One Hundred Years of Quantum Physics Author(s): Daniel Kleppner and Roman Jackiw Source: Science, New Series, Vol. 289, No. 5481 (Aug. 11, 2000), pp. 893-898 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/3077316 Accessed: 26/02/2010 09:45
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http://www.jstor.org JANUARY PATHWAYS OF DISCOVERY "ScienceWars"
One Hundred Years of FEBRUARY Planetary Sciences Quantum Physics MARCH Daniel Kleppnerand RomanJackiw Genomics
An informedlist of the mostprofound scientific developments of the 20thcentury is likelyto include generalrelativity, quantum mechanics, big bangcosmology, the unravelingof the geneticcode, evolu- tionarybiology, and perhapsa few othertopics of the reader'schoice. Among these, quantumme- APRIL chanicsis uniquebecause of its profoundlyradical quality. Quantum mechanics forced physicists to Infectious theirideas of to rethinkthe natureof at the and to revisetheir reshape reality, things deepestlevel, Diseases conceptsof positionand speed, as well as theirnotions of causeand effect. Althoughquantum mechanics was createdto describean abstractatomic world far removed from dailyexperience, its impacton our daily lives couldhardly be greater.The spectacularadvances in chemistry,biology, and medicine-and in essentiallyevery otherscience-could MAY not haveoccurred without the tools thatquantum mechanics made possible. Materials Withoutquantum mechanics there would be no globaleconomy to speak Science of, becausethe electronicsrevolution that brought us the computerage is a childof quantummechanics. So is the photonicsrevolution that broughtus the InformationAge. The creationof quantumphysics JUNE has transformedour world, bringing with it all the benefits-and Cloningand therisks-of a scientificrevolution.' StemCells Unlikegeneral relativity, which grew out of a brilliantinsight into the connectionbetween gravity and geometry,or the deci- pheringof DNA, whichunveiled a new worldof biology,quan- tummechanics did not springfrom a single step.Rather, it was JULY createdin one of thoserare concentrations of geniusthat occur Communications fromtime to time in history.For 20 yearsafter their introduc- andScience tion, quantumideas were so confusedthat there was littlebasis for progress;then a small groupof physicistscreated quantum mechanics in three tumul- AUGUST tuous years. These scientists Quantum o "Quantum were troubled by what they Physics |: weredoing and in somecases dis- theory is the tressedby whatthey had done. The uniquesituation of this most precisely crucialyet elusivetheory is per- Papa Quanta. In 1900, Max SEPTEMBER t haps best summarizedby the Planckstarted the quantum- TheCell Cycle |tested and most following observation:Quan- mechanicalsnowball. tumtheory is themost precisely successful testedand most successful theory in thehistory of science.Never- theory in the theless,not onlywas quantummechanics deeply disturbing to its OCTOBER theory in te founders,today-75 yearsafter the theorywas essentiallycast in Atmospheric historyof its currentform-some of the luminariesof scienceremain dis- Sciences satisfiedwith its foundationsand its interpretation,even as they www.sciencemag.org SCIENCE VOL289 11 AUGUST2000 893 PATHWAYS OF DISCOVERY tially every other measurableproperty of matter,such as It shouldhave been possibleto understandthe shapeof viscosity,elasticity, electrical and thermal conductivity, co- the spectrumby combiningconcepts from thermodynamics efficients of expansion,indices of refraction,and thermo- andelectromagnetic theory, but all attemptsfailed. However, elasticcoefficients. Spurred by the energyof the Victorian by assumingthat the energiesof the vibratingelectrons that work ethic and the developmentof ever more ingenious radiatethe light are quantized,Planck obtained an expres- =s1~11 experimental methods, sion thatagreed beautiful- ;I?r? knowledgeaccumulated at ly with experiment.But as a prodigiousrate. he recognizedall too well, 3~ICII What is most striking the theorywas physically to the contemporaryeye, absurd,"an act of desper- however,is that the com- ation," as he later de- r_II pendious descriptions of scribedit. the properties of matter Planck applied his were essentiallyempirical. quantumhypothesis to the Thousands of pages of .?~ ,?.