sc Ha thr ref 94 enl ere ere the l6l toi sch I/tgrigfj$ tF!pft/1a;:'i{9PIJg; l+rJf.l3 s:ltcjr'9-r JJc)flt, DLJ!.f). f t'\wglJfl?la)JfJe*? 6f up Ina conceptmapof physicsthe studyof lighcstandsat all the maiorintersections. Insightsinto light illuminate the whole of physics,just as scattered light rays illuminatea whole house.This article is not a scholarly history but an illustrativeoverview written with hindsight,of the central role of Iight in makingconnections. 'awakening In 1267RogerBacon.with whom the post-medieval began,"[2JpublishedOpusMalus.In BookV rhe Oprlcssection of that encyclopedicwork Baconwrote,[3] "lt is possible that some other science may be more useful, but no other science has so much sweetress and beautg of utility. Therefore ir is rhe flower of the whole of philosophq and through it, and not without it, can other sciencesbe known." Sevenhundredyearslater this motif was madeexplicitby JacobBronowski:[4] "We see matter by light; we are aware of the presence of light bq the interruption of matter. And that thought makesup the world of every great phgsicist, who finds that he cannot deepen his understandingof one without the other," Letus beginat the beginning. Ceometrical Optics 'About 10 monthsagoa rumor cameto our earsthat a had beenmade . . . Thisfinally causedme to apply sPyglass myselftotally to investigatingtheprinciplesandfiguring out the meansby whichI might arrive at the inventionof a similarinstrument,which I achievedshortlyafterwardon the basisof thescienceof refraction" -Galileo Galilei [5] Navigationand surveyinghavelong dependedon the straightnessof light rays.Through the practicalexperienceprovidedby theseactivities,the opticallawsof rectilinearpropagationand 20 Radiations Fall2014 reflectionbecameknown in antiquity.The first unifled theory in physicscamefrom Hero of Alexandria(c. l0-70 CE), who set forth the principlethat light raysfbllow the path of minimum distance; rectilinearpropagationand the law of ref'lectionfbllow as consequences. [6] Refractionhas beenknown qualitativelyfrom time immemorial. A partially immersedstick appearingto be sharplybent at the water'ssurfacewas mentionedin Plato'sRepublic(c. 360 BCE). "Burning glasses," lensesfor startingfiresby focusingsunlight, were part of ancienttechr-rology, as documentedby artifactssuch asa magnifierfound in the ruins of the palaceof AssyrianKing (708-681BCE).Refractionwasmadea quantitative Sennacherib i tion abr fixe for l to tt esti the 1 moc whe Io ar inte dist mat the I Ler that docl Isaa shul exp( can( eme ,ns one colo whit ratel ye sl affir opt trur whit anal [ : l e g r r n t ( , o n r r c c t i < l n . si n I ) h ) ' s i c sI sciencein the Middle Agesby Muslirn scholarssuchas Ibn alHaytham(c. 965-1040),known to us asAlhazen,who introduced the practiceof measuringanglesfrom the normal for reflectedand refractedrays.Alhazen'scontemporaryAbu Sid al-Ali ibn Sahl(c. 940-1000)expressed the law of refractionin terms of the hypotenusesof right triangles.[7] Willebrord Snellius(or Snell)rediscoveredin 1621the law of refraction,which RendDescartesrediscoveredagainand publishedin its well-known sine form in 1637. Refractionmadepossiblethe lens,which made the cell and of the starsaccessible to human senses.Galileo'sStarryMessenger l6l0 arrdRobertHooke'sMicrographiaof 1665openednew worlds to investigation.They deepenedthe questions,and not only for scholars: . . . He burnedhis housedownfor thefire insurance And spenttheproceedson a telescope To satisfya li_felong curiosity About our placeamongthe infinities. -Robert Frost,"The Star-Splitter" Hero'sprincipleof rninimum distancedoesnot explainrefractior-r.That gap was rernediedby Pierrede Fermatin 1657through a broaderunifying principle:Of all possiblepathsconnectingtwo fixed points,the path followedby a light ray minimizesthe time for light to go betweenthe points. Fermat'sprinciple requireslight to travelat finite speed.Astronomy offeredthe first meaningful estimateof this speedin 1676when Ole Romer usedas a clock the periodic emergenceof Io from behind fupiter'sshadow (The moon hasan orbital period of 42.5hours.)During the time of year when Earth recedesfrom the |upiter-Io system,after eachorbit of Io around fupiter the clock is seenfrom Earth to run slow.Romer interpretedthe delayasthe time light took to travelthe additional infordistancebetweenEarth and