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SHADOWLANDSQuest for Mirror Matter

in the Universe

.

The pictureon the cover shows thecrabnebula – theremnantsof a starwhich explodedin 1054.This pho-tographwastakenat theEuropeanSouthernObserva-tory. (Credit: FORSTeam,8.2-metreVLT ESO).

SHADOWLANDSQuest for Mirror Matter

in the Universe

Robert Foot

[email protected] of Physics

University of MelbourneVictoria 3010 Australia

Preface

My purposein writing this book is two-fold. First, many non-specialistsaskmeto explainthemirror matterideaandthescientificevidencefor it. Second,scienceis so specializedthesedaysthatmany peoplewhoknow a lot aboutonefield oftenknow little aboutanother. Mirror matter, if it exists, would leadto ratherimportantimplicationsfor severalscientificfields,including: particlephysics,astrophysics,cosmology, meteoriticsandplanetaryscience.Thus,itseemedto methataninterestingchallengewouldbeto write abookexplaining themotivation for mirror matterandits evidencewhichcouldusefullyserve thesetwo communities(thatis, bothspecialistsandnon-specialistsalike). Sucha venture,though,is not withoutrisks of variouskinds. Let me stateat the outsetthat the mirrormatterideais not establishedfact; it is anexampleof cutting-edgesciencein progress.It is my hopethat peoplewho readthis bookwill be infectedby, or at leastunderstand,my enthusiasmfor thissubject,andwhy I think it is oneof themostinterestingquestionsinscienceat themoment.

Theprocessof writing this bookgave metheopportunityto re-think many of theoriginalarguments.Some‘gaps’in my knowledgewerefilled in, anda few new directionsexplored.Somematerialisthereforecompletelynew, althoughmostof it hasappearedin thetechnicalscientificliteraturepreviously. I have only cited this sci-entific literaturesparingly, but neverthelessI have endeavouredtoproperlycreditthepeopleresponsiblefor themainoriginal ideas.

It seemsonly yesterdaythat I learnedasa studentthat mirrorreflectionsymmetrywasnot respectedby the fundamentalinterac-tions of nature. Electronsandotherelementaryparticlesare, in asense,‘left-handed’.Althoughmostscientistshave simply cometoacceptthatGodis ‘left-handed’,somehow it alwaysbotheredme....

ii Preface

Onesunny afternoonin May 1991a ratherremarkablethoughtoccurredto me. While playing with an unrelatedidea,it suddenlystruckmethattherewasasubtleyetsimpleway in whichmirror re-flectionsymmetrycouldstill exist. Nature’s mirror couldbeunbro-ken if eachtypeof ordinaryparticlehasa shadowy mirror partner.Theleft-handednessof theordinaryparticlescouldthenbebalancedby the right-handednessof the mirror particles.So thereyou haveit, mirror reflectionsymmetrycanexist but requiressomethingpro-foundly new. It requirestheexistenceof a completelynew form ofmattercalled‘mirror matter’.

At first, it seemedtoo fantasticto really exist. Yet,over thelastfew yearsit appearsthatalmosteveryastrophysicalandexperimen-tal predictionof themirror mattertheoryhasactuallybeenobservedby observationsandexperiments:Thereis fascinatingevidenceformirror matterin the Universefrom astronomicalobservationssug-gestingthatmostof our galaxyis composedof exotic darkmaterialcalled‘dark matter’. Recentparticlephysicsexperimentshave re-vealedunexpectedpropertiesof ghostlyparticlescalled‘neutrinos’andweird matteranti-matteratoms. This unexpectedbehaviour isexpectedif mirror matterexists. Most remarkableof all is theevi-dencethatour planetis frequentlybombardedby mirror matteras-teroid or cometsizedobjects,causingpuzzlingeventssuchasthehuge1908Siberianexplosionwhich felled morethantwo thousandsquarekilometresof nativeforestswithoutleaving asinglemeteoritefragmentbehind! AltogetherI will discussseven major puzzlesinastrophysicsandparticlephysicseacharguing in favour of themir-ror matterhypothesis.Thereareindeedsevenwondersof themirrorworld...

New datafrom currentandfutureexperimentswill keepcomingin even as this book is being printed. Unfortunately, I am not afortuneteller anddo not know what thesefuture experimentsandobservationswill find. However, I canpredictwhatthey will find ifmirror reflectionsymmetryandhencemirror matterexists.Thecasefor mirror matterwill thereforeeitherstrengthenor weakenasnewdatacomesin andfuture experimentsaredone. In the meantime,I adviseyou to sit back, relax and let me take you on a journeyexploring oneof theboldestscientificideasever proposed.

Preface iii

No scientistworks in isolationand I am no exception. I havehadfruitful collaborationson mirror matterwith a numberof verycreative people,including Sergei Gninenko, SashaIgnatiev, HenryLew, ZurabSilagadze,RayVolkasandT. L. Yoon. I have enjoyedinterestingcorrespondenceon someaspectsof this subjectwithSergei Blinnikov, ZdenekCeplechaandAndrei Ol’khovatov. In ad-dition, I wouldliketoacknowledgeinvaluablesupportovertheyearsfrom many friendsandcolleaguesincluding in particular, PasqualeDi Bari, JohnEastman,GregFilewood,DaveHowland,GirishJoshi,Matthew Tully, andNick Whitelegg. I amalsogreatfulto many oftheabovepeople,andalsoJaciAndersonandGlenDeenfor provid-ing mewith usefulcommentson themanuscriptandTony Nguyenfor helpingwith thecover.

Of course,I thankmy family mostof all. It is to themthat Idedicatethisbook.

RobertFootAugust2001

.

for CarolynandJames

Contents

Part I: Why Mirror Matter?

1. Introduction 3

2. ElementaryParticlesandForces 17

Part II: Evidencefor Mirror Matterin theUniverse

3. Discoveryof Mirror Stars? 39

4. Discoveryof Mirror Planets? 67

Part III: Evidencefor Mirror Matterin theLaboratoryandSolarSystem

5. Mirror MatterandPositronium 107

6. Mirror MatterandtheTunguskaEvent 137

Part IV: Evidencefor Mirror Matterfrom DeepUnderground

7. TheMysteryof theDisappearingNeutrinos 175

8. More MissingNeutrinos 205

9. Reflectionson theMirror World 221

Furtherreading

Notes

There aremore thingsinHeavenandEarth,Horatio, thanare dreamtof in your philosophy.

William Shakespeare– Hamlet.

PART I

Why Mirror Matter?

Chapter 1

Introduction

Shortly beforehis deathin 1727, IsaacNewton reflecteduponhislife andwrote

�:

I don’t know what I may appearto the world, but, as tomyself I seemto have beenonly like a boy playing on theseashore,and diverting myself in now and then finding asmootherpebbleor a prettiershell thanordinary, whilst thegreatoceanof truth lay all undiscoveredbeforeme.

More recently in StephenHawking’s a brief history of time, it iswritten

�:

I still believe that therearegroundsfor cautiousoptimismthatwe may now be nearthe endof the searchfor the ulti-matelawsof nature.

The contrastbetweenthe currentLucasianProfessorand the for-merholderof thatpositionis striking. Hawking is not alonein hisprophecy. It hasbeenrepeatedwith monotonousregularitysincethedaysof Maxwell (1865).Onedayit maycometrue,but thatdayis alongwayoff. I believethatarevolutionin sciencemaybeimminent.In fact, over the last decade,remarkableevidencefrom astronomy(studiesof the very big) to studiesof the elementaryparticles(thevery small) suggestthat a completelynew type of matterexists –‘mirror matter’. Thebestideasin scienceareusuallyvery simple,

3

4 Introduction

andfortunatelymirror matterbelongsto thiscategory. I believe thatthe ideasandtheevidencecanbeappreciatedby anyoneinterestedin science.

In the processof uncovering mirror matterwe will encountermany recentandunexpecteddiscoveries,including:

� Invisible starswhich reveal their presenceby gravitationallybendingthelight from moredistantstarsbehindthem. I willarguethattheseinvisiblestarsaremadeof mirrormatterwhichcansimply explainwhy we don’t seethem.

� Planetsorbiting nearbystarswhich areeight timesclosertotheir star than the distanceMercury orbits the Sun. I willsuggestthattheseunexpectedplanetsareexpectedif they aremadeof mirror matter.

� Bizarre, apparentlyfree-floatingplanetswanderingthroughspace. They can be more naturally interpretedas ordinaryplanetsorbitingmirror stars,but I couldbewrong!

� Strangeandunexpectedpropertiesof elementaryparticlessuchastheghostlyneutrinos.Theseparticlesareemittedfrom theSunandin otherprocesses.However, half of themaremiss-ing! The missingneutrinosmay have beentransformedintomirror neutrinosasI will explain.

� I will alsodiscussastrangeclassof ‘meteoriteevents’suchasthe hugeSiberian1908explosionandothersimilar suchex-plosions. Thereis evidencethat theseexplosionsarecausedby the randomcollisionsof our planetwith orbiting ‘mirrormatterspace-bodies’.Most remarkableof all is the realpos-sibility thatmirror matterremnantsmaystill bein thegroundtoday! Needlessto saythepossibleusesof this new typeofmatterarenotevenimagined...

By the way, this is a (generally)seriousscientificbook. However,unlike other ‘seriousscientificbooks’ this book doesnot claim toreveal the ‘mind of God’. In fact,not many ridiculouslygrandiosestatementswill be madeat all. Rather, it is simply a book aboutmirror reflectionsymmetry– andits far reachingimplications.

Introduction 5

Symmetryis a word frequentlyusedin everydaylanguageandweareall awareof whatit means.Examplesof symmetricalobjectsabound:flowers,butterflies,snowflakes,soccerballsandsoon... Infact, assomeof theseexamplesillustrate,symmetryis often asso-ciatedwith beautyandvice versa.It is perhapsnot surprisingthenthat symmetryplaysa pivotal role in our understandingof theele-mentaryparticlesandtheir forces,but let mestartat thebeginning.

Therearemany distinct typesof symmetry. The symmetryofa mushroomis completelydifferent to the symmetryof a butterflywhich in turn is completelydifferent to the symmetryof a soccerball. A butterfly is an exampleof the most familiar symmetry–‘left-right’ symmetry. This symmetryoccurswhentwo equalpor-tions of a whole are the mirror imageof eachother. For obviousreasons,this symmetryis alsocalled‘mirror’ symmetry. A soccerball is anexampleof anothertypeof symmetry– rotationalsymme-try. In fact,it is anexampleof anobjectwith threedimensionalro-tationalsymmetrybecauserotationsaroundany axisdo not changethe appearanceof the ball. Finally a straightfenceor railway lineareexamplesof objectswhich displayanothertypeof symmetry–translationalsymmetry. A railway line or fencelooks the sameaswe movealongit.

Fortunatelythe everydayusageof the conceptof symmetryisexactly the sameas its technicalusagein science. Although it isoftenusefulto describesymmetryin amathematicalway– thisneednot concernus. Herewe needonly discussthe ideasandconceptswhich is enoughto glimpsethe beautifulworld of the elementaryparticlesandtheir interactions.

Most peopleareaware that ordinarymatter: you, me andev-erythingelsewe see,exceptlight itself, is composedof atoms.Al-thoughatomsare very tiny, approximatelyone ten millionth of amillimetre in size,they arestill not themostfundamentalbuildingblocksof matter. Atomsarenotelementaryentities.EachindividualAtom is madeup of electronsanda compactnucleus,which in turnis madefrom protonsandneutrons.Thereareabout100 differenttypesof atomsdependingon thenumberof electronsthatthey con-tain. Thescienceof atoms,how they interactwith eachotherto formmoleculesandhow differentmoleculesinteractwith eachotheris of

6 Introduction

coursethescienceof chemistry. However, we will not be involvedso muchwith chemistrybut with the mostfundamentalof the sci-ences– physics. One thing that physicsis concernedwith is themostbasicquestionsthat canbeasked. For example,whatarethepropertiesof the elementaryparticles:protons,neutrons,electronsfrom whichall matteris made?How do theseparticlesinteractwitheachotherandwith light?

Onethingthathasbeenlearnedovertheyearsis thattheinterac-tionsof elementaryparticlesdisplaya varietyof symmetries.Someof thesesymmetriesarequite familiar suchasrotationalsymmetryand translationalsymmetry. Thus, the laws of physicsremainthesamewhetherwearein Melbourneor in Moscow, whichmeansthatRussianphysicstext booksare useful in Australia and vice versa(afterthey aretranslated...).In additionto translationsin space(andtranslationsin language!)we canimaginetranslationsin time. Thelaws of physicsarethe sametodayasyesterdayor even a centuryago, however our knowledgeof theselaws generallyimproves astime goesby. Hence,physicstext booksare not the sametodayasa centuryago,yet the laws of physicsare the same. Therearestill othermoreabstractsymmetriesof theelementaryparticleinter-actions. Thesearecalled‘Lorentz symmetry’and‘gaugesymme-try’, which areneverthelessquiteelegantandnaturalonceyou getto know them.

Progressin scienceis rarelyasmoothcomfortablejourney. Rapidprogressgenerallyoccursin brief intervalsusuallythroughnew andunexpectedexperimentalresultsandsometimesthroughnovel the-oretical ideas. Of courseprogressis most rapid when theoryandexperimentmove togetherin harmony. Oneof themostremarkabletheoreticalideasof the

��centurywas the discovery of relativ-

ity theory in 1905 by Albert Einstein. Spaceand time were uni-fied with time becomingthe fourth dimension.Einsteinsuggestedthat the laws of physicswere symmetricalunderrotationsin thisfour dimensionalspace-time,ratherthanjust the threedimensionsof space.Thepredictionsof this theory, suchasmoving clocksmustrunmoreslowly, havebeenexperimentallyverifiedwith tremendousprecision. This is possiblebecauseEinstein’s theorynot only tellsus that moving clocksrun moreslowly, but it tells us exactly how

Introduction 7

muchmoreslowly! This four dimensionalrotationalsymmetryofspace-timeis called‘Lorentzsymmetry’* .