~~~~?energy of the vibratorsin r3I spectraldata listed precise _ - H U the walls of a radiating _I values for the wavelengths body. Quantumphysics mighthave endedthere if of the elements, but no- _B body knew why spectral _. :1 in 1905 a novice-Albert lines occurred,much less Einstein-had not reluc- )I)_I whatinformation they con- _ tantlyconcluded that if a veyed.Thermal and electri- vibrator'senergy is quan- S _lrl cal conductivitieswere in- tized, then the energy of terpreted by suggestive the electromagneticfield colorful NISTin 1995, emerged from that it U_J1C models that fitted roughly Superatom. These datal,fr rom radiates-light- half of the facts. There measurementsof rubidiumatom IS C:oalescing into the firstdocument- must also be quantized. were numerousempirical ed Bose-Einsteincondensate. Einstein thus imbued laws,but they were not sat- light with particlelikebe- isfying.For instance, the Dulong-Petitlaw established a sim- havior,notwithstanding that James Clerk Maxwell's theory, ple relationbetween specific heat and the atomicweight of a and over a centuryof definitiveexperiments, testified to material.Much of the time it worked;sometimes it didn't. light'swave nature.Experiments on the photoelectriceffect The massesof equalvolumes of gas werein the ratiosof in- in the followingdecade revealed that when light is absorbed tegers-mostly. The PeriodicTable, which provideda key its energyactually arrives in discretebundles, as if carried r_~l organizingprinciple for the flourishingscience of chemistry, by a particle.The dualnature of light-particlelikeor wave- hadabsolutely no theoreticalbasis. like dependingon whatone looks for-was the firstexam- Amongthe greatestachievements of therevolution is this: ple of a vexingtheme that would recur throughout quantum nIlr Quantummechanics has provideda quantitativetheory of physics.The dualityconstituted a theoreticalconundrum for matter.We now understandessentially every detail of atomic thenext 20 years. )ll _~~ _I~LI~I structure;the Periodic Table has a simpleand natural explana- The first step towardquantum theory had been precipi- tion;and the vast arrays of spectraldata fit intoan elegantthe- tatedby a dilemmaabout radiation. The second step was oreticalframework. Quantum theory permits the quantitative precipitatedby a dilemmaabout matter. It was knownthat understandingof molecules,of solidsand liquids, and of con- atoms containpositively and negativelycharged particles. ductorsand semiconductors.It explainsbizarre phenomena But oppositelycharged particles attract. According to elec- suchas superconductivityand superfluidity, and exotic forms tromagnetictheory, therefore, of mattersuch as the stuffof neutronstars and Bose-Einstein they should spiral into each condensates,in whichall the atomsin a gas behavelike a sin- Atoms_~~ i; other,radiating light in a broad gle superatom.Quantum mechanics provides essential tools 1913,_Nesor r spectrumuntil they collapse. forall of thesciences and for every advanced technology. Once again, the door to I Quantumphysics actually encompasses two entities.The progress was opened by a o firstis the theoryof matterat the atomiclevel: quantum me- of_n~ ao anovice:Niels Bohr. In 1913, chanics.It is quantummechanics that allows us to under- _Bohr proposed a radical hy- standand manipulate the materialworld. The secondis the pothesis:Electrons in an atom quantumtheory of fields. Quantumfield theoryplays a to- probleoaoisb_ exist only in certainstationary _ tallydifferent role in science,to whichwe shallreturn later. states,including a groundstate. Electronschange their energy Quantum Mechanics dictions, betweenby "jumping" thesta- The clue that triggeredthe quantumrevolution came not .p..--- the ..- tionary states, emittingb light fromstudies of matterbut froma problemin radiation.The Atoms go quantum. In whose wavelengthdepends on specific challengewas to understandthe spectrumof light 1913, Niels Bohrushered the energydifference. By com- , emitted by hot bodies: blackbody radiation. The phe- quantumphysics into world biningknown laws with bizarre ~' nomenonis familiarto anyonewho has staredat a fire. Hot of atoms. assumptionsabout quantum be- X o g:lr matterglows, and the hotterit becomesthe brighterit glows. havior, Bohr swept away the The spectrumof the light is broad,with a peak that shifts problemof atomicstability. Bohr's theory was full of contra-g fromred to yellow and finallyto blue (althoughwe cannot dictions,but it provideda quantitativedescription of the F see that)as thetemperature is raised. spectrumof the hydrogenatom. He recognizedboth the suc- 894 11 AUGUST2000 VOL289 SCIENCEwww.sciencemag.org PATHWAYS OF DISCOVERY cess andthe shortcomingsof his model.With uncanny fore- * Diraclaid the foundationsof quantumfield theoryby sight,he ralliedphysicists to createa new physics.His vision providinga quantumdescription of the electromagneticfield. was eventuallyfulfilled, although it took 12 yearsand a new * Bohr announcedthe complementarityprinciple, a generationof youngphysicists. philosophicalprinciple that helped to resolveapparent