Io. Astronomy,which possesses mation carried from the heavensto us by light, now gaveback from the heavensinformation about light itself. Lensesand Spectra "I procuredme a triangular glass-prisme,to try therewith the celebrated Phenomenaof Colours.. ." -lsaac Newton The edgeof everylensforms a prism. The rainbow of colors that emergesfrom prismswas familiar in Aristotle'stime. Received . doctrineheld that a prism somehowmodifiesthe color of light. IsaacNewton had to investigate. He made a hole in his window shutterto let in a fine beam of sunlight.The prism producedthe expectedcolorsof the rainbow,but Nervtonnoticedthe significanceof somethingelse:the circularbeam that enteredthe prism emergedas an elongatedellipse.Eachcolor refractedat a different angle.[8] With a secondapertureNewton could selectfrom this rainbow one color to entera secondprism. This prism did not changethe color.Allowing all the colorsto enterthe secondprism produced white light on its far side.A prism did not modify light but sepa'A ratedit. Newton wrote, naturalistwould scarceexpectto see ye scienceof thosecoloursbecomernathematical,and yet I dare affirm that there is as much certaintyin it as in any other part of white light into a specOptiksJ'[9]This imageof a prism separating trum and the inverseoperationof synthesizingdistinct colorsinto white light, illustratesvisuallythe mathematicsof synthesisand analysis,suchasthe harmonic seriesof Fourier'stheorem. William Herscheland his sisterCarolinemade someof the first catalogsof stars,discoveringmany binary systemsand the planet Uranus.While testinga red filter for observingsunspots,William happenedto placehis hand at the focal point of his reflectingtelescopeand noticed the region to be unexpectedlywarm. To study the temperatureof light, in 1800William insertedthermometers into the separatecolorsof the sun'sspectrum.He noticedthat in going from violet to red, the temperatureincreased.Intrigued,he placeda thermometerbeyondthe red, and there found the highest temperature.Herschelcalledthis warm invisiblelight beyondthe red'taloric rays,"which we know as infrared.Herschel'sresults were anticipatedby 63 yearsby Emilie du ChAtelet.This remarkable woman essentiallydiscoveredthe work-energytheorem,translated Newton'sPrincipiainto the Frenchtranslationusedto this day, and collaboratedwith Voltaireacrossmany years.Her opuswas Eldmentsde Ia Philosophie de Newton (1738),which went deepinto the philosophicalfoundationsof mechanicsand was influential in shifting Frenchscientistsfrom the mechanicsof Descartesto that of Newton. In 1737du Chdteletenteredan essaycompetition on the natureof fire. In her essay"Dissertationon the Natureand Propagationof Fire,"shearguedthat fire is not a materialsubstance,and differentcolorsof light transportdifferentquantities was to line up of heat.The way to demonstratethis, shesuggested, an arrayof thermometers,one insertedinto eachof the separated colorsof the spectrum,which was preciselywhat William Herschel did in 1800.du ChAteletwas not ableto perform the experiment herselffor lack of thermorneters.[10] fosephvon Fraunhofersupervisedglassmelting and grinding processes in his Munich optical institute.He neededto measure the refractiveindicesfor differentcolorsin variouskinds of glass. In one of his experiments,light from an oil lamp flame passed Fraunhofer through a prism to be viewedthrough a telescope. noted dark lines in the spectrum.Intrigued,he looked for generalizations.RepeatingNewton'sexperimenton sunlightwith his telescope-equipped prism,in l8l4-15 dark lineswererevealedin the solarspectrum. In 1857the "daring and resourcefulexperimenter"Robert Bunsen inventeda burner that produceda colorlessflame.Il 1] With Bunsen'sburner the spectraof chemicalsplacedin the flame could be cleanlyseparated.His collaboratorGustavKirchhoff added a prism to completethe basictool of modern spectroscopy, the Payoffscamequickly.In 1860Bunsenand Kirchhoff spectroscope. discoveredrubidium and cesiumin a sampleof Diikheim mineral from Franceand water.In 1868two astronomers,PierreJanssen Norman Lockyerfrom England,independentlyreporteda yellow line in the solarspectrumthat fit no known element.Interpreting it as an unknown element,Lockyernamedit after helios,Greek for "the SunJ'[12]Terrestrialhelium wasnot confirmeduntil 1895 when William Ramseyisolatedit as a byproductof uranium ore. In 1907ErnestRutherfordand Thomas Roydscollectedalpha particlesemittedby radioactivedecay,examinedtheir spectra,and showedthat the particleswerehelium. C l a s s i c aM l echanics "Followingin thefootstepsof Hero and Fermat,he IMaupertuisl thenproclaimedthat thissimplicitycausesnatureto act in sucha way as to rendera certainquantity,whichhe namedthe'action,'a ntinimum."