There are four known fundamentalforces in nature: gravity,electromagnetism,weakandstrongnuclearforces.Gravity is quitefamiliar to mostof us. It keepsour feeton theground,it keepsourplanetandall theotherplanetsin our solarsystemin orbit aroundtheSunandit keepstheSunin orbit aroundthecentreof ourgalaxy.Electromagnetismis nolessimportant– while it is gravity thatholdsus down, it is electromagnetismthat stopsus from falling throughthe floor. It is also the force responsiblefor electricity andmag-netism.While theweakandstrongnuclearforcesarelessfamiliar,they areneverthelessequallyfundamentalandimportantastheothermore familiar forces. For example, the weak and strongnuclearforceprovidestheenergy whichpowerstheSun,withoutwhichourplanetwouldbetoocold to sustainlife.

Todayweknow thatthreeof theseforces,electromagnetism,theweakandstrongnuclearforcesare,mathematically, verysimilarandfairly well understood.Gravity, on theotherhand,is quitedifferentand its relation to the other forcesis somewhat mysterious. Onereasonis thatgravity canbedescribedin geometricaltermsasacur-vatureof four dimensionalspace-timewhile the otherthreeforcesaredescribedin termsof symmetriesonanabstract‘internal’ space,whichis nothingto dowith ordinaryspace-timethatweknow about.Thesepeculiarsymmetriesof theelectromagnetic,weakandstrongnuclearforcesarecalled‘gaugesymmetries’.

Evidently, symmetriesareratherimportantin understandingtheelementaryparticlesand their forces. However, it is pertinenttorecall that thesesymmetrieswerenot alwayssoobvious. I have al-readymentionedthecaseof Lorentzsymmetry– theratherabstractidea that spaceand time can be treatedmathematicallyas a fourdimensionalspace-time.In fact,afterthediscovery of relativity the-ory andLorentzsymmetry, an EnglishPhysicistcalledPaul Diracuncoveredabig problem.In thelate1920’s Diracnoticedthatami-croscopicmathematicaldescriptionof the electronconsistentwith

* In additionto Albert Einstein’sinsight,importantcontributionsto therelativitytheoryweremadeby others,including:HendrikLorentz,HermannMinkowski andHenri Poincare.

8 Introduction

Lorentzsymmetrywasnot possible,unlesssomethingcompletelynew existed.Nothingshortof a new form of matterwasrequiredtoreconcileEinstein’s relativity theorywith the quantummechanicaltheoryof theelectron.Thisnew form of matter, called‘anti-matter’wastherebytheoreticallypredictedto exist.

Specifically, Diracpredictedthatin additionto theparticlesthatmakeupordinarymatter– theelectrons,protonsandneutrons,anti-particlescalled ‘positrons’ (or anti-electrons),‘anti-protons’ and‘anti-neutrons’shouldall exist. The symmetryrequiredeachtypeof anti-particleto have the samemassas the correspondingpar-ticle. Positronsandanti-nuclei(madefrom anti-protonsandanti-neutrons)shouldform ‘anti-atoms’. However, anti-particlesshouldannihilatewhenthey meetordinaryparticlesproducinggammarays(high frequency light). History tells usthatexperimentsshortlyfol-lowed which dramaticallyconfirmedthe existenceof Dirac’s anti-particles.First, thediscovery of thepositronin 1932,andlater, thediscovery of anti-protonsin the 1950’s. Anti-matter is not sciencefantasybut sciencereality. Clearly, the ideaof symmetrycanhaveremarkableimplications.

This bookthough,is concernednot with Lorentzsymmetrybutwith left-rightor mirror reflectionsymmetry. Letusnow briefly lookat thehistoryof thissymmetry. Before1956physicistshadassumedthat thelaws of physicsweresymmetricunderleft-right symmetry.This would meanthat for every fundamentalmicroscopicprocessthat is known to occur, themirror imageprocessshouldalsooccur.In factleft-right symmetryis suchafamiliarandplausiblesymmetryof naturethatit wasneverseriouslyquestioneduntil variousexperi-mentalpuzzlesbeganappearingin the1950’s. Thesepuzzlesled T.D. LeeandC. N. Yangto suggestthat theweaknuclearforcedoesnot display left-right symmetry. They proposedan experimenttodirectly testtheideainvolving the -decayof anunstableisotope...

At the time, mostscientistsdidn’t expectthatmirror symmetrycould really bebroken. Theprevailing scepticismwassummedupby WolfgangPauli whenhewrotein December1956 :

I amhoweverpreparedto betthattheexperimentwill bede-cidedin favourof mirror invariance.For in spiteof YangandLee,I don’t believe thatGodis aweakleft-hander.

Introduction 9

However, Pauli wasnotsofoolishasto let hisbeliefsgetin thewayof science.He did agreethatexperimentsshouldbedoneto checkit � :

I believein reflectioninvariancein contrasttoYangandLee...Betweenbelieving andknowing is adifferenceandin thelastendsuchquestionsmustbedecidedexperimentally.

The experimentsuggestedby Lee andYangwasperformedin1957by C. S.Wu andcollaborators.In thisexperimentanumberofcobalt-60atomswerecooleddown to nearabsolutezeroKelvin (thelowestpossibletemperature)andplacedin a strongmagneticfield.Cobalt-60is anunstableisotope.Ordinarily, Cobalt-60decaysemit-ting an electronwith any directionequally likely. However, undertheseextremeconditions,the electronsshouldbe equally likely toemergefrom thetwo polesof themagneticfield – if thefundamen-tal decayprocessdisplayedmirror symmetry. Yet, it wasobservedthat moreelectronscameout from onedirectionthanthe other. Ifwe observed only onenuclei decayingwe could not sayanything.Mirror symmetrydoesnot meanthateachsingleinteractionor de-cayprocessis thesameasits mirror image– it is not. Mirror sym-metry meansthat the mirror imageprocesscan occur and shouldoccurwith equalprobability. Therefore,by observinga largenum-berof decaysof Cobalt-60we caneasilydeterminewhethermirrorsymmetryis violated.Theremarkableconclusionwasthat thefun-damentallaws of physicsappearto be ‘left-handed’. This is reallyverystrange.Everyotherplausiblesymmetry, suchasrotationalandtranslationalsymmetry, arefoundto bemicroscopicsymmetriesofparticleinteractions.Cannaturereally beleft-handed?

Are they, the fundamentallaws of physicsthat is, really left-handedor do they only appearto be left-handed?RememberourearliercommentsaboutLorentzsymmetry. At onetime this sym-metry did not appear to be a symmetryat all. This was becauseanti-matterhadyet to bediscovered.Only whenyou have particlesand anti-particlesis it possibleto write down a consistentmicro-scopictheoryfor theinteractionsof theelectron,protonandneutronwhich respectsLorentzsymmetry. Remarkably, it turnsout thatit isstill possiblefor particleinteractionsto besymmetricundermirror

10 Introduction

or left-right symmetry. JustasLorentzsymmetryrequiredthe ex-istenceof anti-matter, left-right symmetrycanexist if andonly if anew form of matterexists– mirror matter.

Often,it seemsthatnatureis moresubtleandbeautifulthenfirstimagined.It couldbethatnature’s mirror is of amoreabstractkind.Imaginethat for eachtype of ordinaryparticle thereis a separate‘mirror particle’. That is, not only do we have photons,electrons,positrons,protonsetc., but also mirror photons,mirror electrons,mirror positrons,mirror protonsetc.Wecanimaginethatin nature’smirror notonly spaceis reflectedbut alsoparticlesarereflectedintothesemirror particles.Therelationshipbetweenordinaryandmirrormatteris somewhat like therelationshipbetweentheletters‘b’ and‘d’. The mirror imageof ‘b’ is the letter ‘d’ andthe mirror imageof ‘d’ is the letter ‘b’. Thus,while neither‘b’ nor ‘d’ is symmetric(in a sensethey eachhave the oppositehandedness),together‘bd’is in factmirror symmetric,with thetwo lettersinterchangingin themirror image� . Try it with a mirror andsee!Still, themirror reflec-tion of an objectappearsvery similar to the original. It is perhapsnot surprising,therefore,that the propertiesof the mirror particlesturnout to beverysimilar to theordinaryparticles.For example,themirror particlesmusthave thesamemassandlifetime aseachof theordinaryparticles,otherwisethemirror symmetrywouldbebroken.

In somewaysmirror particlesresembleanti-particles.However,thereis a crucialdifference.Unlike anti-particles,themirror parti-clesinteractwith ordinaryparticlespredominatelyby gravity only.The threenon-gravitational forcesact on ordinaryandmirror par-ticles completelyseparately. For example,while ordinaryphotons(that is, ordinarylight) interactwith ordinarymatter(which is justthe microscopicpictureof the electromagneticforce), they do notinteractwith mirror matter. Similarly, the ‘mirror image’ of thisstatementmustalsohold, that is, themirror photon(that is, mirrorlight) interactswith mirror matterbut doesnotinteractwith ordinarymatter. Theupshotis thatwecannotseemirror photonsbecausewearemadeof ordinarymatter. Themirror photonswouldsimplypassright throughuswithout interactingatall!

The mirror symmetrydoesrequirethoughthat the mirror pho-tonsinteractwith mirror electronsandmirror protonsin exactly the

Introduction 11

samewayin whichordinaryphotonsinteractwith ordinaryelectronsandordinaryprotons.A directconsequenceof this is thata mirroratommadefrom mirror electronsandamirror nucleus,composedofmirror protonsandmirror neutronscanexist. In fact,mirror mattermadefrom mirror atomswould alsoexist with exactly thesamein-ternalpropertiesasordinarymatter, but would becompletelyinvis-ible to us! If you hada rock madeof mirror matteron your hand,itwouldsimplyfall throughyourhandandthenthroughtheEarth,andit wouldenduposcillatingabouttheEarth’scentre* . Wecansafelyconcludethat if therewasa negligible amountof mirror matterinour solarsystem,we would hardly be awareof its existenceat all.Thus,theapparent left-right asymmetryof the laws of naturemaybedueto thepreponderanceof ordinarymatterin our solarsystemratherthandueto a fundamentalasymmetryin thelaws themselves.

Do mirror particlesreally make the laws of physicsleft-rightsymmetric?Let usconsidera simpleandlight-hearted‘thoughtex-periment’involving againtheCobalt-60decay. Imaginetherewasamirror planetorbiting a mirror starin a distantpartof our Universe(note that thereis only one space-time– thereis no ‘mirror Uni-verse’).Let’scall thishypotheticalplanet‘Miros’. Miros is aplanetmadeof mirror matter– atomscomposedof mirror electronsandmirror protonsandmirror neutrons.Miros is somewhatdifferenttoEarththough.It’s a bit smallerwith deeperoceans,but thereis lifeonMiros. Thepeopleof Miros areabit strange,they haveverylargefeetandonly haveoneeye– but they areveryhappy. They havewiseleaderswhowouldnever dreamof puttingnuclearmissilesin spaceandthey realisedveryearlytheimportanceof reducinggreenhousegases.On Miros a football teamcalled ‘Collingwood’ often winsthefootball. Thus,Miros isn’t muchlikeEarthwhich just illustratesthatmicroscopicsymmetryof particleinteractionsdoesnottranslateinto a macroscopicsymmetry.

* LaterI will discussthepossibilitythatanew typeof interaction(or force)couldexist couplingordinarymatterto mirror matter. If this is thecase,it mayactuallybepossibleto pick upamirror rock,althoughit wouldstill beinvisible. Clearly, theconsequencesof suchaforceareveryimportantandit will beconsideredin chapter5. However, in orderto keepthis introductorydiscussionassimpleaspossible,thispossibilityhasbeenignored.

12 Introduction

Anyway, our mirror matter friends on Miros realisedthe im-portanceof purescience;their wisegovernmentalwaysmadesurethatfinancialsupportwasgivento thosemirror scientistswhohadaresearchrecordconsistingof interestingandinnovative ideas.OnedaysomeoneonMiros hadtheideathatthey shouldtestwhetherthefundamentallawsof naturearemirror symmetricor not. Sothey setup their Cobalt-60experimentwith a similar experimentalsetup aswasdoneby peoplehereonEarthin 1957.But whatthey foundwassomethingquitedifferent.They foundthemirror imageresult.Thatis, they foundthatthemirror electronsweremostlyemittedfrom thedecayingCobalt-60mirror nucleusin theoppositedirectionaswasfoundhereonEarth.Ourmirror friendsonMiros concludedthatthelaws of physicswereright-handed.

Thelawsof physicscannotbothbeleft-handedandright-handed.Ordinaryparticlesform a left-handedsector, mirror particlesforma right-handedsector. Taken together, neitherleft nor right is sin-gledout,sinceordinaryandmirror particlesareotherwiseidentical.(This is muchlike theletters‘b’ and‘d’; ‘b’ representstheordinaryparticlesand interactionsand ‘d’ the mirror particlesand interac-tions).However, if mirror particlesdon’t exist anywherein theUni-versethenthe laws of physicsareindeedleft-handed.Similarly iftheUniversewasfull of mirror particleswith noordinaryones,thenthelaws of physicswould beright-handed,but if bothordinaryandmirror particlesexist togetherthenleft-right symmetryis restored.

Thebasicgeometricpointis illustratedin thefollowing diagram.

Nature’s Mirror

The left-handsideof this figure representsthe interactionsof theknown elementaryparticles.Theforcesaremirror symmetriclike aperfectsphere,exceptfor theweakinteraction,which is represented

Introduction 13

asa left hand.Also shown is nature’s mirror - theverticalline downthe middle. Clearly, the reflectionis not the sameasthe original,signifying the fact that the interactionsof the knownparticlesarenot mirror symmetric. If therewerea right handaswell asa lefthandthenmirror symmetrywouldbeunbrokenwithout theneedfornew particles:

However, this doesn’t correspondto naturesinceno right-handedweakinteractionsareseenin experiments.