para- At first, attemptsto advanceBohr's quantum ideas-the doxesof quantumtheory, particularly wave-particle duality. ! -( so-calledold quantumtheory-suffered one defeatafter an- The principalplayers in the creationof quantumtheory ?I other.Then a series of developmentstotally changedthe were young. In 1925, Pauli was 25 years old, Heisenberg courseof thinking. and EnricoFermi were 24, and Dirac and Jordanwere 23. In 1923 Louis de Broglie,in his Ph.D.thesis, proposed Schrodinger,at age 36, was a late bloomer.Born and Bohr thatthe particlebehavior of lightshould wereolder still, and it is significantthat their 3lI haveits counterpartin the wavebehav- contributionswere largely interpretative. ior of particles.He associateda wave- The profoundlyradical nature of the intel- _I[I~111E lengthwith the momentum of a particle: lectualachievement is revealedby Einstein's The higherthe momentumthe shorter reaction.Having invented some of the key thewavelength. The idea was intriguing, concepts that led to quantumtheory, Ein- ?I but no one knewwhat a wave stein it. His on Bose-Einstein particle's rejected paper -11[ naturemight signify or how it relatedto statisticswas his last contributionto quan- atomic structure. Nevertheless, de tumphysics and his last significantcontribu- Broglie'shypothesis was an important tionto physics. precursorfor events soon to takeplace. i~!'^ ~ Thata new generationof physicistswas In the summerof 1924, there was needed to create quantummechanics is another N. Lord Kelvin described yet precursor.Satyendra hardly surprising. -lI Bose proposeda totallynew way to ex- why in a letterto Bohr congratulatinghim the Planckradiation law. He treat- plain on his 1913paper on hydrogen.He saidthat ;I ed light as if it were a gas of massless therewas muchtruth in Bohr'spaper, but he particles(now called photons)that do would never understandit himself. Kelvin not obey the classical laws of Boltz- Getting weirder. Louisde Broglie recognizedthat radically new physicswould ;Lra_III I mannstatistics but behaveaccording to saidthat if wavelike. light can behave needto come fromunfettered minds. E11 a new type of statisticsbased on parti- likeparticles, then particlescan be- In 1928, the revolutionwas finishedand cles' indistinguishablenature. Einstein havelike waves. the foundationsof quantummechanics were -~~II immediatelyapplied Bose's reasoning essentiallycomplete. The freneticpace with to a realgas of massiveparticles and obtaineda new law- which it occurredis revealedby an anecdoterecounted by l)ll - I11 to become known as the Bose-Einsteindistribution-for the late AbrahamPais in InwardBound. In 1925, the con- how energyis sharedby theparticles in a gas. Undernormal cept of electron spin had been proposed by Samuel U(ll[ circumstances,however, the new and old theoriespredicted Goudsmitand GeorgeUhlenbeck. Bohr was deeplyskepti- the samebehavior for atomsin a gas. Einsteintook no fur- cal. In December,he traveledto Leiden,the Netherlands,to -~1111~ ther interest,and the resultlay undevelopedfor more attend the jubilee of thana decade.Still, its key idea,the indistinguishability Hendrik A. Lorentz's of particles,was aboutto becomecritically important. d octorate. Pauli met Suddenly,a tumultuousseries of events occurred, the trainatHamburg, culminatingin a scientificrevolution. In the 3-yearpe- tofind out Germany, -I riodfrom January 1925 to January1928: Bohr'sopinion a bout * WolfgangPauli proposed the exclusionprinciple, the possibilityele of c- providinga theoreticalbasis for the PeriodicTable. tronspin. Bohr said the * WernerHeisenberg, with Max Born and Pascual proposal was "very, Jordan,discovered matrix mechanics, the first version very interesting," his of quantummechanics. The historicalgoal of under- well-known put-down standingelectron motion within atoms was abandoned phrase. Later at Lei- in favorof a systematicmethod for organizingobserv- den, Einsteinand Paul UVC able spectral lines. Ehrenfest met Bohr's ?l * Erwin Schrddingerinvented wave mechanics,a train, also to discuss t secondform of quantummechanics in whichthe state spin. There, Bohr ex- |L 2 of a systemis describedby a wave function,the solu- plained his objection, < is tion to Schrodinger'sequation. Matrix mechanics and but Einstein showed a www.sciencemag.orgSCIENCE VOL 289 11 AUGUST2000 895 PATHWAYS OF DISCOVERY The creation of quantum mechanics triggered a scien- function. Consequently, the concept of an electron having a _na tific gold rush. Among the early achievements were these: particular location and a particular momentum loses its Heisenberg laid the foundations for atomic structure foundation.The this: To [ E -~~~~~I theory Uncertainty