadagutthatwentrightthrough,likewehave,itwoulddividetheanimalintwo,andthepoorcreaturewouldfallapart.Sotwoflatdirectionsarenot enough for anything as complicated as intelligent life. There issomething special about three space dimensions. In three dimensions,planets can have stable orbits around stars. This is a consequence ofgravitation obeying the inverse square law, as discovered by RobertHooke in 1665 and elaborated on by Isaac Newton. Think about thegravitational attraction of two bodies at a particular distance. If thatdistanceisdoubled,thentheforcebetweenthemisdividedbyfour.Ifthedistance is tripled thenthe force isdividedbynine, ifquadrupled, thenthe force isdividedby sixteenandsoon.This leads to stableplanetaryorbits. Now let’s think about four space dimensions. There gravitation

would obey an inverse cube law. If the distance between two bodies isdoubled,thenthegravitationalforcewouldbedividedbyeight,tripledbytwenty-sevenandifquadrupled,bysixty-four.Thischangetoaninversecube law prevents planets fromhaving stable orbits around their suns.Theywouldeitherfallintotheirsunorescapetotheouterdarknessandcold. Similarly, the orbits of electrons in atomswouldnot be stable, somatter as we know it would not exist. Thus although the multiple-histories idea would allow any number of nearly flat directions, onlyhistorieswiththreeflatdirectionswillcontainintelligentbeings.Onlyinsuch histories will the question be asked, “Why does space have threedimensions?”One remarkable feature of the universe we observe concerns the

microwavebackgrounddiscoveredbyArnoPenziasandRobertWilson.Itis essentially a fossil record of how the universewaswhen very young.Thisbackgroundisalmostthesameindependentlyofwhichdirectiononelooksin.Thedifferencesbetweendifferentdirectionsisaboutonepartin100,000.Thesedifferencesare incrediblytinyandneedanexplanation.Thegenerallyacceptedexplanationforthissmoothnessisthatveryearlyin the history of the universe it underwent a period of very rapidexpansion,bya factorofat leastabillionbillionbillion.Thisprocess isknownasinflation,somethingthatwasgoodfortheuniverseincontrasttoinflationofpricesthattoooftenplaguesus.Ifthatwasalltherewastoit,themicrowaveradiationwouldbetotallythesameinalldirections.Sowheredidthesmalldiscrepanciescomefrom?In early 1982, Iwrote a paper proposing that these differences arose

fromthequantumfluctuationsduringthe inflationaryperiod.Quantumfluctuations occur as a consequence of the Uncertainty Principle.Furthermore, these fluctuations were the seeds for structures in ouruniverse: galaxies, stars and us. This idea is basically the samemechanism as so-called Hawking radiation from a black hole horizon,whichIhadpredictedadecadeearlier,exceptthatnowitcomesfromacosmologicalhorizon, thesurface thatdivided theuniversebetween theparts thatwe can see and the parts thatwe cannot observe.Weheld aworkshop inCambridgethatsummer,attendedbyall themajorplayersinthefield.Atthismeeting,weestablishedmostofourpresentpictureofinflation,includingtheall-importantdensityfluctuations,whichgiverisetogalaxyformationandsotoourexistence.Severalpeoplecontributedto

thefinalanswer.Thiswastenyearsbeforefluctuationsinthemicrowavesky were discovered by the COBE satellite in 1993, so theory was wayaheadofexperiment.Cosmologybecameaprecisionscienceanothertenyearslater,in2003,

with the first results from the WMAP satellite. WMAP produced awonderful map of the temperature of the cosmic microwave sky, asnapshotoftheuniverseataboutone-hundredthof itspresentage.Theirregularitiesyouseearepredictedbyinflation,andtheymeanthatsomeregions of the universe had a slightly higher density than others. Thegravitational attraction of the extra density slows the expansion of thatregion,andcaneventuallycauseittocollapsetoformgalaxiesandstars.Solookcarefullyatthemapofthemicrowavesky.Itistheblueprintforall the structure in the universe. We are the product of quantumfluctuationsintheveryearlyuniverse.Godreallydoesplaydice.SupersedingWMAP, today there is the Planck satellite, with amuch

higher-resolutionmapof theuniverse. Planck is testing our theories inearnest,andmayevendetecttheimprintofgravitationalwavespredictedbyinflation.Thiswouldbequantumgravitywrittenacrossthesky.There may be other universes. M-theory predicts that a great many

universes were created out of nothing, corresponding to the manydifferent possible histories. Each universe has many possible historiesandmanypossiblestatesastheyagetothepresentandbeyondintothefuture.Mostofthesestateswillbequiteunliketheuniverseweobserve.There is still hope that we see the first evidence forM-theory at the

LHCparticleaccelerator,theLargeHadronCollider,atCERNinGeneva.FromanM-theoryperspective,itonlyprobeslowenergies,butwemightbe lucky and see a weaker signal of fundamental theory, such assupersymmetry.Ithinkthediscoveryofsupersymmetricpartnersfortheknownparticleswouldrevolutioniseourunderstandingoftheuniverse.In 2012, the discovery of theHiggs particle by the LHC at CERN in

Genevawasannounced.Thiswasthefirstdiscoveryofanewelementaryparticleinthetwenty-firstcentury.ThereisstillsomehopethattheLHCwilldiscoversupersymmetry.ButeveniftheLHCdoesnotdiscoveranynewelementaryparticles,supersymmetrymightstillbefoundinthenextgenerationofacceleratorsthatarepresentlybeingplanned.ThebeginningoftheuniverseitselfintheHotBigBangistheultimate

high-energy laboratory for testing M-theory, and our ideas about thebuildingblocksofspace–timeandmatter.Differenttheoriesleavebehinddifferent fingerprints in the current structure of the universe, soastrophysicaldatacangiveuscluesabouttheunificationofalltheforcesofnature.Sotheremaywellbeotheruniverses,butunfortunatelywewillneverbeabletoexplorethem.We have seen something about the origin of the universe. But that

leavestwobigquestions.Willtheuniverseend?Istheuniverseunique?Whatthenwillbethefuturebehaviourofthemostprobablehistories

of the universe? There seem to be various possibilities, which arecompatiblewiththeappearanceofintelligentbeings.Theydependontheamountofmatterintheuniverse.Ifthereismorethanacertaincriticalamount,thegravitationalattractionbetweenthegalaxieswillslowdowntheexpansion.Eventually theywill then start falling towards eachother andwill all

cometogetherinaBigCrunch.Thatwillbetheendofthehistoryoftheuniverse, in real time.When Iwas in the Far East, I was asked not tomention the Big Crunch, because of the effect it might have on themarket.Butthemarketscrashed,somaybethestorygotoutsomehow.InBritain,peopledon’tseemtooworriedaboutapossibleendtwentybillionyears inthe future.Youcandoquitea lotofeating,drinkingandbeingmerrybeforethat.If thedensityof theuniverse isbelowthecriticalvalue,gravity is too

weaktostopthegalaxiesflyingapartforever.Allthestarswillburnout,andtheuniversewillgetemptierandemptier,andcolderandcolder.So,again, thingswill come to an end, but in a less dramaticway. Still,wehaveafewbillionyearsinhand.Inthisanswer,Ihavetriedtoexplainsomethingoftheorigins,future

andnatureofouruniverse.Theuniverseinthepastwassmallanddenseand so it is quite like the nutshell with which I began. Yet this nutencodeseverythingthathappensinrealtime.SoHamletwasquiteright.Wecouldbeboundedinanutshellandcountourselveskingsofinfinitespace.

WhatcamebeforetheBigBang?

Accordingtotheno-boundaryproposal,askingwhatcamebeforetheBigBangismeaningless—likeaskingwhatis

southoftheSouthPole—becausethereisnonotionoftimeavailabletoreferto.Theconceptoftimeonlyexistswithin

ouruniverse.

3

ISTHEREOTHERINTELLIGENTLIFEINTHEUNIVERSE?

I would like to speculate a little on the development of life in theuniverse,andinparticularonthedevelopmentof intelligent life.Ishalltakethisto includethehumanrace,eventhoughmuchof itsbehaviourthroughouthistoryhasbeenpretty stupidandnot calculated toaid thesurvival of the species. Two questions I shall discuss are “What is theprobabilityoflifeexistingelsewhereintheuniverse?”and“Howmaylifedevelopinthefuture?”It is amatter of commonexperience that things getmoredisordered

and chaotic with time. This observation even has its own law, the so-calledsecondlawofthermodynamics.Thislawsaysthatthetotalamountof disorder, or entropy, in the universe always increases with time.However,thelawrefersonlytothetotalamountofdisorder.Theorderinone body can increase provided that the amount of disorder in itssurroundingsincreasesbyagreateramount.Thisiswhathappensinalivingbeing.Wecandefinelifeasanordered

systemthatcankeepitselfgoingagainstthetendencytodisorderandcanreproduce itself. That is, it canmake similar, but independent, orderedsystems. To do these things, the system must convert energy in someordered form—like food, sunlight or electric power—into disorderedenergy, in the form of heat. In this way, the system can satisfy therequirement that the total amount of disorder increases while, at thesame time, increasing the order in itself and its offspring. This soundslikeparents living inahousewhichgetsmessierandmessiereachtimetheyhaveanewbaby.A living being like you or me usually has two elements: a set of

instructionsthattellthesystemhowtokeepgoingandhowtoreproduceitself, and amechanism to carry out the instructions. In biology, thesetwopartsarecalledgenesandmetabolism.But it isworthemphasisingthat there need be nothing bio-logical about them. For example, acomputervirusisaprogramthatwillmakecopiesofitselfinthememoryofacomputer,andwilltransferitselftoothercomputers.ThusitfitsthedefinitionofalivingsystemthatIhavegiven.Likeabiologicalvirus,itisaratherdegenerateform,becauseitcontainsonlyinstructionsorgenes,anddoesn’thaveanymetabolismof itsown. Instead, it reprograms themetabolismof thehostcomputer,orcell.Somepeoplehavequestionedwhether viruses should count as life, because they are parasites, andcannot exist independently of their hosts. But thenmost forms of life,ourselves included, are parasites, in that they feed off and depend fortheir survival on other forms of life. I think computer viruses shouldcountaslife.Maybeitsayssomethingabouthumannaturethattheonlyform of life we have created so far is purely destructive. Talk aboutcreating life in our own image. I shall return to electronic formsof lifelateron.Whatwenormallythinkofas“life”isbasedonchainsofcarbonatoms,

with a few other atoms such as nitrogen or phosphorus. One canspeculate thatonemighthave lifewithsomeotherchemicalbasis,suchassilicon,butcarbonseemsthemostfavourablecase,becauseithastherichest chemistry. That carbon atoms should exist at all, with theproperties that they have, requires a fine adjustment of physicalconstants, such as the QCD scale, the electric charge and even thedimension of space–time. If these constants had significantly differentvalues,eitherthenucleusofthecarbonatomwouldnotbestableortheelectrons would collapse in on the nucleus. At first sight, it seemsremarkable that the universe is so finely tuned.Maybe this is evidencethat the universe was specially designed to produce the human race.However, one has to be careful about such arguments, because of theAnthropicPrinciple,theideathatourtheoriesabouttheuniversemustbecompatiblewithourownexistence.Thisisbasedontheself-evidenttruththat if theuniversehadnotbeensuitable for lifewewouldn’tbeaskingwhy it is so finely adjusted. One can apply the Anthropic Principle ineither its Strong orWeak versions. For the StrongAnthropic Principle,onesupposesthattherearemanydifferentuniverses,eachwithdifferent

valuesofthephysicalconstants.Inasmallnumber,thevalueswillallowtheexistenceofobjectslikecarbonatoms,whichcanactasthebuildingblocksoflivingsystems.Sincewemustliveinoneoftheseuniverses,weshould not be surprised that the physical constants are finely tuned. Ifthey weren’t, we wouldn’t be here. The Strong form of the AnthropicPrincipleisthusnotverysatisfactory,becausewhatoperationalmeaningcanonegivetotheexistenceofallthoseotheruniverses?Andiftheyareseparate fromour ownuniverse, how canwhat happens in them affectouruniverse?Instead,IshalladoptwhatisknownastheWeakAnthropicPrinciple. That is, I shall take the values of the physical constants asgiven.But I shall seewhatconclusionscanbedrawn fromthe fact thatlifeexistsonthisplanetatthisstageinthehistoryoftheuniverse.TherewasnocarbonwhentheuniversebeganintheBigBang,about

13.8billionyearsago.Itwassohotthatallthematterwouldhavebeeninthe formofparticles calledprotonsandneutrons.Therewould initiallyhave been equal numbers of protons and neutrons. However, as theuniverse expanded, it cooled. About a minute after the Big Bang, thetemperature would have fallen to about a billion degrees, about ahundredtimesthetemperatureintheSun.Atthistemperature,neutronsstarttodecayintomoreprotons.If thishadbeenall thathadhappened,all thematter in theuniverse

wouldhaveendedupasthesimplestelement,hydrogen,whosenucleusconsistsofasingleproton.However,someoftheneutronscollidedwithprotons and stuck together to form the next simplest element, helium,whosenucleusconsistsoftwoprotonsandtwoneutrons.Butnoheavierelements, like carbon or oxygen, would have been formed in the earlyuniverse.Itisdifficulttoimaginethatonecouldbuildalivingsystemoutofjusthydrogenandhelium—andanywaytheearlyuniversewasstillfartoohotforatomstocombineintomolecules.The universe continued to expand and cool. But some regions had

slightly higher densities than others and the gravitational attraction ofthe extra matter in those regions slowed down their expansion, andeventuallystoppedit.Instead,theycollapsedtoformgalaxiesandstars,startingfromabouttwobillionyearsaftertheBigBang.SomeoftheearlystarswouldhavebeenmoremassivethanourSun;theywouldhavebeenhotter than the Sun andwould have burned the original hydrogen andhelium into heavier elements, such as carbon, oxygen and iron. This

couldhavetakenonlyafewhundredmillionyears.Afterthat,someofthestarsexplodedassupernovaeandscatteredtheheavyelementsbackintospace,toformtherawmaterialforlatergenerationsofstars.Otherstarsaretoofarawayforustobeabletoseedirectlyiftheyhave

planetsgoingroundthem.However, therearetwotechniquesthathaveenabledustodiscoverplanetsaroundotherstars.Thefirst is to lookatthe star and see if the amount of light coming from it is constant. If aplanetmoves in frontof thestar, the light fromthestarwillbe slightlyobscured. The star will dim a little bit. If this happens regularly, it isbecause a planet’s orbit is taking it in front of the star repeatedly. Asecond method is to measure the position of the star accurately. If aplanetisorbitingthestar,itwillinduceasmallwobbleinthepositionofthe star.This canbeobservedandagain, if it is a regularwobble, thenone deduces that it is due to a planet in orbit around the star. Thesemethods were first applied about twenty years ago and by now a fewthousand planets have been discovered orbiting distant stars. It isestimated that one star in five has an Earth-like planet orbiting it at adistancefromthestartobecompatiblewithlifeasweknowit.Ourownsolarsystemwasformedaboutfourandahalfbillionyearsago,oralittlemore thanninebillionyearsafter theBigBang, fromgascontaminatedwiththeremainsofearlierstars.TheEarthwasformedlargelyoutoftheheavierelements,includingcarbonandoxygen.Somehow,someoftheseatomscametobearrangedintheformofmoleculesofDNA.Thishasthefamousdouble-helixform,discoveredinthe1950sbyFrancisCrickandJamesWatsoninahutontheNewMuseumsite inCambridge.Linkingthetwochainsinthehelixarepairsofnitrogenousbases.Therearefourtypesofnitrogenousbases—adenine,cytosine,guanineandthymine.Anadenine on one chain is always matched with a thymine on the otherchain,andaguaninewithacytosine.Thus thesequenceofnitrogenousbases on one chain defines a unique, complementary sequence on theotherchain.Thetwochainscanthenseparateandeachactsasatemplatetobuild furtherchains.ThusDNAmolecules can reproduce thegeneticinformation coded in their sequences of nitrogenous bases. Sections ofthe sequence can also be used to make proteins and other chemicals,which can carry out the instructions, coded in the sequence, andassembletherawmaterialforDNAtoreproduceitself.AsIsaidearlier,wedonotknowhowDNAmolecules firstappeared.

As the chances against aDNAmolecule arisingby random fluctuationsareverysmall,somepeoplehavesuggestedthatlifecametoEarthfromelsewhere—for instance, brought here on rocks breaking off fromMarswhile the planets were still unstable—and that there are seeds of lifefloatingroundinthegalaxy.However,itseemsunlikelythatDNAcouldsurviveforlongintheradiationinspace.Iftheappearanceoflifeonagivenplanetwasveryunlikely,onemight

have expected it to take a long time. More precisely, one might haveexpectedlifetoappearaslateaspossiblewhilestillallowingtimeforthesubsequentevolutiontointelligentbeings, likeus,beforetheSunswellsupandengulfstheEarth.Thetimewindowinwhichthiscouldoccuristhe lifetime of the Sun—about ten billion years. In that time, anintelligentformoflifecouldconceivablymasterspacetravelandbeabletoescapetoanotherstar.Butifnoescapeispossible,lifeonEarthwouldbedoomed.ThereisfossilevidencethattherewassomeformoflifeonEarthabout

threeandahalfbillionyearsago.Thismayhavebeenonly500millionyearsafter theEarthbecamestableandcoolenoughfor life todevelop.But life couldhave taken sevenbillion years todevelop in theuniverseandstillhavelefttimetoevolvetobeingslikeus,whocouldaskabouttheoriginoflife.Iftheprobabilityoflifedevelopingonagivenplanetisverysmall,why did it happen onEarth in about one-fourteenth of the timeavailable?The early appearance of life on Earth suggests that there is a good

chance of the spontaneous generation of life in suitable conditions.MaybetherewassomesimplerformoforganisationwhichbuiltupDNA.OnceDNAappeared,itwouldhavebeensosuccessfulthatitmighthavecompletelyreplacedtheearlierforms.Wedon’tknowwhattheseearlierformswouldhavebeen,butonepossibilityisRNA.RNA is like DNA, but rather simpler, and without the double-helix

structure. Short lengths of RNA could reproduce themselves likeDNA,andmight eventually build up to DNA.We cannotmake these nucleicacids in the laboratory fromnon-livingmaterial.But given 500millionyears,andoceanscoveringmostoftheEarth,theremightbeareasonableprobabilityofRNAbeingmadebychance.AsDNAreproduceditself,therewouldhavebeenrandomerrors,many

ofwhichwouldhavebeenharmfulandwouldhavediedout.Somewouldhavebeenneutral—theywouldnothaveaffectedthefunctionofthegene.Andafewerrorswouldhavebeenfavourabletothesurvivalofthespecies—thesewouldhavebeenchosenbyDarwiniannaturalselection.Theprocessofbiologicalevolutionwasveryslowatfirst.Ittookabout

two and ahalf billion years before the earliest cells evolved intomulti-cellularorganisms.Butittooklessthananotherbillionyearsforsomeofthesetoevolveintofish,andforsomeofthefish,inturn,toevolveintomammals.Thenevolutionseemstohavespeededupevenmore.Ittookonlyaboutahundredmillionyearstodevelopfromtheearlymammalstous.Thereasonisthattheearlymammalsalreadycontainedtheirversionsoftheessentialorganswehave.Allthatwasrequiredtoevolvefromearlymammalstohumanswasabitoffine-tuning.Butwiththehumanraceevolutionreachedacriticalstage,comparable

inimportancewiththedevelopmentofDNA.Thiswasthedevelopmentoflanguage,andparticularlywrittenlanguage.Itmeantthatinformationcouldbepassedonfromgenerationtogeneration,otherthangeneticallythroughDNA.Therehas been somedetectable change inhumanDNA,brought about by biological evolution, in the 10,000 years of recordedhistory, but the amount of knowledge handed on from generation togeneration has grown enormously. I have written books to tell yousomethingofwhatIhavelearnedabouttheuniverseinmylongcareerasascientist,andindoingsoIamtransferringknowledgefrommybraintothepagesoyoucanreadit.TheDNA in ahumaneggor spermcontains about threebillionbase

pairsofnitrogenousbases.However,muchof the informationcoded inthissequenceseemstoberedundantorisinactive.Sothetotalamountofuseful information in our genes is probably something like a hundredmillionbits.Onebitofinformationistheanswertoayes/noquestion.Bycontrast, a paperback novel might contain two million bits ofinformation.Therefore,ahumanisequivalenttoaboutfiftyHarryPotterbooks,andamajornationallibrarycancontainaboutfivemillionbooks—orabout ten trillionbits.Theamountof informationhandeddown inbooksorviatheinternetis100,000timesasmuchasthereisinDNA.Evenmore important is the fact that the information inbookscanbe

changed,andupdated,muchmorerapidly.Ithastakenusseveralmillion

years to evolve from less advanced, earlier apes. During that time, theuseful information in our DNA has probably changed by only a fewmillionbits,sotherateofbiologicalevolutioninhumansisaboutabitayear. By contrast, there are about 50,000 new books published in theEnglishlanguageeachyear,containingoftheorderofahundredbillionbits of information.Of course, the greatmajority of this information isgarbage andno use to any formof life. But, even so, the rate atwhichuseful information canbe added ismillions, if notbillions,higher thanwithDNA.Thismeans that we have entered a new phase of evolution. At first,

evolutionproceededbynaturalselection—fromrandommutations.ThisDarwinianphaselastedaboutthreeandahalfbillionyearsandproducedus,beingswhodeveloped language toexchange information.But in thelast10,000yearsorsowehavebeeninwhatmightbecalledanexternaltransmissionphase. In this, the internal recordof information, handeddowntosucceedinggenerationsinDNA,haschangedsomewhat.Buttheexternal record—in books and other long-lasting forms of storage—hasgrownenormously.Some people would use the term “evolution” only for the internally

transmitted genetic material and would object to it being applied toinformation handed down externally. But I think that is too narrow aview. We are more than just our genes. We may be no stronger orinherently more intelligent than our caveman ancestors. But whatdistinguishesus fromthem is theknowledge thatwehaveaccumulatedoverthelast10,000years,andparticularlyoverthelast300.Ithinkitislegitimate to take a broader view and include externally transmittedinformation,aswellasDNA,intheevolutionofthehumanrace.The timescale forevolution in theexternal transmissionperiod is the

timescaleforaccumulationofinformation.Thisusedtobehundreds,oreventhousands,ofyears.Butnowthistimescalehasshrunktoaboutfiftyyearsor less.On theotherhand, thebrainswithwhichweprocess thisinformationhaveevolvedonlyontheDarwiniantimescale,ofhundredsof thousands of years. This is beginning to cause problems. In theeighteenthcentury,therewassaidtobeamanwhohadreadeverybookwritten. But nowadays, if you read one book a day, it would take youmanytensofthousandsofyearstoreadthroughthebooksinanationallibrary.Bywhichtime,manymorebookswouldhavebeenwritten.