Thereare two remainingpossibilities: We caneitherchopthehandoff – but this is toodrasticandis thereforenotshown. It corre-spondsto having no weakinteractionsat all, againin disagreementwith observations. This lastpossibilityconsistsof addinganentirenew figure with the handon the otherside. Everythingis doubledeventhesymmetricpart,which is clearlymirror symmetricasindi-catedin thefollowing diagram:

It is this lastpossibilitythatmaycorrespondto nature.While themirror mattertheoryis simple,elegant,andthe idea

hasbeenknown for a long time, it is only in thepastdecadethatex-perimentalandobservationalevidencefor mirror matterhasgrownto thepoint whereastrongcasecanbemadethatit actuallyexists–andhencethemotivationfor thisbook.Theevidencefor mirror mat-ter is diverse,rangingfrom studiesof the lightestandmostelusive

14 Introduction

of theknown elementaryparticles– theneutrinos,to observationsofthelargestsystems– galaxiesof starsin theUniverse.After readingthis book the readerwill be awareof the evidenceandmay makehis or herown judgement.At thevery least,thequestionof theex-istenceof mirror matteris oneof the mostinterestingquestionsinscienceat themoment,andit shouldbe(hopefully)answeredin thenext fiveyears.

In the following chapters,I will provide the generalpictureofhow wecanfind outif mirror matteractuallyexistsandwhy thecasefor its existencecurrentlyseemsso strong. Therearebroadlytwodifferentstrategieswhich canbe usedto test the theory. First, be-causemirror matteris stableandbehavesmuchlikeordinarymatter,it shouldexist in theUniversetoday. If onebelievesthatthebig bangtheoryis the correctdescriptionof the origin of the Universe,andthereis someevidencefor that,thenmirror mattershouldhavebeencreatedalongwith ordinarymatterwhentheUniversewasborn. Infact,independentlyof whetherthebig bangtheoryis corrector not,themicroscopicsymmetrybetweenordinaryandmirror mattersug-geststhatwhatever mechanismcreatedordinarymattershouldalsocreatemirror matter. In other words, an almostinevitable conse-quenceof theideathatthefundamentallawsof physicsdisplayleft-right symmetryis thatmirror mattermustexist in theUniverse.Fur-thermore,like ordinarymatter, mirror mattercanform stars,planetsandasteroidsizedobjectswhich canpopulatethe heavens. How-ever, suchmirror stars,planetsandthelike wouldbeinvisible to us,sincemirror matterwould only radiateor reflectmirror light whichdoesn’t interactat all with us ordinarypeople,andour telescopesmadefrom ordinarymatter. Thus, the first main predictionof themirror mattertheoryis that invisible or dark mattershouldexist intheUniverse.

Onemight think that invisible dark matterwould be unobserv-able,andthisit literally is,howevertherearesimplewaysof demon-stratingin quitea compellingway thatit really exists. In fact,thereis a lot of astronomicalevidencethat the Universeis full of suchinvisible dark matter. In the following chaptersthis evidencewillbe presentedand discussed.The evidencenot only suggeststhatmostof ourgalaxyis madeof darkmatter, but thatnearbystarshave

Introduction 15

mirror planetsandevenmoreremarkable,thatoursolarsystemcon-tains mirror matter‘spacebodies’ (that is, asteroidsizedobjects)whicharefrequentlybombardingourown planetEarth.

Theothermainstrategy for searchingfor theexistenceof mirrormatteris throughtheimplicationsfor microscopicprocessessuchasparticleinteractions.This is becauseit is actuallypossiblefor newsmall forcesto exist, which (like gravity) act on bothordinaryandmirror matter. However, becausewe know that the laws of micro-scopicparticle interactionsobey certainsymmetries,suchasrota-tional,Lorentzandgaugesymmetries,thereareonly a few possibleways in which small forcescan coupleordinary to mirror matter.Onepossibleforceis asmallcouplingof ordinaryphotonsto mirrorphotons.I will explain in subsequentchaptersmorepreciselywhatthis statementmeans,however theeffect of this tiny force, it turnsout, is to makeorthopositronium(aweird typeof ‘atom’ madefromanelectronandapositron)decayfasterthanwewouldotherwiseex-pect– aneffect which hasalreadybeenobserved in anexperiment.A moredramaticeffect is thatit canmake mirror matterspacebod-iesvisibleasthey travel throughtheatmosphere.They maynotonlybevisible but mayexplodeleadingto devastatingconsequences.Infact, theremnantsof suchcosmicbodiesmaystill be in thegroundatvariousimpactsitesbecausethesmallforcebetweenordinaryandmirror mattercanbelargeenoughto opposetheforceof gravity.

Anotherway in which microscopicforcescancoupleordinaryandmirror matteris througha typeof ‘mixing’ of neutrinos.Again,I will explain more preciselywhat this statementmeanslater on,however, theeffectof it is to makeordinaryneutrinostransformintomirror neutrinos,therebycausingthemto effectively disappear. AsI will discuss,thereis remarkableevidencethatneutrinosdo indeeddisappear, moreover, therateatwhichthey areobservedto disappearis predictedpreciselyin themirror mattertheory.

Chapter 2

Elementary Particles andForces

This chapterdidn’t appearin thefirst draft of this book. Includingtoo muchbackgroundmaterialcanbe dangerouslyboring. On theonehand,I wantedto get straightinto the ‘interestingstuff ’, andon theotherhand,someconfusionmayarisefor peopleunfamiliarwith someof thebasicconcepts.I have thereforeincludedthisbriefsummaryof someof thebasic‘particle physics’concepts,andalsoemphasisedagainhow this is extendedto includethehypothesisofmirror symmetry. Let me startby sizing up the variousscientificdisciplines...

Nature’s distanceladder

Onecoulddefinethevariousscientificdisciplines:physics,chem-istry, biology, geology, astronomyandcosmologyby thecharacter-istic distancesize or scale involved. The distancescalescover ahugerangefrom onetenmillion billionth of a centimetre– thedo-mainof particlephysics,to distancesof order100billion light years– thesizeof thevisibleUniverse.Onelight yearis thedistancethatlight travels in oneyearwhich is itself a very largedistance– about10,000billion kilometres.

Mathematiciansarevery clever people. They quickly invented

17

18 ElementaryParticlesandForces

a very simpleway of expressingvery largenumbersandvery smallnumbers.In scientificnotationlargenumbersareexpressedaspow-ersof 10. For example, �� ! ��"� � � � ��

# ��%$ �� # ��'& ��(�

In this notationthesizeof thevisible Universeis

� �)�light years–

or

� �)*centimetres.

We canalsousescientificnotationto expressvery small num-bers,like thesizeof anatom– aboutonehundredmillionth of acen-timetre. Onehundredmillionth is the fraction

,+" �� # �� # �� . Inscientificnotation,

,+" �� # �� # ��-� ,+" ��.which is convention-

ally expressedas

�"/ .. Thus, the sizeof an atomis simply

�"/ .cm. With this environmentallyfriendly papersaving notation,wecanconvenientlyexpressthecharacteristicdistancescalesof nature,seeFigure 2.1. This figurealsoillustratestheconceptof a logarith-mic scaleor simply, ‘log’ scale. In a log scaleeachfactorof 10 isequallyspaced,but let us not worry too muchaboutthat. Insteadlet’s go straightto theheartof (the)matter.

Whatis an elementaryparticle?

It is essentiallythesamequestionasasking“what is everythingmadeof?”. Things aroundus, as well as us, are madeof atoms.But whatareatomsmadeof? At onetime it wasthoughtthatatomsweren’t madeof anything, they were indivisible – the basicbuild-ing blocksof matter. Atomsareabout

� / .cm in size.Eventually,

it wasfoundthat therewereevensmallerparticles.Electronswerediscovered(1897-1899),which we now know arepoint-like downto distanceslessthanabout

� / �10cm. Not longafterthatdiscovery

it wasproposedthat atomswerecomposedof electronsembeddedin atypeof jelly, or rather, plumpudding(astheEnglishwouldhave

ElementaryParticlesandForces 19

Man

10

10

10

10

10

10

10

10

10

10

10

10

-8

-4

0

4

8

12

16

20

24

28

-16

-12

= 1

Cosmology

Astronomy

Geology

Biology

Chemistry

Physics

Visible Universe

Galaxy diameter

Nearest star

Distance to the sun

Earth’s diameter

Mt. Everest

Snail

DNA

Atom

Proton

Smallest distanceprobed in acceleratorexperiments

Figure2.1: Nature’sdistanceladder(in centimetres).

it) of positive charge. The plum puddingmodel,asit wasknown,did not longendure.It wasdevouredby Rutherfordin 1911.

How did Rutherforddo it? Early in 1909Rutherfordsuggestedto twocolleagues,HansGeiger(of ‘Geigercounter’fame)andErnestMarsdento scattera beamof 2 particlesoff a metal foil ( 2 parti-cles are just helium nuclei andareemittedby variousradioactiveelements).The 2 particleswerevery energetic, travelling at about10,000km/s. In theplumpuddingmodel,thesevery fast 2 particlesshouldjust deviateonly slightly whenpassingthoughthefoil – buttheexperimentsuggestedotherwise.It wasfoundthata small frac-tion of alphaparticlesactuallybouncedback! This meansthat the


plum puddingwasno ordinarypudding– it suggeststhatlargepipswerepresent.Rutherfordlaterremarked

0:

It wasquitethemostincredibleeventthathaseverhappenedto mein my life. It wasalmostasincredibleasif you fireda15-inchshellat a pieceof tissuepaperandit camebackandhit you.

It took a short time for Rutherfordto realisethat the implicationof thesescatteringexperimentswasthatthepositive chargewasnotspreadoutin apudding,but concentratedatthecentrein a‘nucleus’.Atomsarein factmostlyemptyspace.

Insight into the ratherstrangebehaviour of electronsin atomsbeganin 1913with theBohr modelof theatom. It wasfound thatthe traditional or ‘classical’ conceptswere completelyinadequateto describethe domainof microscopicphenomena.A completelydifferenttypeof theorywasneeded.In short,a sortof physicsrev-olution occurred,andby the late1920’s thequantumtheoryof theelectronhad arrived. Ambitious and occasionallyeven grandiosestatementswereheardfrom all quarters.Oneof the leadingphysi-cistsof theday, PaulDirac (theanti-matterman)proclaimed3 :

Theunderlyingphysicallawsnecessaryfor themathematicaltheoryof a largepartof physicsandthewholeof chemistryarecompletelyknown.

Thestructureof thenucleuswasalsoa greatproblemfor manyyears. Most peoplethoughtthat it wascomposedof electronsandprotons– but they could never get it to work. Thingsweregreatlyclarifiedby thediscovery of theneutronin 1932. Thenucleuscon-sistedof protonsandneutronsboundtogetherby anew force,calledthestrongnuclearforce.

Oneimportantlessonfrom history is thatalmostall progressinscienceis drivenby experiments.Purethoughtseldomgetsvery far– unlessit is coupledwith experiments. Without experimentswemight aswell sit aroundin a hot tub andconcludethat all matterconsistsof four elements:fire, water, air, but what’s theotherone?I guessit mustbeearth,but thenwhatarepeoplemadeof? At thispoint,I shouldprobablysurrenderandtakeoutmy encyclopaedia,if


I hadone,andtry to follow thethinkingof theancients.But perhapsthis procedure,apparentlyfollowed by nearlyall writers of popu-lar bookson science,is missingthepoint. Sciencereally beganinearnestwhenpeoplefinally gotoutof their tubsandstartedto inves-tigate– do carefulobservations,get their handsdirty andactuallydo experiments.Sometimeshistorytravels in circlesthough.Thereis an interestingrecenttrendin particlephysics. Somepeoplearereturningto their hot tubsandarguingthateverythingis madefrom‘strings’ about

�"/ � cm long which live in 10 dimensionalspace-time... Theideathattheworld is a flat platethatsitson a tortoiseisin many waysabettertheory. It’sa lot simplerandcanbetested.Ofcourse,weall know thatit cannotexplainvariousestablishedthingssuchasthefactthatpeoplewho buy round-the-world airline ticketsusuallyreturnsafely. In contrast,string theoryappearsto have notestableconsequences.But that’s anotherstory.

In additionto theprotonsandneutrons(whichtogetherarecalled‘nucleons’ sincethey make up the nucleus)and the electron,onemoretypeof particlewasinferredto exist which is calledtheneu-trino. Neutrinosarealmost‘nothing’. They have no electriccharge,almost no mass,and interact with the other particlesextremelyweakly by a new force called the ‘weak nuclearforce’. Yet theyexist. Indeedatonetimeit wasthoughtthatthiswasall therewasasfaraselementaryparticleswereconcerned.In 1947GeorgeGamowpublisheda book (One,Two, Three... Infinity) which summedupthesituationat thattime

.:

“But is this the end?”you may ask. “What right do we have toassumethatnucleons,electrons,andneutrinosarereally elemen-tary andcannotbesubdivided into still smallerconstituentparts?Wasn’t it assumedonly half a centuryagothattheatomswerein-divisible?Yetwhatacomplicatedpicturethey presenttoday!” Theansweris that,althoughthereis, of course,no way to predictthefuture developmentof the scienceof matter, we have now muchsounderreasonsfor believing thatourelementaryparticlesareac-tually the basicunits andcannotbe subdivided further. Whereasallegedlyindivisibleatomswereknown to show a greatvarietyofrather complicatedchemical,optical, and other properties,the


propertiesof elementaryparticlesof modernphysicsareextremelysimple;in factthey canbecomparedin theirsimplicity to theprop-ertiesof geometricalpoints. Also, insteadof a ratherlargenum-ber of “indivisible atoms” of classicalphysics,we are now leftwith only threeessentiallydifferententities: nucleons,electrons,andneutrinos. And, in spiteof the greatdesireandeffort to re-duceeverythingto its simplestform, onecannotpossiblyreducesomethingto nothing.Thus,it seemsthatwe haveactuallyhit thebottomin our searchfor thebasicelementsfrom which matterisformed.