Principle quantifies lo- by obtaining an approximate solution to Schrodinger's cate a particle precisely, the wave function must be sharply : r -~~~~~~~; for the helium atom in and tech- not a re- - ~~~~~sl~ equation 1927, general peaked (that is, spread out). However, sharp peak niques for calculating the structures of atoms were created quires a steep slope, and so the spread in momentum will be soon after by John Slater, Douglas Rayner Hartree, and great. Conversely, if the momentum has a small spread, the Vladimir Fock. The structureof the hydrogen molecule was slope of the wave function must be small, which means that solved by Fritz London and Walter Heitler; Linus Pauling it must spread out over a large volume, thereby portraying built on their results to found theoretical chemistry. Arnold the particle'slocation less exactly. Sommerfeld and Pauli laid the fbundations of the theory of Waves can interfere. Their heights add or subtract de- 11;: N111 : electrons in metals, and Felix Bloch created band structure pending on their relative phase. Where the amplitudes are in _3111~ theory. Heisenberg explained the origin of ferromagnetism. phase, they add; where they are out of phase, they subtract. The enigma of the random nature of radioactive decay by If a wave can follow several paths from source to receiver,as alpha particle emission was explained in 1928 by a light wave undergoing two-slit interference,then the illu- IllrI Gamow, who showed that it occurs mination will interference Parti- _;3Z_I? George by quantum- generally display fringes. - ~~~~~l mechanical tunneling. In the following years Hans Bethe cles obeying a wave equation will do likewise, as in electron laid the foundations for nuclear physics and explained the diffraction.The analogy seems reasonable until one inquires _GI1111~1~ energy source of stars. With about the nature of the wave. A wave is generally 1Z1111~ these developments atomic, thought of as a disturbancein a medium. In quantum molecular, solid state, and mechanics there is no medium, and in a sense there is nuclear physics entered the no wave, as the wave function is fundamentally a &1z&s1t- modern age. ', - statementof our knowledge of a system. -~l~ - Symmetryand identity. A helium atom consists of Controversy and Confusion ; a nucleus surrounded by two electrons. The wave Alongside these advances, function of helium describes the position of each however, fierce debates v electron. However, there is no way of distinguishing P]qv=s weretaking place on the in- ) which electron is which. Consequently, if the elec- 1-11 terpretation and validity of trons are switched the system must look the same, quantum mechanics. Fore- which is to say the probability of finding the elec- most among the protago- trons in given positions is unchanged. Because the nists were Bohr and Heisen- Omniscient math. It's probability depends on who embraced the tough berg, j to solve, but ErwinSchrod- the sqare of the mag- new and Einstein theory, inger's famous equation nitudeofthewavefunc- and Schrodinger, who were (shown in one of its many tion, the wave function dissatisfied. To appreciate for the system with the I; forms)describes every observ- the reasons for such tur- ablestate of a physicalsystem. interchanged particles moil, one needs to must be related to the understand some original wave function of the key features /t 0 ; I' (V ) w in one oftwo r(vt) J -?'l) it ways: *K"t-t-]-t 1Ls_~ of quantum theo- ? y - S7 Either is identical to 11k;s1111?1 1 ry, which we sum- the original wave func- marize here. (For tion, or its sign is sim- simplicity, we describe the Schrodinger version of quantum ply reversed, i.e., it is multiplied by a factor of-1. Which mechanics, sometimes called wave mechanics.) one is it? :1I;xU-1 'i I Fundamental description: the wavefinction. The behav- One of the astonishing discoveries in quantummechanics _1111? ior of a system is described by Schrodinger'sequation. The is that for electrons the wave function always changes sign. solutions to Schrodinger'sequation are known as wave func- The consequences are dramatic, for if two electrons are in tions. The complete knowledge of a system is described by the same quantum state, then the wave function has to be its its wave function, and from the wave function one can cal- negative opposite. Consequently, the wave function must culate the possible values of every observable quantity.The vanish. Thus, the probability of finding two electrons in the probabilityof finding an electron in a given volume of space same state is zero. This is the Pauli exclusion principle. All is proportionalto the square of the magnitude of the wave particles with