Thishasmeant thatnoonepersoncanbe themasterofmore thanasmallcornerofhumanknowledge.Peoplehavetospecialise,innarrowerandnarrowerfields.Thisis likelytobeamajorlimitationinthefuture.We certainly cannot continue, for long, with the exponential rate ofgrowth of knowledge that we have had in the last 300 years. An evengreaterlimitationanddangerforfuturegenerationsisthatwestillhavethe instincts, and in particular the aggressive impulses, thatwe had incaveman days. Aggression, in the form of subjugating or killing othermen and taking their women and food, has had definite survivaladvantage up to the present time. But now it could destroy the entirehumanraceandmuchoftherestoflifeonEarth.Anuclearwarisstillthemost immediate danger, but there are others, such as the release of ageneticallyengineeredvirus.Orthegreenhouseeffectbecomingunstable.There is no time to wait for Darwinian evolution to make us more

intelligent andbetternatured.Butwearenowenteringanewphaseofwhatmightbecalledself-designedevolution,inwhichwewillbeabletochangeandimproveourDNA.WehavenowmappedDNA,whichmeanswehaveread“thebookoflife,”sowecanstartwritingincorrections.Atfirst,thesechangeswillbeconfinedtotherepairofgeneticdefects—likecystic fibrosis and muscular dystrophy, which are controlled by singlegenesandsoarefairlyeasytoidentifyandcorrect.Otherqualities,suchasintelligence,areprobablycontrolledbyalargenumberofgenes,anditwill be much more difficult to find them and work out the relationsbetween them.Nevertheless, I am sure that during this century peoplewill discover how to modify both intelligence and instincts likeaggression.Lawswillprobablybepassedagainstgeneticengineeringwithhumans.

Butsomepeoplewon’tbeabletoresistthetemptationtoimprovehumancharacteristics,suchassizeofmemory,resistancetodiseaseandlengthof life. Once such superhumans appear, there are going to be majorpolitical problemswith the unimproved humans, whowon’t be able tocompete.Presumably,theywilldieout,orbecomeunimportant.Instead,there will be a race of self-designing beings, who are improvingthemselvesatanever-increasingrate.Ifthehumanracemanagestoredesignitself,toreduceoreliminatethe

risk of self-destruction, it will probably spread out and colonise other

planetsandstars.However,long-distancespacetravelwillbedifficultforchemicallybasedlifeforms—likeus—basedonDNA.Thenaturallifetimeforsuchbeingsisshortcomparedwiththetraveltime.Accordingtothetheoryof relativity,nothing can travel faster than light, so a round tripfrom us to the nearest star would take at least eight years, and to thecentreofthegalaxyabout50,000years.Insciencefiction,theyovercomethisdifficultybyspacewarps,or travel throughextradimensions.ButIdon’t think these will ever be possible, no matter how intelligent lifebecomes.Inthetheoryofrelativity,ifonecantravelfasterthanlight,onecanalsotravelbackintime,andthiswouldleadtoproblemswithpeoplegoingbackandchangingthepast.Onewouldalsoexpecttohavealreadyseen large numbers of tourists from the future, curious to look at ourquaint,old-fashionedways.ItmightbepossibletousegeneticengineeringtomakeDNA-basedlife

survive indefinitely, or at least for 100,000 years. But an easier way,which is almost within our capabilities already, would be to sendmachines. These could be designed to last long enough for interstellartravel.When they arrived at a new star, they could land on a suitableplanetandminematerialtoproducemoremachines,whichcouldbesentontoyetmorestars.Thesemachineswouldbeanewformoflife,basedonmechanical and electronic components rather thanmacromolecules.They could eventually replace DNA-based life, just as DNA may havereplacedanearlierformoflife.

•

Whatarethechancesthatwewillencountersomealienformoflifeasweexplore the galaxy? If the argument about the timescale for theappearanceoflifeonEarthiscorrect,thereoughttobemanyotherstarswhoseplanetshavelifeonthem.Someofthesestellarsystemscouldhaveformed five billion years before the Earth—so why is the galaxy notcrawling with self-designing mechanical or biological life forms? Whyhasn’ttheEarthbeenvisitedandevencolonised?Bytheway,Idiscountsuggestions thatUFOs contain beings fromouter space, as I think thatany visits by aliens would be much more obvious—and probably also

muchmoreunpleasant.So why haven’t we been visited? Maybe the probability of life

spontaneously appearing is so low that Earth is the only planet in thegalaxy—or in the observable universe—on which it happened. Anotherpossibility is that there was a reasonable probability of forming self-reproducingsystems,likecells,butthatmostoftheseformsoflifedidnotevolve intelligence. We are used to thinking of intelligent life as aninevitable consequence of evolution, butwhat if it isn’t?TheAnthropicPrincipleshouldwarnustobewaryofsucharguments.Itismorelikelythatevolutionisarandomprocess,withintelligenceasonlyoneofalargenumberofpossibleoutcomes.It isnotevenclear that intelligencehasany long-termsurvivalvalue.

Bacteria,andothersingle-cellorganisms,mayliveonifallother lifeonEarth iswipedout by our actions.Perhaps intelligencewas anunlikelydevelopmentforlifeonEarth,fromthechronologyofevolution,asittookaverylongtime—twoandahalfbillionyears—togofromsinglecellstomulti-cellular beings, which are a necessary precursor to intelligence.ThisisagoodfractionofthetotaltimeavailablebeforetheSunblowsup,soitwouldbeconsistentwiththehypothesisthattheprobabilityforlifetodevelopintelligenceislow.Inthiscase,wemightexpecttofindmanyotherlifeformsinthegalaxy,butweareunlikelytofindintelligentlife.Anotherway inwhich lifecould fail todevelop toan intelligent stage

wouldbeifanasteroidorcometweretocollidewiththeplanet.In1994,weobserved the collisionof a comet,Shoemaker–Levy,withJupiter. Itproduced a series of enormous fireballs. It is thought the collision of arathersmallerbodywiththeEarth,aboutsixty-sixmillionyearsago,wasresponsible for the extinction of the dinosaurs. A few small earlymammalssurvived,butanythingaslargeasahumanwouldhavealmostcertainly beenwipedout. It is difficult to sayhowoften such collisionsoccur, but a reasonable guess might be every twenty million years, onaverage. If this figure is correct, it would mean that intelligent life onEarth has developed only because of the lucky chance that there havebeennomajorcollisionsinthelastsixty-sixmillionyears.Otherplanetsin the galaxy, on which life has developed, may not have had a longenoughcollision-freeperiodtoevolveintelligentbeings.A third possibility is that there is a reasonable probability for life to

formandtoevolvetointelligentbeings,butthesystembecomesunstableand the intelligent life destroys itself. Thiswould be a very pessimisticconclusionandIverymuchhopeitisn’ttrue.Ipreferafourthpossibility:thatthereareotherformsofintelligentlife

outthere,butthatwehavebeenoverlooked.In2015Iwasinvolvedinthelaunchof theBreakthroughListenInitiatives.BreakthroughListenusesradio observations to search for intelligent extraterrestrial life, and hasstate-of-the-art facilities, generous funding and thousands of hours ofdedicated radio telescope time. It is the largest ever scientific researchprogramme aimed at finding evidence of civilisations beyond Earth.BreakthroughMessageisaninternationalcompetitiontocreatemessagesthatcouldbereadbyanadvancedcivilisation.Butweneedtobewaryofanswering back until we have developed a bit further.Meeting amoreadvancedcivilisation,atourpresentstage,mightbeabitliketheoriginalinhabitants of America meeting Columbus—and I don’t think theythoughttheywerebetteroffforit.

IfintelligentlifeexistssomewhereelsethanonEarth,woulditbesimilartotheformsweknow,ordifferent?

IsthereintelligentlifeonEarth?Butseriously,ifthereisintelligentlifeelsewhere,itmustbeaverylongwayawayotherwiseitwouldhavevisitedEarthbynow.AndIthinkwewould’veknownifwehadbeenvisited;itwouldbelikethe

filmIndependenceDay.

4

CANWEPREDICTTHEFUTURE?

Inancienttimes,theworldmusthaveseemedprettyarbitrary.Disasterssuchas floods,plagues, earthquakesor volcanoesmusthave seemed tohappenwithoutwarningorapparentreason.Primitivepeopleattributedsuch natural phenomena to a pantheon of gods and goddesses, whobehavedinacapriciousandwhimsicalway.Therewasnowaytopredictwhat they would do, and the only hope was to win favour by gifts oractions.Manypeoplestillpartiallysubscribetothisbeliefandtrytomakeapactwithfortune.TheyoffertobehavebetterorbekinderifonlytheycangetanA-gradeforacourseorpasstheirdrivingtest.Graduallyhowever,peoplemusthavenoticedcertainregularitiesinthe

behaviourofnature.Theseregularitiesweremostobviousinthemotionoftheheavenlybodiesacrossthesky.Soastronomywasthefirstsciencetobedeveloped.ItwasputonafirmmathematicalbasisbyNewtonmorethan300yearsago,andwestillusehis theoryofgravity topredict themotion of almost all celestial bodies. Following the example ofastronomy, it was found that other natural phenomena also obeyeddefinite scientific laws. This led to the idea of scientific determinism,whichseemsfirsttohavebeenpubliclyexpressedbytheFrenchscientistPierre-Simon Laplace. I would like to quote to you Laplace’s actualwords,butLaplacewas rather likeProust in thathewrote sentencesofinordinate length and complexity. So I have decided to paraphrase thequotation. In effect what he said was that if at one time we knew thepositionsandspeedsofalltheparticlesintheuniverse,thenwewouldbeabletocalculatetheirbehaviouratanyothertimeinthepastor future.There is a probably apocryphal story that when Laplace was asked by

NapoleonhowGod fitted into this system, he replied, “Sire, I havenotneededthathypothesis.”Idon’tthinkthatLaplacewasclaimingthatGoddidn’t exist. It is just that God doesn’t intervene to break the laws ofscience.Thatmustbethepositionofeveryscientist.Ascientificlawisnotascientificlawifitonlyholdswhensomesupernaturalbeingdecidestoletthingsrunandnotintervene.Theideathatthestateoftheuniverseatonetimedeterminesthestate

atallothertimeshasbeenacentraltenetofscienceeversinceLaplace’stime. It implies thatwe can predict the future, in principle at least. Inpractice,however,ourabilitytopredict thefuture isseverely limitedbythe complexity of the equations, and the fact that they often have apropertycalledchaos.AsthosewhohaveseenJurassicParkwillknow,thismeansa tinydisturbance inoneplacecancauseamajorchange inanother. A butterfly flapping its wings in Australia can cause rain inCentralPark,NewYork.Thetroubleis,itisnotrepeatable.Thenexttimethebutterflyflapsitswingsahostofotherthingswillbedifferent,whichwillalsoinfluencetheweather.Thischaosfactoriswhyweatherforecastscanbesounreliable.Despitethesepracticaldifficulties,scientificdeterminismremainedthe

official dogma throughout the nineteenth century. However, in thetwentiethcenturythereweretwodevelopmentsthatshowthatLaplace’svision,ofacompletepredictionofthefuture,cannotberealised.Thefirstof thesedevelopmentswaswhat iscalledquantummechanics.Thiswasput forward in 1900by theGermanphysicistMaxPlanckas anadhochypothesis, to solve an outstanding paradox. According to the classicalnineteenth-centuryideasdatingbacktoLaplace,ahotbody,likeapieceofred-hotmetal,shouldgiveoffradiation.Itwouldloseenergyinradiowaves,theinfra-red,visiblelight,ultra-violet,X-raysandgammarays,allatthesamerate.Thiswouldmeannotonlythatwewouldalldieofskincancer, but also that everything in the universe would be at the sametemperature,whichclearlyitisn’t.However,Planck showedonecouldavoid thisdisaster ifonegaveup

theideathattheamountofradiationcouldhavejustanyvalue,andsaidinsteadthatradiationcameonlyinpacketsorquantaofacertainsize.Itisabit likesaying thatyoucan’tbuysugar loose in thesupermarket, ithastobeinkilogrambags.Theenergyinthepacketsorquantaishigherforultra-violetandX-raysthanforinfra-redorvisiblelight.Itmeansthat

unlessabodyisveryhot,liketheSun,itwillnothaveenoughenergytogiveoffevenasinglequantumofultra-violetorX-rays.That iswhywedon’tgetsunburnfromacupofcoffee.Planck regarded the idea of quanta as just amathematical trick, and

notashavinganyphysicalreality,whateverthatmightmean.However,physicistsbegantofindotherbehaviourthatcouldbeexplainedonly interms of quantities having discrete or quantised values rather thancontinuously variable ones. For example, it was found that elementaryparticlesbehaved rather like little tops, spinningaboutanaxis.But theamountofspincouldn’thavejustanyvalue.Ithadtobesomemultipleofabasic unit.Because this unit is very small, onedoesnotnotice that anormaltopreallyslowsdowninarapidsequenceofdiscretesteps,ratherthanasacontinuousprocess.But,fortopsassmallasatoms,thediscretenatureofspinisveryimportant.It was some time before people realised the implications of this

quantumbehaviour fordeterminism. Itwasnotuntil 1927 thatWernerHeisenberg, another German physicist, pointed out that you couldn’tmeasuresimultaneouslyboththepositionandspeedofaparticleexactly.To seewhere a particle is, onehas to shine light on it.But byPlanck’sworkonecan’tuseanarbitrarilysmallamountoflight.Onehastouseatleastonequantum.Thiswilldisturbtheparticleandchangeitsspeedinaway that can’t be predicted. To measure the position of the particleaccurately,youwillhavetouselightofshortwavelength,likeultra-violet,X-raysorgammarays.Butagain,byPlanck’swork,quantaoftheseformsof light have higher energies than those of visible light. So they willdisturbthespeedoftheparticlemore.Itisano-winsituation:themoreaccurately you try to measure the position of the particle, the lessaccuratelyyoucanknowthespeed,andviceversa.ThisissummedupintheUncertaintyPrinciplethatHeisenbergformulated;theuncertaintyinthe position of a particle times the uncertainty in its speed is alwaysgreater than a quantity called Planck’s constant, divided by twice themassoftheparticle.Laplace’s vision of scientific determinism involved knowing the

positions and speeds of the particles in the universe, at one instant oftime. So it was seriously undermined by Heisenberg’s UncertaintyPrinciple.Howcouldonepredictthefuture,whenonecouldnotmeasureaccurately both the positions and the speeds of particles at the present

time?Nomatterhowpowerfulacomputeryouhave,ifyouputlousydatainyouwillgetlousypredictionsout.Einsteinwasveryunhappyaboutthisapparentrandomnessinnature.

His views were summed up in his famous phrase “God does not playdice.”He seemed to have felt that the uncertaintywas only provisionaland that therewasanunderlying reality, inwhichparticleswouldhavewell-defined positions and speeds and would evolve according todeterministiclawsinthespiritofLaplace.ThisrealitymightbeknowntoGod,butthequantumnatureof lightwouldpreventusseeingit,exceptthroughaglassdarkly.Einstein’sviewwaswhatwouldnowbecalledahiddenvariabletheory.

Hidden variable theories might seem to be the most obvious way toincorporatetheUncertaintyPrincipleintophysics.Theyformthebasisofthementalpictureoftheuniverseheldbymanyscientists,andalmostallphilosophers of science. But these hidden variable theories are wrong.TheBritishphysicistJohnBell,devisedanexperimental test that couldfalsify hidden variable theories. When the experiment was carried outcarefully, the results were inconsistent with hidden variables. Thus itseems that evenGod is boundby theUncertaintyPrinciple and cannotknowboththepositionandthespeedofaparticle.Alltheevidencepointsto God being an inveterate gambler, who throws the dice on everypossibleoccasion.Other scientists were much more ready than Einstein to modify the

classical nineteenth-century view of determinism. A new theory,quantummechanics,wasputforwardbyHeisenberg,ErwinSchrödingerfrom Austria and the British physicist Paul Dirac. Dirac was mypredecessor but one as the Lucasian Professor in Cambridge. Althoughquantummechanicshasbeenaround fornearly seventyyears, it is stillnotgenerallyunderstoodorappreciated,evenbythosewhouseit todocalculations. Yet it should concern us all, because it is completelydifferentfromtheclassicalpictureofthephysicaluniverse,andofrealityitself.Inquantummechanics,particlesdon’thavewell-definedpositionsand speeds. Instead, they are represented by what is called a wavefunction.This is anumber at eachpoint of space.The sizeof thewavefunction gives the probability that the particle will be found in thatposition.Therateatwhichthewavefunctionvariesfrompointtopointgivesthespeedoftheparticle.Onecanhaveawavefunctionthatisvery

stronglypeakedinasmallregion.Thiswillmeanthattheuncertaintyinthepositionissmall.Butthewavefunctionwillvaryveryrapidlynearthepeak,upononesideanddownontheother.Thustheuncertaintyinthespeed will be large. Similarly, one can have wave functions where theuncertainty in the speed is small but the uncertainty in the position islarge.Thewavefunctioncontainsallthatonecanknowoftheparticle,both

itspositionanditsspeed.Ifyouknowthewavefunctionatonetime,thenitsvaluesatothertimesaredeterminedbywhatiscalledtheSchrödingerequation.Thusonestillhasakindofdeterminism,butit isnotthesortthatLaplaceenvisaged.Insteadofbeingabletopredictthepositionsandspeedsof particles, allwe canpredict is thewave function.Thismeansthat we can predict just half what we could according to the classicalnineteenth-centuryview.Although quantum mechanics leads to uncertainty when we try to

predictboththepositionandthespeed,itstillallowsustopredict,withcertainty, one combination of position and speed. However, even thisdegreeofcertaintyseemstobethreatenedbymorerecentdevelopments.The problem arises because gravity canwarp space–time somuch thattherecanberegionsofspacethatwecan’tobserve.Such regions are the interiors of black holes. That means that we

cannot,eveninprinciple,observetheparticlesinsideablackhole.Sowecannotmeasuretheirpositionsorvelocitiesatall.Thereisthenanissueofwhetherthisintroducesfurtherunpredictabilitybeyondthatfoundinquantummechanics.To sum up, the classical view, put forward by Laplace, was that the

futuremotionofparticleswascompletelydetermined, ifoneknewtheirpositions and speeds at one time. This view had to be modified whenHeisenberg put forward his Uncertainty Principle, which said that onecouldnotknowboth thepositionandthespeedaccurately.However, itwasstillpossible topredictonecombinationofpositionandspeed.Butperhapseventhislimitedpredictabilitymightdisappearifblackholesaretakenintoaccount.

Dothelawsgoverningtheuniverseallowustopredictexactlywhatisgoingtohappentousinthefuture?

Theshortanswerisno,andyes.Inprinciple,thelawsallowustopredictthefuture.Butinpracticethecalculationsare

oftentoodifficult.