-Wolfgang Yourgrauand StanleyMandelstamI l3] 2l F al l 2Ol 4Radi ari ons I ElegantConnectionsin Physics After Newton revolutionizedopticshe turned to mechanics. Generalizinginductively from specificproblems solved in quantitative detail [14]-Archimedes on the lever,Galileoon projectiles, Huygenson the pendulum,and Newton himself on gravitation-he postulatedin 1687three lawsof motion that turned mechanicsinto an axiomatic system.As the laws of geometricaloptics could be derived from Fermat'sleasttime principle, could the samebe done for mechanics?Severalproposalswere forthcoming. Theseincluded fohann Bernoulli's 1717principle of virtual work for statics,extended to dynamicsby |ean le Rond dAlembert in 1743. Around 1740PierreLouis Moreaude Maupertuis(who tutored young Emilie du ChAteletin calculus)suggestedthat a particle acted on by specificforcesmovesin a way that minimizes the "action." This approachwas successfullydemonstratedfor central forcesby LeonhardEuler in 1744.Inhis MicaniqueAnalytiqueof 1788,Joseph LagrangegeneralizedMaupertuis'principle to all conservative 'hction" forcesand clarified as the line integral of momentum. The generalizationof this principle to all of mechanics(later extended to most of physics)was published in two papersby William R. Hamilton in 1834-35.[15]Hamilton'sprinciple postulatesthat of all L r s e d b v p e r n r i s s i < - r nf r o n r \ A P I . 2 o l 4 t l i ( t l r S c l r o o l P h r s i t s P l r o t o C o r r t e s t . " (Ci k rxllliirrttqq R e f r a c t i o r r , " b v C l a i r e l r r r r : rl s a b e l l e S a l o f f - C o s t e . l t h a r a l l i q h S r l t o o l . the conceivabletrajectorieswhereby a particle might travel between two fixed points, the trajectory actually followed minimizes the time-averageddifferencebetweenthe particle'skinetic and potential energies.The principles of Hamilton and Fermat arosefrom similar motivations,but a logicalconnectionbetweenthem would haveto await generalrelativity. Ontology "From the multitude of experiences it [science]selectsa few simpleforms, and constructsfrom them, by thought,an objective world of things."-Max Born [16] "Youknow somethingand then the qualitystimulushits . . . , but to defineit all you'vegot to work with what you know. So your definition is made up of what you know. It\ an analogue to whatyou alreadyknow."-Robert Pirsig [17] A debateabout the ultimate realityof light beganin the time of Plato and the Sophists.By the time of Newton and Huygens,those arguingthe question"What is light?" faceda binary choice:What 22 Radiarions Fall 2Ol4 is light-wave or particle?Robert Hooke'sMicrographic describes how colors of thin films dependedon a film's thickness,suggesting a standing wave condition. Christaan Huygensarguedthat the tremendous speedof light would be feasibleonly if light was a disturbancethrougha medium, not the bulk motion of a medium. He gavethe wave hypothesispredictive power by postulating that each point on a wave front behavesas the sourceof another wave.If that were so, then light should radiate into regionsthat would otherwise remain in geometric shadow.Hooke and FrancescoGrimaldi had noticed diffraction in the fine structure of shadowscastby a needle. Initially ambivalent ("I make no hypotheses"),Newton eventually argued that light was a beam of particles.While acknowledging that somethingperiodicoccurswith waves(and discovering an interferencepattern called "Newton'srings"), he interpreted the periodicity as something that matter does fo light. To Newton, the diffraction reports did not require light to be a wave.Gravity acts betweenseparatedmassivebodies,so matter could bestow its periodic influenceon light from a distance. Refraction offered one way to decidethe question.When light passesfrom air into water the ray bends toward the normal. If light consistsof waves,the speedof light in water would be lessthan its speedin air. If light consistsof particlesthe reversewould happen. In 1800Thomas Young demonstratedthat the interferenceof light passedthrough a double aperture.Sucha pattern could be interpreted only as the superpositionof waves.Augustin Fresnel worked out a comprehensivetheory of diffraction basedon the assumptionthat light consistsof waves,and his predictions were vindicated, famously so with the notorious "Poissont spot,"a bright spot, due to wave diffraction, in the shadowbehind an illuminated disk. In 1850Ldon Foucault measuredthe speedof light in water and found it to be lessthan the speedof light in air. The riddle "What is light?" seemedanswered.