BeforeGamow’sink coulddry, ahostof new unstableparticleswerediscoveredstartingwith themuonin 1947,aparticlewhichappearedto have the broadcharacteristicsof a ‘heavy electron’– about200timesheavier in fact.

Any physicalprocessthatwe observe canalwaysbereducedtothe microscopicinteractionsof elementaryparticles. In a certainliteral sense,elementaryparticlesand their interactionsare at the‘heart of thematter’. In theprevious chapter, I mentionedthat theelectromagneticforcesavesusfrom falling throughthefloor (threecheersfor electromagnetism!).This ‘foot feat’ is accomplishedbytheatomsin our feet repellingagainsttheatomsin the floor – andthis is not due to the odour of smelly feet! Electronsdon’t havefeelingsbut do have electriccharge,andanything with chargeis in-fluencedby theelectromagneticforce. Like chargesrepel,oppositechargesattract. As the electronsfrom the atomsin the outer sur-faceof our feet pushagainstthe electronsfrom the atomson theouter layersof the floor strongelectromagneticrepulsiontakesef-fect. This leadsto a reactionforcewhichopposestheforceof grav-ity.

MatterParticlesandForceparticles

At this point it is usefulto distinguishbetweentwo broadtypesof elementaryparticles.‘Matter particles’and‘force particles’. As


we have seen,matterparticlesconsistof theelectron,proton,neu-tron(whicharetheconstituentsof atoms)andalessfamiliarparticlecalledtheneutrino.For eachof theseelementaryparticlesthereis adistinctanti-particle(whichcanalsobeclassifiedasatypeof matterparticle).

Even thoughanti-particlesare stable,you can’t dig them outof the groundsincethey would have vanishedin a ‘puff of light’longagoif therewasany initial anti-matterin oursolarsystem(andmaybeeven in the Universeaswell). Neverthelessthey exist andplay importantroles in astrophysics,suchas in supernova explo-sions. Therearealsoa large numberof unstable,shortlived parti-cles,which have beendiscoveredin the late1940’s andthefollow-ing decades.Eachtypeof elementaryparticlehasvariousintrinsicpropertiessuchastheir massandelectriccharge. They alsohave acertainamountof ‘spin’. Roughlyspeaking,the elementaryparti-clesareeachsomewhat like a spinningtop. Thematterparticlesallhave thesameamountof spin,which in standardunitshasthevalue1/2,while the‘Forceparticles’havetwiceasmuchspin,thatis, theyhave spin1.

In the1960’s - 1970’s it wasrealisedthat theprotonsandneu-tronsarenot really elementary. They canbeviewedasbeingcom-posedof moreelementaryconstituentscalled‘quarks’. Quarkswerefirst introducedby JamesJoycein hisbookFinnegansWake: ‘Threequarksfor MusterMark’. However, Joyce did not realisethat theideawasmoreuniversal– notonly did MusterMark getthreequarksbut every proton and neutrontoo. This was first conjecturedbyGeorgeZweig andMurray Gell-Mannin 1963. Gell-Mann’s paperwasrejectedby the JournalPhysicalReview Letters while GeorgeZweig’s paperwasonly distributedasa preprint.TheauthoritiesatCERN,wherehewasworking at the time, declaredthat it wastoocrazyto besubmittedfor publication.

What is it aboutquarksthat is so crazy? The problemis thattheseprotonandneutronconstituentparticleswerenever seen. Ifquarksreallyexist whycan’t webreakopentheneutronsandprotonsandisolatethe threequarks?By contrast,the constituentparticlesof atomsare the electrons,protonsandneutronswhich canall beisolated. The strangebehaviour of quarkswas finally understood


whenasuccessfultheoryof thestronginteractionswasput togetherduringtheearly1970’s. This theorysuggeststhatquarkscanneverbe isolatedandcanonly exist in protonsandneutronsandalsoinvariousothershort-livedparticles.Fortunatelythough,for thethingsthatI will discussin thisbook,thesedetailsarejust that,details.Wedo not needto know anything aboutthe detailedpropertiesof thestrongnuclearforce (or indeedquarks)except for the fact that itbindsprotonsandneutronstogetherinto nuclei. The propertiesofthestablematterparticlesaresummarizedin Table2.1* .

Actually freeneutronsarenot stable,but decaywith anaveragelifetime of about12 minutes.However, within thenucleusthey arequitestableunlessthenucleusis radioactive. A radioactive nucleusis onewhich spontaneouslydecaysafter a certaintime. Thereareseveraltypesof decayprocesses,but theonewhichwewill bemostinterestedin is called -decay.

Matter Mass 465 � Electric Strong WeakParticle Charge Force Force

Proton(7 ) 938MeV +1 Yes YesAnti-proton( 87 ) 938MeV –1 Yes YesNeutron( 9 ) 940MeV 0 Yes YesAnti-neutron( 89 ) 940MeV 0 Yes YesElectron( : ) 0.51MeV –1 No YesPositron( 8: ) 0.51MeV +1 No YesNeutrino( ; ) < 5 eV 0 No YesAnti-neutrino( 8; ) < 5 eV 0 No Yes

Table 2.1: Some propertiesof the (stable) matter particles (and theanti-particles).

* The commonunit of energy is the electronVolt, or eV. 1 eV is the energygainedby anelectronaftertravelling throughapotentialof 1 Volt. 1 MeV = =)> ? eV.Also, I haveexpressedthemassin termsof its energy equivalentthroughEinstein’sfamousrelation, @BADCFE)G . In this equation,@ is theenergy, C is themassand Eis thespeedof light in vacuum.


In the -decayprocessa neutronis convertedinto a protonandviceversa:

9 H 7JIK:LIM8;� /ON �QP�R,ST�

7 H 9UIV8:WIK;�YX N �QP�R,ST�(Z

Fortunately X decayis not observed to occurfor free(that is, iso-lated)protonswhich canbeunderstoodfrom energy conservation–lighterparticlescannotdecayintoheavier ones.Putmoresimply, wecannotgainweightwithout eating.Theneutronis heavier thantheprotonsofreeprotonsarequitestable.However, within thenucleusthingsaremorecomplicatedbecauseelectromagneticpotentialen-ergy canbegainedwhenelectricallychargedprotonsareconvertedinto electricallyneutralneutrons.For this reasonit is possibleforprotonsto decay, but only in certainnuclei. Whetheror not a givennucleusundergoes -decay, andthe type of decay( X or /

) de-pendson the proportionof neutronsto protonswithin the nucleus.I will talk a little moreabout -decayin a moment,but let mefirstintroducethenotionof a force.

What is a ‘force’? Macroscopicallyit is a type of intrinsic at-tractionor repulsionbetweenobjects.Without any forcean objectwould move in uniform motion without changingits speedor itsdirection. Likewise when objectschangetheir speedor directionthenthis is dueto a force. In everydaylife, we areawareof manyapparentlydifferenttypesof forces: kicking the football impartsaforce to the ball, bumpingour headon the wall, acceleratingin acar, etc. However, microscopicallythereareonly four known ‘fun-damental’forces. They are ‘fundamental’in the sensethat all theother forcesresult from them at the microscopiclevel. The fourfundamentalforcesaregravity, electromagnetism,strongandweaknuclearforces. Thesefour forcesare further distinguishedby therangeof their effect. Gravity andelectromagnetismarelong rangeforces– they aregenerallybelievedto exert their influenceover ar-bitrary large distances.While the strongandweaknuclearforcesareobserved to be very shortrange– they only have a measurableeffectovermicroscopicdistances.Despitethis,microscopically, theforcesof electromagnetism,strongandweaknuclearforcesareall


Figure2.2: Microscopicpictureof the electromagneticforce. Theelec-tronemitsaphoton( [ ) whichcausestheelectronto changedirection.Thephotonis absorbedby anotherelectronat a later time. In this andin othersuchdiagrams,time (t) runsup the pageanddistance(x) runsacrossthepage.

fairly well understood.Eachforcecanbemicroscopicallydescribedin termsof theactionof a ‘force particle’.

Taking the electromagneticforce for example,microscopicallythefundamentalprocessinvolvedis theinteractionsof electronsandprotonswith photons. The photon is the force particle for elec-tromagnetism.Consideringthe electron,it has,at any give time acertainchanceof emitting a photon. This ‘interaction’ causestheelectronto changedirectionandspeedif the photonis eventuallyabsorbedby anotherdistinct matterparticle. This can be vieweddiagramaticallyasshown in Figure 2.2 above.

This typeof diagramwasfirst usedby RichardFeynmanin the1940s. In the technicalliteraturethis type of diagramis called a‘Feynmandiagram’,but in thisbookI will usemoredescriptive lan-guageandcall it an‘interactiondiagram’.Anyway, microscopicallytheelectromagneticforceresultsfrom photon-electroninteractions.


This is the fundamentalprocessbehind the force of electromag-netism. Actually thingsareslightly morecomplicatedbecausetheexchangedphotonin Figure2.2is notexactlythesameasarealpho-ton – it is calleda ‘virtual photon’in thetechnicalliterature.Againthough,we don’t needto bothertoo much abouttechnicaldetailssuchasthis.

Broadly speaking, the weak and strong nuclear forces aresimilar to electromagnetism,but they eachhave certainimportantdifferencesas well. Consideringthe weak interactions,therearenot one but three force particles called \ X # \ / #(] �

. Theseparticleswerefirst predictedto exist in 1961andfinally discoveredin an experimentin 1983. One interestingthing about \_^ parti-clesis that whenthey areemittedor absorbedthey alwayschangethe identity of the matterparticle. For example,considerthe -decayprocessof thedecayof aprotonin thenucleus:7`Ha9bIc8:"IU; ,which can be viewed diagramaticallyas shown in Figure 2.3below. As the diagramillustrates,the proton is converted into aneutronasit emitsa \ X particle,which later turnsinto a positronandaneutrino.

nW

Figure2.3: Microscopicpictureof d -decay. Theproton(e ) is transformedinto a neutron( f ) by emitting a gih . The gih then transformsinto apositron( jk ) andaneutrino( l ).


Turning now to the strongnuclearforce, the protonsandneu-tronsexchangenotthreebut eightforceparticlescalledgluons.Thereareadditionalcomplicationssincethesegluonsarebelieved to in-teractwith thepoint-like particlescalled‘quarks’within theprotonsandneutrons.Fortunately, complicateddetailssuchasthisneednotconcernusatall sinceweonly needto beawarethatthestronginter-actionsbind thequarksinto nucleonsandit alsobindsnucleonsintonuclei.Nuclei of coursecancombinewith electronsvia theelectro-magneticforceto form atoms.Atomscancombinetogetherto formmolecules(also via the electromagneticforce) and moleculescancombinetogetherto form youandme.But of courseweareperhapsmorethanabunchof atoms...

The‘force particles’aresummarizedin thetablebelow.

Force Forceparticle

Electromagnetism m (photon)WeakNuclearForce \ ^ #6] �StrongNuclearForce npo (Gluons)

We don’t needto know muchaboutthesedetails. The readeronlyneedsto beawarethattheold but goodideaof forcescanbeviewedmicroscopicallyasdueto the exchangeof force particles,andthattheforceparticlefor electromagnetismis just thephoton,thephotonis of coursetheparticlewhichmakesup ordinarylight.

One final commentis that gravity is not well understoodmi-croscopically. It is temptingto postulatethe existenceof a forceparticle for gravity – called the ‘graviton’, but the fine detailsarenot known. Whatis known is thatgravity is quitedifferentfrom theotherforces.How to reconcilegravity with microscopicphysicsisa deepmystery. Luckily, we don’t needto worry too muchaboutthis becausewe only considertheeffectsof gravity on largeobjectssuchasasteroids,planetsandstarsetc.For suchlargeobjectsNew-ton’s or Einstein’s ‘classical’ theoryof gravity suffices. I will talkmoreaboutNewtonandEinsteinlater, but for now let’s returnto themicroscopicparticleinteractionsof thenon-gravitational forces.


Mirror ParticlesandMirror Forces

The ideaof mirror matterarisesfrom the interactionsof theel-ementaryparticles. Theseinteractionsareknown to possessmanysymmetries,but themostobvioussymmetryof all, left-right or mir-ror symmetry, is not a symmetryof the knownelementaryparti-cles. The weak nuclearforce is the culprit which is, in a sense,left-handed.As discussedin the previous chapter, this remarkablefactwasfirst demonstratedin 1957using -decayexperiments.

Thisapparentleft-handednessof thefundamentallawsof physicsis particularly striking for neutrinos. The neutrino is an elusiveelementaryparticle which is emitted along with the positron (orelectron)in -decay. As I have alreadymentioned, -decaycanbe viewed asthe elementaryprocessof proton(or neutron)decaywithin radioactive nuclei (suchasCobalt-60),7qH 9rI 8:sIV; .Like mostelementaryparticles,suchasthe electronor proton,theneutrinoalwayshasacertainamountof ‘spin’. I have alreadymen-tioned that this meansthat eachelectroncan be viewed, roughlyspeaking,asa ‘spinningtop’. Spinis anintrinsicpropertylikemassor charge. Every neutrino,electronor proton,alwayshasthesameamountof spin,althoughit maypoint in differentdirections.

A remarkableobservationthoughis thatin -decay, or any otherprocessthat producesneutrinos, the neutrinos ( ; ) always havetheir spin axis orientatedin the samedirection relative to theirdirection of motion. If the neutrino were coming towards you,you’d seeit spinningclockwise,in otherwords,it twists like a left-handedcorkscrew. Justas a clockwisespinningtop becomesananti-clockwisespinningtop if viewed in a mirror, the left-handedneutrinobecomesa right-handedonewhenviewed in a mirror (seeFigure 2.4 on the following page). Thus,mirror symmetrywouldsuggesttheneutrinoshouldbeemittedwith aright-handedspinhalfof the time. Yet, nobodyhasever observed a single right-handedneutrino.In contrast,anti-neutrinosarealwaysobservedto beright-handed.