5

WHATISINSIDEABLACKHOLE?

It is said that fact is sometimes stranger than fiction, and nowhere isthatmore true than in the caseofblackholes.Blackholesare strangerthananythingdreamedupbyscience-fictionwriters,buttheyarefirmlymattersofsciencefact.The firstdiscussionofblackholeswas in1783,byaCambridgeman,

JohnMichell.Hisargumentranasfollows.Ifonefiresaparticle,suchasa cannon ball, vertically upwards, it will be slowed down by gravity.Eventually, the particle will stop moving upwards, and will fall back.However, if the initial upwards velocitywere greater than some criticalvalue,calledtheescapevelocity,gravitywouldneverbestrongenoughtostoptheparticle,anditwouldgetaway.Theescapevelocityisjustover11kilometrespersecondfortheEarth,andabout617kilometrespersecondfortheSun.Bothofthesearemuchhigherthanthespeedofrealcannonballs.Buttheyarelowcomparedtothespeedoflight,whichis300,000kilometres per second. Thus light can get away from the Earth or Sunwithout much difficulty. However, Michell argued that there could bestars that were much more massive than the Sun which had escapevelocities greater than the speed of light.Wewould not be able to seethem,becauseanylighttheysentoutwouldbedraggedbackbygravity.Thus they would be what Michell called dark stars, what we now callblackholes.Tounderstandthem,weneedtostartwithgravity.Gravityisdescribed

byEinstein’sgeneral theoryofrelativity,which isa theoryofspaceandtimeaswellasgravity.ThebehaviourofspaceandtimeisgovernedbyasetofequationscalledtheEinsteinequationswhichEinsteinputforward

in 1915. Although gravity is by far the weakest of the known forces ofnature,ithastwocrucialadvantagesoverotherforces.First,itactsoveralong range. The Earth is held in orbit by the Sun, ninety-threemillionmilesaway,andtheSunisheldinorbitaroundthecentreofthegalaxy,about 10,000 light years away. The second advantage is that gravity isalwaysattractive,unlikeelectric forceswhichcanbeeitherattractiveorrepulsive. These two featuresmean that for a sufficiently large star thegravitational attraction between particles can dominate over all otherforcesandleadtogravitationalcollapse.Despitethesefacts,thescientificcommunity was slow to realise thatmassive stars could collapse in onthemselvesundertheirowngravityandtofigureouthowtheobjectleftbehind would behave. Albert Einstein even wrote a paper in 1939claimingthatstarscouldnotcollapseundergravity,becausemattercouldnot be compressed beyond a certain point. Many scientists sharedEinstein’sgutfeeling.TheprincipalexceptionwastheAmericanscientistJohnWheeler,whoinmanyways is theheroof theblackholestory.Inhiswork in the1950sand1960s,heemphasisedthatmanystarswouldeventuallycollapse,andexploredtheproblemsthisposedfortheoreticalphysics. He also foresaw many of the properties of the objects whichcollapsedstarsbecome—thatis,blackholes.Duringmostofthelifeofanormalstar,overmanybillionsofyears,it

willsupport itselfagainst itsowngravitybythermalpressurecausedbynuclear processes which convert hydrogen into helium. Eventually,however, thestarwillexhaust itsnuclear fuel.Thestarwillcontract. Insome cases, it may be able to support itself as a white dwarf star, thedense remnants of a stellar core. However, SubrahmanyanChandrasekharshowedin1930thatthemaximummassofawhitedwarfstar is about 1.4 times that of the Sun. A similar maximummass wascalculatedbytheRussianphysicistLevLandauforastarmadeentirelyofneutrons.What would be the fate of those countless stars with a greatermass

thanthemaximummassofawhitedwarforneutronstaroncetheyhadexhausted nuclear fuel? The problem was investigated by RobertOppenheimer of later atom bomb fame. In a couple of papers in 1939,with George Volkoff and Hartland Snyder, he showed that such a starcouldnotbesupportedbypressure.Andthatifoneneglectedpressure,auniformsphericallysystematicsymmetricstarwouldcontracttoasingle

point of infinite density. Such a point is called a singularity. All ourtheories of space are formulated on the assumption that space–time issmoothandnearlyflat,sotheybreakdownatthesingularity,wherethecurvatureofspace–timeisinfinite.Infact,itmarkstheendofspaceandtimeitself.ThatiswhatEinsteinfoundsoobjectionable.Then the Second World War intervened. Most scientists, including

RobertOppenheimer,switchedtheirattentiontonuclearphysics,andtheissue of gravitational collapse was largely forgotten. Interest in thesubject revivedwith thediscoveryofdistantobjectscalledquasars.Thefirst quasar, 3C273,was found in 1963.Many other quasarswere soondiscovered. They were bright despite being at great distances from theEarth. Nuclear processes could not account for their energy output,because they release only a small fraction of their rest mass as pureenergy. The only alternative was gravitational energy released bygravitationalcollapse.Gravitational collapse of stars was rediscovered.When this happens,

thegravityoftheobjectdrawsallitssurroundingmatterinwards.Itwasclear that a uniform spherical starwould contract to a point of infinitedensity, a singularity. But what would happen if the star isn’t uniformandspherical?Couldthisunequaldistributionofthestar’smattercauseanon-uniformcollapseandavoidasingularity?Inaremarkablepaper in1965,RogerPenroseshowedtherewouldstillbeasingularity,usingonlythefactthatgravityisattractive.The Einstein equations can’t be defined at a singularity. Thismeans

that at this point of infinite density one can’t predict the future. Thisimpliesthatsomethingstrangecouldhappenwheneverastarcollapsed.We wouldn’t be affected by the breakdown of prediction if thesingularities are not naked—that is, they are not shielded from theoutside. Penrose proposed the cosmic censorship conjecture: allsingularities formedby the collapse of stars or otherbodies arehiddenfromviewinsideblackholes.Ablackholeisaregionwheregravityissostrong that light cannot escape. The cosmic censorship conjecture isalmostcertainly true,becauseanumberofattempts todisprove ithavefailed.When John Wheeler introduced the term “black hole” in 1967, it

replacedtheearliernameof“frozenstar.”Wheeler’scoinageemphasised

that the remnants of collapsed stars are of interest in their own right,independently of how they were formed. The new name caught onquickly.Fromtheoutside,youcan’t tellwhat is insideablackhole.Whatever

you throw in, orhowever it is formed,blackholes look the same. JohnWheeler is known for expressing this principle as “A blackhole hasnohair.”A black hole has a boundary called the event horizon. It is where

gravity is just strong enough to drag light back and prevent it fromescaping.Becausenothingcantravelfasterthanlight,everythingelsewillgetdraggedbackalso.FallingthroughtheeventhorizonisabitlikegoingoverNiagaraFallsinacanoe.IfyouareabovetheFalls,youcangetawayif youpaddle fast enough, but once you are over the edge you are lost.There’snowayback.AsyougetnearertheFalls,thecurrentgetsfaster.This means it pulls harder on the front of the canoe than the back.There’sadangerthatthecanoewillbepulledapart.It is thesamewithblack holes. If you fall towards a black hole feet first, gravity will pullharder on your feet than your head, because they are nearer the blackhole.Theresultisthatyouwillbestretchedoutlengthwise,andsquashedin sideways. If the black hole has amass of a few times our Sun, youwould be torn apart and made into spaghetti before you reached thehorizon.However,ifyoufellintoamuchlargerblackhole,withamassofmore than amillion times the Sun, the gravitational pullwould be thesameonthewholeofyourbodyandyouwouldreachthehorizonwithoutdifficulty.So,ifyouwanttoexploretheinsideofablackhole,makesureyou choose a big one. There is a black hole with amass of about fourmilliontimesthatoftheSunatthecentreofourMilkyWaygalaxy.Althoughyouwouldn’tnoticeanything inparticularasyou fell intoa

blackhole, someonewatchingyou fromadistancewouldneverseeyoucross the event horizon. Instead, you would appear to slow down andhover just outside. Your image would get dimmer and dimmer, andredderandredder,untilyouwereeffectivelylostfromsight.Asfarastheoutsideworldisconcerned,youwouldbelostforever.ShortlyafterthebirthofmydaughterLucyIhadaeurekamoment.I

discovered the area theorem. If general relativity is correct, and theenergy density of matter is positive, as is usually the case, then the

surfaceareaof theeventhorizon, theboundaryofablackhole,has thepropertythatitalwaysincreaseswhenadditionalmatterorradiationfallsinto the black hole. Moreover, if two black holes collide andmerge toform a single black hole, the area of the event horizon around theresulting black hole is greater than the sum of the areas of the eventhorizonsaroundtheoriginalblackholes.Theareatheoremcanbetestedexperimentally by the Laser Interferometer Gravitational-WaveObservatory(LIGO).OnSeptember14,2015,LIGOdetectedgravitationalwaves from the collision and merger of two black holes. From thewaveform, one can estimate the masses and angular momenta of theblack holes, and by the no-hair theorem these determine the horizonareas.Thesepropertiessuggestthatthereisaresemblancebetweenthearea

of the event horizon of a black hole and conventional classical physics,specifically the concept of entropy in thermodynamics. Entropy can beregarded as ameasure of thedisorder of a system, or equivalently as alack of knowledge of its precise state. The famous second law ofthermodynamics says that entropy always increases with time. Thisdiscoverywasthefirsthintofthiscrucialconnection.The analogy between the properties of black holes and the laws of

thermodynamicscanbeextended.Thefirstlawofthermodynamicssaysthat a small change in the entropy of a system is accompanied by aproportional change in the energy of the system. Brandon Carter, JimBardeenandIfoundasimilarlawrelatingthechangeinmassofablackhole to a change in the area of the event horizon. Here the factor ofproportionality involvesaquantitycalled thesurfacegravity,which isameasureofthestrengthofthegravitationalfieldattheeventhorizon.Ifone accepts that the area of the event horizon is analogous to entropy,thenitwouldseemthatthesurfacegravityisanalogoustotemperature.Theresemblanceisstrengthenedbythefactthatthesurfacegravityturnsout to be the same at all points on the event horizon, just as thetemperatureisthesameeverywhereinabodyatthermalequilibrium.Althoughthere isclearlyasimilaritybetweenentropyandtheareaof

the event horizon, it was not obvious to us how the area could beidentifiedastheentropyofablackhole itself.Whatwouldbemeantbytheentropyofablackhole?Thecrucialsuggestionwasmadein1972byJacobBekenstein,whowasagraduatestudentatPrincetonUniversity.It

goes like this.When a blackhole is createdby gravitational collapse, itrapidlysettlesdowntoastationarystate,whichischaracterisedbythreeparameters:themass,theangularmomentumandtheelectriccharge.This makes it look as if the final black hole state is independent of

whether thebody thatcollapsedwascomposedofmatterorantimatter,orwhetheritwassphericalorhighlyirregularinshape.Inotherwords,ablackholeofagivenmass,angularmomentumandelectricchargecouldhave been formed by the collapse of any one of a large number ofdifferentconfigurationsofmatter.Sowhatappearstobethesameblackholecouldbeformedbythecollapseofalargenumberofdifferenttypesof star. Indeed, if quantum effects are neglected, the number ofconfigurations would be infinite since the black hole could have beenformed by the collapse of a cloud of an indefinitely large number ofparticlesofindefinitelylowmass.Butcouldthenumberofconfigurationsreallybeinfinite?QuantummechanicsfamouslyinvolvestheUncertaintyPrinciple.This

statesthatitisimpossibletomeasureboththepositionandspeedofanyobject. If one measures exactly where something is, then its speed isundetermined.Ifonemeasuresthespeedofsomething,thenitspositionisundetermined. Inpractice, thismeans that it is impossible to localiseanything.Supposeyouwanttomeasurethesizeofsomething,thenyouneed to figure out where the ends of this moving object are. You canneverdothisaccurately,becauseitwillinvolvemakingameasurementofboththepositionsofsomethinganditsspeedatthesametime.Inturn,itisthenimpossibletodeterminethesizeofanobject.Allyoucandoistosay that the Uncertainty Principlemakes it impossible to say preciselywhat the size of something really is. It turns out that the UncertaintyPrinciple imposes a limit on the size of something. After a little bit ofcalculation, one finds that for a given mass of an object, there is aminimumsize.Thisminimumsizeissmallforheavyobjects,butasonelooks at lighter and lighter objects, the minimum size gets bigger andbigger.Thisminimumsizecanbethoughtofasaconsequenceofthefactthatinquantummechanicsobjectscanbethoughtofeitherasawaveoraparticle.Thelighteranobjectis,thelongeritswavelengthisandsoitismorespreadout.Theheavieranobjectis,theshorteritswavelengthandsoitwillseemmorecompact.Whentheseideasarecombinedwiththoseofgeneralrelativity, itmeansthatonlyobjectsheavierthanaparticular

weightcan formblackholes.Thatweight isabout thesameas thatofagrainofsalt.Afurtherconsequenceoftheseideasisthatthenumberofconfigurations that could form a black hole of a given mass, angularmomentum,andelectriccharge,althoughvery large,mayalsobefinite.Jacob Bekenstein suggested that from this finite number, one couldinterpret the entropy of a black hole. This would be a measure of theamountof informationthatseems irretrievably lost,duringthecollapsewhenablackholeiscreated.TheapparentlyfatalflawinBekenstein’ssuggestionwasthat,ifablack

hole has a finite entropy that is proportional to the area of its eventhorizon, it also ought to have a non-zero temperature which would beproportional to its surface gravity. This would imply that a black holecouldbeinequilibriumwiththermalradiationatsometemperatureotherthan zero. Yet according to classical concepts no such equilibrium ispossiblesincetheblackholewouldabsorbanythermalradiationthatfellon it but by definitionwouldnot be able to emit anything in return. Itcannotemitanything,itcannotemitheat.Thiscreatedaparadoxaboutthenatureofblackholes,theincredibly

denseobjectscreatedbythecollapseofstars.Onetheorysuggestedthatblack holes with identical qualities could be formed from an infinitenumber of different types of stars. Another suggested that the numbercould be finite. This is a problem of information—the idea that everyparticleandeveryforceintheuniversecontainsinformation.Becauseblackholeshavenohair,asthescientistJohnWheelerputit,

onecan’ttellfromtheoutsidewhatisinsideablackhole,apartfromitsmass, electric charge and rotation. This means that a black hole mustcontain a lot of information that is hidden from the outsideworld.Butthereisalimittotheamountofinformationonecanpackintoaregionofspace. Information requires energy, and energy hasmass by Einstein’sfamousequation,E=mc2.So,ifthere’stoomuchinformationinaregionofspace, itwillcollapse intoablackhole,andthesizeof theblackholewill reflect the amount of information. It is like pilingmore andmorebooksintoalibrary.Eventually,theshelveswillgivewayandthelibrarywillcollapseintoablackhole.Iftheamountofhiddeninformationinsideablackholedependsonthe

sizeofthehole,onewouldexpectfromgeneralprinciplesthattheblack

holewouldhaveatemperatureandwouldglowlikeapieceofhotmetal.Butthatwasimpossiblebecause,aseveryoneknew,nothingcouldgetoutofablackhole.Orsoitwasthought.This problem remained until early in 1974, when I was investigating

what the behaviour of matter in the vicinity of a black hole would beaccordingtoquantummechanics.Tomygreatsurprise,Ifoundthattheblackholeseemedtoemitparticlesatasteadyrate.Likeeveryoneelseatthat time, I accepted the dictum that a black hole could not emitanything.Ithereforeputquitealotofeffortintotryingtogetridofthisembarrassingeffect.ButthemoreIthoughtaboutit,themoreitrefusedtogoaway,sothat intheendIhadtoaccept it.Whatfinallyconvincedme itwasarealphysicalprocesswas that theoutgoingparticleshaveaspectrumthatispreciselythermal.Mycalculationspredictedthatablackhole creates and emits particles and radiation, just as if it were anordinaryhotbody,withatemperaturethatisproportionaltothesurfacegravity and inversely proportional to the mass. This made theproblematicsuggestionofJacobBekenstein,thatablackholehadafiniteentropy, fully consistent, since it implied that a black hole could be inthermalequilibriumatsomefinitetemperatureotherthanzero.Since that time, the mathematical evidence that black holes emit

thermalradiationhasbeenconfirmedbyanumberofotherpeoplewithvariousdifferentapproaches.Oneway tounderstand theemission isasfollows.Quantummechanicsimpliesthatthewholeofspaceisfilledwithpairs of virtual particles and antiparticles that are constantlymaterialising in pairs, separating and then coming together again, andannihilatingeachother.Theseparticlesarecalledvirtual,because,unlikereal particles, they cannotbe observeddirectlywith aparticledetector.Their indirect effects can nonetheless bemeasured, and their existencehas been confirmed by a small shift, called the Lamb shift, which theyproduce in the spectrum energy of light from excited hydrogen atoms.Now, in the presence of a black hole, onemember of a pair of virtualparticles may fall into the hole, leaving the other member without apartner with which to engage in mutual annihilation. The forsakenparticleorantiparticlemayfallintotheblackholeafteritspartner,butitmayalsoescape to infinity,where itappears tobe radiationemittedbytheblackhole.Anotherwayof lookingat theprocess is toregardthememberof the

pairofparticlesthatfallsintotheblackhole,theantiparticlesay,asbeingreallyaparticlethatistravellingbackwardsintime.Thustheantiparticlefallingintotheblackholecanberegardedasaparticlecomingoutoftheblack hole but travelling backwards in time.When the particle reachesthepointatwhichtheparticle–antiparticlepairoriginallymaterialised,itisscatteredbythegravitationalfield,sothatittravelsforwardintime.AblackholeofthemassoftheSunwouldleakparticlesatsuchaslowratethat it would be impossible to detect. However, there could be muchsmallerminiblackholeswiththemassof,say,amountain.Thesemighthave formed in the very early universe if it had been chaotic andirregular.Amountain-sizedblackholewouldgiveoffX-raysandgammarays, at a rate of about ten million megawatts, enough to power theworld’selectricitysupply.Itwouldn’tbeeasy,however,toharnessaminiblackhole.Youcouldn’tkeepitinapowerstationbecauseitwoulddropthroughthefloorandendupatthecentreoftheEarth.Ifwehadsuchablackhole,about theonlyway tokeepholdof itwouldbe tohave it inorbitaroundtheEarth.Peoplehavesearchedforminiblackholesofthismass,buthavesofar

not found any. This is a pity because, if they had, I would have got aNobel Prize. Another possibility, however, is that we might be able tocreate micro black holes in the extra dimensions of space–time.According to some theories, the universe we experience is just a four-dimensional surface in a ten- or eleven-dimensional space. The movieInterstellar gives some idea ofwhat this is like.Wewouldn’t see theseextra dimensions, because light wouldn’t propagate through them butonly through the four dimensions of our universe. Gravity however,wouldaffect theextradimensions,andwouldbemuchstrongerthaninouruniverse.Thiswouldmakeitmucheasiertoformalittleblackholeintheextradimensions.ItmightbepossibletoobservethisattheLHC,theLarge Hadron Collider, at CERN in Switzerland. This consists of acircular tunnel, twenty-seven kilometres long. Two beams of particlestravel round this tunnel inoppositedirections andaremade to collide.Some of the collisions might create micro black holes. These wouldradiateparticlesinapatternthatwouldbeeasytorecognise.SoImightgetaNobelPrizeafterall.*

As particles escape from a black hole, the hole will lose mass andshrink.Thiswillincreasetherateofemissionofparticles.Eventually,the

blackholewillloseallitsmassanddisappear.Whatthenhappenstoallthe particles and unlucky astronauts that fell into the black hole? Theycan’t just re-emergewhen the black hole disappears. The particles thatcomeoutofablackholeseemtobecompletelyrandomandtobearnorelationtowhatfellin.Itappearsthattheinformationaboutwhatfellinislost,apartfromthetotalamountofmassandtheamountofrotation.Butifinformationislost,thisraisesaseriousproblemthatstrikesattheheartofourunderstandingofscience.Formorethan200years,wehavebelieved in scientific determinism—that is, that the laws of sciencedeterminetheevolutionoftheuniverse.If informationwere really lost in black holes,wewouldn’t be able to

predict the future, because a black hole could emit any collection ofparticles.Itcouldemitaworkingtelevisionsetoraleather-boundvolumeofthecompleteworksofShakespeare,thoughthechanceofsuchexoticemissions isvery low. It ismuchmore likely toemit thermal radiation,like theglow fromred-hotmetal. Itmight seemthat itwouldn’tmatterverymuch if we couldn’t predict what comes out of black holes. Therearen’t any black holes near us. But it is a matter of principle. Ifdeterminism, thepredictabilityof theuniverse,breaksdownwithblackholes, it could break down in other situations. There could be virtualblackholesthatappearasfluctuationsoutofthevacuum,absorbonesetof particles, emit another and disappear into the vacuum again. Evenworse, ifdeterminismbreaksdown,wecan’tbesureofourpasthistoryeither.Thehistorybooksandourmemoriescould justbe illusions.It isthepastthattellsuswhoweare.Withoutit,weloseouridentity.It was therefore very important to determine whether information

really was lost in black holes, or whether in principle it could berecovered.Many scientists felt that information should not be lost, butforyearsnoonesuggestedamechanismbywhichitcouldbepreserved.Thisapparentlossofinformation,knownastheinformationparadox,hastroubled scientists for the last forty years, and still remains one of thebiggestunsolvedproblemsintheoreticalphysics.Recently, interest in possible resolutions of the information paradox

has been revived as new discoveries have been made about theunification of gravity and quantummechanics. Central to these recentbreakthroughsistheunderstandingofthesymmetriesofspace–time.