[18] Lingering questionsremained,as they alwaysdo with important questionsthat have multiple layers.First, supposinglight to be a wave,what is waving?Second,acousticalwavesrequire a medium; what servesas the medium for light, the "aether"?Third, light had been found to be polarized by bifringent crystals.Reconciling polarization and the rapid speedof light with our ability to breeze freely through the aether offered a perplexing situation. Elecrromagnetism "Maxwell shewedlight to be an electromagnetic phenomenon, so that the wholescienceof Opticsbecamea branchof Electromagnetism. . .." -famesJeans[19] Hints at a connection betweenelectricity and magnetismcame when Hans Christian Orsted showedthat moving electriccharge makesmagnetismand when Michael Faradayshowedthat changing magnetismmakeselectricity.A unified theory of electromagnetism was written by IamesMaxwell in 1862.Action at a distance,which servedwell for staticinteractions,was replacedwith the dynamic conceptof the field, a function of spaceand time. The interactionsof matter proceedthrough fields.On one hand, local fields tell a particle of matter how to move. Newton'ssecond law with the Lorentz force,for instance,predicts the motion of a chargedparticle in responseto electromagneticfields.On the other hand, matter determinesthe fields around it. Maxwell'sequations relatethe electric and magnetic fields to their chargedparticle sourcesand relatethe fields to eachother. When a chargedparticle Maxwell'sequationssaythe fields it producesmust accelerates, I C t s ll c t f I r li tl s L,legantConnectionsin PhvsicsI change.A changingelectricfield producesa magneticfield that also changes,and the changingmagneticfield producesa changing electricfield. Togetherthe changingfields rnakea self-propagating wavemoving at the speedof light. In responseto the "What is waving?"question,light must thus field! The equationsdescribingthis be a wavein the electromagnetic wavehaveno restriction on the frequency,suggestingthe existence of a continuouselectromagneticspectrum of harmonics whose frequenciesrangefrom zero to infinity. The equationsalso saythat the propagatingfields are transverseto the direction of wavetravel,implying polarization and explaining the effectsof bifringent crystals. In 1886-89 Heinrich Hertz affirmed Maxwell by broadcasting and detectingradio wavesin the laboratory.While doing so the alert Hertz noticeda spuriousglitch in his apparatus.Radiationof low intensity but sufficiently high frequencyimmediately stimulatesan electriccurrent in certain materials;at low frequenciesthe incoming light producesno current even at high intensity.Dubbed the photoelectriceffect,this anomaly in the interaction of light with matter did not fit Maxwell'stheory. For two decadesit remaineda mystery. Maxwell had answeredimportant questionsabout light, but others remained.The equationssaythat electromagneticwaves need no medium, that they travel in empty spaceat the speedof light, c, but the equationsare silent on the frame of reference.In 1895l6-year-oldAlbert Einsteinwonderedwhat he would observe if he rode on a beam of light. Intuition said that Einstein'slightwave surfer should observea stationary crest of the electromagnetic wave.But Maxwell'sequationsinsist that electromagneticwaves travel at speedc even from the surfer'sperspective!This paradox, like all paradoxes,suggestedthat the question should be restated. Einsteinheld the questionin his mind for l0 years.Then the 26-year-oldEinsteinwrote "On the Electrodynamicsof Moving Bodiesl'noting that "Maxwell'selectrodynamics-as usuallyunderstood at the present-when appliedto moving bodies,leadsto asymmetriesthat do not seemto be inherentin the phenomena."