In the introductionI pointedout that the fundamentalinterac-tionsof naturecouldexhibit mirror symmetryonly if asetof mirrorparticlesexist. Onemight wonder, though,whethermirror particles


Figure2.4: A clockwisespinningtop (or left-handedneutrino)becomesananti-clockwisespinningtop (or right-handedneutrino)whenviewedina mirror.

arereallynecessary. Couldnature’smirror reflectparticlesinto anti-particlesaswell asreflectingspace?This seemspossiblebecauseall neutrinosareobserved to beleft-handedwhile all anti-neutrinosareobservedto beright-handed.Suchmirror symmetrywouldhavemany other implications. For a while this ideaappearedto work.Theanti-particleprocessesdid appearto behave like themirror im-ageof the ordinary particle processes.However, this anti-mattermirror wasshatteredafter just sevenyears.In 1964,anexperimentdemonstratedthatthis typeof mirror wasalsobroken.

The 1964experimentinvolved a ratherstrangeshort lived par-ticle known asa kaon. Becausekaonslive only a very short timebeforethey decay– lessthana millionth of a second– they mayseemquite unimportant.Neverthelessthey exist andtheir interac-tionsmustdisplaythesymmetriesof nature,whatever they happento be. Indeed,ourcurrentunderstandingof elementaryparticlesnotonly describesthestableparticlessuchastheprotonsandelectrons,but alsostrangeshortlivedparticles(of which therearemany) suchasthe kaons. Anti-kaonsalsoexist andaredistinct particles. The1964experimentdemonstratedthat kaonsdo not display left-right


symmetryevenwhenkaonsarealsoreflectedinto anti-kaons.Thisamountsto nothinglessthantheapparentbreakdown of any form ofleft-right symmetryin nature– ordoesit? Theprospectthatthemostnaturalsymmetryimaginable– mirror symmetry– is not a symme-try of nature,while everyotherobvioussymmetrysuchasrotationalsymmetryandtranslationalsymmetryareindeedsymmetriesseemsrathersurprisingto saytheleast.

Remarkablythough,asI mentionedin thepreviouschapterandwill expanduponhere,it turnsout thatit is still possiblefor particleinteractionsto exhibit alsomirror symmetryif anew form of matter,called ‘mirror matter’ exists. As just discussedabove, having ourmirror reflectparticlesinto anti-particlesaswell asthe mandatoryspacereflectionsimply doesn’t work. It was a logical possibilitybut it didn’t agreewith experiments. If it doesn’t agreewith ex-perimentsit can’t describenature.Instead,imaginehaving a mirrorthatreflectseveryparticle(includingtheiranti-particles)into acom-pletelynew typeof particle–whichwemightcall a‘mirror particle’.In otherwords,I amproposingthat for eachtypeof ordinaryparti-cle, suchasthephoton,electron,positron,proton,anti-protonetc.,thereis a correspondingmirror particlewhich is a distinctphysicalparticle.

This typeof mirror seemsabit differentto theonein yourbath-room,sincebathroommirrorsdo not changetheidentity of thepar-ticles,or do they? Actually though,your bathroommirror changesleft-handedparticlesinto right-handedones,so the reflectedimageis, microscopicallyor ‘quantummechanically’,composedof differ-ent particlesor ‘states’. It is thereforean a priori possibility thatnature’s mirror could reflectordinaryparticlesinto distinct mirrorparticles.This mirror is illustratedin Figure 2.5 (on the followingpage)with themirror particlesbeingdistinguishedfrom theordinaryoneswith aprime( t ).

The mirror symmetryinterchangesthe ordinaryparticleswiththemirror particlesaswell asreflectingspace,sothatthepropertiesof themirror particlescompletelymirror thoseof theordinaryparti-cles. This meansthat themirror particlesmusthave thesamemassandlifetime aseachof the ordinaryparticlesotherwisethe mirrorsymmetrywould be broken. It alsomeansthat while the ordinary


n n

n n

W, ZW, Z

Figure 2.5: Nature’s mirror might reflect eachordinary particle into adistinctmirror particle.

particlesappearin certainprocessesto be left-handed,the mirrorparticlesappearin the correspondingmirror processesto be right-handed.For example,the -decayprocess,

7`Hu9UIV8:WIK;�vwherethe‘L’ remindsusthat theneutrinois observed to bealwaysleft-handedlyspinning,implies theexistenceof the ‘mirror image’process

7 t Ha9 t IV8: t I-; twwith themirror neutrinospinningright-handedly.

ImportantlyI assumedthateachof theforceparticlesalsohasadistinctmirror partner. This is a crucialassumptionandit is neces-saryto explain why mirror particlesarenot producedin laboratoryexperiments. Ordinary particlesinteractwith other ordinary par-ticles throughthe exchangeof ordinary force particles. Similarly,mirror particlesinteractwith othermirror particlesthroughtheex-changeof mirror force particles. Thereareno ‘cross interactions’


connectingordinaryandmirror particlesfromany of theknown non-gravitational forces* .

Clearly, just as ordinary atomscan form by the electromag-netic force betweenprotonsandelectrons,mirror atomscan formby the mirror electromagneticforce betweenmirror protonsandmirror electrons.However, ordinaryandmirror atomsdo not inter-act with eachother– exceptby thevery feeblegravitational force.Thus,if therewasa rock madeof mirror matterin front of our eyesthenwe couldn’t seeit becauseit doesn’t emit or reflectordinarylight. It couldemit mirror photonsif it washot but mirror photonswould passright throughus without interacting.Conversely, if weshoneordinarylight on it thentheordinaryphotonswould just passthroughit. As I alreadymentionedin chapter1, we couldn’t pickit up becauseit would simply fall throughour handundertheforceof gravity andthenthroughtheEarth(assumingherethat thereareno new interactionsconnectingordinaryandmirror matter, seethepreviousfootnote).Wecansafelyconcludethatif therewasanegli-gible amountof mirror matterin our solarsystemwe would hardlybeawareof its existenceatall.

Anotherwayof illustratingtheconsequencesof themirror sym-metry connectingordinary and mirror particlesis by consideringthe following ‘thought experiment’. I alreadydiscussedonesuchthoughtexperimentin theintroduction–aboutadistantmirrorplanetcalledMiros. Now imaginethatthereisawizardmorepowerful thanHarry Potter, so powerful in fact that hecould easilychangeeveryparticle in our entiresolarsysteminto mirror particles. Would wenotice?We would still beherebut madeof mirror atomsinsteadofordinaryones,gravity wouldholdourfeetdown,mirror electromag-netismwouldstopusfrom falling throughthefloor (madeof mirrormatter)and the Sunwould produceenergy via the mirror nuclearforcewhichwouldbeconvertedinto mirror light via mirror electro-magneticinteractions.Theonly observabledifferencewouldbethat

* Actually, lateron in chapter5 I will discussthepossibilityof tiny new forcesconnectingtheordinaryandmirror particles.However, thesearenew forceswhicharecompletelyindependentfrom the four known forces. For thepurposesof thispreliminarydiscussiona detailedexaminationof possiblesmall forcesconnectingordinaryandmirror particlesis ignored.


the starsin the night sky would look different– if we aremadeofmirror matterwewouldseemirror starsinsteadof theordinaryones.Thus,assumingthat therearebothordinaryandmirror starsin thesky, we would seea differentsetof starsif we weremadeof mir-ror matter. Theonly otherdifferencewould be that in -decaythemirror neutrinoswouldall beright-handedinsteadof left-handed...

Whosecrazyidea?

Scientistsoftenamusethemselvesby arguingaboutthepriorityof ideas– everyoneneedsa hobby! Who did what, when. In the ,x ��

centuryNewtonandLeibnizhadlotsof fun arguingaboutwhoreally discovered calculus. In the caseof mirror matter the ideashoulddatefrom sometimeafter 1956,sincebeforethis everyonegenerallyassumedthat the fundamentalinteractionswere alreadymirror symmetricsotherewouldhavebeennoreasonfor postulatingtheexistenceof mirror matter. It is somewhatsurprisingto learnthatthe ideaof mirror matterdidn’t take long to beproposed.The ideafirst appearedin thescientificliteraturein 1956,thesameyearthatitwassuggestedthattheordinaryinteractionsdid notrespectleft-rightsymmetry. In fact,not only in thesameyear, but alsoby thesameauthors(LeeandYang)andalsoin thesamepaper! While theLeeandYangpaperwasdevotedto arguingthatleft-right symmetrymaybebrokenby theweakinteractionsof theordinaryparticles,thelasttwo paragraphssuggestedthatit couldbeunbrokenif mirror matterexisted.In thewordsof LeeandYang(from their 1956paper

*):

As is well known,parityy violationimpliestheexistenceof aright-left asymmetry. We have seenin the above somepossibleexper-imental testsof this asymmetry. Theseexperimentstestwhetherthe presentelementaryparticlesexhibit asymmetricalbehaviourwith respectto theright andtheleft. If suchasymmetryis indeed

* AuthorsNote: ‘parity’ is anotherterm usedin the technicalliteratureto de-scribemirror symmetry. ‘Parity violation’ means‘violation of mirror symmetry’.


found,thequestioncouldstill beraisedwhethertherecouldnotex-ist correspondingelementaryparticlesexhibiting oppositeasym-metry suchthat in the broadersensetherewill still be over-allright-left symmetry. If this is the case,it shouldbe pointedout,theremustexist two kindsof protonse"z ande|{ , theright-handedoneandthe left-handedone.Furthermore,at thepresenttime theprotonsin the laboratorymust be predominatelyof one kind inorderto producethesupposedlyobservedasymmetry,.....

In sucha picturethesupposedlyobservedright andleft asym-metryis thereforeascribednot to a basicnon-invarianceunderin-version,but to a cosmologicallylocal preponderanceof, say, e {over e"z , a situationnot unlike that of the preponderanceof thepositive protonover the negative. Speculationsalongtheselinesareextremely interesting,but arequite beyond the scopeof thisnote.

LeeandYangneverreturnedto themirror matterideaandwerecon-tent with receiving the Nobel prize for their work suggestingthatmirror symmetrywasbroken. In fact,theideawaslargely forgottenwith only a handfulof paperswritten on thesubjectduringthefol-lowing threedecades.My colleagues,HenryLew, RayVolkasandI,blissfully unawareof thelasttwo paragraphsof LeeandYang’s pa-per, rediscoveredtheideain 1991andput it into a moderncontext.More recently, Zurab Silagadzealso rediscovered the idea whilereadingthe ‘Encyclopaediaof AnomalousPhenomena’– I’ ll haveto getacopy of thatbook! I have alsobeentold by akind Professorfrom India thattheideafirst appearedseveralthousandyearsagointheancientbookthe‘Upanishads’.....

In physics many seemingly simple and elegant ideas areproposedonly to be eventually discardedwhen they arecarefullycheckedby experimentsandobservations.This is somethingwhichdistinguishessciencefrom otherdisciplines.The fateof themirrormattertheorythereforerestswith experimentsandastronomicalob-servations– it cannotbedecidedby purethought.It is time now toexaminetheevidence....

It’ s goodto haveanopenmind,but not soopenthat yourbrainsfall out.

BertrandRussell

PART II

Evidence for Mirror Matterin the Universe

Chapter 3

Discovery of Mirror Stars?

If mirror matterreally doesexist then it is reasonableto supposethat it exists in our galaxyandin othergalaxies.Yet, becauseit isinvisible, neitheremittingnor reflectingordinarylight, it would becompletelydark.Thisdoesnotmean,though,thatit cannothaveob-servableconsequencesbecauseeveninvisibledarkmattercanmakeits presenceknown to usby its gravitationaleffects. A famoushis-torical exampleof thepower of gravity is thediscovery of our } ��planet,Neptune.

Thediscoveryof Neptune

The first six planetshave beenobserved since ancienttimes.The

x ��planet,Uranus,wasdiscoveredby theEnglishastronomer

William Herschelin 1781usinga homemadereflectingtelescope.Prior to the discovery of Uranus,the most distant known planetwasSaturn,which orbits the Sunat a distanceof about1.4 billionkilometres– nearly10 timesthedistanceat which theEarthorbitsthe Sun. Uranus,it turnsout, orbits at a distanceof about2.9 bil-lion kilometers,takingapproximately84 yearsto completeanorbitaroundthesun.

The } �� planetNeptunewasdiscovered65yearslater, but unlikeUranus,whosediscovery wasaccidental,thediscovery of Neptunewasno accident.Indeed,Neptune’s discovery is a ratherimpressiveexampleof the power of the scientificmethod. This discovery isillustratedin Figure 3.1.

39

40 Discovery of Mirror Stars?

Path of Uranus

in 1800Uranus

UranusUranus

Uranus

In 1810

In 1840

In 1830

Neptune In 1800

NeptuneNeptune1830

Neptune1840

in 1810

Path of Neptune

Figure3.1: Therelativepositionsof UranusandNeptuneduringtheperiod1800-1840.Between1800-1810Uranuswasmoving towardsNeptuneandthegravitationalinfluenceof NeptunecausedUranusto travel faster. Whilebetweentheyears1830-1840Uranuswasmoving awayfrom Neptuneandthegravitationalinfluenceof NeptunecausedUranusto travel slower.

Newton showed us how to calculatethe orbits of the planets.Put anotherway, they mustobey Newton’s laws of motion. How-ever, Herschel’s discovery of the

x ��planetUranuseventually led

to somethingodd. Its orbit did not follow exactly theexpectationsfrom Newton. This meansthateithera) Newton’s laws weresome-how wrong* , b) thereweremistakesin theobservationsof Uranus,

* Actually, Einsteinshowed muchlater that they do in fact breakdown undercertainconditions,but thiswasnot thereasonfor theanomaliesin Uranus’s orbit.