Supposetherewasnogravityandspace–timewascompletelyflat.Thiswouldbelikeacompletelyfeaturelessdesert.Suchaplacehastwotypesofsymmetry.Thefirstiscalledtranslationsymmetry.Ifyoumovedfromonepointinthedeserttoanother,youwouldnotnoticeanychange.Thesecond symmetry is rotation symmetry. If you stood somewhere in thedesert and started to turn around, you would again not notice anydifference in what you saw. These symmetries are also found in “flat”space–time,thespace–timeonefindsintheabsenceofanymatter.If one put something into this desert, these symmetries would be

broken. Suppose therewas amountain, an oasis and some cacti in thedesert, it would look different in different places and in differentdirections. The same is true of space–time. If one puts objects into aspace–time,thetranslationalandrotationalsymmetriesgetbroken.Andintroducingobjectsintoaspace–timeiswhatproducesgravity.Ablackholeisaregionofspace–timewheregravityisstrong,space–

timeisviolentlydistortedandsooneexpectsitssymmetriestobebroken.However,asonemovesawayfromtheblackhole,thecurvatureofspace–time gets less and less. Very far away from the black hole, space–timelooksverymuchlikeflatspace–time.Backinthe1960s,HermannBondi,A.W.KennethMetzner,M.G.J.

vanderBurgandRainerSachsmadethetrulyremarkablediscoverythatspace–time far away from any matter has an infinite collection ofsymmetries known as supertranslations. Each of these symmetries isassociated with a conserved quantity, known as the supertranslationcharges. A conserved quantity is a quantity that does not change as asystem evolves. These are generalisations of more familiar conservedquantities. For example, if space–time does not change in time, thenenergy isconserved. If space–time looks thesameatdifferentpoints inspace,thenmomentumisconserved.Whatwasremarkableabout thediscoveryofsupertranslations is that

there are an infinite number of conserved quantities far from a blackhole. It is theseconservation laws thathavegivenanextraordinaryandunexpectedinsightintoprocessingravitationalphysics.In 2016, together with my collaborators Malcolm Perry and Andy

Strominger, I was working on using these new results with theirassociated conserved quantities to find a possible resolution to the

information paradox.We know that the three discernible properties ofblack holes are theirmass, their charge and their angularmomentum.Thesearetheclassicalchargesthathavebeenunderstoodforalongtime.However, black holes also carry a supertranslation charge. So perhapsblackholeshavea lotmoretothemthanwefirst thought.Theyarenotbald orwith only three hairs, but actually have a very large amount ofsupertranslationhair.Thissupertranslationhairmightencodesomeoftheinformationabout

what is inside the black hole. It is likely that these supertranslationcharges do not contain all of the information, but the rest might beaccounted for by some additional conserved quantities, superrotationcharges, associated with some additional related symmetries calledsuperrotations,whichare,asyet,notwellunderstood.Ifthisisright,andall the informationaboutablackholecanbeunderstoodintermsof its“hairs,” then perhaps there is no loss of information. These ideas havejustreceivedconfirmationwithourmostrecentcalculations.Strominger,Perry and myself, together with a graduate student, Sasha Haco, havediscovered that these superrotation charges an account for the entireentropy of any black hole. Quantummechanics continues to hold, andinformationisstoredonthehorizon,thesurfaceoftheblackhole.The black holes are still characterised only by their overall mass,

electricchargeandspinoutsidetheeventhorizonbuttheeventhorizonitselfcontainstheinformationneededtotellusaboutwhathasfallenintotheblackhole inaway thatgoesbeyond these threecharacteristics theblackholehas.Peoplearestillworkingontheseissuesandthereforetheinformationparadoxremainsunresolved.ButIamoptimisticthatwearemovingtowardsasolution.Watchthisspace.

* NobelPrizescannotbeawardedposthumously,sosadlythisambitionwillnowneverberealised.

Isfallingintoablackholebadnewsforaspace

traveller?

Definitelybadnews.Ifitwereastellarmassblackhole,you

wouldbemadeintospaghettibeforereachingthehorizon.Ontheotherhand,ifitwereasupermassiveblackhole,youwouldcrossthehorizonwithease,butbecrushedoutof

existenceatthesingularity.

6

ISTIMETRAVELPOSSIBLE?

In science fiction, space and time warps are commonplace. They areusedforrapidjourneysaroundthegalaxyorfortravelthroughtime.Buttoday’s science fiction is often tomorrow’s science fact. Sowhat are thechancesoftimetravel?Theideathatspaceandtimecanbecurvedorwarpedisfairlyrecent.

For more than 2,000 years the axioms of Euclidean geometry wereconsidered tobe self-evident.As thoseof youwhowere forced to learngeometry at school may remember, one of the consequences of theseaxiomsisthattheanglesofatriangleaddupto180degrees.However,inthelastcenturypeoplebegantorealisethatotherformsof

geometrywerepossibleinwhichtheanglesofatriangleneednotaddupto 180 degrees. Consider, for example, the surface of the Earth. ThenearestthingtoastraightlineonthesurfaceoftheEarthiswhatiscalledagreatcircle.Thesearetheshortestpathsbetweentwopointssotheyaretheroutes thatairlinesuse.Considernowthe triangleon thesurfaceoftheEarthmadeupoftheequator,thelineof0degreeslongitudethroughLondonand the lineof90degrees longtitudeeast throughBangladesh.The two lines of longitude meet the equator at a right angle, or 90degrees.ThetwolinesoflongitudealsomeeteachotherattheNorthPoleatarightangle,or90degrees.Thusonehasa trianglewith threerightangles. The angles of this triangle add up to 270 degrees which isobviouslygreaterthanthe180degreesforatriangleonaflatsurface.Ifonedrewa triangleonasaddle-shapedsurfaceonewould find that theanglesaddeduptolessthan180degrees.ThesurfaceoftheEarthiswhatiscalledatwo-dimensionalspace.That

is, you canmove on the surface of the Earth in two directions at rightangles to each other: you can move north–south or east–west. But ofcoursethereisathirddirectionatrightanglestothesetwoandthatisupor down. In other words the surface of the Earth exists in three-dimensionalspace.The three-dimensionalspace is flat.That is tosay itobeysEuclideangeometry.Theanglesofatriangleaddupto180degrees.However, one could imagine a race of two-dimensional creatures whocouldmoveaboutonthesurfaceoftheEarthbutwhocouldn’texperiencethe third direction of up or down. They wouldn’t know about the flatthree-dimensionalspaceinwhichthesurfaceoftheEarthlives.Forthemspacewouldbecurvedandgeometrywouldbenon-Euclidean.But just as one can think of two-dimensional beings living on the

surface of the Earth, so one could imagine that the three-dimensionalspaceinwhichwelivewasthesurfaceofasphereinanotherdimensionthatwedon’tsee.Ifthespherewereverylarge,spacewouldbenearlyflatandEuclideangeometrywouldbeaverygoodapproximationoversmalldistances.ButwewouldnoticethatEuclideangeometrybrokedownoverlarge distances. As an illustration of this imagine a team of paintersaddingpainttothesurfaceofalargeball.Asthethicknessofthepaintlayerincreased,thesurfaceareawouldgo

up. If the ball were in a flat three-dimensional space one could go onadding paint indefinitely and the ball would get bigger and bigger.However, if the three-dimensional space were really the surface of asphereinanotherdimensionitsvolumewouldbelargebutfinite.Asoneaddedmore layersofpaint theballwouldeventually fillhalf the space.After that thepainterswould find that theywere trapped ina regionofever-decreasingsize,andalmostthewholeofspacewouldbeoccupiedbytheballanditslayersofpaint.Sotheywouldknowthattheywerelivinginacurvedspaceandnotaflatone.Thisexampleshowsthatonecannotdeducethegeometryoftheworld

from first principles as the ancientGreeks thought. Instead one has tomeasure the spacewe live in and find out its geometry by experiment.However,althoughawaytodescribecurvedspaceswasdevelopedbytheGerman Bernhard Riemann in 1854, it remained just a piece ofmathematicsforsixtyyears.Itcoulddescribecurvedspacesthatexistedin the abstract, but there seemedno reasonwhy the physical spacewelived inshouldbecurved.Thisreasoncameonly in1915whenEinstein

putforwardthegeneraltheoryofrelativity.General relativity was a major intellectual revolution that has

transformedthewaywethinkabouttheuniverse.Itisatheorynotonlyof curved space but of curved or warped time as well. Einstein hadrealisedin1905thatspaceandtimeareintimatelyconnectedwitheachother, which is when his theory of special relativity was born, relatingspaceandtimetoeachother.Onecandescribethelocationofaneventbyfour numbers. Three numbers describe the position of the event. TheycouldbemilesnorthandeastofOxfordCircusandtheheightabovesealevel.Onalargerscaletheycouldbegalacticlatitudeandlongitudeanddistancefromthecentreofthegalaxy.Thefourthnumberisthetimeoftheevent.Thusonecanthinkofspace

and time togetheras a four-dimensional entity called space–time.Eachpointofspace–timeis labelledbyfournumbersthatspecify itspositioninspaceandintime.Combiningspaceandtimeintospace–timeinthiswaywouldberathertrivialifonecoulddisentangletheminauniqueway.Thatistosayiftherewasauniquewayofdefiningthetimeandpositionofeachevent.However, inaremarkablepaperwrittenin1905whenhewasaclerkintheSwisspatentoffice,Einsteinshowedthatthetimeandpositionatwhichone thoughtaneventoccurreddependedonhowonewasmoving.Thismeantthattimeandspacewereinextricablyboundupwitheachother.Thetimesthatdifferentobserverswouldassigntoeventswouldagree

if theobserverswerenotmovingrelative toeachother.But theywoulddisagreemorethefastertheirrelativespeed.Soonecanaskhowfastdoesone need to go in order that the time for one observer should gobackwardsrelativetothetimeofanotherobserver.Theanswerisgiveninthefollowinglimerick:

TherewasayoungladyofWightWhotravelledmuchfasterthanlightShedepartedonedayInarelativewayAndarrivedonthepreviousnight.

So all we need for time travel is a spaceship that will go faster thanlight.Unfortunately in the samepaperEinstein showed that the rocketpowerneededtoaccelerateaspaceshipgotgreaterandgreaterthenearer

itgottothespeedoflight.Soitwouldtakeaninfiniteamountofpowertoacceleratepastthespeedoflight.Einstein’spaperof1905seemedtoruleouttimetravelintothepast.It

alsoindicatedthatspacetraveltootherstarswasgoingtobeaveryslowand tediousbusiness. Ifonecouldn’tgo faster than light the round tripfrom us to the nearest star would take at least eight years and to thecentreofthegalaxyabout50,000years.Ifthespaceshipwentverynearthespeedoflightitmightseemtothepeopleonboardthatthetriptothegalactic centre had taken only a few years. But that wouldn’t bemuchconsolation if everyone you had known had died and been forgottenthousandsofyearsagowhenyougotback.Thatwouldn’tbemuchgoodfor science-fiction novels either, so writers had to look for ways to getroundthisdifficulty.In1915,Einsteinshowedthattheeffectsofgravitycouldbedescribed

bysupposingthatspace–timewaswarpedordistortedbythematterandenergy in it, and this theory is known as general relativity. We canactuallyobservethiswarpingofspace–timeproducedbythemassoftheSunintheslightbendingoflightorradiowavespassingclosetotheSun.This causes the apparent position of the star or radio source to shift

slightlywhen theSun isbetween theEarthand the source.The shift isverysmall,aboutathousandthofadegree,equivalenttoamovementofaninchatadistanceofamile.Neverthelessitcanbemeasuredwithgreataccuracyanditagreeswiththepredictionsofgeneralrelativity.Wehaveexperimentalevidencethatspaceandtimearewarped.Theamountofwarpinginourneighbourhoodisverysmallbecauseall

thegravitational fields inthesolarsystemareweak.However,weknowthatverystrongfieldscanoccur,forexampleintheBigBangorinblackholes. So can space and time be warped enough tomeet the demandsfromsciencefictionforthingslikehyperspacedrives,wormholesortimetravel?At first sightall these seempossible.Forexample, in 1948KurtGödel founda solution toEinstein’s fieldequationsofgeneral relativitythat represents a universe inwhich all thematterwas rotating. In thisuniverse it would be possible to go off in a spaceship and come backbeforeyouhadsetout.Gödelwasat theInstituteofAdvancedStudyinPrinceton,whereEinsteinalsospenthislastyears.Hewasmorefamousfor proving you couldn’t prove everything that is true even in such an

apparently simple subject as arithmetic. But what he proved aboutgeneral relativity allowing time travel really upset Einstein, who hadthoughtitwouldn’tbepossible.WenowknowthatGödel’ssolutioncouldn’trepresenttheuniversein

whichwelivebecauseitwasnotexpanding.Italsohadafairlylargevalueforaquantitycalledthecosmologicalconstantwhichisgenerallybelievedto be very small.However, other apparentlymore reasonable solutionsthat allow time travel have since been found. A particularly interestingonefromanapproachknownasstringtheorycontainstwocosmicstringsmovingpasteachotherataspeedveryneartobutslightlylessthanthespeedoflight.Cosmicstringsarearemarkableideaoftheoreticalphysicswhich science-fictionwritersdon’t really seem tohavecaughton to.Astheirnamesuggeststheyarelikestringinthattheyhavelengthbutatinycross-section.Actuallytheyaremorelikerubberbandsbecausetheyareunderenormoustension,somethinglikeahundredbillionbillionbilliontonnes. A cosmic string attached to the Sun would accelerate it fromnoughttosixtyinathirtiethofasecond.Cosmic strings may sound far-fetched and pure science fiction, but

therearegoodscientificreasonstobelievetheycouldhaveformedinthevery early universe shortly after the Big Bang. Because they are undersuchgreattensiononemighthaveexpectedthemtoacceleratetoalmostthespeedoflight.What both the Gödel universe and the fast-moving cosmic-string

space–timehaveincommonisthattheystartoutsodistortedandcurvedthatspace–timecurvesbackonitselfandtravelintothepastwasalwayspossible.Godmighthavecreatedsuchawarpeduniverse,butwehavenoreasontothinkthathedid.All theevidence is that theuniversestartedout in theBigBangwithout thekindofwarpingneeded toallow travelinto the past. Since we can’t change the way the universe began, thequestion of whether time travel is possible is one of whether we cansubsequentlymake space–time so warped that one can go back to thepast.I thinkthis isanimportantsubjectforresearch,butonehastobecareful not to be labelled a crank. If one made a research grantapplicationtoworkontimetravelitwouldbedismissedimmediately.Nogovernmentagencycouldaffordtobeseentobespendingpublicmoneyonanythingaswayout as time travel. Insteadonehas touse technicaltermslikeclosedtime-likecurves,whicharecodefortimetravel.Yetitis

a very serious question. Since general relativity can permit time travel,doesitallowitinouruniverse?Andifnot,whynot?Closely related to time travel is the ability to travel rapidly fromone

position in space to another. As I said earlier, Einstein showed that itwouldtakeaninfiniteamountofrocketpowertoaccelerateaspaceshiptobeyondthespeedoflight.Sotheonlywaytogetfromonesideofthegalaxy to the other in a reasonable timewould seem to be if we couldwarpspace–timesomuchthatwecreatedalittletubeorwormhole.Thiscould connect the two sides of the galaxy and act as a short cut to getfromone to theotherandbackwhileyour friendswerestillalive.Suchwormholeshavebeenseriouslysuggestedasbeingwithinthecapabilitiesofafuturecivilisation.Butifyoucantravelfromonesideofthegalaxytotheotherinaweekortwoyoucouldgobackthroughanotherwormholeandarrivebackbeforeyouhadsetout.Youcouldevenmanagetotravelbackintimewithasinglewormholeifitstwoendsweremovingrelativetoeachother.Onecanshowthattocreateawormholeoneneedstowarpspace–time

in the oppositeway to that inwhich normalmatterwarps it. Ordinarymatter curves space–time back on itself, like the surface of the Earth.However,tocreateawormholeoneneedsmatterthatwarpsspace–timeintheoppositeway,likethesurfaceofasaddle.Thesameistrueofanyotherwayofwarpingspace–timetoallowtraveltothepastiftheuniversedidn’tbeginsowarpedthatitallowedtimetravel.Whatonewouldneedwouldbematterwithnegativemassandnegativeenergydensitytomakespace–timewarpinthewayrequired.Energy is rather likemoney. Ifyouhaveapositivebankbalance,you

candistributeitinvariousways.But,accordingtotheclassicallawsthatwerebelieveduntilquiterecently,youweren’tallowedtohaveanenergyoverdraft.So these classical lawswouldhave ruledoutusbeingable towarptheuniverseinthewayrequiredtoallowtimetravel.However,theclassical laws were overthrown by quantum theory, which is the othergreat revolution in our picture of the universe apart from generalrelativity. Quantum theory is more relaxed and allows you to have anoverdraft on one or two accounts. If only the banks were asaccommodating. In other words, quantum theory allows the energydensitytobenegativeinsomeplacesprovideditispositiveinothers.

Thereasonquantumtheorycanallowtheenergydensitytobenegativeis that it is based on the Uncertainty Principle. This says that certainquantities likethepositionandspeedofaparticlecan’tbothhavewell-definedvalues.Themoreaccurately thepositionofaparticle isdefinedthegreateristheuncertaintyinitsspeed,andviceversa.TheUncertaintyPrinciple also applies to fields like the electromagnetic field or thegravitationalfield.Itimpliesthatthesefieldscan’tbeexactlyzeroeveninwhatwethinkofasemptyspace.Foriftheywereexactlyzerotheirvalueswouldhavebothawell-definedpositionatzeroandawell-definedspeedwhich was also zero. This would be a violation of the UncertaintyPrinciple. Instead the fields would have to have a certain minimumamount of fluctuations. One can interpret these so-called vacuumfluctuations as pairs of particles and antiparticles that suddenly appeartogether,moveapartandthencomebacktogetheragainandannihilateeachother.These particle–antiparticle pairs are said to be virtual because one

cannotmeasurethemdirectlywithaparticledetector.However,onecanobservetheireffectsindirectly.OnewayofdoingthisisbywhatiscalledtheCasimireffect.Imaginethatyouhavetwoparallelmetalplatesashortdistance apart. The plates act likemirrors for the virtual particles andanti-particles.Thismeansthattheregionbetweentheplatesisabitlikean organ pipe and will only admit light waves of certain resonantfrequencies. The result is that there are a slightly different number ofvacuumfluctuationsorvirtualparticlesbetweentheplatesthanthereareoutsidethem,wherevacuumfluctuationscanhaveanywavelength.Thedifferenceinthenumberofvirtualparticlesbetweentheplatescomparedwithoutsidetheplatesmeansthattheydon’texertasmuchpressureononesideoftheplatescomparedwiththeother.Thereisthusaslightforcepushing the plates together. This force has been measuredexperimentally. So, virtual particles actually exist and produce realeffects.Because there are fewer virtual particles or vacuum fluctuations

between theplates, theyhavea lowerenergydensity than in theregionoutside.Buttheenergydensityofemptyspacefarawayfromtheplatesmust be zero. Otherwise it would warp space–time and the universewouldn’tbenearly flat.So theenergydensity in theregionbetweentheplatesmustbenegative.