[20] The relativemotion betweena magnetand a coil of conducting wire illustratesthe issue.Whatever the referenceframe, the relative motion resultsin a forceon the chargecarriers,driving an electric current in the coil. An observeraboardthe coil seesa changing magneticflux as the magnet sweepsby. Faraday'slaw saysan electricfield E getsinducedin the coil, producingthe force4E on the charges.An observeraboard the magnet seesa different picture. The coil sweepsby with velocity v, carrying the chargedparticles through the magneticfield B. Each chargeq feelsthe force qvxB. Thus do distinct mechanismsdescribethe sameresult,an asymmetry in the explanationnot inherentin the phenomena.Einstein wondered what principle would unify the two explanations. The thought experimentabout light surfing suggesteda clue in light itself.If you ride on the beam of light that bouncesoff a clock at 10:00am, then you staywith the information that says the time is l0 o'clock.[21]For the light-wavesurfer,time stands still. Newtonian relativity of inertial framespostulatesthe separate invarianceof length and time intervals;as a consequence,the speed of light must be relative.Einstein replacedthose assumptionswith the postulateof the invarianceof the speedof light betweeninertial frames,which requiresspaceand time intervals to be relative. Mechanicshad to adaptto light, insteadof the light adapting to mechanics. Specialrelativity,which linked light to spaceand time, also linked light to massand energy.Energy and momentum became the time and spacecomponentsof a vectorin four-dimensional space-time.Its geometrywas not Euclideanbut hyperbolic. The squareof the energy-momentum four-vector was given by a difference,not a sum, with the particle'smassas the vector'smagnitude. For a free particle,E - (pc)' - (mct)t. Thermodynamics and Quantum Physics "By 1906or 1908Planck had cometo seethat his compromise over cavity radiation had loosedsomethingbrand new and -J.L. Heilbron l22l menacinginto the world of physics." The thermodynamics of light motivated the extensionof Newtonianmechanicsto quantum mechanics.Macroscopic thermodynamicsservesas a boundary condition on microscopic statisticalmechanics.After many triumphs with enginesand phase changesand the kinetic theory of gases,statisticalthermodynamics confronted the question of finding the energy density of light as a function of frequency.Light and matter in thermal equilibrium was produced in the laboratory by a metal box held at temperature T. The atoms in the box walls are made of oscillating chargedparticles and radiatelight. According to Newtonian mechanics,the energy l)lroto t i)luI('s! oI rvww.tlrr'llrtllli< rlrrnr.ritt.rtt't. of a harmonic oscillator is proportional to the frequencysquared. The sum over all microscopic states,a procedurerequired by statisticalmechanics,thus predicts an energy density that diverges as frequencycubed,the "ultravioletcatastropheJ'Although the experimentalspectrumof light in thermal equilibrium with matter goesas the frequencycubed at low frequencies,as the frequency increasesthe distribution mapped by data reachesa peak and then slidestoward zero at the highest frequencies. Max Planck realizedthat the predicted distribution function could be made to peak and trail off at high frequenciesif the energy of an oscillatorof frequency/waslinear in/and exhibitedonly a harmonic seriesof discreteovertones,so that E" = nhf, where n = 0,1,2,3,.. . with h a constantto be fit to data.The distribution function that resultedhad the right shape,whateverthe value of Planck's constanth.Itfit the data preciselyif hhad the astonishinglysmall value6.6xl0-34J.s.Planckhad solvedthis important problem,but at the price of making an ad hoc hypothesisabout energyquantization, a drastic move which at the time pointed to nothing else. Five yearsafter Planck'shypothesisEinstein revisitedthe thermodynamics of light. He calculatedthe entropy of radiation and compared the result to the entropy of a box filled with ideal gas molecules.Then camethe hereticalpunch line. The entropyof the Fall2Ol4 Radiations 23 radiationmatchesthe errtropyof the molecules,said Einstein,if a light waveof frequency/correspondsto a swarm of particles,each carryingenergyE = h.f.Accordingto Einstein,light itself is quantized.He showedhow this corpuscleinterpretationof light solved outstandingmysteriesir-rthe interactionof matter and radiation. Most famously,the photoelectriceffectmade senseas a collision betweena light corpuscleand an electronif Einstein'sft has the /r. Planck'sconstantft pointed to something samevalueas Plar-rck's deep.