Discovery of Mirror Stars? 41

or c) therewassomethingnew. In October1845 the EnglishmanJohnAdamsandindependentlyon theothersideof thechannelUr-bainLeverrier(in June1846)proposedthatanhithertounseenplanet(Neptune)mustexist furtherfrom theSun.Not only did they predictthat it mustexist, but themathematicsthroughwhich physicallawsaredescribedallowed themto predict its positionvery accurately.In fact, the two independentcalculationsof Adamsand Leverrieragreedwith eachother to within 1 degreefor their positioningofNeptune.

JohnAdamswasspectacularlyunsuccessfulat convincing theastronomersto searchfor Neptune– they eitherdidn’t understandhiscalculationsor didn’t botherto. Leverrier’seffortsmetwith moresuccess.Thenight afterreceiving a letterfrom Leverriersuggestingthat he shouldlook for the new planet,JohannGalle of the BerlinObservatoryfoundNeptunein September1846.But Galle’s job wasmadeeasyfor him – he wastold whereto look. Clearly, Neptunemadeits presenceknown first by its gravitational effects andwaslater observed directly. Galle’s boss,JohannEncke, who initiallythought that the searchwas a wild goosechase(or in the case–‘planetchase’)wroteto Leverrier

� �:

Allow me, Sir, to congratulateyou most sincerelyon thebrilliant discoverywith whichyouhaveenrichedastronomy.Your namewill be forever linkedwith themostoutstandingconceivableproofof thevalidity of universalgravitation...

Of course,in bookssuchas this, authorssuchas myself arealways wheeling out successfulhistorical examples. The readershould be aware that for every theoreticalsuccessthere are alsomany failures. The failures,however, arenot usuallyemphasisedandareoftenquickly forgotten* . Still, thesuccessfulcasesdo show

* Onesuch‘f ailure’ whichhasnotquitebeenforgottenis thestoryof theplanet‘Vulcan’. Buoyedby his ‘discovery’ of Neptune,Leverrierwenton to arguethatanew planet– Vulcan– wasrequiredto explainananomalyin theorbit of Mercury.Searchesfor Vulcanfailed to find it, or rather, many searchesfound it, but it wasnever confirmed.Theanomalywaslaterexplainedby Einsteinin 1915;Mercury’sorbital misbehaviour wasnot dueto a new planetor dueto mistakenobservations,but due to the modificationof gravity predictedby Einstein’s generalrelativitytheory.


thatat leastsometimespeoplegetthingsright. With thiscautionarynotein mind, let menow continuethestory.

Distribution of matterin theheavens

In the Universematteris not uniformly distributed. From ob-servationsweknow thatmatterbunchestogetherto form stars,starsbunchtogetherto formgalaxies,andgalaxiesbunchtogetherto formgalaxyclusters.This is our currentpictureof the observableUni-verse.Exactlyhow theUniversecameto belike this is certainlyaninterestingbut verydifficult problem.It is aproblemwhich is at theforefrontof modernresearch.Needlessto sayit is evennow notun-derstoodandthat’swhyit’sat theforefrontof modernresearch.For-tunately, themostcompellingargumentsfor theexistenceof mirrormatterin our galaxyareessentiallyobservationbased.They do notrequireknowledgeof the physicsof galaxyformationor completeunderstandingof the evolution of the Universefrom its beginning,assumingit hasone,to thepresenttime. Of course,suchknowledgewouldbevery useful,but we canlearnmuchwithout it.

Newton’s lawsof gravitationaresimpleandpowerful. Anythingwith masswill influencethe motion of any otherbody with mass.The influenceis greaterthecloserthe two bodiesare. Equally im-portantis that theeffectsof gravity aregreatestfor bodiesof largermass. Of coursethis effect is well known to Moon walkers. NeilArmstrongcould jump higheron theMoon thanon theEarth.Thiswasnot just becauseof his greatjoy at beingthefirst personon theMoon, or becausehehadjust boughta Toyota. Rather, it wassim-ply becausetheforceof gravity ontherelatively light Moonis muchlessthantherelatively heavy Earth.

Thereis goodreasonsto believe that gravity is universal. Theorbitsof theMoonandman-madesatellitesaroundtheEarth,theor-bitsof theplanets,cometsandasteroidsaroundtheSunall obey thesameuniversallaw. Indeed,the power of Newton’s laws hasbeenquite spectacularlydemonstratedwith our discussionof Neptune.What abouton very small distances?Small distancescalescanbestudiedin carefullaboratoryexperimentsusinga typeof pendulum


Figure 3.2: Orbital speedof the planetsversustheir distancefrom theSun.

calleda ‘torsion pendulum’.Suchexperimentshave confirmedthatgravity is describedby thesamerulesonveryshortdistancesasit isover largedistances.For example,a recentlaboratoryexperiment

�)�hasmeasuredthe gravitational attractionbetweenobjectsjust 0.2millimetresapart,againshowing thatNewton’s laws areupheld.

Thegravitational forceon theplanetsis duemainly to themassof the Sun. This is becausethe Sun containsmore than ~�~�� ofthe massof the solar system. The closera planet is to the Sun,the greaterthegravitational attractive force which theplanetfeels.Thelarger theforce,thelarger is theplanet’s orbital velocity. Con-versely, the more distantthe orbit the weaker the hold of gravity,which meansthatdistantplanetsmustmove moreslowly otherwisethey would beflung into spacenever to return. Figure 3.2 (above)showshow theorbitalspeedof theplanetsvariesfrom theirdistanceto the Sun. [The unit of distanceis the AstronomicalUnit or AU.


1 AU is theEarth-Sundistance].As thefigureshows, thevelocityrangesfrom about50km/sfor ourclosestplanet– Mercury, to about5 km/sfor ourmostdistantknown planet– Pluto.

Evidently, thereis a strongconnectionbetweenorbital velocityandthe forceof gravity. In fact,knowing Newton’s laws we couldput constraintson the distribution of massin our solar systembystudyingtheorbitsof theplanetsandtheotherorbiting bodiessuchas cometsand asteroids;the massdistribution directly affects thegravitational forcewhich dictatestheorbital motionof theplanets.For our solarsystemthereis not muchroomfor a large proportionof invisible mirror matter, or any othertypeof invisible matter. Anynearbymirror matterplanetin our solarsystemwould have madeits presenceknown via its gravitationaleffectson themotionof theotherplanetsor comets,in muchthe sameway that Neptune’s ex-istencewasrevealedfrom its gravitational effect on the motion ofUranus.

Nevertheless,smallbodies(for example,asteroidor cometsizedobjects)madeof mirror matterarepossiblebecausethegravitationalinfluenceof thesebodieswould be far too small to have beende-tected. Also, a planetaryor even star sizedmirror object is alsopossibleif its orbit is distantenough.In fact,in chapter6 I will dis-cussfascinatingevidencethatthereareindeedmirror matterobjectsout therein oursolarsystem.Thereis explosive evidencethatsmallasteroidor cometsizedmirror matterobjectsexist andoccasionallycollide with theEarthaswell asindependentevidencefor planetorstarsizedmirror matterobjectsin distantorbitsfrom theSun.

In any case,within the orbit of Pluto thereis not much roomfor a large amountof mirror matter. Let us move on to larger dis-tances.If we wereto look at our solarsystemfrom a greatdistanceaway– sogreatthatoursolarsystemappearedasatiny pointsourceof light, thenwe would noticethat it is alsoin motion. It is orbit-ing aroundthecentreof our galaxyin a roughlycircularorbit. Thehugedistanceinvolved meansthat it orbits thecentreof thegalaxyonly aboutonceevery 200 million years. If we move even furtheraway, suchthatour galaxywasonly thesizeof a bright point, thenour solar systemwould be in motion around the neighbouringgalaxies.


But beforewe go away any further, let us comeback a step.While I have arguedthat thereis no evidencefor a large amountof mirror matter in our solar system(althoughlater I will arguethat thereis interestingevidencefor a small amount),what abouton larger distancescales?Could therebea large amountof mirrormatterin ourgalaxy?Onemight think thatbecauseNewton tellsustherecannotbe muchmirror matterin our solarsystem,it followsthat therecannotbe much mirror matter in our galaxy. Still, wemustbecareful,thegalaxyis somuchlarger thanthesolarsystem.It might bepossiblefor ordinaryandmirror matterto bedistributedquite independently;a sortof cosmicsegregation,a bit of ordinarymatterhere,abit of mirror matterthere...

Actually, it turnsout to beveryeasyto understandwhy ordinaryandmirror mattershouldbe separatedon relatively small distancescaleslike our solarsystem.However, I will postponea discussionof this for later. In the meantimeonecould just keepin mind thepossibilitythatthedistribution of mirror matterandordinarymattercandependverymuchonthedistancescaleinvolved. In fact,asjustaboutany ancientpersonfrom Mongoliaor Tibetwouldsurelyhavetestified,becausetheir nearbyregion containsonly land and theyandnobodyelsethey knew ever saw any oceans,the whole worldmustbemadeof land...They might have beensurprisedto discoverthattheEarth’s surfaceis coveredby morethan70%ocean...

Galaxiescontainan invisiblesphericalhaloof dark matter

It is now time to take a closer look at our own galaxy – TheMilk y Way. Our galaxyappearsto be a typical spiral galaxycon-tainingof order100billion starswhicharedistributedin a flat disk,with a small sphericalbulgeat thecentre.Obviously (with currenttechnology)we can’t view our entire galaxy from a distance,butwe cantake picturesof othersimilar galaxies. Figure 3.3 (on thefollowing page)shows picturesof threetypical spiralgalaxieswithdifferentorientations.


Justlike themassdistribution in our solarsystemcouldbe de-terminedby looking at the motionsof the planetsaroundthe Sun,we candeterminethemassof thegalaxy, aswell asobtaininginfor-mationaboutthedistribution of masswithin thegalaxy, by measur-ing thevelocity of starsat variouslocationsanddistancesfrom thegalacticcentre.We might expectthat mostof themassis nearthecentralregionof thegalaxybecausethat’swheremostof thelight is.If this werethecasethenour galaxywould bedynamicallysimilarto our solarsystem– but on a muchlarger scale. This meansthattheorbitsof starsshouldshow a significantdecreasein their orbitalvelocity asoneobservesstarsorbiting furtherandfurther from thegalacticcentre.Surprisinglythoughthis is not thecase.In fact,thevelocity is moreor lessconstantasonelooksat objectswith largerandlargerorbits (Figure 3.4

�1�). This is trueof starsat theedgeof

thediskaswell asstarsandcompactgroupsof starscalled‘globularclusters’distributedoutof theplaneof thedisk.

The conclusionis that thereis much more to our galaxy thanmeetstheeye. And this is in fact literally true. The massanddis-tribution of light emitting/reflectingmatteris completelydifferentto the massand distribution of matter inferred dynamically fromtheeffectsof gravity throughNewton’s laws. Theupshotis thatour

FIGURE3.3


Figure3.4: Observedorbiting velocitiesof stars(verticalaxis)in thespi-ral galaxyM33, superimposedon its optical image. The horizontalaxisis thedistancefrom thecentreof thegalaxyin kiloparsecs(1 kiloparsecsis 3.3 thousandlight years). The poor agreementbetweenthe expectedvelocitiesandtheactualonesprovidesstrongevidencefor invisible ‘darkmatter’.

galaxy(andsimilar resultshave alsobeenfoundfor othergalaxies)extendswell beyondthevisible edge.Evenmoreinterestingis thatthe massis distributed spherically, roughly like a (3-D) sphereorball, which is calledthehalo,despitethefactthatthevisiblemassispredominatelydistributedin a flat disk.

Althoughthis invisible massdistribution is calleda ‘halo’, it isnot much like the halo aroundthe headsof the saints. Rather, itis a threedimensionalsphericaldistribution which startsfrom thegalacticcentreandextendsbeyond the visible edgeof the galaxy.The amountof massin this threedimensionalhalo is estimatedtobeat leastseveraltimestheamountof massin thedisk. Thus,in the


100 thousand light years

us

Halo

Galactic center

Figure3.5: Inferreddistributionof massin ourgalaxy. Thevisiblematterforms a disk, viewed edge-onin this figure, which is surroundedby aninvisible threedimensionalsphericalhaloof matter.

caseof galaxies,whatwe seeis notwhatwe get.Theinferredmassdistribution of ourgalaxyis illustratedin Figure 3.5 (above).

This result is not somethingthat wasfound yesterdayor eventhedaybefore.Theevidencehasbuilt up over many decades;it isnotevenconceivablethattheresultscouldbedueto mistakenobser-vations.Theobservationshave beenrepeatedby many independentgroupsin many countriesfor many galaxies,all reachingthesameembarrassingconclusions. We may thereforesay with somecer-tainty that either invisible dark matterexists or Newton’s laws arewrong.

Weknow thatNewton’s lawscannotbecompletelywrong.They


have beenverified for objectsin our solarsystemwith tremendousaccuracy. The only known examples(in our solar system)wherethey breakdown area tiny anomalyin the motion of Mercury andalso the bendingof light aroundthe Sun. However, both of theseexamplesarecompletelyunderstoodandwereexplained,or in thecaseof thebendingof light aroundtheSun,predictedby Einsteinin1915.Einstein’s theoryof gravity goesby thetitle of ‘generalrela-tively’. Generalrelativity is not simply a modificationof Newton’slaws,but averydifferentsortof theory. However, Einstein’sgeneralrelativity theorydoesagreewith Newton’s theorywhenthegravita-tional force is not too strong,asis almostalwaysthe casein mostpracticalexamples.For this reason,Newton’s laws areconsideredtodayasa very usefulapproximation. In the caseof starsorbitingaroundthegalacticcentre,Einstein’s theorygivesthesameresultsasNewton.