We thus have experimental evidence from the bending of light thatspace–time is curved and confirmation from theCasimir effect thatwecanwarpitinthenegativedirection.Soitmightseemthatasweadvanceinscienceand technologywemightbeable toconstructawormholeorwarpspaceandtimeinsomeotherwaysoastobeabletotravelintoourpast. If thiswere the case itwould raise awhole host of questions andproblems.Oneoftheseisiftimetravelwillbepossibleinthefuture,whyhasn’tsomeonecomebackfromthefuturetotellushowtodoit.Evenif thereweresoundreasonsforkeepingusinignorance,human

nature being what it is it is difficult to believe that someone wouldn’tshowoffandtelluspoorbenightedpeasantsthesecretoftimetravel.Ofcourse,somepeoplewouldclaimthatwehavealreadybeenvisitedfromthe future. They would say that UFOs come from the future and thatgovernmentsareengagedinagiganticconspiracytocoverthemupandkeepforthemselvesthescientificknowledgethatthesevisitorsbring.AllIcansayisthatifgovernmentswerehidingsomethingtheyaredoingapoor job of extracting useful information from the aliens. I’m prettyscepticalofconspiracytheories,asIbelievethatcock-uptheoryismorelikely. The reports of sightings of UFOs cannot all be caused by extra-terrestrialsbecausetheyaremutuallycontradictory.But,onceyouadmitthatsomearemistakesorhallucinations,isn’titmoreprobablethattheyallare thanthatwearebeingvisitedbypeople fromthe futureor fromtheothersideof thegalaxy?If theyreallywanttocolonisetheEarthorwarnusofsomedangertheyarebeingratherineffective.Apossiblewaytoreconciletimetravelwiththefactthatwedon’tseem

tohavehadanyvisitorsfromthefuturewouldbetosaythatsuchtravelcanoccuronlyinthefuture.Inthisviewonewouldsayspace–timeinourpastwasfixedbecausewehaveobserveditandseenthatitisnotwarpedenoughtoallowtravelintothepast.Ontheotherhandthefutureisopen.Sowemightbeabletowarpitenoughtoallowtimetravel.Butbecausewecanwarpspace–timeonlyinthefuture,wewouldn’tbeabletotravelbacktothepresenttimeorearlier.This picture would explain why we haven’t been overrun by tourists

fromthe future.But itwould still leaveplentyofparadoxes.Suppose itwerepossibletogooffinarocketshipandcomebackbeforeyouhadsetoff. What would stop you blowing up the rocket on its launch pad orotherwise preventing yourself from setting out in the first place?There

are other versions of this paradox, like going back and killing yourparentsbeforeyouwereborn,buttheyareessentiallyequivalent.Thereseemtobetwopossibleresolutions.One iswhat I shall call theconsistent-historiesapproach. It says that

onehas to findaconsistent solutionof theequationsofphysicseven ifspace–timeissowarpedthatitispossibletotravelintothepast.Onthisviewyoucouldn’tsetoutontherocketshiptotravelintothepastunlessyouhadalreadycomebackandfailedtoblowupthelaunchpad.It isaconsistent picture, but it would imply that we were completelydetermined:wecouldn’tchangeourminds.Somuchforfreewill.TheotherpossibilityiswhatIcallthealternative-historiesapproach.It

has been championed by the physicist David Deutsch and it seems tohave beenwhat the creator ofBack to the Future had inmind. In thisview, in one alternative history there would not have been any returnfromthefuturebeforetherocketsetoffandsonopossibilityofitbeingblown up. But when the traveller returns from the future he entersanotheralternativehistory.Inthisthehumanracemakesatremendouseffort to build a spaceship but just before it is due to be launched asimilarspaceship,appearsfromtheothersideofthegalaxyanddestroysit.David Deutsch claims support for the alternative-histories approach

fromthesum-over-historiesconceptintroducedbythephysicistRichardFeynman. The idea is that according to quantum theory the universedoesn’t justhaveauniquesinglehistory.Insteadtheuniversehaseverysingle possible history, each with its own probability. Theremust be apossible history in which there is a lasting peace in the Middle East,thoughmaybetheprobabilityislow.Insomehistoriesspace–timewillbesowarpedthatobjectslikerockets

will be able to travel into their pasts. But each history is complete andself-contained, describing not only the curved space–time but also theobjects in it. So a rocket cannot transfer to another alternative historywhenitcomesroundagain.Itisstillinthesamehistorywhichhastobeself-consistent.ThusdespitewhatDeutschclaimsI thinkthesum-over-historiesideasupportstheconsistent-historieshypothesisratherthanthealternative-historiesidea.It thus seems thatwe are stuckwith the consistent-histories picture.

However,thisneednotinvolveproblemswithdeterminismorfreewillifthe probabilities are very small for histories in which space–time is sowarped that time travel is possible over a macroscopic region. This iswhat I call the Chronology Protection Conjecture: the laws of physicsconspiretopreventtimetravelonamacroscopicscale.It seems that what happens is that when space–time gets warped

almost enough to allow travel into the past virtual particles can almostbecome real particles following closed trajectories. The density of thevirtualparticlesandtheirenergybecomeverylarge.Thismeansthattheprobability of thesehistories is very low.Thus it seems theremaybe aChronology Protection Agency at work making the world safe forhistorians.Butthissubjectofspaceandtimewarpsisstillinitsinfancy.AccordingtoaunifyingformofstringtheoryknownasM-theory,whichisourbesthopeofunitinggeneralrelativityandquantumtheory,space–time ought to have eleven dimensions, not just the four that weexperience.Theideaisthatsevenoftheseelevendimensionsarecurledupintoaspacesosmallthatwedon’tnoticethem.Ontheotherhandtheremainingfourdirectionsarefairlyflatandarewhatwecallspace–time.Ifthispictureiscorrectitmightbepossibletoarrangethatthefourflatdirections get mixed up with the seven highly curved or warpeddirections.Whatthiswouldgiverisetowedon’tyetknow.But itopensexcitingpossibilities.Inconclusion,rapidspacetravelandtravelbackintimecan’tberuled

out according to our present understanding. They would cause greatlogical problems, so let’s hope there’s a Chronology Protection Law toprevent people going back and killing their parents. But science-fictionfansneednotloseheart.There’shopeinM-theory.

Isthereanypointinhostingapartyfortimetravellers?

Wouldyouhopeanyonewouldturnup?

In2009Iheldapartyfortimetravellersinmycollege,GonvilleandCaiusinCambridge,forafilmabouttimetravel.Toensurethatonlygenuinetimetravellerscame,Ididn’t

sendouttheinvitationsuntilaftertheparty.Onthedayofthe

party,Isatincollegehoping,butnoonecame.Iwasdisappointed,butnotsurprised,becauseIhadshownthatifgeneralrelativityiscorrectandenergydensityispositive,

timetravelisnotpossible.Iwouldhavebeendelightedifoneofmyassumptionshadturnedouttobewrong.

7

WILLWESURVIVEONEARTH?

InJanuary2018,theBulletinoftheAtomicScientists,ajournalfoundedbysomeof thephysicistswhohadworkedon theManhattanProject toproduce the first atomic weapons, moved the Doomsday Clock, theirmeasurement of the imminence of catastrophe—military orenvironmental—facingourplanet,forwardtotwominutestomidnight.Theclockhasan interestinghistory. Itwas started in 1947,at a time

whentheatomicagehadonlyjustbegun.RobertOppenheimer,thechiefscientistfortheManhattanProject,saidlaterofthefirstexplosionofanatomicbombtwoyearsearlier inJuly1945,“Weknewtheworldwouldnotbethesame.Afewpeople laughed,a fewpeoplecried,mostpeoplewere silent. I remembered the line from the Hindu scripture, theBhagavad-Gita,‘Now,IambecomeDeath,thedestroyerofworlds.’ ”In1947,theclockwasoriginallysetatsevenminutestomidnight.Itis

now closer toDoomsday than at any time since then, save in the early1950sat thestartof theColdWar.Theclockand itsmovementsare,ofcourse, entirely symbolicbut I feel compelled topointout that suchanalarmingwarningfromotherscientists,promptedatleastinpartbytheelectionofDonaldTrump,mustbetakenseriously.Istheclock,andtheideathattimeistickingorevenrunningoutforthehumanrace,realisticoralarmist?Isitswarningtimelyortime-wasting?Ihaveaverypersonalinterestintime.Firstly,mybestsellingbook,and

themain reason that I am known beyond the confines of the scientificcommunity,wascalledABriefHistoryofTime.Sosomemightimaginethat Iamanexperton time,althoughof course thesedaysanexpert isnotnecessarilyagoodthingtobe.Secondly,assomeonewhoattheageof

twenty-onewastoldbytheirdoctorsthattheyhadonlyfiveyearstolive,andwhoturnedseventy-six in2018, Iamanexperton time inanothersense,amuchmorepersonalone.Iamuncomfortably,acutelyawareofthepassageoftime,andhavelivedmuchofmylifewithasensethatthetimethatIhavebeengrantedis,astheysay,borrowed.Itiswithoutdoubtthecasethatourworldismorepoliticallyunstable

thanatanytimeinmymemory.Largenumbersofpeoplefeelleftbehindbotheconomicallyandsocially.Asaresult,theyareturningtopopulist—or at least popular—politicians who have limited experience ofgovernmentandwhoseabilitytotakecalmdecisionsinacrisishasyettobetested.SothatwouldimplythataDoomsdayClockshouldbemovedcloser to a critical point, as theprospect of careless ormalicious forcesprecipitatingArmageddongrows.TheEarthisunderthreatfromsomanyareasthatitisdifficultforme

tobepositive.Thethreatsaretoobigandtoonumerous.First,theEarthisbecomingtoosmallforus.Ourphysicalresourcesare

beingdrainedatanalarmingrate.Wehavepresentedourplanetwiththedisastrous gift of climate change.Rising temperatures, reduction of thepolaricecaps,deforestation,over-population,disease,war,famine, lackofwateranddecimationofanimal species; theseareall solvablebut sofarhavenotbeensolved.Globalwarmingiscausedbyallofus.Wewantcars,travelandabetter

standard of living. The trouble is, by the time people realise what ishappening, it may be too late. As we stand on the brink of a SecondNuclear Age and a period of unprecedented climate change, scientistshave a special responsibility, once again, to inform the public and toadvise leaders about the perils that humanity faces. As scientists, weunderstandthedangersofnuclearweapons,andtheirdevastatingeffects,andwearelearninghowhumanactivitiesandtechnologiesareaffectingclimatesystemsinwaysthatmayforeverchangelifeonEarth.Ascitizensof theworld,we have a duty to share that knowledge, and to alert thepublic to the unnecessary risks thatwe livewith every day.We foreseegreatperilifgovernmentsandsocietiesdonottakeactionnow,torendernuclearweaponsobsoleteandtopreventfurtherclimatechange.At the same time, many of those same politicians are denying the

reality of man-made climate change, or at least the ability of man to

reverseit, justatthemomentthatourworldisfacingaseriesofcriticalenvironmental crises. The danger is that global warming may becomeself-sustaining,ifithasnotbecomesoalready.ThemeltingoftheArcticandAntarcticicecapsreducesthefractionofsolarenergyreflectedbackintospace,andsoincreasesthetemperaturefurther.ClimatechangemaykillofftheAmazonandotherrainforestsandsoeliminateoneofthemainwaysinwhichcarbondioxideisremovedfromtheatmosphere.Theriseinsea temperaturemay trigger the releaseof largequantitiesof carbondioxide. Both these phenomena would increase the greenhouse effect,and soexacerbateglobalwarming.Botheffects couldmakeour climatelike that of Venus: boiling hot and raining sulphuric acid, with atemperatureof250degreescentigrade(482degreesFahrenheit).Humanlifewouldbeunsustainable.Weneed to gobeyond theKyotoProtocol,the international agreementadopted in 1997, and cut carbonemissionsnow.Wehavethetechnology.Wejustneedthepoliticalwill.Wecanbeanignorant,unthinkinglot.Whenwehavereachedsimilar

crisesinourhistory,therehasusuallybeensomewhereelsetocolonise.Columbus did it in 1492when he discovered theNewWorld. But nowthereisnonewworld.NoUtopiaaroundthecorner.Wearerunningoutofspaceandtheonlyplacestogotoareotherworlds.The universe is a violent place. Stars engulf planets, supernovae fire

lethalraysacrossspace,blackholesbumpintoeachotherandasteroidshurtlearoundathundredsofmilesasecond.Granted,thesephenomenado notmake space sound very inviting, but these are the very reasonswhy we should venture into space instead of staying put. An asteroidcollisionwouldbesomethingagainstwhichwehavenodefence.Thelastbigsuchcollisionwithuswasaboutsixty-sixmillionyearsagoandthatisthoughttohavekilledthedinosaurs,anditwillhappenagain.Thisisnotsciencefiction;itisguaranteedbythelawsofphysicsandprobability.Nuclear war is still probably the greatest threat to humanity at the

present time. It is a danger we have rather forgotten. Russia and theUnited States are no longer so trigger-happy, but suppose there’s anaccident,orterroristsgetholdoftheweaponsthesecountriesstillhave.Andtheriskincreasesthemorecountriesobtainnuclearweapons.Evenafter the end of the ColdWar, there are still enough nuclear weaponsstockpiledtokillusall,severaltimesover,andnewnuclearnationswilladd to the instability.With time, the nuclear threatmay decrease, but

otherthreatswilldevelop,sowemustremainonourguard.One way or another, I regard it as almost inevitable that either a

nuclearconfrontationorenvironmentalcatastrophewillcrippletheEarthat somepoint in thenext 1,000yearswhich,asgeological timegoes, isthemereblinkofaneye.BythenIhopeandbelievethatour ingeniousrace will have found a way to slip the surly bonds of Earth and willthereforesurvivethedisaster.Thesameofcoursemaynotbepossibleforthemillionsofother species that inhabit theEarth, and thatwill beonourconscienceasarace.Ithinkweareactingwithrecklessindifferencetoourfutureonplanet

Earth.Atthemoment,wehavenowhereelsetogo,butinthelongrunthehumanraceshouldn’thaveallitseggsinonebasket,orononeplanet.IjusthopewecanavoiddroppingthebasketbeforewelearnhowtoescapefromEarth.Butweare,bynature,explorers.Motivatedbycuriosity.Thisisauniquelyhumanquality.ItisthisdrivencuriositythatsentexplorerstoprovetheEarthisnotflatanditisthesameinstinctthatsendsustothe stars at the speed of thought, urging us to go there in reality. Andwhenever we make a great new leap, such as the Moon landings, weelevate humanity, bring people and nations together, usher in newdiscoveries and new technologies. To leaveEarth demands a concertedglobal approach—everyone should join in. We need to rekindle theexcitementoftheearlydaysofspacetravelinthe1960s.Thetechnologyis almost within our grasp. It is time to explore other solar systems.Spreadingoutmaybe theonly thingthatsavesus fromourselves. Iamconvinced that humans need to leave Earth. If we stay, we risk beingannihilated.

•

So,beyondmyhopeforspaceexploration,whatwillthefuturelooklikeandhowmightsciencehelpus?Thepopularpictureofscienceinthefutureisshowninscience-fiction

series likeStarTrek.TheproducersofStarTrek evenpersuadedme totakepart,notthatitwasdifficult.That appearance was great fun, but I mention it to make a serious

point.NearlyallthevisionsofthefuturethatwehavebeenshownfromH.G.Wellsonwardshavebeenessentiallystatic.Theyshowasocietythatis inmostcases far inadvanceofours, inscience, in technologyand inpolitical organisation. (The last might not be difficult.) In the periodbetween now and then theremust have been great changes, with theiraccompanying tensions and upsets. But, by the timewe are shown thefuture,science, technologyandtheorganisationofsocietyaresupposedtohaveachievedalevelofnear-perfection.Iquestionthispictureandaskifwewilleverreachafinalsteadystate

ofscienceandtechnology.Atnotimeinthe10,000yearsorsosincethelast IceAgehas the human race been in a state of constant knowledgeandfixedtechnology.Therehavebeenafewsetbacks,likewhatweusedtocalltheDarkAgesafterthefalloftheRomanEmpire.Buttheworld’spopulation,whichisameasureofourtechnologicalabilitytopreservelifeand feedourselves,has risensteadily,witha fewhiccups like theBlackDeath.Inthelast200yearsthegrowthhasattimesbeenexponential—andtheworldpopulationhasjumpedfrom1billiontoabout7.6billion.Other measures of technological development in recent times areelectricity consumption, or the number of scientific articles. They tooshow near-exponential growth. Indeed, we now have such heightenedexpectations that somepeople feel cheated by politicians and scientistsbecausewehavenotalreadyachievedtheUtopianvisionsof thefuture.Forexample,thefilm2001:ASpaceOdysseyshoweduswithabaseontheMoonandlaunchingamanned,orshouldIsaypersonned, flighttoJupiter.There is no sign that scientific and technological development will

dramaticallyslowdownandstopinthenearfuture.CertainlynotbythetimeofStarTrek,which is only about350years away.But thepresentrateofgrowthcannotcontinueforthenextmillennium.Bytheyear2600theworld’s populationwouldbe standing shoulder to shoulder and theelectricity consumption would make the Earth glow red hot. If youstackedthenewbooksbeingpublishednexttoeachother,atthepresentrateofproductionyouwouldhavetomoveatninetymilesanhourjusttokeep up with the end of the line. Of course, by 2600 new artistic andscientificworkwillcomeinelectronicformsratherthanasphysicalbooksand papers. Nevertheless, if the exponential growth continued, therewouldbe tenpapersasecond inmykindof theoreticalphysics,andno

timetoreadthem.Clearlythepresentexponentialgrowthcannotcontinueindefinitely.So

what will happen? One possibility is that we will wipe ourselves outthrough some disaster such as a nuclear war. Even if we don’t destroyourselvescompletelythereisthepossibilitythatwemightdescendintoastateofbrutalismandbarbarity,liketheopeningsceneofTerminator.How will we develop in science and technology over the next

millennium?Thisisverydifficulttoanswer.Butletmestickmyneckoutandoffermypredictionsforthefuture.Iwillhavesomechanceofbeingrightaboutthenexthundredyears,buttherestofthemillenniumwillbewildspeculation.Ourmodern understanding of science began about the same time as

the European settlement of North America, and by the end of thenineteenthcentury it seemed thatwewereabout toachievea completeunderstanding of the universe in terms of what are now known asclassicallaws.But,aswehaveseen,inthetwentiethcenturyobservationsbegantoshowthatenergycameindiscretepacketscalledquantaandanew kind of theory called quantummechanics was formulated byMaxPlanckandothers.Thispresentedacompletelydifferentpictureofrealityin which things don’t have a single unique history, but have everypossible history eachwith its ownprobability.When one goes down tothe individual particles, the possible particle histories have to includepaths that travel faster than light and even paths that go back in time.However,thesepathsthatgobackintimearenotjustlikeangelsdancingonapin.Theyhaverealobservationalconsequences.Evenwhatwethinkofasemptyspaceisfullofparticlesmovinginclosedloopsinspaceandtime. That is, theymove forwards in time on one side of the loop andbackwardsintimeontheotherside.Theawkwardthingisthatbecausethere’saninfinitenumberofpoints

inspaceandtime,there’saninfinitenumberofpossibleclosedloopsofparticles.Andaninfinitenumberofclosedloopsofparticleswouldhaveaninfiniteamountofenergyandwouldcurlspaceandtimeuptoasinglepoint. Even science fiction did not think of anything as odd as this.Dealingwiththisinfiniteenergyrequiressomereallycreativeaccounting,andmuchoftheworkintheoreticalphysicsinthelasttwentyyearshasbeenlookingforatheoryinwhichtheinfinitenumberofclosedloopsin

spaceandtimecanceleachothercompletely.Onlythenwillwebeabletounify quantum theory with Einstein’s general relativity and achieve acompletetheoryofthebasiclawsoftheuniverse.Whataretheprospectsthatwewilldiscoverthiscompletetheoryinthe

next millennium? I would say they were very good, but then I’m anoptimist. In 1980 I said I thought there was a 50–50 chance that wewould discover a complete unified theory in the next twenty years.Wehavemade some remarkable progress in the period since then, but thefinaltheoryseemsaboutthesamedistanceaway.Will theHolyGrailofphysicsbealwaysjustbeyondourreach?Ithinknot.Atthebeginningofthetwentiethcenturyweunderstoodtheworkings

ofnatureonthescalesofclassicalphysicsthataregooddowntoaboutahundredthofamillimetre.Theworkonatomicphysicsinthefirstthirtyyears of the century took our understanding down to lengths of amillionth of a millimetre. Since then, research on nuclear and high-energyphysicshastakenustolengthscalesthataresmallerbyafurtherfactorofabillion.Itmightseemthatwecouldgoonforeverdiscoveringstructuresonsmallerandsmallerlengthscales.However,thereisalimittothisseriesaswithaseriesofnestedRussiandolls.Eventuallyonegetsdowntoasmallestdoll,whichcan’tbetakenapartanymore.InphysicsthesmallestdolliscalledthePlancklengthandisamillimetredividedbya 100,000 billion billion billion. We are not about to build particleacceleratorsthatcanprobetodistancesthatsmall.Theywouldhavetobelargerthanthesolarsystemandtheyarenotlikelytobeapprovedinthepresent financial climate. However, there are consequences of ourtheoriesthatcanbetestedbymuchmoremodestmachines.It won’t be possible to probe down to the Planck length in the

laboratory, though we can study the Big Bang to get observationalevidenceathigherenergiesandshorterlengthscalesthanwecanachieveon Earth. However, to a large extent we shall have to rely onmathematical beauty and consistency to find the ultimate theory ofeverything.TheStarTrekvisionofthefutureinwhichweachieveanadvancedbut

essentiallystaticlevelmaycometrueinrespectofourknowledgeofthebasiclawsthatgoverntheuniverse.ButIdon’tthinkwewilleverreachasteadystate intheuseswemakeof these laws.Theultimatetheorywill

placenolimitonthecomplexityofsystemsthatwecanproduce,anditisin this complexity that I think themost importantdevelopmentsof thenextmillenniumwillbe.