half-integer spin, including electrons, behave _ function. Consequently, the location of the particle is this way and are called fermions. For particles with integer : "spreadout" over the volume of the wave function. The mo- spin, including photons, the wave function does not change mentum of a particle depends on the slope of the wave func- sign. Such particles are called bosons. Electrons in an atom ^ tion: The greater the slope, the higher the momentum. Be- arrange themselves in shells because they are fermions, but cause the slope varies from place to place, momentum is light from a laser emerges in a single superintense beam- , also "spreadout."' The need to abandona classical picture in essentially a single quantum state--because light is com- which position and velocity can be determined with arbi- posed of bosons. Recently, atoms in a gas have been cooled | traryaccuracy in favor of a blurredpicture of probabilitiesis to the quantum regime where they form a Bose-Einstein U at the heart of quantummechanics. condensate, in which the system can emit a superintense 2 Measurementsmade on identical systems that are identi- matterbeam-forming an atom laser. cally preparedwill not yield identical results. Rather,the re- These ideas apply only to identical particles, because if a suits will be scattered over a range described by the wave different particles are interchanged the wave function will ? 896 11 AUGUST2000 VOL289 SCIENCE www.sciencemag.org PATHWAYS OF DISCOVERY certainlybe different.Consequently, particles behave like TheSecond Revolution fermionsor like bosonsonly if theyare totally identical. The Duringthe freneticyears in the mid-1920swhen quantum absoluteidentity of likeparticles is amongthe mostmysteri- mechanics was being invented,another revolution was ous aspectsof quantummechan- underway. The foundationswere being laid ics. Amongthe achievementsof for the second branchof quantumphysics- quantumfield theory is that it quantumfield theory.Unlike quantumme- canexplain this mystery. chanics,which was createdin a short flurry Whatdoes it mean? Ques- of activity and emerged essentially com- tions such as whata wave func- plete, quantumfield theory has a tortuous tion "really is" and what is history that continuestoday. In spite of the meant by "makinga measure- difficulties,the predictionsof quantumfield ment"were intenselydebated in theoryare the most precisein all of physics, the earlyyears. By 1930, how- and quantum field theory constitutes a ever,a moreor less standardin- paradigmfor some of the most crucialareas terpretation of quantum me- of theoreticalinquiry. chanicshad been developedby The problem that Bohrand his colleagues,the so- motivated quantum field called Copenhageninterpreta- theory was the questionof tion. The key elementsare the how an atom radiateslight probabilisticdescription of mat- as its electrons "jump" ter and events, and reconcilia- from excited states to the tion of the wavelikeand parti- groundstate. Einsteinpro- ^ : clelikenatures of thingsthrough posed such a process, Bohr'sprinciple of complemen- called spontaneous emis- ^ tarity.Einstein never accepted IHI^H sion, in 1916,but he had no quantumtheory. He and Bohr Quantumwebs. By creatingF )articles that way to calculate its rate. B E debatedits principlesuntil Ein- share quantumstates, such ais these "en- Solving the problem re- stein'sdeath in 1955. tangled" photons at the inte.rsections of quired developing a fully ^ _~ theselaser-generated rings, re; searchers are relativisticquantum theory _ I A central issue in the de- developing new quanturun encryption of electromagneticfields, a 2 bates on quantummechanics schemesand quantum comrn ters. quantumtheory of light. Quantummechan- ' was whetherthe wave function ics is the theory of matter.Quantum field z containsall possibleinformation about a systemor if there theory,as its name suggests, is the theory of fields, not < mightbe underlyingfactors-hidden variables-thatdeter- only electromagneticfields but otherfields thatwere sub- | minethe outcomeof a particularmeasurement. In the mid- sequentlydiscovered. | 1960sJohn S. Bell showedthat if hiddenvariables existed, In 1925 Born, Heisenberg,and Jordanpublished some _ experimentallyobserved probabilities would have to fall initial ideas for a theory of light, but the seminal steps | below certainlimits, dubbed"Bell's inequalities." Experi- were taken by Dirac-a young and essentiallyunknown S ments were carried out by a numberof groups, which physicist working in isolation-who presented his ~-found that the inequalitieswere violated.Their collective field theory in 1926. The t data came down decisivelyagainst the possibilityof hid- theory