Of course,it is possibleto imaginethat both Newton andEin-steinarewrong.Nobodyis perfect.Maybethesetheoriesonly workover relatively small distancessuchasthesizeof our solarsystem– amerefew billion kilometres,while over largerdistancesthey be-comemodifiedin sucha way to explain themotionsof thestarsinourgalaxywithoutany embarrassinginvisiblematter. Certainlythisis possible,but so far nobodyhasmanagedto find a very eleganttheorywhich doesthis. Obviously, thenon-existenceof sucha the-ory cannotberigorouslyshown either. Nevertheless,at thepresenttime,themostreasonableinterpretationof theobservationsseemstobethatinvisible darkmatterreally doesexist, andin factdominatesthemassof ourgalaxy.

Thenature of thebeast

If we acceptthatinvisible darkmatterreally doesexist, thenthenext logicalquestionto askis whatis thenatureof thisdarkmatter?Is it somethingstandardmadeof ordinarymatter? Maybe it is intheform of faint deadstarscalled‘white dwarfs’ or smallstarsthatnever gethot enoughto burn hydrogencalled‘brown dwarfs’ ? Oris it somethingelse,somethingmoreexotic?


While someof it is surelyin theform of dustandgasthiscannotexplain theinferreddarkmatterin thehalo.Astronomershave beenableto gatherinformationontheoveralldistribution of dustandgaswithin our galaxyby measuringtheeffectsof theabsorptionof starlight andby tell-tale radio (wavelength)emissions.They concludethatdustandgascontributeanegligible amountto thegalactichalo,althoughtheremaybea significantcomponentin thedisk. In fact,every conventionalpossibility for thedarkmatterrunsinto seriousproblemsfor onereasonor another.

As anotherexample,let mebriefly mentionwhitedwarfs.Whitedwarfsarefaint deadstarswhich have usedup all their nuclearfuelandno longersustainnuclearreactions.Our sunis destinedto be-comeawhite dwarf oneday. Currently, it is amiddleagedstarwithnomid-life crisesin sightsowedon’t have to worry toomuchat themoment. Anyway, whena starstopsburning nuclearfuel its cen-tral pressureis no longergreatenoughto supportits hugeweight.The effect of this is that the starbecomesgravitationally unstable.Theinnerpartof thestarcollapsesunderits own weightwith awhitedwarf astheendproduct.Typically awhitedwarf hasasizeassmallastheEarthbut with amasscomparableto thatof theSun.

Initially white dwarfsarequitehot,but sincethey areno longerburningnuclearfuel andproducingenergy, they slowly cool. How-ever, becausethey aresosmall they arevery faint. Indeed,their lu-minosity is proportionalto their surfaceareawhich is about10,000timessmallerthananordinarystarlikeourSun.Becauseof theirex-tremefaintnessthey canonly beobservedin thevery nearbyregionof our galaxy, typically lessthana few hundredlight yearsfrom us(although,theyoungestandhottestwhitedwarfscanbeseensignifi-cantlyfurtherawaythanthis). Thepopulationof whitedwarfscouldthereforebevery numerous– perhapsnumerousenoughto accountfor themysteriousinvisible massin our galaxy. Still, thereareverybig problemswith this ideadespiteits obviousmerits.

Themainproblemwith ‘white dwarf darkmatter’ is that in thecollapseprocesswherethey areformed,theouterlayersof thestarareejectedinto space.This would leadto observableconsequenceswhich arenot seen.For instance,I alreadydiscussedthe fact thatobservationsappearto excludeany significantamountof gasor dust


in thehaloof our galaxy. Evenif theejectedgascollapsesontothegalacticdisk dueto collisional processes,its estimatedabundancewould begreaterthantheentireinferredmassof thedisk. Further-more, this ejectedmaterial is rich in heavy elementssuchas car-bonandnitrogen(in astrophysicsany elementheavier thanheliumis calleda ‘heavy element’)which do not seemto be particularlyabundantin ourgalaxy.

Otherpossibilitiesfor thehalodarkmatter, suchasblackholesandneutronstars,suffer similarproblemssincetheir formationalsoleadsto heavy elementpollution andother tell-tale signs. In fact,everyconventionalcandidatefor thedark matteris in seriouscon-flict with observations. Not surprisinglythen,themysteriousnatureof thedarkmatteris widely consideredasthegreatestof all puzzlesin astrophysicsat themoment.

At theendof thedayweareleft with theremarkableconclusionthat, not only is mostof the massin galaxiesinvisible, but galax-ies it seemsare not predominatelymadefrom ordinary matteratall. Galaxiesseemto bepredominatelymadefrom somethingcom-pletely unknown, somethingthat is, in a very literal sense,not ofthisworld...

Entermirror matter

Imaginethatasignificantpartof ourgalaxywasindeedmadeofmirror matter. Could that explain the mysteryof the inferreddarkmatter? Clearly mirror matteris dynamicallyvery similar to ordi-nary matter, it would form stars,planetsetc., but would not emitany ordinary light. It would emit mirror light (that is, mirror pho-tons),but wecan’t detectthat. In shortit wouldbeinvisible– whichis just what’s required. So far so good. But what aboutthe dis-tribution? Becauseordinaryandmirror matteronly interactswitheachothervia gravity, their distribution canbecompletelydifferent(dependingon their initial conditionssuchaschemicalcompositionandangularmomentum).But could this really explain why mirrormatterdoesn’t form in adisk like ordinarymatter?


Figure3.6: M87 – an exampleof an Elliptical Galaxy. (Credit: Anglo-AustralianTelescope,David Malin).

Perhapsa relevantpieceof informationis theobservationalfactthat in somegalaxiesordinarymatteris distributedroughlyspheri-cally ratherthanin a disk. Suchgalaxiesarecalledelliptical galax-iesandonesuchexampleis shown in Figure 3.6. This immediatelysuggeststhat mirror mattercould, in principle, also form a spher-ical distribution. So maybeour galaxyandother similar galaxieshave their ordinarymatterembeddedinto anapproximatelyspheri-cal mirror galaxy. But observationstell us that every spiral galaxysimilar to our galaxy always seemsto have an invisible approxi-matelysphericalhalo. Why shouldit alwaysoccur?Thefactthat italwaysseemsto occursuggeststhatit probablycannotbeexplainedjust from somerandominitial conditions,suchasangularmomen-tum. I feel thattheanswermight lie in theinitial chemicalmake upof theUniverse,asI will explain in amoment.

Galaxiesarebelieved to be formed from a giant collectionofparticlesheldtogetherby gravity. In otherwords,they wereonceahugegascloud.Within thesehugesystems,particlesarecontinually


colliding off eachother. Thesecollisionsdotwo things.They createapressurewhichcanresisttheforceof gravity. If this is all thattheydid, thegaswould never collapse;it would just sit there.However,the pressurecanbe reducedover time if the collisionsareable toexcite the atoms/moleculesinto higherenergy levels. The excitedatomssubsequentlyradiatephotons(which eventuallyescapefromthe gas)andthe atomsmove backto their lowestenergy state. Inthis way heatcanbe removed from the gasallowing it to becomemoretightly compressed,thatis, to collapse.

Importantly, thecollapseprocessoccursquiteindependentlyfortheordinarymatterandmirror mattercomponents.Why? Becausecollisionsareanelectromagneticprocess,whichactsindependentlyon ordinaryandmirror matter: Ordinaryparticlescancollide withordinaryparticles,mirror particlescancollidewith mirror particles,but ordinaryandmirrorparticlescannotcollidewith eachother. Thismeansthatthetemperatureandpressureprofilesof theordinaryandmirror mattercomponentsare,in general,completelydifferentandevolve differently. The dynamicsof sucha self gravitating two-componentcollapsinghugegascloud is very complicated. Com-plicatedenoughperhapsto explain the vast array of galaxiesandstructuresthatareseenin theUniverse.

Onething that is known thoughis that theway in which suchathing evolvesdependsquitesensitively on its initial chemicalcom-position.Chemicalcompositionrefersto theproportionsof thedif-ferentelementsandmoleculesthatarepresent.Theinitial chemicalcompositionof theordinarymattercouldbequitedifferentfrom theinitial chemicalcompositionof themirror matter. Why?Theanswermay endup beingdue to the initial conditionsat the very instantwhentheUniversewascreated– duringthe‘big bang’– andI willsaya few morewordsaboutthis lateron. This meansthat theevo-lution of theordinaryandmirror mattercomponentscouldbequitedifferent.Onecouldeasilyimaginethat therateof collapseof mir-ror matterinto compactsystemssuchasmirror stars/planetsis muchfasterthanordinarymatterwhich reducesthe‘friction’ betweenthemirror mattercomponentsin thegalaxytherebypreventingcollapseof the mirror matter into a galacticdisk. In other words, mirrorstarsandplanetsmightcondenseoutof theprimordialgalacticsoup


beforethemirror matterhastime to collapseontoadisk.It thereforeseemspossiblefor a roughlysphericalhalopredom-

inatelymadeof mirror matterto exist. It wouldcontainmirror stars,dustandmaybealso large gasclouds... But how canwe test thisidea?

Waiting for an explodingmirror star...

Thereareseveralwaysof testingthis ideathatourgalaxyis fullof mirror matterobjectssuchasmirror stars.First,old massivestarsdonotjustfadeaway;they collapsewith abangin atitanicexplosioncalledasupernova. Theseexplosionsaresopowerful thatthey mayevenoutshinethegalaxyin which they appear. Sucheventsthougharequiterare,occurringin ourgalaxyaboutonceeveryfew hundredyearsor so.Oneof themostspectaculartookplaceonthe

& ��of July

in 1054.Chineseobservationsat the time recordedthat the starwasso

bright that it waseven visible during theday-time,andwasnearlyasbright astheMoon at night-time.Curiously, strangelights in thesky arealsoreportedevery yearin theUnitedStatesalsoon the

& ��of July, but the origin of thesemorerecenteventsis undoubtlyofterrestrialorigin.... Theremnantsof the1054supernova explosion,known astheCrabnebula, still exists(Figure 3.7) andis oneof myfavourite astronomicalpictures. [My most favourite astronomicalpictureis, of course,VincentVanGogh’s Starrynight].

The last recordedsupernova event in our galaxy occurredin1604. Sowe shouldbeoverduefor another... In fact in 1987a su-pernova in anearbygalaxyexplodedandwasvisiblewith thenakedeye as well as in undergroundexperiments,as I will explain in amoment.

How doesastargetinto trouble?Starsevolvepeacefullyfor mil-lions of years,however nothinglastsforever (includingdiamonds!)and eventually the star runs out of nuclearfuel. When this hap-pensthecoreof thestarcollapsesunderits own weight in lessthana second. If the star’s massis lessthan abouteight solar massesthen the end product is a white dwarf – an object about the sizeof the Earthwith a massof aboutthe Sun. However, if the star’s


.

———————————————— ——————-

Figure3.7here

———————————————— ——————-

massexceedsabout8 solarmassesthentheendproductof this gi-ganticexplosionis somethingevenweirder. Whensuchaheavy starcollapses,thepressurebecomessogreatthatall of theelectronsarepressedinto theprotonswhich combineto form neutronsandneu-trinos,

7�I-:FHa9�IK; ZThis is a weak interactionprocess,which only becomesenergeti-cally possiblebecauseof thehugegravitationalpressure.Anyway,


theneutrinosescapeleaving behindatightly compressedballof neu-trons– calleda ‘neutronstar’. An objectheavier thanour sun,butwith a radiusof only about15 kilometres.Onespoonfulof thisma-terialwouldweighmorethanNew York City... Clearlysuchmaterialshouldbehandledwith care.If swallowedseekmedicaladvice...

In the caseof the 1987supernova explosion,not only wasthelargeincreasein brightnessobserved,but theburstof neutrinoswasdetectedin undergroundlaboratoriesin Japanandthe USA. A to-tal of 20 neutrinoeventsover a time scaleof just 12 secondswererecorded.As I will discussin laterchapters,neutrinosareexpectedto beawindow into themirror world. A tiny mixing forceis allowedandwould have theeffect of changinghalf of thesupernova neutri-nos into mirror neutrinos.Unfortunately, becauseof variouslargeuncertainties,the initial numberof supernova neutrinosthatwouldbeexpectedto arriveat theEarthcannotbepreciselydetermined,sowecan’t tell whetherhalf of theneutrinosaremissing.Thisunsatis-factorysituationmayimprove in thefuturewith thegreatadvancesin neutrinodetectorsthatnow exist.

Evenmoreinteresting,though,is theimplicationsof theconver-sionof mirror neutrinosintoordinaryneutrinoswhichwouldhappenif a mirror star explodes. This could make a mirror supernova ef-fectively ‘visible’ even if no visible light is emitted. It would bea phantomexplosion detectableonly with neutrinos* . (Actuallyanotherpossibility, which I will discussin chapter5, is that a tinyforceallowing for photonmirror photontransitionscouldmakemir-ror stellarexplosionsproduceanobservableburstof photonsaswellasaneutrinoburst).

Althoughthereis asyetnoevidenceof suchphantomstellarex-plosionsthis is not unexpectedsincewe know thatnearbyordinarysupernova explosionsarerelatively rareevents. Theestimatedrate

* It is possiblethatanordinarysupernova explosioncouldmimic anexplodingmirror starif thelight from theordinarysupernova wereblockedoutby interstellardust. However, even this possibility canbe testedby looking at the directionoftheneutrinos.Thedirectionof theneutrinosfrom anexplodingordinarysupernovashouldcomefrom within thediskof our galaxy(if it happensto beoneof thefewhaloordinarystarsit certainlywon’t beobscuredby dust).On theotherhand,theneutrinosfrom anexplodingmirror starwill mostlikely comefrom thehalo,thatis, in a directionout of theplaneof thegalacticdisk.


of ordinarysupernovaexplosionsis roughlyonceeveryfew hundredyears– soit maynotbesurprisingif therateof mirror supernovaex-plosionsalsoturnsout to below.

Discoveryof mirror stars?