•

By far themostcomplexsystems thatwehaveareourownbodies.LifeseemstohaveoriginatedintheprimordialoceansthatcoveredtheEarthfourbillionyearsago.Howthishappenedwedon’tknow.Itmaybethatrandom collisions between atoms built up macro-molecules that couldreproduce themselves and assemble themselves into more complicatedstructures.WhatwedoknowisthatbythreeandahalfbillionyearsagothehighlycomplicatedmoleculeDNAhademerged.DNAisthebasisforall life on Earth. It has a double-helix structure, like a spiral staircase,which was discovered by Francis Crick and James Watson in theCavendishlabatCambridgein1953.Thetwostrandsofthedoublehelixare linked by pairs of nitrogenous bases like the treads in a spiralstaircase. There are four kinds of nitrogenous bases: cytosine, guanine,adenineandthymine.Theorderinwhichthedifferentnitrogenousbasesoccur along the spiral staircase carries the genetic information thatenables the DNA molecule to assemble an organism around it andreproduceitself.AstheDNAmadecopiesofitselftherewouldhavebeenoccasionalerrorsintheorderofthenitrogenousbasesalongthespiral.InmostcasesthemistakesincopyingwouldhavemadetheDNAunabletoreproduce itself. Such genetic errors, or mutations as they are called,woulddieout.Butinafewcasestheerrorormutationwouldincreasethechances of the DNA surviving and reproducing. Thus the informationcontentinthesequenceofnitrogenousbaseswouldgraduallyevolveandincrease in complexity. This natural selection of mutations was firstproposedby anotherCambridgeman,CharlesDarwin, in 1858, thoughhedidn’tknowthemechanismforit.Becausebiologicalevolutionisbasicallyarandomwalkinthespaceof

allgeneticpossibilities,ithasbeenveryslow.Thecomplexity,ornumberof bits of information that are coded in DNA, is given roughly by thenumberofnitrogenousbasesinthemolecule.Eachbitofinformationcan

bethoughtofastheanswertoayes/noquestion.Forthefirsttwobillionyearsorsotherateofincreaseincomplexitymusthavebeenoftheorderof one bit of information every hundred years. The rate of increase ofDNAcomplexitygraduallyrosetoaboutonebitayearoverthelast fewmillionyears.ButnowweareatthebeginningofanewerainwhichwewillbeabletoincreasethecomplexityofourDNAwithouthavingtowaitfor the slow process of biological evolution. There has been relativelylittlechangeinhumanDNAinthelast10,000years.Butitislikelythatwewillbeabletoredesignitcompletelyinthenextthousand.Ofcourse,many people will say that genetic engineering on humans should bebanned.But I rather doubt that theywill be able to prevent it.Geneticengineeringonplantsandanimalswillbeallowedforeconomicreasons,andsomeoneisboundtotryitonhumans.Unlesswehaveatotalitarianworldorder,someonewilldesignimprovedhumanssomewhere.Clearly developing improved humans will create great social and

political problems with respect to unimproved humans. I’m notadvocating human genetic engineering as a good thing, I’m just sayingthatitislikelytohappeninthenextmillennium,whetherwewantitornot.ThisiswhyIdon’tbelievesciencefictionlikeStarTrekwherepeopleareessentiallythesame350yearsinthefuture.Ithinkthehumanrace,anditsDNA,willincreaseitscomplexityquiterapidly.In a way, the human race needs to improve itsmental and physical

qualitiesifitistodealwiththeincreasinglycomplexworldarounditandmeet new challenges like space travel. And it also needs to increase itscomplexity ifbiologicalsystemsaretokeepaheadofelectronicones.Atthemoment computers have an advantage of speed, but they show nosignofintelligence.Thisisnotsurprisingbecauseourpresentcomputersarelesscomplexthanthebrainofanearthworm,aspeciesnotnotedforitsintellectualpowers.ButcomputersroughlyobeyaversionofMoore’sLaw,which says that their speed and complexitydouble every eighteenmonths. It is one of these exponential growths that clearly cannotcontinueindefinitely,andindeedithasalreadybeguntoslow.However,the rapid pace of improvement will probably continue until computershave a similar complexity to the human brain. Some people say thatcomputerscannevershowtrueintelligence,whateverthatmaybe.Butitseemstomethat ifverycomplicatedchemicalmoleculescanoperate inhumans to make them intelligent, then equally complicated electronic

circuitscanalsomakecomputersactinanintelligentway.Andiftheyareintelligenttheycanpresumablydesigncomputersthathaveevengreatercomplexityandintelligence.This iswhy I don’t believe the science-fictionpicture of an advanced

but constant future. Instead, I expect complexity to increase at a rapidrate, inboththebiologicalandtheelectronicspheres.Notmuchof thiswill happen in the next hundred years, which is all we can reliablypredict.Butbytheendofthenextmillennium,ifwegetthere,thechangewillbefundamental.LincolnSteffens once said, “I have seen the future and itworks.”He

wasactually talkingabout theSovietUnion,whichwenowknowdidn’tworkverywell.Nevertheless,Ithinkthepresentworldorderhasafuture,butitwillbeverydifferent.

Whatisthebiggestthreattothefutureofthisplanet?

Anasteroidcollisionwouldbe—athreatagainstwhichwehavenodefence.Butthelastbigsuchasteroidcollisionwasaboutsixty-sixmillionyearsagoandkilledthedinosaurs.Amoreimmediatedangerisrunawayclimatechange.Ariseinoceantemperaturewouldmelttheicecapsandcausethereleaseoflargeamountsofcarbondioxide.Botheffects

couldmakeourclimatelikethatofVenuswithatemperatureof250degreescentigrade(482degreesFahrenheit).

8

SHOULDWECOLONISESPACE?

Whyshouldwegointospace?Whatisthejustificationforspendingallthateffortandmoneyongettingafewlumpsofmoonrock?Aren’ttherebettercauseshereonEarth?Theobviousanswerisbecauseit’sthere,allaroundus.NottoleaveplanetEarthwouldbelikecastawaysonadesertislandnot trying toescape.Weneedtoexplore thesolarsystemto findoutwherehumanscouldlive.Inaway,thesituationislikethatinEuropebefore1492.Peoplemight

wellhavearguedthatitwasawasteofmoneytosendColumbusonawildgoose chase. Yet the discovery of the New World made a profounddifference to theOld. Just think,wewouldn’t havehad theBigMacorKFC. Spreading out into space will have an even greater effect. It willcompletely change the future of thehuman race, andmaybedeterminewhetherwehave any future at all. Itwon’t solve any of our immediateproblemsonplanetEarth,but itwillgiveusanewperspectiveonthemand cause us to look outwards rather than inwards. Hopefully, it willuniteustofacethecommonchallenge.Thiswouldbealong-termstrategy,andbylongtermImeanhundreds

or even thousands of years.We could have a base on theMoonwithinthirtyyears,reachMarsinfiftyyearsandexplorethemoonsoftheouterplanetsin200years.Byreach,Imeaninspacecraftwithhumansaboard.Wehavealreadydriven roversonMarsand landedaprobeonTitan,amoonofSaturn,but ifweareconsidering the futureof thehumanracewehavetogothereourselves.Going into space won’t be cheap, but it would take only a small

proportion of world resources. NASA’s budget has remained roughly

constant in real terms since the time of theApollo landings, but it hasdecreasedfrom0.3percentofUSGDPin1970toabout0.1percentin2017.Evenifweweretoincreasetheinternationalbudgettwentytimes,tomakeaseriousefforttogointospace,itwouldonlybeasmallfractionofworldGDP.There will be those who argue that it would be better to spend our

money solving the problems of this planet, like climate change andpollution,ratherthanwasting itonapossibly fruitlesssearchforanewplanet. I’m not denying the importance of fighting climate change andglobalwarming,butwecandothatandstillspareaquarterofapercentofworldGDPforspace.Isn’tourfutureworthaquarterofapercent?We thought space was worth a big effort in the 1960s. In 1962,

PresidentKennedycommittedtheUStolandingamanontheMoonbytheendofthedecade.OnJuly20,1969,BuzzAldrinandNeilArmstronglandedon thesurfaceof theMoon. It changed the futureof thehumanrace. Iwas twenty-seven at the time, a researcher at Cambridge, and Imissedit.IwasatameetingonsingularitiesinLiverpoolandlisteningtoa lecture by René Thom on catastrophe theory when the landing tookplace. There was no catch-up TV in those days, and we didn’t have atelevision,butmysonagedtwodescribedittome.The space race helped to create a fascination with science and

accelerated our technological progress.Many of today’s scientists wereinspiredtogointoscienceasaresultoftheMoonlandings,withtheaimofunderstandingmoreaboutourselvesandourplaceintheuniverse.Itgave us new perspectives on our world, prompting us to consider theplanetasawhole.However,afterthelastMoonlandingin1972,withnofuture plans for further manned space flight, public interest in spacedeclined.ThiswentalongwithageneraldisenchantmentwithscienceintheWest,becausealthoughithadbroughtgreatbenefitsithadnotsolvedthesocialproblemsthatincreasinglyoccupiedpublicattention.Anewcrewedspaceflightprogrammewoulddoalottorestorepublic

enthusiasm for space and for science generally. Robotic missions aremuch cheaper and may provide more scientific information, but theydon’t catch the public imagination in the same way. And they don’tspreadthehumanraceintospace,whichI’marguingshouldbeourlong-termstrategy.Agoalofabaseon theMoonby2050,andofamanned

landingonMarsby2070,wouldreignitethespaceprogramme,andgiveit a sense of purpose, in the sameway that PresidentKennedy’sMoontargetdidinthe1960s.Inlate2017,ElonMuskannouncedSpaceXplansfor a lunar base and a Mars mission by 2022, and President Trumpsigned a space policy directive refocusing NASA on exploration anddiscovery,soperhapswe’llgetthereevensooner.A new interest in space would also increase the public standing of

sciencegenerally.Thelowesteeminwhichscienceandscientistsareheldishaving serious consequences.We live ina society that is increasinglygoverned by science and technology, yet fewer and fewer young peoplewant to go into science.A new and ambitious space programmewouldexcite the young and stimulate them into entering a wide range ofsciences,notjustastrophysicsandspacescience.Thesameistrueforme.Ihavealwaysdreamedofspaceflight.Butfor

somanyyearsIthoughtitwasjustthat,adream.ConfinedtoEarthandin a wheelchair, how could I experience the majesty of space exceptthroughimaginationandmyworkintheoreticalphysics.IneverthoughtIwouldhave the opportunity to see our beautiful planet from space orgazeoutintotheinfinitybeyond.Thiswasthedomainofastronauts,theluckyfewwhogettoexperiencethewonderandthrillofspaceflight.ButI had not factored in the energy and enthusiasm of individuals whosemission it is to take that first step in venturing outside Earth. And in2007Iwasfortunateenoughtogoonazero-gravityflightandexperienceweightlessnessforthefirsttime.Itonlylastedforfourminutes,butitwasamazing.Icouldhavegoneonandon.IwasquotedatthetimeassayingthatIfearedthehumanraceisnot

goingtohaveafutureifwedon’tgointospace.Ibelieveditthen,andIbelieveitstill.AndIhopeIdemonstratedthenthatanyonecantakepartin space travel. I believe it is up to scientists like me, together withinnovative commercial entrepreneurs, to do all we can to promote theexcitementandwonderofspacetravel.But can humans exist for long periods away from the Earth? Our

experiencewiththeISS,theInternationalSpaceStation,showsthatitispossibleforhumanbeingstosurviveformanymonthsawayfromplanetEarth.However,thezerogravityoforbitcausesanumberofundesirablephysiological changes, including a weakening of the bones, as well as

creatingpracticalproblemswithliquidsandsoon.Onewouldthereforewantanylong-termbaseforhumanbeingstobeonaplanetormoon.Bydiggingintothesurface,onewouldgetthermalinsulation,andprotectionfrommeteorsandcosmicrays.Theplanetormooncouldalsoserveasasourceof therawmaterials thatwouldbeneededif theextra-terrestrialcommunitywastobeself-sustaining,independentofEarth.Whatarethepossiblesitesofahumancolonyinthesolarsystem?The

mostobviousistheMoon.Itisclosebyandrelativelyeasytoreach.Wehavealready landedonit,anddrivenacross it inabuggy.Ontheotherhand,theMoonissmall,andwithoutatmosphere,oramagneticfieldtodeflect the solar-radiation particles, like on Earth. There is no liquidwater, although theremaybe ice in the craters at theNorth andSouthPoles.Acolonyon theMooncoulduse thisasa sourceofoxygen,withpowerprovidedbynuclearenergyorsolarpanels.TheMooncouldbeabasefortraveltotherestofthesolarsystem.Marsistheobviousnexttarget.ItishalfasfaragainastheEarthfrom

theSun, and so receiveshalf thewarmth. It oncehadamagnetic field,but it decayed four billion years ago, leaving Mars without protectionfrom solar radiation. This stripped Mars of most of its atmosphere,leavingitwithonly1percentofthepressureoftheEarth’satmosphere.However,thepressuremusthavebeenhigherinthepast,becauseweseewhat appear to be run-off channels and dried-up lakes. Liquid watercannotexiston thesurfaceofMarsnow. Itwouldvaporise in thenear-vacuum.This suggests thatMarshadawarmwetperiod,duringwhichlifemight have appeared, either spontaneously or through panspermia(thatis,broughtfromsomewhereelseintheuniverse).ThereisnosignoflifeonMarsnow,but ifwe foundevidence that lifehadonceexisted itwouldindicatethattheprobabilityoflifedevelopingonasuitableplanetwas fairly high.Wemust be careful, though, thatwe don’t confuse theissue by contaminating the planet with life from Earth. Similarly, wemustbeverycarefulnot tobringbackanyMartian life.Wewouldhavenoresistancetoit,anditmightwipeoutlifeonEarth.NASA has sent a large number of spacecraft to Mars, starting with

Mariner4in1964.Ithassurveyedtheplanetwithanumberoforbiters,the latest being the Mars reconnaissance orbiter. These orbiters haverevealed deep gulleys and the highest mountains in the solar system.NASAhasalsolandedanumberofprobesonthesurfaceofMars,most

recently the two Mars rovers. These have sent back pictures of a drydesert landscape. Like on the Moon, water and oxygen might beobtainablefrompolarice.TherehasbeenvolcanicactivityonMars.Thiswouldhavebroughtmineralsandmetals to thesurface,whichacolonycoulduse.TheMoonandMarsarethemostsuitablesitesforspacecoloniesinthe

solar system.MercuryandVenusare toohot,whileJupiter andSaturnare gas giantswithno solid surface.ThemoonsofMars are very smallandhavenoadvantagesoverMars itself.Someof themoonsofJupiterandSaturnmightbepossible.Europa,amoonofJupiter,hasafrozenicesurface. But theremay be liquid water under the surface in which lifecould have developed. How can we find out? Do we have to land onEuropaanddrillahole?Titan, amoon of Saturn, is larger andmoremassive than ourMoon

andhasadenseatmosphere.TheCassini–HuygensmissionofNASAandtheEuropeanSpaceAgencyhaslandedaprobeonTitanwhichhassentbackpicturesof the surface.However, it is very cold,being so far fromtheSun,andIwouldn’tfancylivingnexttoalakeofliquidmethane.But what about boldly going beyond the solar system? Our

observations indicate that a significant fraction of stars have planetsaround them. So far,we candetect only giant planets, like Jupiter andSaturn,but it is reasonable toassumethat theywillbeaccompaniedbysmaller,Earth-likeplanets.SomeofthesewilllieintheGoldilockszone,wherethedistancefromthestarisintherightrangeforliquidwatertoexist on their surface. There are around a thousand stars within thirtylightyearsofEarth.If1percentofthesehaveEarth-sizedplanetsintheGoldilockszone,wehavetencandidateNewWorlds.Take Proxima b, for example. This exoplanet, which is the closest to

Earthbut still four andahalf light years away, orbits the starProximaCentauri within the solar system Alpha Centauri, and recent researchindicatesthatithassomesimilaritiestoEarth.Travellingtothesecandidateworldsisn’tpossibleperhapswithtoday’s

technology,butbyusingourimaginationwecanmakeinterstellartravela long-termaim—in thenext200 to500years.The speedatwhichwecansendarocketisgovernedbytwothings,thespeedoftheexhaustandthefractionofitsmassthattherocketlosesasitaccelerates.Theexhaust

speed of chemical rockets, like the ones we have used so far, is aboutthreekilometrespersecond.Byjettisoning30percentoftheirmass,theycanachieveaspeedofabouthalfakilometrepersecondandthenslowdown again. According to NASA, it would take as little as 260 days toreachMars,giveortaketendays,withsomeNASAscientistspredictingas little as 130days.But itwould take threemillion years to get to thenearest star system. To go fasterwould require amuch higher exhaustspeed thanchemical rocketscanprovide, thatof light itself.Apowerfulbeamof light from the rear coulddrive the spaceship forward.Nuclearfusion could provide 1 per cent of the spaceship’s mass energy, whichwouldaccelerateittoatenthofthespeedoflight.Beyondthat,wewouldneedeithermatter–antimatterannihilationorsomecompletelynewformofenergy.Infact,thedistancetoAlphaCentauriissogreatthattoreachitinahumanlifetimeaspacecraftwouldhavetocarryfuelwithroughlythe mass of all the stars in the galaxy. In other words, with currenttechnology interstellar travel is utterly impractical. Alpha Centauri canneverbecomeaholidaydestination.Wehaveachancetochangethat,thankstoimaginationandingenuity.

In 2016 I joined with the entrepreneur Yuri Milner to launchBreakthrough Starshot, a long-term research and developmentprogrammeaimedatmaking interstellar travel a reality. Ifwe succeed,wewillsendaprobetoAlphaCentauriwithinthelifetimeofpeoplealivetoday.ButIwillreturntothisshortly.How do we start this journey? So far, our explorations have been

limited to our local cosmic neighbourhood. Forty years on, our mostintrepid explorer, Voyager, has just made it to interstellar space. Itsspeed,elevenmilesasecond,meansitwouldtakeabout70,000yearstoreachAlphaCentauri.Thisconstellationis4.37lightyearsaway,twenty-fivetrillionmiles.IftherearebeingsaliveonAlphaCentauritoday,theyremainblissfullyignorantoftheriseofDonaldTrump.Itisclearweareenteringanewspaceage.Thefirstprivateastronauts

will bepioneers, and the first flightswill behugely expensive, but overtime it ismyhope that space flightwill becomewithin the reachof farmoreof theEarth’spopulation.Takingmoreandmorepassengers intospace will bring new meaning to our place on Earth and to ourresponsibilitiesasitsstewards,anditwillhelpustorecogniseourplaceandfutureinthecosmos—whichiswhereIbelieveourultimatedestiny

lies.Breakthrough Starshot is a real opportunity for man to make early

forays into outer space, with a view to probing and weighing thepossibilitiesofcolonisation.Itisaproof-of-conceptmissionandworksonthree concepts: miniaturised spacecraft, light propulsion and phase-lockedlasers.TheStarChip,afullyfunctionalspaceprobereducedtoafew centimetres in size, will be attached to a light sail. Made frommetamaterials, the light sail weighs no more than a few grams. It isenvisagedthatathousandStarChipsandlightsails,thenanocraft,willbesent intoorbit.On theground,anarrayof lasersat thekilometre scalewill combine into a single, verypowerful lightbeam.Thebeam is firedthroughtheatmosphere,strikingthesailsinspacewithtensofgigawattsofpower.Theideabehindthisinnovationisthatthenanocraftrideonthelight

beammuchasEinsteindreamedaboutridingalightbeamattheageofsixteen.Notquitetothespeedoflight,buttoafifthofit,or100millionmiles an hour. Such a system could reach Mars in less than an hour,reach Pluto in days, pass Voyager in under a week and reach AlphaCentauriinjustovertwentyyears.Oncethere,thenanocraftcouldimageanyplanetsdiscoveredinthesystem,testformagneticfieldsandorganicmolecules and send thedataback toEarth in another laserbeam.Thistinysignalwouldbereceivedbythesamearrayofdishesthatwereusedto transit the launch beam, and return is estimated to take about fourlightyears.Importantly,theStarChip’strajectoriesmayincludeafly-byofProximab, theEarth-sizedplanet that is in thehabitable zoneof itshost star, in Alpha Centauri. In 2017, Breakthrough and the EuropeanSouthern Observatory joined forces to further a search for habitableplanetsinAlphaCentauri.There are secondary targets for Breakthrough Starshot. It would

explore the solar system and detect asteroids that cross the path ofEarth’sorbitaroundtheSun.Inaddition,theGermanphysicistClaudiusGroshasproposed that this technologymayalsobeused to establishabiosphereofunicellularmicrobesonotherwiseonlytransientlyhabitableexoplanets.Sofar,sopossible.However,therearemajorchallenges.Alaserwitha

gigawatt of powerwouldprovide only a fewnewtons of thrust.But the

nanocraftcompensateforthisbyhavingamassofonlyafewgrams.Theengineeringchallengesareimmense.Thenanocraftmustsurviveextremeacceleration, cold, vacuum and protons, as well as collisions with junksuch as space dust. In addition, focusing a set of lasers totalling 100gigawatts on the solar sails will be difficult due to atmosphericturbulence.Howdowecombinehundredsof lasers throughthemotionoftheatmosphere,howdowepropelthenanocraftwithoutincineratingthem and how dowe aim them in the right direction? Thenwewouldneed to keep the nanocraft functioning for twenty years in the frozenvoid,sotheycansendbacksignalsacrossfourlightyears.Buttheseareengineeringproblems, and engineers’ challenges tend, eventually, to besolved.Asitprogressesintoamaturetechnology,otherexcitingmissionscanbeenvisaged.Evenwithlesspowerful laserarrays, journeytimestootherplanets, to theoutersolarsystemorto interstellarspacecouldbevastlyreduced.Ofcourse,thiswouldnotbehumaninterstellartravel,evenifitcould

bescaleduptoacrewedvessel.Itwouldbeunabletostop.Butitwouldbe themoment when human culture goes interstellar, when we finallyreachoutintothegalaxy.AndifBreakthroughStarshotshouldsendbackimagesofahabitableplanetorbitingourclosestneighbour,itcouldbeofimmenseimportancetothefutureofhumanity.In conclusion, I return to Einstein. If we find a planet in the Alpha

Centauri system, its image, capturedbyacamera travellingata fifthoflightspeed,willbeslightlydistortedduetotheeffectsofspecialrelativity.Itwouldbethefirsttimeaspacecrafthasflownfastenoughtoseesucheffects.Infact,Einstein’stheoryiscentraltothewholemission.Withoutitwewouldhaveneitherlasersnortheabilitytoperformthecalculationsnecessary forguidance, imaginganddata transmissionover twenty-fivetrillionmilesatafifthoflightspeed.Wecanseeapathwaybetweenthatsixteen-year-oldboydreamingof

ridingonalightbeamandourowndream,whichweareplanningtoturnintoareality,ofridingourownlightbeamtothestars.Wearestandingatthethresholdofanewera.Humancolonisationonotherplanetsisnolongersciencefiction.Itcanbesciencefact.Thehumanracehasexistedasaseparatespeciesforabouttwomillionyears.Civilisationbeganabout10,000 years ago, and the rate of development has been steadilyincreasing. If humanity is to continue for another million years, our

futureliesinboldlygoingwherenooneelsehasgonebefore.Ihopeforthebest.Ihaveto.Wehavenootheroption.