was full of pitfalls: v den variables.For most scientists,this resolvedany doubt formidable calculational E aboutthe validityof quantummechanics. complexity, predictions of - Nevertheless,the natureof quantumtheory continues to infinite quantities,and ap- M attractattention because of the fascinationwith what is parentviolations of the cor- p sometimesdescribed as "quantumweirdness." The weird respondenceprinciple. o propertiesof quantumsystems arise from what is knownas In the late 1940s a new 5 entanglement.Briefly, a quantumsystem, such as an atom, approach to the quantum . can existin anyone of a numberof stationarystates but also theory of fields, QED (for in a superposition-orsum-of suchstates. If one measures quantumelectrodynamics), M someproperty such as the energyof an atomin a superposi- was developed by Richard tion state, in general the result is sometimes one value, Feynman,Julian Schwinger, - sometimesanother. So far,nothing is weird. and Sin-Itiro Tomonaga. | It is also possible,however, to constructa two-atomsys- They sidesteppedthe infini- v tem in an entangledstate in which the propertiesof both ties by a procedure,called < atomsare shared with each other. If the atomsare separated, Fields go quantum. Paul renormalization,which es- I informationabout one is shared,or entangled,in the stateof Diracspearheaded work lead- sentially subtractsinfinite z the other.The behavioris unexplainableexcept in the lan- ing to quantumfield theory quantitiesso as to leavefi- ^guage of quantummechanics. The effectsare so surprising as wellas discoveriessuch as nite results.Because there is v thatthey are the focusof studyby a smallbut active theoret- antimatter. no exact solution to the | ical andexperimental community. The issuesare not limited complicatedequations of the . to questionsof principle,as entanglementcan be useful.En- theory,an approximateanswer is presentedas a series of g tangledstates have already been employedin quantumcom- termsthat become more and more difficult to calculate.Al- - municationsystems, and entanglement underlies all propos- though the terms become successively smaller,at some e als forquantum computation. pointthey shouldstart to grow,indicating the breakdownof www.sciencemag.org SCIENCE VOL289 11 AUGUST2000 897 PATHWAYS OF DISCOVERY the approximation. In spite of these Eachmonth, Britann fica .com enhancesthe Today,the quest to understandthe ul- perils, QED ranks among the most bril- Pathways of Discove*ry essay with links to timate nature of matter is the focus of relevantitems withinlan id without Encylope- liant successes in the history of physics. diaBritannica's vast st tore .s of information.To an intense scientific study that is remi- Its prediction of the interaction strength accessthis month'sIPatl hways essay andall niscent of the frenzied and miraculous between an electron and a magnetic previous ones, go tc) www.britannica.com days in which quantum mechanics was andclick on the Scieince field has been experimentally con- created, and whose outcome may be ?I firmed to a precision of two parts in even morefar-reaching. The effortis in- -l~I 1,000,000,000,000. extricablybound to the questfor a quantumdescription of Notwithstandingits fantasticsuccesses, QED harbors gravity.The procedurefor quantizingthe electromagnetic -I enigmas.The view of emptyspace-the vacuum-that the field thatworked so brilliantlyin QED has failed to work theory providesinitially seems preposterous.It turnsout for gravity,in spiteof a half-centuryof effort.The problem :I- that empty space is not really empty.Rather, it is filled is critical,for if generalrelativity and quantummechanics rI with small,fluctuating electromagnetic fields. Thesevacu- are both correct,then they mustultimately provide a con- um fluctuationsare essential for explainingspontaneous sistentdescription for the same events.There is no contra- emission.Furthermore, they producesmall but measurable diction in the normalworld around us, becausegravity is shifts in the energies of atoms and certainproperties of so fantasticallyweak comparedto the electricalforces in particlessuch as the electron.Strange as they seem, these atoms that quantumeffects are negligibleand a classical effects have been confirmedby some of the most precise descriptionworks beautifully. But for a system such as a experiments ever carried out. black hole where gravity is incredibly strong, we have no At the low energies of the world reliable way to predict quantumbehavior. around us, quantum mechanics is fantas- One century ago our understandingof the physical world tically accurate. But at high energies was empirical. Quantumphysics gave us a theory of matter where