On quite a different tack, even invisible starscan reveal theirpresencethroughtheir gravitational effectson light. In 1986,Bo-hdanPaczynskihada goodidea

� . His ideawasto mounta searchfor darkmatterbasedontheideathatamassiveobjectcouldactasasortof lens.Evenif we can’t seethemassive object,its gravity canbendthe light comingfrom a moredistantstararoundit, in muchthesameway that light getsbentasit passesthrougha magnifyingglass.If thereareinvisible bodiesfloating in thehaloof our galaxyit is possiblethat they shouldpassbetweenusandour line of sightto a backgroundstar. If this happensthenthegravity magnifiesthelight from thebackgroundstarasthe light passesaroundthe invis-ible object– causingthebackgroundstarto brighten.This effect isillustratedin Figure 3.8.

Paczynski

Invisible Star(= Mirror Star?)

Background star in aneighbouring galaxy

Bohdan

Figure 3.8: Magnificationof a star’s light by the gravity of an invisible‘star’ (Mirror star?). The light bendsaroundthe invisible star just likelight bendsin a magnifyingglass.


Becausethe invisible star, the backgroundstar, and our solarsystem,areall in relativemotion,themagnificationcanonly lastfora finite time. The moremassive the invisible object(the lens),thestrongeritsgravitationaleffectandthelongertheperiodof increasedbrightness.For massesin therangefrom planetsizeto severaltimestheSun’s mass,thebrightnesslastsfrom a few hoursto asmuchasa yearor so. However, even if the halo of our galaxywasfull ofinvisiblemassiveobjects,thechancethatoneof theseobjectswouldpassbetweenusandaparticularbackgroundstaris verylow. In fact,it canbeestimatedthatthechanceis aboutonein a million, but theoddscanbeimprovedby simultaneouslymonitoringa largenumberof starsover severalyears.

Four teamsof researchesbegan the searchnearly a decadeago.Firstoff theblockswastheFrenchcollaborationExperiencedeRecherched’ObjectsSombres(EROS),whichwascloselyfollowedby the Optical Gravitational LensingExperiment(OGLE), run byPaczynskiand colleaguesat the University of Warsaw in Poland,thelargeAustralian- USMassiveCompactHaloObjects(MACHO)project, and a smaller French effort called Disk Unseenobjects(DUO). All four usegroundbasedtelescopesand a largeamountof computermemoryto storethebrightnessmeasurementsof millions of stars. When theseprojectsstartedit was expectedthatthemostlikely candidatefor theinvisiblehaloobjectswerelowmassstarscalledbrown dwarfsweighinglessthan10%themassoftheSun.Suchlightweightswouldbetoo faint to seeandthusmightbetheinvisiblecomponentof ourgalaxy. However, whatthey foundwasnot lightweightsbut stellar-weightobjects,with a typical massof abouthalf themassof theSun,andenougheventsto accountfornearlyhalf theestimatedmassof thehalo. Severalsucheventsareshown onthefollowing pagein Figure 3.9 obtainedby theMACHOexperiment�� .

Thus,insteadof finding what they mostexpected,theobserva-tionswereableto excludelightweightobjectsfrom beinga signif-icant componentof the halo. More interestingthough,is that theresultscan be viewed as evidencefor mirror matter, sincemirrorstarsshouldhave a typical masssimilar to that of ordinarystars–which is closeto abouthalf the massof the Sun. In otherwords,


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MACHO LMC 2-year Events

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Figure3.9: Eightdistinct‘MA CHOevents’.Thefiguresshow theintensityof light from a backgroundstar(verticalaxis) asa functionof time. Thelight from thebackgroundstargetsamplifiedfor acertainduration,whichoccursbecausean invisible starpassesin front of thebackgroundstar. Ineachof thesecasesthebackgroundstaris in theLargeMagellanicCloud(LMC) – anearbygalaxy.

the datais roughly consistentwith what you could expect with amirror matterhalo. TheMACHOsarejust mirror stars,or perhapsmirror white dwarfs... Putanotherway, MACHOsarereally Mas-sive AstrophysicalCompactHalo Mirror Objects(MACHMOs).

Evenmoreremarkableis theinferredtotalmassof theMACHOsfoundin theexperiments.Theoutcomeis that theMACHOsmakeup only half or a bit lessof the inferredmassof thehalo�1� . Recall


our earlierdiscussion– themassof thehalocouldbeinferredfromthemotionof starsatvariousdistancesfrom thecentreof thegalaxy.Thefactthattheresultsfrom theMACHOexperimentsfallsshortinaccountingfor all of themassof thehalosuggeststhattheremustbeanothercomponentto thehalowhich doesnot show up in theMA-CHO experiments.Actually this featurecanalsobe plausiblyex-plainedby themirror mattertheory. Mirror starsdon’t make up theentirehalo simply becausemirror matter, like ordinarymatter, ex-istsin two forms: In theform of mirror starswhicharetheMACHOeventsobtainedin theexperiments,aswell asin theform of mirrordustandgaswhich do not leave any observablesignal. [Thesepar-ticular experimentswereonly sensitive to compactsystemssuchasstar-sizedobjects,while thegravitationaleffectof cloudsof gasanddustwould betoodispersedto have beenobserved].

While plausiblyexplaining the resultsof theseexperimentsisonething– rigorousproof is another. Obviously it is difficult to rig-orouslyprove that theMACHOs‘observed’ in theexperimentsaremirror stars(unlessoneof themhappensto explodein our galaxy,leadingto an observableburst of neutrinos).On the otherhand,ifwe take themirror mattertheoryseriously, MACHOsarepredictedto exist, andif they really arethedarkmatterthentheresultsof theMACHO experimentsreally had to find a positive result for MA-CHOs abouthalf the massof the Sun. The fact that the resultsfrom the MACHO experimentsareconsistentwith this predictionis strongevidencefor the theory. Still, if it wasthe only evidencefor mirror matter, thenthecasefor its existencewould be far fromcompelling. In short, it would be nothing to write homeabout–let alonea book! However, I will identify seven major puzzlesinastronomyandparticlephysics,eachplausiblysuggestingthatmir-ror matterexists.

MACHOsor WIMPs?

Let usfinish this chapterwith a brief discussionof themainal-ternative model for the dark matter. While mirror mattercan leadto theformationof mirror stars,whichareanexampleof a MassiveAstrophysicalCompactHalo Object (MACHO), the main alterna-


tive candidatefor the invisible dark matteris appropriatelycalledWIMPs. WIMPs arehypotheticalWeakly InteractingMassive Par-ticles. Their invisibility arisesbecauseit is assumedthat they don’tcoupleto photons. In fact, they interactonly by extremelyweakshort-rangeinteractionsandconsequentlythey seldomcollide witheachother (or with ordinary matter)which meansthat they can’tcollapseto form starsizedobjects. They cannotthereforeexplaintheMACHOevents.Actually they appearto have greatdifficulty inexplaining many of the specificobservationson the natureof darkmatter. For example,WIMP darkmattermakesspecificpredictionsfor thedensityprofile of darkmatterin galaxieswhich seemsto bein strongdisagreementwith observations�1� . Furthermore,WIMPsseemunableto explain the inferredcomplexity of dark matter. Ofcourseit is possiblethatI maybebiased!In defenceof WIMP theo-riesonecansaythatthey areverypopularamongparticlephysicists.

In view of theirpopularity, many experimentershavebeensearch-ing for WIMPs for a long time. Theideais that theseweakly inter-acting particlescould make an observable signal in purpose-builtundergrounddetectors. Insteadof going into the boring technicaldetails,let mesaysomethingabouttheflavourof theWIMP search.Evenbetter, let mequoteoneof theWIMP enthusiaststhemselves.Twelve yearsago(1989),L. Krausseloquentlycapturedtheexcite-mentof thehuntwhenhewrotein colourful language�� :

You area graduatestudentin physics. It’s lateSaturdaynightandyou would muchratherbeat a party. Instead,you area mileunderground,in a cavernousenclosure,entertainedonly by thesoundof acoolingfanwhirring in thedesk-topminicomputerthatis monitoringpulsesreceivedfrom thegargantuandevice locatedin the main chambernext door. It hasbeena boring eight-hourshift andyou longto taketheelevatorrideupthemineshaftto thesurface,to breaththefreshair andto watchthenightsky, thestarstwinkling, andthecool, evanescentglow of themoonbathingtheearth’ssurface.Youare,afterall, studyingto beanastrophysicist,not a geologist. Whenyou forsooka lucrative programmingjobin orderto returnto graduateschool,you envisionedworking at ahugeradiotelescopeaimedat theheavens,sensingthefaintpulses


emittedby quasarsbillions of light-yearsaway. Yet hereyou are,deepunderground,monitoringa new experimentbuilt by a col-laborationamongfour universitieslocatedon threecontinents.Inorderto passthetimeyouwatchthecalibrationpulsesappearwithclocklike regularity on your monitor, notinghow eachexactly re-producesthelast.

Suddenly, almosttoo fastto sense,you noticesomethingmo-mentarilydifferentaboutthesignal.Youhalt theon-lineoutputonthecomputerandcall up theprogramthatsingle-stepsthroughthedata. While the programloadson themachine,your mind races.Thereis asmallchancethatthepulseyousaw, or imagined,is theinfinitesimally small signal from an elementaryparticle makingupatotally new typeof matterneverbeforeobservedonearththatinteractedin your detector. If so, this could be the first time thisparticlehasinteractedin thetento fifteenbillion yearssinceit wascreatedin thefiery Big Bang.Youmaybelookingatasignalfromthebeginningof time! Suchparticlesmayconstituteonehundredtimesmorematerial,by weight, thaneverythingwe canseeputtogether, therebygoverningthestructure,evolution, andeventualfateof theuniverse!Yourdiscoverycouldaffect thewaywe thinkaboutthe universeasdramaticallyashadCopernicus’s assertionthattheearthmovesaboutthesun...

Or perhapsit is just abit of noisein thedetector...

This waspublishedin 1989, andunluckily for that poor graduatestudent,it wasin facta ‘bit of noisein thedetector’;he’s still downthatmineshaftwaitingfor asignalfrom thedawn of time. He’s longgivenuphopeof any excitementandregretsnot takingthatlucrativeprogrammingjob...

Despitemorethana decadeof dedicatedsearches,no WIMPshave beendetected. While I suspectthat WIMPs do not exist, Idefinitelysupportsuchexperiments– so long asI don’t have to dothem! It’s the only way of knowing for sure. Experimentsaretheessenceof science...


BeautyandtheBeast

Themostpopularmanifestationof WIMPs comesfrom a parti-cle physicstheorycalled‘supersymmetry’.Supersymmetryis oneof thosegoodideaswhich doesnot seemto beusedin nature.Su-persymmetryis asymmetrywhichconnectseachof theknown typesof elementaryparticlewith a hypothetical‘superpartner’of a dif-ferent spin. That sucha symmetrycan exist is quite non-trivial.For themathematicallymindedsupersymmetryholdsmuchcharm.However, its implementationasa symmetryof particleinteractionsis very troublesomefor experiments. Most important is that thesupposed‘superpartners’of eachof theknown elementaryparticlesmusthave the samemassasthe ordinaryparticles. This featureisvery similar to the propertiesof mirror particlesor anti-particles;they alsohave thesamemassastheir correspondingordinaryparti-cles. That’s what the symmetrytells us. The problemwith super-symmetryis thatif they did havethesamemass,thenthesuperparti-cleswould have beenexperimentallydiscoveredmany yearsagoinlaboratoryexperiments.Thereis simply no known way of makingtheminvisible in theLab – exceptby breakingthesymmetry. It isvery sad. Supersymmetryis probablywhatThomasHuxley hadinmind whenhewrote,‘The greattragedyof science:theslayingof abeautifulhypothesisby anugly fact’, or maybenot.

Although it is possible to write down theories withhypothetical superparticles which have ‘broken symmetry’,they tendto be very complicatedbecausethereareessentiallyun-limited waysof breakingthesymmetry. Theresultingconstructionis calledthe ‘minimal supersymmetricstandardmodel’, which hasmorethan100freeparameters– andthat’s theminimalmodel!Nev-ertheless,supersymmetryis very popularamongparticlephysicistsbecausethey canwrite lots of paperspredictingall of the experi-mentaleffects that these100 parametersallow. Anyway, becauseof all theseparametersit is possibleto arrangethings so that thelightestsupersymmetricparticleis neutralandstable,andcanthere-fore bethedarkmatterof theUniverse.It seemsto methough,thatthis scenariois not very compellingbecauseit is ad hoc. For ex-ample,thereis no theoreticalreasonfor any of thesupersymmetric


particlesto be stableoncethe symmetryis broken. Overall, it hasalways seemedto me that supersymmetricmodelsare very ugly,principally becausethey aresocomplicatedandarbitrary.

Thisis in sharpcontrastto themanifestbeautyof mirror symme-try which canbecompletelyunbroken if mirror matterexists. Themicroscopicpropertiesof the mirror particlesare thencompletelyfixed without any problemsfor existing experiments. One couldalsoimaginebreakingmirror symmetryby giving themirror parti-clesheavier or lighter masses– but this leadsto sevenyearsof badluck! I shouldknow, I toyed with sucha model in 1994,exactlyseven yearsago. Of course,beautyis not necessarilythe sameastruth. Beautyalways involves somesubjective judgement. In theendwe musteachfollow our own judgement,the truth of the(mir-ror) matterwill bedecidedby carefulexperimentsandobservations.

* * *

If mirror matterdoesindeedexist in our galaxy, then binarysystemsconsistingof ordinary and mirror mattershouldalso ex-ist. Although systemscontainingapproximatelyequalamountsofordinaryandmirror matterareunlikely dueto, for example,thedif-fering ratesof collapsefor ordinaryandmirror matter(leadingto alocalsegregationof ordinaryandmirror matter),systemscontainingpredominatelyordinarymatterwith a smallamountof mirror mat-ter andvice versa,shouldexist. Interestingly, thereis remarkableevidencefor theexistenceof suchsystemscomingfrom extrasolarplanetastronomy, thesubjectof thenext chapter.

shadowlands - school of physicsfoot/3chap.pdf · the ideas and the evidence can be appreciated by...

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