[23]The name of the light corpuscle,the photon, cameyears later,in 1926.[24] With the conceptof the photon in mind, one can look againto specialrelativity,which requiresany particlemoving at the speed of light to carry zero mass.With zero mass,the energy-mornentum relation for a photon simplifies to E = pc. Togetherwith E = hf and c = )rf, it follows that a light wave of wavelengthl. correspondsto a swarm of photons,eachcarrying momentum p = hllt. This idea, rigorousfor massless particles,was boldly postulatedby Louis de Brogliein 1924to hold for massiveparticlestoo. Thus did the stained-glass thermodynamicsof light-along with spectroscopy's window into the atom-lead the way into quantum mechanics. CeneralRelarivity 'Another importantconsequence of the theory,which can be testedexperimentally,has to do with thepath of raysof light . . . We can thereforedraw the conclusionfrom this, that a ray of light passingnear a largemassis deflected. . . The existence of this deflection,whichamountsto 1.7. . . was confirmed,with remarkableaccuracy,by the EnglishSolar EclipseExpeditionin 1919... ." -Albert Einstein[25] Between1905and 1915Einsteinextendedspecialrelativity frames.Thanksto the principle of the to arbitrarilyaccelerated equivalenceof gravitationaland inertial rnass,generalrelativity Early testsof generalrelativity servesas a theory of gravitatior-r. checkedits predictionsfor the behaviorof light, irrcludingthe and deflectionof a light ray grazingthesun, gravitatior-ralredshift, radar echo delay. David Hilbert realizedthat Einstein'sgravitationalfield equations could be derivedin analogyto Ferrnat'sprinciple:Of all the possibletrajectoriesthat a particlernight follow betweentwo eventsin space-time,the trajectoryactuallyfollowedmaximizesthe particle'sproper time for the trip. In the limiting caseof a particle moving slowlyin a weak gravitationalfield, this "Fermat'sprinciple for gravity" reducesto Hamilton'sprinciple of classicalmechanics. Newtoniancosmologyhad origir-rally er-rvisioned a static, everlasting,infinite universe.However,the Newtonianuniversewas unstableand paradoxical-how could a universefilled to infinity In 1917,with with starsshowa dark sky at night (Olbers'paradox)? his new tool expressinggravitationas the curvatureof space-time, Einsteinsolvedthe cosmologicalproblem at infirrity by abolishing infinity. He postulatedthe three-dirnensional universeto be the surfaceof a staticsphereembeddedin four-dirnensionalEuclidearr space.AlexanderFriedrnannand GeorgesLemaitreaskedwhy the universemust be static.Their equationspredicteda universein which spacecould contractor stretchto show a velocity-distance would relation.At the cosmicscalethe relativespeedof two poir-rts be proportionalto their separatior-t. Measuringastronomicaldistancesrequiresthe light of standard candles.HenriettaSwanLeavittprovidedcrucial candlesirr l9l2 when shediscovereda relationshipbetweenthe periodsand lurni24 Radiarions Fall2014 nositiesof Cepheidvariablestars.Edwin Hubble usedCepheidsin 1924to probe distancesto spiral nebulae,which turned out to be millions of light-yearsaway.The ur-riverse suddenlybecamevery big. By applyingthe Cepheiddistanceindicatorsand Doppler shifis to the spectraof galaxies,in 1929he offeredthe first evidencefor the cosmicvelocity-distancerelation.The journey toward big-bang cosrnologywas underway. In a universethat begir-rs in the big-bangscenario,after the primordial gasof relativisticparticlescoolssufficientlyfor atoms to form, an afterglowof photonsmust remain.The wavelengthsof thosephotonsare continuouslystretchedby the cosmicexpansion. In l94B the existencein our universeof this backgroundradiation was predictedby Ralph Alpher and Robert Hermann.Their first estimate placedits temperaturetoday near 5 K. Alpher and Hermann tried throughout the 1950sto convinceradio astronomersto look for the afterglow.