relativistic effects come into play, and fields, and that knowledge transformedour world. Look- :II a more general approach is needed. ing to the next century,quantum mechanics will continue to Quantum field theory was invented to provide fundamentalconcepts and essential tools for all of reconcile quantum mechanics with spe- the sciences. We can make such a predictionconfidently be- cial relativity. cause for the world around us quantumphysics provides an has this )II The towering role that quantum field exact and complete theory. However, physics today 51 theoryplays in physicsarises from the an- in common with physics in 1900: It remains ultimately em- swers it provides to some of the most pro- pirical-we cannot fully predictthe propertiesof the elemen- -lI found questions about the nature of mat- tary constituentsof matter,we must measurethem. ter. Quantum field theory explains why Perhaps string theory-a generalization of quantum -l~ there are two fundamentalclasses of par- field theory that eliminates all infinities by replacing ticles-fermions and bosons-and how pointlike objects such as the electron with extended their properties are related to their intrin- objects-or some theory only now being conceived, will sic spin. It describes how particles-not solve the riddle. Whatever the outcome, the dream of ulti- only photons, but electrons and positrons mate understandingwill continue to be a driving force for (antielectrons)-are created and annihi- new knowledge, as it has been since the dawn of science. B | ~~~~~lated.It explains the mysterious nature of One century from now, the consequences of pursuing that _SHH| identity in quantum mechanics-how dream will belie our imagination. _IUjju identical particles are absolutely identical because they are created by the same un- FurtherReading derlying field. QED describes not only B. Bederson, Ed., More Things in Heaven and Earth: A Celebration of Physics at the electron but the class of particles called leptons that in- the Millennium (Springer Verlag, New York, 1999). J. S. Bell, Speakable and Unspeakable in Quantum Mechanics: Collected Papers cludes the muon, the tau meson, and their antiparticles. on Quantum Mechanics (reprint edition) (Cambridge University Press, Because QED is a theory for leptons, however, it cannot Cambridge, 1989). describe more complex particles called hadrons. These in- L. M. Brown, A. Pais, B. Pippard, Eds., Twentieth Century Physics (Institute of Physics, Philadelphia 1995). clude protons, neutrons, and a wealth of mesons. For D. Cassidy, Uncertainty: The Life and Science of Werner Heisenberg (W. H. hadrons, a new theory had to be invented, a generalization Freeman, New York, 1993). of QED called quantum chromodynamics, or QCD. Analo- A. Einstein, Born-Einstein Letters, trans. Irene Born (Macmillan, London, 1971). H. Kragh, Dirac: A Scientific Biography (Cambridge University Press, Cam- gies abound between QED and QCD. Electrons are the con- bridge, 1990). stituents of atoms; quarks are the constituents of hadrons. W. Moore, Schrodinger: Life and Thought (Cambridge University Press, Cam- In QED the force between charged particles is mediated by bridge, 1989). the photon; in the force between is mediated A. Pais, Inward Bound: Of Matter and Forces in the Physical World (Oxford QCD quarks University Press, Oxford, 1986). by the gluon. In spite of the parallels, there is a crucial dif- A. Pais, Niels Bohr's Times: In Physics, Philosophy, and Polity (Oxford Univer- ference between QED and QCD. Unlike leptons and pho- sity Press, Oxford, 1991). tons, quarks and gluons are forever confined within the hadron.They cannot be liberatedand studied in isolation. DanielKleppner is LesterWolfProfessor of Physicsand Acting Director of QED and QCD are the cornerstonesfor a grand synthesis the ResearchLaboratory of Electronicsat the MassachusettsInstitute of known as the StandardModel. The StandardModel has suc- Technology.His researchinterests include atomic physics,quantum op- cessfully accounted for every particle experimentcarried out tics, ultraprecisespectroscopy, and Bose-Einsteincondensation. to date. However, for many physicists the StandardModel is RomanJackiw is JerroldJacharias Professor of Physicsat MIT.His research i inadequate,because data on the masses, charges, and other interestsinclude applying quantum field theory to physicalproblems, the- z propertiesof thefundamental particles need to be foundfrom oreticalparticle physics, and the searchfor unexpected,subtle effects that | experiments.An idealtheory would predict all of these. mayapply to particle,condensed matter, and gravitational physics. 8 898 11 AUGUST2000 VOL289 SCIENCE www.sciencemag.org