Theeraofcivilianspacetraveliscoming.Whatdoyou

thinkitmeanstous?

Ilookforwardtospacetravel.Iwouldbeoneofthefirsttobuyaticket.Iexpectthatwithinthenexthundredyearswewillbeabletotravelanywhereinthesolarsystem,except

maybetheouterplanets.Buttraveltothestarswilltakeabitlonger.Ireckonin500years,wewillhavevisitedsomeofthenearbystars.Itwon’tbelikeStarTrek.Wewon’tbeabletotravelatwarpspeed.Soaroundtripwilltakeatleastten

yearsandprobablymuchlonger.

9

WILLARTIFICIALINTELLIGENCEOUTSMARTUS?

Intelligence is central towhat itmeans to be human. Everything thatcivilisationhastoofferisaproductofhumanintelligence.DNA passes the blueprints of life between generations. Ever more

complexlifeformsinputinformationfromsensorssuchaseyesandearsandprocesstheinformationinbrainsorothersystemstofigureouthowtoactand thenacton theworld,byoutputting information tomuscles,for example. At some point during our 13.8 billion years of cosmichistory, somethingbeautifulhappened.This informationprocessinggotso intelligent that life forms became conscious. Our universe has nowawoken,becomingawareofitself.Iregarditatriumphthatwe,whoareourselvesmerestardust,havecometosuchadetailedunderstandingoftheuniverseinwhichwelive.I think there isnosignificantdifferencebetweenhow thebrainofan

earthworm works and how a computer computes. I also believe thatevolutionimpliestherecanbenoqualitativedifferencebetweenthebrainofanearthwormandthatofahuman.Itthereforefollowsthatcomputerscan,inprinciple,emulatehumanintelligence,orevenbetterit.It’sclearlypossible forsomething to acquirehigher intelligence than its ancestors:weevolved tobe smarter thanourape-likeancestors,andEinsteinwassmarterthanhisparents.IfcomputerscontinuetoobeyMoore’sLaw,doublingtheirspeedand

memorycapacityeveryeighteenmonths,theresultisthatcomputersarelikely to overtake humans in intelligence at some point in the nexthundredyears.Whenanartificial intelligence (AI)becomesbetter thanhumans at AI design, so that it can recursively improve itself without

humanhelp,wemayfaceanintelligenceexplosionthatultimatelyresultsinmachineswhose intelligenceexceedsoursbymorethanoursexceedsthat of snails. When that happens, we will need to ensure that thecomputers have goals aligned with ours. It’s tempting to dismiss thenotion of highly intelligent machines as mere science fiction, but thiswouldbeamistake,andpotentiallyourworstmistakeever.For the last twentyyearsor so,AIhasbeen focusedon theproblems

surroundingtheconstructionofintelligentagents,systemsthatperceiveandactinaparticularenvironment.Inthiscontext,intelligenceisrelatedtostatisticalandeconomicnotionsofrationality—thatis,colloquially,theability to make good decisions, plans or inferences. As a result of thisrecent work, there has been a large degree of integration and cross-fertilisation among AI, machine-learning, statistics, control theory,neuroscience and other fields. The establishment of shared theoreticalframeworks,combinedwiththeavailabilityofdataandprocessingpower,has yielded remarkable successes in various component tasks, such asspeech recognition, image classification, autonomous vehicles,machinetranslation,leggedlocomotionandquestion-answeringsystems.As development in these areas and others moves from laboratory

researchtoeconomicallyvaluabletechnologies,avirtuouscycleevolves,wherebyevensmallimprovementsinperformanceareworthlargesumsofmoney,promptingfurtherandgreaterinvestmentsinresearch.ThereisnowabroadconsensusthatAIresearchisprogressingsteadilyandthatitsimpactonsocietyislikelytoincrease.Thepotentialbenefitsarehuge;we cannot predict what we might achieve when this intelligence ismagnified by the tools AImay provide. The eradication of disease andpovertyispossible.BecauseofthegreatpotentialofAI,itisimportanttoresearchhowtoreapitsbenefitswhileavoidingpotentialpitfalls.SuccessincreatingAIwouldbethebiggesteventinhumanhistory.Unfortunately, itmightalsobe the last,unlesswe learnhowtoavoid

the risks.Usedas a toolkit,AI canaugmentour existing intelligence toopenupadvances in everyareaof scienceand society.However, itwillalso bring dangers. While primitive forms of artificial intelligencedeveloped so far have proved very useful, I fear the consequences ofcreating something that canmatch or surpass humans. The concern isthatAIwouldtakeoffonitsownandredesignitselfatanever-increasingrate. Humans, who are limited by slow biological evolution, couldn’t

competeandwouldbesuperseded.AndinthefutureAIcoulddevelopawill of its own, a will that is in conflict with ours. Others believe thathumans can command the rate of technology for a decently long time,andthatthepotentialofAItosolvemanyoftheworld’sproblemswillberealised.AlthoughIamwellknownasanoptimistregardingthehumanrace,Iamnotsosure.Inthenearterm,forexample,worldmilitariesareconsideringstarting

an arms race in autonomous weapon systems that can choose andeliminate their own targets.While theUN is debating a treaty banningsuchweapons,autonomous-weaponsproponentsusuallyforgettoaskthemost important question.What is the likely end-point of an arms raceand is that desirable for the human race? Dowe really want cheap AIweaponstobecometheKalashnikovsoftomorrow,soldtocriminalsandterrorists on the black market? Given concerns about our ability tomaintainlong-termcontrolofevermoreadvancedAIsystems,shouldwearm them and turn over our defence to them? In 2010, computerisedtrading systems created the stock-market Flash Crash; what would acomputer-triggeredcrashlooklikeinthedefencearena?Thebesttimetostoptheautonomous-weaponsarmsraceisnow.In themedium term, AImay automate our jobs, to bring both great

prosperityandequality.Lookingfurtherahead,therearenofundamentallimits to what can be achieved. There is no physical law precludingparticlesfrombeingorganisedinwaysthatperformevenmoreadvancedcomputations than the arrangements of particles in human brains. Anexplosivetransitionispossible,althoughitmayplayoutdifferentlythaninthemovies.AsmathematicianIrvingGoodrealisedin1965,machineswithsuperhumanintelligencecouldrepeatedlyimprovetheirdesignevenfurther,inwhatscience-fictionwriterVernorVingecalledatechnologicalsingularity. One can imagine such technology outsmarting financialmarkets, out-inventing human researchers, out-manipulating humanleaders and potentially subduing us with weapons we cannot evenunderstand. Whereas the short-term impact of AI depends on whocontrolsit,thelong-termimpactdependsonwhetheritcanbecontrolledatall.Inshort,theadventofsuper-intelligentAIwouldbeeitherthebestor

theworst thing ever tohappen tohumanity.The real riskwithAI isn’tmalicebutcompetence.Asuper-intelligentAIwillbeextremelygoodat

accomplishingitsgoals,andifthosegoalsaren’talignedwithourswe’reintrouble.You’reprobablynotanevilant-haterwhostepsonantsoutofmalice,butifyou’reinchargeofahydroelectricgreen-energyprojectandthere’sananthill in theregion tobe flooded, toobad for theants.Let’snotplacehumanityinthepositionofthoseants.Weshouldplanahead.Ifasuperioraliencivilisationsentusatextmessagesaying,“We’llarriveinafewdecades,”wouldwejustreply,“OK,calluswhenyougethere,we’llleave the lights on”? Probably not, but this is more or less what hashappened with AI. Little serious research has been devoted to theseissuesoutsideafewsmallnon-profitinstitutes.Fortunately, this is now changing. Technology pioneers Bill Gates,

SteveWozniakandElonMuskhaveechoedmyconcerns,andahealthyculture of risk assessment and awareness of societal implications isbeginning to take root in the AI community. In January 2015, I, alongwithElonMuskandmanyAIexperts,signedanopenletteronartificialintelligence,callingforseriousresearchintoitsimpactonsociety.Inthepast, Elon Musk has warned that superhuman artificial intelligence iscapable of providing incalculable benefits, but if deployed incautiouslywill have an adverse effect on the human race. He and I sit on thescientificadvisoryboardfortheFutureofLifeInstitute,anorganisationworkingtomitigateexistentialrisksfacinghumanity,andwhichdraftedtheopenletter.ThiscalledforconcreteresearchonhowwecouldpreventpotentialproblemswhilealsoreapingthepotentialbenefitsAIoffersus,and is designed to get AI researchers and developers to pay moreattention to AI safety. In addition, for policymakers and the generalpublictheletterwasmeanttobeinformativebutnotalarmist.Wethinkit is very important that everybody knows that AI researchers areseriouslythinkingabouttheseconcernsandethicalissues.Forexample,AI has the potential to eradicate disease and poverty, but researchersmustworktocreateAIthatcanbecontrolled.InOctober2016,IalsoopenedanewcentreinCambridge,whichwill

attempt to tacklesomeof theopen-endedquestionsraisedby therapidpace of development in AI research. The Leverhulme Centre for theFuture of Intelligence is a multi-disciplinary institute, dedicated toresearching the future of intelligence as crucial to the future of ourcivilisation and our species. We spend a great deal of time studyinghistory, which, let’s face it, is mostly the history of stupidity. So it’s a

welcome change that people are studying instead the future ofintelligence.Weareawareofthepotentialdangers,butperhapswiththetools of this new technological revolutionwewill even be able to undosomeofthedamagedonetothenaturalworldbyindustrialisation.Recent developments in the advancement of AI include a call by the

European Parliament for drafting a set of regulations to govern thecreationofrobotsandAI.Somewhatsurprisingly,thisincludesaformofelectronic personhood, to ensure the rights and responsibilities for themost capable andadvancedAI.AEuropeanParliament spokesmanhascommented that, as a growing number of areas in our daily lives areincreasingly affected by robots,we need to ensure that robots are, andwill remain, in the service of humans. A report presented to theParliament declares that the world is on the cusp of a new industrialrobot revolution. It examines whether or not providing legal rights forrobots as electronic persons, on a par with the legal definition ofcorporate personhood, would be permissible. But it stresses that at alltimes researchers and designers should ensure all robotic designincorporatesakillswitch.This didn’t help the scientists on board the spaceship with Hal, the

malfunctioning robotic computer in Stanley Kubrick’s 2001: A SpaceOdyssey, but that was fiction. We deal with fact. Lorna Brazell, aconsultant at the multinational law firm Osborne Clarke, says in thereportthatwedon’tgivewhalesandgorillaspersonhood,sothere isnoneedtojumpatroboticpersonhood.Butthewarinessisthere.Thereportacknowledgesthepossibilitythatwithina fewdecadesAIcouldsurpasshumanintellectualcapacityandchallengethehuman–robotrelationship.By 2025, therewill be about thirtymega-cities, eachwithmore than

tenmillioninhabitants.Withall thosepeopleclamouringforgoodsandservicestobedeliveredwhenevertheywantthem,cantechnologyhelpuskeeppacewithourcravingfor instantcommerce?Robotswilldefinitelyspeed up the online retail process. But to revolutionise shopping theyneedtobefastenoughtoallowsame-daydeliveryoneveryorder.Opportunities for interacting with the world, without having to be

physicallypresent,areincreasingrapidly.Asyoucanimagine,Ifindthatappealing,not leastbecause city life forall ofus is sobusy.Howmanytimeshaveyouwishedyouhadadoublewhocouldshareyourworkload?

Creating realistic digital surrogates of ourselves is an ambitiousdream,butthelatesttechnologysuggeststhatitmaynotbeasfar-fetchedanideaasitsounds.WhenIwasyounger,theriseoftechnologypointedtoafuturewhere

wewouldallenjoymoreleisuretime.Butinfactthemorewecando,thebusierwebecome.Ourcitiesarealreadyfullofmachinesthatextendourcapabilities,butwhatifwecouldbeintwoplacesatonce?We’reusedtoautomated voices on phone systems and public announcements. Nowinventor Daniel Kraft is investigating how we can replicate ourselvesvisually.Thequestionis,howconvincingcananavatarbe?Interactive tutors couldproveuseful formassiveopenonline courses

(MOOCs)andforentertainment.Itcouldbereallyexciting—digitalactorsthatwould be forever young and able to perform otherwise impossiblefeats.Ourfutureidolsmightnotevenbereal.Howweconnectwiththedigitalworldiskeytotheprogresswe’llmake

inthefuture.Inthesmartestcities,thesmartesthomeswillbeequippedwith devices that are so intuitive they’ll be almost effortless to interactwith.Whenthetypewriterwasinvented,itliberatedthewayweinteractwith

machines.Nearly 150 years later and touch screenshaveunlockednewwaystocommunicatewiththedigitalworld.RecentAIlandmarks,suchasself-drivingcars,oracomputerwinningatthegameofGo,aresignsofwhat is to come. Enormous levels of investment are pouring into thistechnology,whichalreadyformsamajorpartofourlives.Inthecomingdecades it will permeate every aspect of our society, intelligentlysupporting and advising us in many reas including healthcare, work,educationandscience.Theachievementswehaveseensofarwillsurelypaleagainstwhat thecomingdecadeswillbring,andwecannotpredictwhatwemightachievewhenourownmindsareamplifiedbyAI.Perhaps with the tools of this new technological revolution we can

makehumanlifebetter.Forinstance,researchersaredevelopingAIthatwould help reverse paralysis in people with spinal-cord injuries. Usingsiliconchipimplantsandwirelesselectronicinterfacesbetweenthebrainand the body, the technologywould allow people to control their bodymovementswiththeirthoughts.I believe the future of communication is brain–computer interfaces.

Therearetwoways:electrodesontheskullandimplants.Thefirstislikelookingthroughfrostedglass, thesecondisbetterbutrisks infection.IfwecanconnectahumanbraintotheinternetitwillhaveallofWikipediaasitsresource.The world has been changing even faster as people, devices and

information are increasingly connected to each other. Computationalpowerisgrowingandquantumcomputingisquicklybeingrealised.Thiswillrevolutioniseartificialintelligencewithexponentiallyfasterspeeds.Itwill advance encryption. Quantum computers will change everything,even human biology. There is already one technique to edit DNAprecisely,calledCRISPR.Thebasisofthisgenome-editingtechnologyisabacterial defence system. It can accurately target and edit stretches ofgeneticcode.Thebestintentionofgeneticmanipulationisthatmodifyinggenes would allow scientists to treat genetic causes of disease bycorrectinggenemutations.Thereare,however,lessnoblepossibilitiesformanipulating DNA. How far we can go with genetic engineering willbecomeanincreasinglyurgentquestion.Wecan’tseethepossibilitiesofcuringmotorneuronediseases—likemyALS—withoutalsoglimpsingitsdangers.Intelligence ischaracterisedas theability toadapt tochange.Human

intelligenceistheresultofgenerationsofnaturalselectionofthosewiththeabilitytoadapttochangedcircumstances.Wemustnotfearchange.Weneedtomakeitworktoouradvantage.We all have a role to play in making sure that we, and the next

generation,havenotjusttheopportunitybutthedeterminationtoengagefullywith thestudyof scienceatanearly level, so thatwecangoon tofulfilourpotential andcreateabetterworld for thewholehumanrace.We need to take learning beyond a theoretical discussion of how AIshouldbeand tomakesureweplan forhow it canbe.Weallhave thepotentialtopushtheboundariesofwhatisaccepted,orexpected,andtothink big. We stand on the threshold of a brave new world. It is anexciting,ifprecarious,placetobe,andwearethepioneers.Whenweinventedfire,wemesseduprepeatedly,theninventedthefire

extinguisher.Withmorepowerfultechnologiessuchasnuclearweapons,syntheticbiologyandstrongartificialintelligence,weshouldinsteadplanahead and aim to get things right the first time, because itmay be the

onlychancewewillget.Ourfutureisaracebetweenthegrowingpowerofourtechnologyandthewisdomwithwhichweuseit.Let’smakesurethatwisdomwins.

Whyarewesoworriedaboutartificialintelligence?Surelyhumansarealwaysabletopulltheplug?

Peopleaskedacomputer,“IsthereaGod?”Andthecomputersaid,“Thereisnow,”andfusedtheplug.

10

HOWDOWESHAPETHEFUTURE?

