Quantum Physics IllusionorReality? Quantum Physics Illusion or Reality? second edition ALASTAIR I. M. RAE SchoolofPhysicsandAstronomy UniversityofBirmingham cambridge university press Cambridge,NewYork,Melbourne,Madrid,CapeTown,Singapore,Sa˜oPaulo, Delhi, Mexico City CambridgeUniversityPress TheEdinburghBuilding,CambridgeCB28RU,UK PublishedintheUnitedStatesofAmericabyCambridgeUniversityPress,NewYork www.cambridge.org Informationonthistitle:www.cambridge.org/9781107604643 (cid:2)C CambridgeUniversityPress1986,2004 Thispublicationisincopyright.Subjecttostatutoryexception andtotheprovisionsofrelevantcollectivelicensingagreements, noreproductionofanypartmaytakeplacewithout thewrittenpermissionofCambridgeUniversityPress. Firstpublished1986 Reprinted1986,1988,1989,1991,1992 Cantoedition1994 Reprinted1996,1998,2000,2002,2004 SecondCantoedition2004 Fifthprinting2009 Reprintedwithupdates2009 Reprinted 2012 Printedand bound by CPI Group (UK) Ltd, Croydon CR0 4YY BritishLibraryCataloguinginPublicationdata Rae,AlastairI.M. Quantumphysics:illusionorreality? 1.Quantumtheory I.Title 530.1(cid:3)2 QC174.12 LibraryofCongressCataloguinginPublicationdata Rae,AlastairI.M. Quantumphysics:illusionorreality? Bibliography Includesindex. 1.Quantumtheory. 2.Physics–Philosophy. I.Title. QC174.12.R335 1985 530.1(cid:3)285–13256 ISBN978-1-107-60464-3Paperback CambridgeUniversityPresshasnoresponsibilityforthepersistenceor accuracyofURLsforexternalorthird-partyinternetwebsitesreferred tointhispublicationanddoesnotguaranteethatanycontentonsuch, websitesis,orwillremain,accurateorappropriate.Informationregarding prices,traveltimetablesandotherfactualinformationgiveninthiswork arecorrectatthetimeoffirstprintingbutCambridgeUniversityPressdoes notguaranteetheaccuracyofsuchinformationthereafter. ToAnn Ilikerelativityandquantumtheories BecauseIdon’tunderstandthem Andtheymakemefeelasifspaceshifted Aboutlikeaswanthatcan’tsettle Refusingtositstillandbemeasured Andasiftheatomwereanimpulsivething Alwayschangingitsmind. D.H.Lawrence Timepresentandtimepast Arebothperhapspresentintimefuture Andtimefuturecontainedintimepast. T.S.Eliot Doyouthinkthethingspeoplemakefoolsof themselvesaboutareanylessrealandtrue thanthethingstheybehavesensiblyabout? BernardShaw Contents Prefacetothefirstedition pageix Prefacetothesecondedition xi · 1 Quantumphysics 1 · 2 Whichwayarethephotonspointing? 17 · 3 Whatcanbehiddeninapairofphotons? 31 · 4 WonderfulCopenhagen? 48 · 5 Isitallinthemind? 62 · 6 Manyworlds 75 · 7 Isitamatterofsize? 88 · 8 Backwardsandforwards 101 · 9 Onlyonewayforward? 111 · 10 Canwebeconsistent? 118 · 11 Illusionorreality? 129 Furtherreading 138 Index 142 vii Preface to the first edition Quantum physics is the theory that underlies nearly all our current under- standingofthephysicaluniverse.Sinceitsinventionsomesixtyyearsago thescopeofquantumtheoryhasexpandedtothepointwherethebehaviour of subatomic particles, the properties of the atomic nucleus and the struc- ture and properties of molecules and solids are all successfully described in quantum terms. Yet, ever since its beginning, quantum theory has been hauntedbyconceptualandphilosophicalproblemswhichhavemadeithard tounderstandanddifficulttoaccept. As a student of physics some twenty-five years ago, one of the prime fascinations of the subject to me was the great conceptual leap quantum physicsrequiredustomakefromourconventionalwaysofthinkingabout the physical world. As students we puzzled over this, encouraged to some extentbyourteacherswhowereneverthelessmoreconcernedtotrainushow toapplyquantumideastotheunderstandingofphysicalphenomena.Atthat timeitwasdifficulttofindbooksontheconceptualaspectsofthesubject– oratleastanythatdiscussedtheproblemsinareasonablyaccessibleway. Some twenty years later when I had the opportunity of teaching quantum mechanics to undergraduate students, I tried to include some references to theconceptualaspectsofthesubjectand,althoughtherewasbythenaquite extensiveliterature,muchofthiswasstillrathertechnicalanddifficultforthe non-specialist.WithexperienceIhavebecomeconvincedthatitispossibleto explaintheconceptualproblemsofquantumphysicswithoutrequiringeither a thorough understanding of the wide areas of physics to which quantum theoryhasbeenappliedoragreatcompetenceinthemathematicaltechniques thatprofessionalsfindsouseful.Thisbookismyattempttoachievethisaim. Thefirstfourchaptersofthebooksetoutthefundamentalideasofquan- tum physics and describe the two main conceptual problems: non-locality, which means that different parts of a quantum system appear to influence eachotherevenwhentheyarealongwayapartandevenalthoughthereis noknowninteractionbetweenthem,andthe‘measurementproblem’,which arises from the idea that quantum systems possess properties only when these are measured, although there is apparently nothing outside quantum physics to make the measurement. The later chapters describe the various solutionsthathavebeenproposedfortheseproblems.Eachoftheseinsome way challenges our conventional view of the physical world and many of ix