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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
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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
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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
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"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
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irnd
[23] t). Neuenschwancler,
Quanta,Entropy,and the
(Fall2004),17-21;"Lasersin
principleswell known fionr introductoryanclintermediatephysics
Photoelectric
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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
,
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[24] G.N. Lewisintroclucecl
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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,
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[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]
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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,
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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
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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),
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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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