A century ago, Albert Einstein revolutionised our understanding ofspace, time, energy and matter. We are still finding awesomeconfirmationsofhispredictions,likethegravitationalwavesobservedin2016 by the LIGO experiment.When I think about ingenuity, Einsteinsprings tomind.Wheredidhis ingenious ideas come from?Ablendofqualities, perhaps: intuition, originality, brilliance. Einstein had theabilityto lookbeyondthesurfacetoreveal theunderlyingstructure.Hewasundauntedbycommonsense, the ideathat thingsmustbethewaytheyseemed.Hehadthecouragetopursueideasthatseemedabsurdtoothers. And this set him free to be ingenious, a genius of his time andeveryother.A key element for Einsteinwas imagination.Many of his discoveries

came from his ability to reimagine the universe through thoughtexperiments.Attheageofsixteen,whenhevisualisedridingonabeamoflight, he realised that from this vantage lightwould appear as a frozenwave.Thatimageultimatelyledtothetheoryofspecialrelativity.Onehundredyearslater,physicistsknowfarmoreabouttheuniverse

than Einstein did. Now we have greater tools for discovery, such asparticleaccelerators,supercomputers,spacetelescopesandexperimentssuch as the LIGO lab’s work on gravitational waves. Yet imaginationremainsourmostpowerfulattribute.With it,wecanroamanywhere inspace and time.We canwitnessnature’smost exotic phenomenawhiledriving in a car, snoozing in bed or pretending to listen to someoneboringataparty.Asaboy,Iwaspassionatelyinterestedinhowthingsworked.Inthose

days,itwasmorestraightforwardtotakesomethingapartandfigureoutthemechanics. I was not always successful in reassembling toys I hadpulledtopieces,butIthinkIlearnedmorethanaboyorgirltodaywould,ifheorshetriedthesametrickonasmartphone.My job now is still to figure out how thingswork, only the scale has

changed.Idon’tdestroytoytrainsanymore.Instead,Itrytofigureouthow the universe works, using the laws of physics. If you know howsomethingworks,youcancontrolit.ItsoundssosimplewhenIsayitlikethat! It isanabsorbingandcomplexendeavour thathas fascinatedandthrilled me throughout my adult life. I have worked with some of thegreatestscientistsintheworld.Ihavebeenluckytobealivethroughwhathasbeenaglorioustimeinmychosenfield,cosmology,thestudyoftheoriginsoftheuniverse.The human mind is an incredible thing. It can conceive of the

magnificenceoftheheavensandtheintricaciesofthebasiccomponentsofmatter.Yetforeachmindtoachieveitsfullpotential,itneedsaspark.Thesparkofenquiryandwonder.Often that sparkcomes froma teacher.Allowme toexplain. Iwasn’t

the easiest person to teach, I was slow to learn to read and myhandwriting was untidy. But when I was fourteen my teacher at myschoolinStAlbans,DikranTahta,showedmehowtoharnessmyenergyandencouragedmetothinkcreativelyaboutmathematics.Heopenedmyeyes tomathsas theblueprintof theuniverse itself. If you lookbehindeveryexceptionalpersonthereisanexceptionalteacher.Wheneachofusthinksaboutwhatwecandoinlife,chancesarewecandoitbecauseofateacher.However, education and science and technology research are

endangerednowmorethaneverbefore.Duetotherecentglobalfinancialcrisis and austerity measures, funding is being significantly cut to allareas of science, but in particular the fundamental sciences have beenbadlyaffected.Wearealsoindangerofbecomingculturallyisolatedandinsular,and increasinglyremote fromwhereprogress isbeingmade.Atthelevelofresearch,theexchangeofpeopleacrossbordersenablesskillsto transfer more quickly and brings new people with different ideas,derived from their different backgrounds. This can easily make forprogress where now this progress will be harder. Unfortunately, we

cannotgobackintime.WithBrexitandTrumpnowexertingnewforcesin relation to immigration and the development of education, we arewitnessing a global revolt against experts, which includes scientists. Sowhatcanwedotosecurethefutureofscienceandtechnologyeducation?Ireturntomyteacher,MrTahta.Thebasisforthefutureofeducation

mustlieinschoolsandinspiringteachers.Butschoolscanonlyofferanelementary framework where sometimes rote-learning, equations andexaminationscanalienatechildrenfromscience.Mostpeoplerespondtoaqualitative,ratherthanaquantitative,understanding,withouttheneedfor complicated equations. Popular science books and articles can alsoputacrossideasaboutthewaywelive.However,onlyasmallpercentageof the population read even the most successful books. Sciencedocumentaries and films reach amass audience, but it is only one-waycommunication.WhenIstartedoutinthefieldinthe1960s,cosmologywasanobscure

and cranky branch of scientific study. Today, through theoretical workand experimental triumphs such as the LargeHadronCollider and thediscoveryof theHiggsboson,cosmologyhasopened theuniverseup tous.Therearebigquestionsstilltoanswerandmuchworkliesahead.Butweknowmorenowandhaveachievedmoreinthisrelativelyshortspaceoftimethananyonecouldhaveimagined.But what lies ahead for those who are young now? I can say with

confidencethattheirfuturewilldependmoreonscienceandtechnologythan any previous generation’s has done. They need to know aboutsciencemorethananybeforethembecauseitispartoftheirdailylivesinanunprecedentedway.Without speculating too wildly, there are trends we can see and

emergingproblems thatweknowmustbedealtwith,nowand into thefuture.Among theproblems I countglobalwarming, finding spaceandresourcesforthemassiveincreaseintheEarth’shumanpopulation,rapidextinctionofotherspecies,theneedtodeveloprenewableenergysources,thedegradationoftheoceans,deforestationandepidemicdiseases—justtonameafew.There are also the great inventions of the future, which will

revolutionisethewayswelive,work,eat,communicateandtravel.Thereis such enormous scope for innovation in every area of life. This is

exciting.We could be mining rare metals on theMoon, establishing ahumanoutpostonMarsandfindingcuresandtreatmentsforconditionswhich currently offer no hope. The huge questions of existence stillremain unanswered—how did life begin on Earth? What isconsciousness?Isthereanyoneoutthereorarewealoneintheuniverse?Thesearequestionsforthenextgenerationtoworkon.Somethinkthathumanitytodayisthepinnacleofevolution,andthat

this is as good as it gets. I disagree. There ought to be something veryspecialabout theboundaryconditionsofouruniverse,andwhatcanbemore special than that there is no boundary. And there should be noboundary to human endeavour.We have two options for the future ofhumanityasIseeit:first,theexplorationofspaceforalternativeplanetsonwhichto live,andsecond,thepositiveuseofartificial intelligencetoimproveourworld.The Earth is becoming too small for us. Our physical resources are

being drained at an alarming rate. Mankind has presented our planetwiththedisastrousgiftsofclimatechange,pollution,risingtemperatures,reductionof thepolar ice caps, deforestation anddecimationof animalspecies.Ourpopulation,too,isincreasingatanalarmingrate.Facedwiththese figures, it is clear thisnear-exponentialpopulationgrowthcannotcontinueintothenextmillennium.Anotherreasontoconsidercolonisinganotherplanetisthepossibility

ofnuclearwar.There isa theory thatsays thereasonwehavenotbeencontacted by extra-terrestrials is that when a civilisation reaches ourstage of development it becomes unstable and destroys itself.We nowhavethetechnologicalpowertodestroyeverylivingcreatureonEarth.Aswe have seen in recent events in North Korea, this is a sobering andworryingthought.But Ibelievewecanavoid thispotential forArmageddon,andoneof

thebestwaysforustodothisistomoveoutintospaceandexplorethepotentialforhumanstoliveonotherplanets.Theseconddevelopmentwhichwill impactonthefutureofhumanity

istheriseofartificialintelligence.Artificial intelligence research is now progressing rapidly. Recent

landmarkssuchasself-drivingcars,acomputerwinningthegameofGoand the arrival of digital personal assistants Siri, Google Now and

Cortana are merely symptoms of an IT arms race, fuelled byunprecedented investments and building on an increasingly mature,theoretical foundation. Such achievements will probably pale againstwhatthecomingdecadeswillbring.But theadventof super-intelligentAIwouldbeeither thebestor the

worst thing ever tohappen tohumanity.We cannot know ifwewill beinfinitely helped by AI, or ignored by it and sidelined, or conceivablydestroyed by it. As an optimist, I believe thatwe can create AI for thegoodoftheworld,thatitcanworkinharmonywithus.Wesimplyneedto be aware of the dangers, identify them, employ the best possiblepractice and management and prepare for its consequences well inadvance.Technology has had a huge impact on my life. I speak through a

computer. I havebenefited fromassisted technology to givemea voicethat my illness has taken away. I was lucky to lose my voice at thebeginningof thepersonalcomputingage. Intelhasbeensupportingmeforovertwenty-fiveyears,allowingmetodowhatIloveeveryday.Overthese years the world, and technology’s impact on it, has changeddramatically.Technologyhaschangedthewayweallliveourlives,fromcommunicationtogeneticresearch,toaccesstoinformation,andmuch,much more. As technology has got smarter, it has opened doors topossibilities that Ididn’teverpredict.The technology that isnowbeingdeveloped to support the disabled is leading theway in breaking downthe communication barriers which once stood in the way. It is often aproving ground for the technology of the future. Voice to text, text tovoice,homeautomation,drivebywire,eventheSegway,weredevelopedfor the disabled, years before they were in everyday use. Thesetechnologicalachievementsareduetothesparkoffirewithinourselves,the creative force. This creativity can take many forms, from physicalachievementtotheoreticalphysics.Butsomuchmorewillhappen.Braininterfacescouldmakethismeans

of communication—used bymore andmore people—quicker andmoreexpressive. I now use Facebook—it allows me to speak directly to myfriends and followers worldwide so they can keep up with my latesttheoriesandseepictures frommy travels. Italsomeans I canseewhatmychildrenarereallyupto,ratherthanwhattheytellmetheyaredoing.

Inthesamewaythattheinternet,ourmobilephones,medicalimaging,satellite navigation and social networks would have beenincomprehensibletothesocietyofonlyafewgenerationsago,ourfutureworld will be equally transformed in ways we are only beginning toconceive.Informationonitsownwillnottakeusthere,butitsintelligentandcreativeusewill.There is somuchmore to come and I hope that this prospect offers

great inspiration to schoolchildren today.Butwehave a role to play inmakingsurethisgenerationofchildrenhavenotjusttheopportunitybutthewishtoengagefullywiththestudyofscienceatanearlylevelsothatthey cangoon to fulfil theirpotential andcreateabetterworld for thewholehumanrace.AndIbelievethefutureoflearningandeducationistheinternet.Peoplecananswerbackandinteract.Inaway,theinternetconnectsusalltogetherliketheneuronsinagiantbrain.AndwithsuchanIQ,whatcannotwebecapableof?WhenIwasgrowingupitwasstillacceptable—nottomebutinsocial

terms—tosaythatonewasnotinterestedinscienceanddidnotseethepointinbotheringwithit.Thisisnolongerthecase.Letmebeclear.Iamnot promoting the idea that all young people should grow up to bescientists. I do not see that as an ideal situation, as the world needspeoplewith awide variety of skills.But I amadvocating that all youngpeopleshouldbe familiarwithandconfidentaroundscientific subjects,whatever they choose to do. They need to be scientifically literate, andinspiredtoengagewithdevelopmentsinscienceandtechnologyinordertolearnmore.A world where only a tiny super-elite are capable of understanding

advanced science and technology and its applications would be, tomymind,adangerousandlimitedone.Iseriouslydoubtwhetherlong-rangebeneficial projects such as cleaningup the oceans or curingdiseases inthedevelopingworldwouldbegivenpriority.Worse,wecouldfindthattechnologyisusedagainstusandthatwemighthavenopowertostopit.Idon’tbelieveinboundaries,eitherforwhatwecandoinourpersonal

livesorforwhatlifeandintelligencecanaccomplishinouruniverse.Westand at a threshold of important discoveries in all areas of science.Withoutdoubt,ourworldwillchangeenormouslyinthenextfiftyyears.We will find out what happened at the Big Bang. We will come to

understandhowlifebeganonEarth.Wemayevendiscoverwhetherlifeexists elsewhere in the universe. While the chances of communicatingwithan intelligentextra-terrestrial speciesmaybeslim, the importanceofsuchadiscoverymeanswemustnotgiveuptrying.Wewillcontinuetoexplore our cosmichabitat, sending robots andhumans into space.Wecannotcontinuetolookinwardsatourselvesonasmallandincreasinglypolluted and overcrowded planet. Through scientific endeavour andtechnological innovation,wemust lookoutwards to thewideruniverse,whilealsostrivingtofixtheproblemsonEarth.AndIamoptimisticthatwe will ultimately create viable habitats for the human race on otherplanets.WewilltranscendtheEarthandlearntoexistinspace.Thisisnottheendofthestory,butjustthebeginningofwhatIhope

willbebillionsofyearsoflifeflourishinginthecosmos.And one final point—we never really know where the next great

scientificdiscoverywillcomefrom,norwhowillmakeit.Openingupthethrill and wonder of scientific discovery, creating innovative andaccessible ways to reach out to the widest young audience possible,greatly increases thechancesof findingand inspiring thenewEinstein.Wherevershemightbe.Soremembertolookupatthestarsandnotdownatyourfeet.Tryto

makesenseofwhatyouseeandwonderaboutwhatmakestheuniverseexist. Be curious. And however difficult life may seem, there is alwayssomethingyoucandoandsucceedat.Itmattersthatyoudon’tjustgiveup.Unleashyourimagination.Shapethefuture.

Whatworld-changingidea,smallorbig,wouldyouliketo

seeimplementedbyhumanity?

Thisiseasy.Iwouldliketoseethedevelopmentoffusionpowertogiveanunlimitedsupplyofcleanenergy,andaswitchtoelectriccars.Nuclearfusionwouldbecomeapracticalpowersourceandwouldprovideuswithan

inexhaustiblesupplyofenergy,withoutpollutionorglobalwarming.

AfterwordLucyHawking

OnthebleakgreynessofaCambridgespringday,wesetoffinacortègeof black cars towards Great St Mary’s Church, the university churchwhere distinguished academics by tradition have their funeral services.Out of term, the streets seemed muted. Cambridge looked empty, notevenawandering tourist in sight.Theonly spikesof colour came fromtheblue flashing lightsof thepolicemotorcycleoutriders, guarding thehearsewithmyfather’scoffininit,stoppingthesparsetrafficaswewent.Andthenweturnedleft.Andsawthecrowds,massedalongoneofthe

most recognisable streets in the world, King’s Parade, the heart ofCambridge itself. I have never seen so many people so silent. Withbanners,flags,camerasandmobilephonesheldaloft,thehugenumbersof people lining the streets stood in quiet respect as theheadporter ofGonvilleandCaius,myfather’sCambridgecollege,dressedceremoniallyinhisbowlerhatandcarryinganebonycane,walkedsolemnlyalongthestreettomeetthehearseandwalkittothechurch.Myauntsqueezedmyhandaswebothburstintotears.“Hewouldhave

lovedthis,”shewhisperedtome.Sincemyfatherdied,therehasbeensomuchhewouldhaveloved,so

much I wish he could have known. I wish he could have seen theextraordinary outpouring of affection towards him, coming from allaround the world. I wish he could have known how dearly loved andrespectedhewasbymillionsofpeoplehehadnevermet.Iwishhehadknownhewouldbe interred inWestminsterAbbey,between twoofhisscientificheroes, IsaacNewtonandCharlesDarwin,and thatashewaslaid torest in theearthhisvoicewouldbebeamedbyaradio telescopetowardsablackhole.

Buthewouldalsohavewonderedwhatallthefusswasabout.Hewasasurprisingly modest man who, while adoring the limelight, seemedbaffledbyhisownfame.Onephraseinthisbookjumpedoffthepageatme as summing up his attitude to himself: “if I have made acontribution.”He is the only personwhowould have added the “if” tothatsentence.Ithinkeveryoneelsefeltprettysurehehad.Andwhatacontributionitis.Bothintheoverarchinggrandeurofhis

work in cosmology, exploring the structure and origins of the universeitselfandinhiscompletelyhumanbraveryandhumourinthefaceofhischallenges.Hefoundawaytoreachbeyondthelimitsofknowledgewhilesurpassingthelimitsofenduranceatthesametime.Ibelieveitwasthiscombination which made him so iconic yet also so reachable, soaccessible. He suffered but he persevered. It was effortful for him tocommunicate—but he made that effort, constantly adapting hisequipmentashefurtherlostmobility.Heselectedhiswordspreciselysothat they would have maximum impact when spoken in that flatelectronic voice which became so oddly expressive when used by him.Whenhespoke,peoplelistened,whetheritwashisviewsontheNHSorontheexpansionoftheuniverse,neverlosinganopportunitytoincludeajoke,deliveredinthemostdeadpanfashionbutwithaknowingtwinkleinhiseyes.My fatherwasalsoa familyman,a fact lostonmostpeopleuntil the

filmThe Theory of Everything came out in 2014. It certainly was notusual, in the 1970s, to find a disabled person who had a spouse andchildrenofhis ownnor onewith such a strong senseof autonomyandindependence.Asasmallchild,Iintenselydislikedthewaystrangersfeltfreetostareatus,sometimeswithopenmouths,asmyfatherpilotedhiswheelchair at insane speeds through Cambridge, accompanied by twomop-hairedblondchildren,oftenrunningalongsidewhiletryingtoeatanicecream.Ithoughtitwasincrediblyrude.IusedtotrytostarebackbutI don’t think my indignation ever hit the target, especially not from achildishfacesmearedwithmeltedlolly.Itwasn’t,byanystretchoftheimagination,anormalchildhood.Iknew

that—andyetatthesametimeIdidn’t.Ithoughtitwasperfectlynormaltoaskgrown-upslotsofchallengingquestionsbecausethisiswhatwedidathome. Itwas onlywhen I allegedly reduced a vicar to tearswithmyclose examinationofhisproofof the existenceofGod that it started to

dawnonmethatthiswasunexpected.Asachild,Ididn’tthinkofmyselfasthequestioningtype—Ibelieved

that was my elder brother, who in the manner of elder brothersoutsmarted me at every turn (and indeed still does). I remember onefamily holiday—which, like so many family holidays, mysteriouslycoincided with an overseas physics conference. My brother and Iattended some of the lectures—presumably to givemymother a breakfromherwraparoundcaringduties.Inthosedays,physicslectureswerenot popular and definitely not for kids. I sat there, doodling on mynotepad,butmybrotherputhisskinnylittle-boyarmintheairandaskeda question of the distinguished academic presenter while my fatherglowedwithpride.Iamoftenasked,“WhatisitliketobeStephenHawking’sdaughter?”

andinevitably,thereisnobriefanswerthatfitsthebill.Icansaythatthehighswereveryhigh,thelowswereprofoundandthatinbetweenexistedaplacewhichweused tocall “normal—forus,”anacceptanceasadultsthatwhatwe foundnormalwouldn’t count as such for anyone else. Astimedullstherawgrief,Ihavereflectedthatitcouldtakemeforevertoprocess our experiences. In a way, I’m not even sure I want to.Sometimes,Ijustwanttoholdontothelastwordsmyfathersaidtome,that I had been a lovely daughter and that I should be unafraid. I willnever be as brave as him—I’m not by nature a particularly courageousperson—buthe showedme that I could try.And that tryingmight turnouttobethemostimportantpartofcourage.Myfathernevergaveup,henevershiedawayfromthefight.Attheage

ofseventy-five,completelyparalysedandable tomoveonlya fewfacialmuscles,hestillgotupeveryday,putonasuitandwenttowork.Hehadstufftodoandwasnotgoingtoletafewtrivialitiesgetinhisway.ButIhave to say, had he known about the police motorcycle outriders whowerepresent athis funeral, hewouldhave requested themeachday tonavigatehimthroughthemorningtrafficfromhishomeinCambridgetohisoffice.Happily, he did know about this book. It was one of the projects he

workedoninwhatwouldturnouttobehislastyearonEarth.Hisideawastobringhiscontemporarywritingstogetherintoonevolume.Likesomanythingsthathavehappenedsincehedied,Iwishhecouldhaveseen

thefinalversion.Ithinkhewouldhavebeenveryproudofthisbookandeven he might have had to admit, in the end, that he had made acontributionafterall.

LucyHawkingJuly2018

Acknowledgements

The Stephen Hawking estate would like to thank Kip Thorne, EddieRedmayne, Paul Davies, Seth Shostak, Dame Stephanie Shirley, TomNabarro,MartinRees,MalcolmPerry,PaulShellard,RobertKirby,NickDavies, Kate Craigie, Chris Simms, Doug Abrams, Jennifer Hershey,Anne Speyer, Anthea Bain, Jonathan Wood, Elizabeth Forrester, YuriMilner, Thomas Hertog, Ma Hauteng, Ben Bowie and Fay Dowker fortheirhelpincompilingthisbook.Stephen Hawking was well known for his scientific and creative

collaborations throughout his career, from working with colleagues onground-breakingsciencepaperstocollaboratingwithscriptwriters,suchas the team from The Simpsons. In his later years, Stephen neededincreasinglevelsofsupportfromthosearoundhimbothtechnicallyandintermsofcommunicationassistance.TheestatewouldliketothankallthosewhohelpedStephentokeepcommunicatingwiththeworld.

ByStephenHawking

ABriefHistoryofTimeBlackHolesandBabyUniversesandOtherEssays

TheIllustratedABriefHistoryofTimeTheUniverseinaNutshell

ABrieferHistoryofTime(withLeonardMlodinow)TheGrandDesign(withLeonardMlodinow)

MyBriefHistoryBriefAnswerstotheBigQuestions

FORCHILDRENGeorge’sSecretKeytotheUniverse(withLucyHawking)George’sCosmicTreasureHunt(withLucyHawking)

GeorgeandtheBigBang(withLucyHawking)GeorgeandtheUnbreakableCode(withLucyHawking)

GeorgeandtheBlueMoon(withLucyHawking)

AbouttheAuthor

STEPHEN HAWKING was the Lucasian Professor of Mathematics at theUniversity ofCambridge for thirty years and the recipient ofnumerousawards and honors including the Presidential Medal of Freedom. HisbooksforthegeneralreaderincludeMyBriefHistory,theclassicABriefHistoryofTime, the essay collectionBlackHoles andBabyUniverses,The Universe in a Nutshell, and, with Leonard Mlodinow, A BrieferHistoryofTimeandTheGrandDesign.Healsoco-authoredaseriesofchildren’sbookswithhisdaughter,beginningwithGeorge’sSecretKeytotheUniverse.StephenHawkingdiedin2018.

hawking.org.uk

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