[26]In 1964it was accidentlyfound by Arno Penziasand RobertWilson. Their measurements gavea temperatureof 2.7 K.l27l Ever since,it has offereda window into the genesisof the universe. Todaylight hasbecomethe most incisiveof tools in cosrnology. Precisionmeasurements of the cosmicafterglowof the big bang heraldedthe era of precisioncosmology;theharmonicsin the affor the terglow'spower spectrumoffer a kind of electrocardiogram earlyuniverse.The irony of our presentstateof fertile ignorance is that the greatestmysteriesat presentare not about the existence of light, but its absence:dark rnatterand dark energy.Could dark erlergybe our aether? and Beyond Quan[umElectrodynamics "Tlte diagramswe make of quarksexchanginggluonsare very similar to thepictureswe drau,for electronsexchanging photons.Sosimilar,in fact, that you might say that thephysicistshaveno imagination-that theyjust copiedthe theory And oJEtantum electrodynamics for thestronginteractions! you\'e right: that'swhat we did, but with a little twist." -Richard Feynrnan[28] In the rnid- 1920s,qLlantummechanicsdevelopedinto the form now taught to physicsmajors.But it took two more decadesto An electron reconcilequantum mechanicswith electrodynamics. is r-rotan idealpoint charge.The "total" electronincludesits ideal "bare"chargeplus the interactionsof the electronwith its own electromagneticfield. An electronernitsand reabsorbsphotons,and someof thosephotonsbriefly turn into electron-positronpairsthat combineback into a photon beforereturning to the original electron. The energybudgetfor producingthesevirtual particlescomes frorn the energyfuzzinessinherentin the Heisenberguncertainty principle.Thus what we seeas "the electron'in the laboratoryincludesa cloud of virtual photonsand electron-positronpairs.This contribis a seriousproblem becausetheseintermediateprocesses ute infinity to the quantum state! The remedyis "renormalizationl'A theory is said to be renorrnalizablewhen all divergentpiecescancelout eachother in perturbationtheory,leavir-rg as a residuethe observedchargeand mass.Accordingto our presentunderstanding,renormalizability presentsa necessary condition for ar-ryser-rsible theory of fundamental interactions. Quantum electrodyrrarnics-theinteractionof light with electricallychargedrnatter-was the first renormalizabletheory of It servesas the templatefor the elementaryparticleinteractior-rs. l.] :,g,t'I t-!. o n trccLitflL:_ll1lllf)'sicsI Universe,5th ed. (W.H. Freeman,New York,NY, 2000),9l-92; "'Ihis Month in PhysicsHistory:The Discoveryof Heliumj'APS News, A u g . / S e p t2. 0 1 4 , 2 - 3 . Vorintiottol I l3] WolfgarrgVrurgrau anclStanleyMancielstar:n, PrinciplesirrI)yrtttrtrics tutd QuantumTheory(Dover Publications, M i n e o l a ,N Y , 2 0 0 7 ) ,1 9 . From quarksto cosmology, light n* 0""" a tool, a model,arndan EirrnestC. Watsont, Me[14] RobertA. Millikan, DuaneRoller,trnc-l inspirationto all of physics.Light hasalsobeena r.netaphorical Nlttlecular Plry,5is5, Hettt,ttrul Stttuul(Ginn and Co., Boston, cltorrics, MA, 1937),historicillnotesfirr platesbetweenpp. 35 anci36. symbolof hopeand wisdom in allcultures.The Hindu fbur-clay fbstivalof lights,Diwali, celebrates the triurnph of knowleclgeover und I l5] LawrenceE. Goodnranand Willianr H. Warner,Sttttics (Waclsworth, ignorance,hopeoverdarkness.In the Book of Genesis, the "poem of I)T,rttttttics Belmont,CA, 1964).A historictrlsection, the dawn"in the fudeo-Christiern mythos,God speaksthe universe clesprite its brevity,precedes Ch. l. into existence one irrlvly(]enerotiorr(Springer-Verlag, by uttering"Let therebe lightl' In Burddhism New [16] Max Born, Pltysics [.et York,NY, I96L)),29. seeksenlightennrent, the lightsof wisdonrand compassion. 2015,the Yearof Light,be a celebrartion of knowledgetrndwisclorn I l7] Robert Pirsig,Zert ond tlrcArt o.l'MotorcycleMdtieno rucc(Wilovercorning povertyanclignorance. l i a r r nM o r r o w & C o . ,N e w Y o r k ,N \ ' , 1 9 7 3 ) , 3 5 1 . [31] Physicsand its technological applications haveessential rolesin achievingtheseencls.May we [ 1 8 ]W i g h t m a nr, e f .B , 1 3 0 - 1 4 - 5 . usethem wiselyand in the serviceof all that lives.May the secular The MatlrctnaticolTheoryo.fElectricityancl [19] Sir larnesJearns, (Canrbriclge world of physicshelp us flnd'bur placeamongthe inflnities"in ir Mognetisrrr Univ. Press,Cambriclge, UK, 1966,reprint f'estivalof light. ,e* o f t g O go r i g i n a l ) ,I - 3 . [20] John Stachel,ed, EinsteitisMiroculousYcor:FivePapersThut (PrincetonUtriversityPress,Princetotr, Acknowledgmenrs Changedtlrc Ftcc o_lPlt1'5is5 "On the ElectrodyNJ, l99B),123.SeealsoD. Neuenschwancler, I thar-rkDevin Powell,Kendra Redmoncl,and l)aniel Golornbekfbr nanricsof Moving Iloclies(PartA: Kinemartics) by Albert Einstein," (Fall2005),l0-15; ancl"C)nthe Electroclynamics suggestions Ilodiotiotrs that resulteclin an inrprovedmirnuscript. of MovirrgBoclies(Part 13:Electrodynarntics) and its Corollary,E = tttd,by (Spring2006),13-22. ReFerences Afbert Einstein,"Ilodiotiotts [2 I ] Bronowski,ref. 4, p. 247. l -5.org. I I ] http://www.light2O l22l1.1,.Heill'rron,I.lteDilenurutso_l'ottUprightMan: Max Plonck rrs "The (Univ.of CaliforniaPress,Berkeley, awakening began witli l{oger Bacon" cor-nes fronr Will Spoke ntton_fitrGernutrtScience [2] '[he (GardenCity PublishingOo.,New Durant, Storyo_l'Plilosoplry cA, 1986),21. "Einstein's York,NY, l93U),I17. In the prresent irrticle,events,persor-rs, irnd [23] t). Neuenschwancler, Quanta,Entropy,and the (Fall2004),17-21;"Lasersin principleswell known fionr introductoryanclintermediatephysics Photoelectric Effbct,"Radiotiotts textbooksarrenot ref'erenced. l 9 l 7-The Stinrulatecl E,nrission of Racliatio nl' Iladiati ons(Spring Phy5icst FrontAntiquityto the 2 0 0 4 ) ,l B - 2 1 . [3] ].D.Bernal,A Historyof'Classicol in Noture the term in a paperpublishecl , Quontum(Barnes& Noble,New York,NY, I 97I ), I 16. For a brief [24] G.N. Lewisintroclucecl biographyof RogerBaconseethe Britnrnicaonline article, D e c e m b e1 r 8 ,1 9 2 6 . http://www.britannica.com/ EBchecked/topic ity, 5th ecl.( Princeton I 48177I Roger-Bacon. [ 25] Albert Einstein,'l'|rcMeottirtgo.fRelotit, Urriv.Press,Princeton,NL IL)56),92-93. [4] IacobBronowski,TheAscentoJ'Mon(Little,Brown,& Co., Boston, MA, 1972),223-224. [26] Victor Alpher,"RalphA. Alpher,RobertC. Herman,anclthe Galileo Galilei, Sidereus Nttncius or The Sidcred Messettger, Albert Cosrrric MicrowaveBackgrounclRacliation," P/rysics in Perspec t ive [5] VarrHelden,tr. (Univ.of ChicagoPress,Chicago,I[., l9B9),36-37. 1 4 , 3 0 0 - 3 3 4( 2 0 t 2 ) . "'Big-bang'cosmologyand Hecht,Optics,4thecl.(Addison-Wesley, New Yrrrk,NY, [6] Er-rgene [27] I RalphAlpher anclRobertHerrttart, 2002),contuinsa splendidhistoricalsur-umaryin its openir-rg pages. cosnric backgrouncl racliationl' Moder n Costnoktgy i tt Ret rosp ect, ll. Ibn Sahlon burning nrirBertotti,R. Bablinot,S. Bergia,anclA. Messina,eds.(Cambridge [7] R. Rashed,A pioneerin anaclastics: rors and lenses,lsis81, 464-491(1990).Seealso"Founclations of Urriv.Press,New York,NY, 1990),147-148.SeealsoD. Neuen"Historyof Big BangCosmology,Part4: The Expanding GeometricitlOptics:Phenonrenology and Principlesj'SPSObserver schwander, (Summer20I 0), http://www.spsobserver.org. (Spring2009),19-25; Universe,Ylem,anclthe CMtlRl' ILodiotiorrs "Historyof Big lletngCosmology,Part6: The Accelerating lderrs(Yale Universe, [8] William P.D.Wightrnan,TJreOrowth o_fScicnti_l'ic (Spring2010), UniversityPress,New Hirven,CT, 1953),120-128. l)ark Energy,anclF.instein'.s Enigmai'Rodiotions l9-23; arrd"l.ight aurclTwo Curves,C)neCenturyApartl' Rodiiliotts [9] Bronowski,ref.4,227. ( S p r i n g2 0 0 2 ) ,t 9 - 2 3 . S/rulbrd Encyclopedia o_l'Philosoplry, Il0] "Emilie du ChAtelet," 'l'|rc http:/iplato.stanford.edu/entries/emilie-clu/chatelet/; [.arurel Corona, [2tt] RicharclP.Feynrnan,QEI): StrongeTlrcoryoJ'Lightond "Heat, Light, and Emilie du Chdtelet,"http://www.wondersandnrrrr- Motter (PrincetonUniv.Press,Princeton,N], 1985),136. 'A 'Star.rdarcl vels.com/201 1/06/heat-light-ancl-emilie-clu-chatalet.html. Herschel's [29] D. Neuenschwancier, CursoryGlanceat the Model' discoveryof IR is well documentediu uuurerousastrol-romy texts of Elenrentary ParticlePhysics," SPSObsen,cr(Sunrrner200tt), and websites.It is not clearto this authorif eitl-rer Willianror Carohttp://www.spsobserver.org. line Herschelwereawareof Emilie du Chitelet: the Herschelswere [30] PierreRanrond,Field Tlrcory:A Modern Pritrrcr(Benjtimin/ from Germanyand emigratedto Englancl. PublishingCo., Reacling, MA, l98l ). Cunrnrir.rgs 'fhis miryalsobe irn opprrlrtuue utorttentto lamentthe thought[1 I ] Wightman,ref.8, 258. [31] lessexcesses tlrc of light pollutionand light trespass. [12] Neil F.Corninsand Williarn). KaufmannIII, Discoverirry other theoriesof elementaryparticlephysics.[29] At its foundation standsa principleof leastaction,adaptedto quantumfleld theory, that tracesits inspirationbackthroughthe analogousprinciplesof HamiltonanclFermat.[30I Fall2Ol4 Radiations 25
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