2010_kleppner_one hundred years of quantum physics

8/10/2019 2010_Kleppner_One Hundred Years of Quantum Physics

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One Hundred Years of Quantum PhysicsAuthor(s): Daniel Kleppner and Roman JackiwSource: Science, New Series, Vol. 289, No. 5481 (Aug. 11, 2000), pp. 893-898Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/3077316

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2/7

P THW YS

O DISCOVERY

n e

undred

e a r s o f

Quantum

Physics

Daniel

Kleppner

and Roman

Jackiw

An informed

ist of the most

profound

cientific

developments

f the 20th

century

s

likely

o include

general

elativity,

uantum

mechanics,

ig bangcosmology,

he

unraveling

f the

genetic

ode,

evolu-

tionary

biology,

and

perhaps

few other

opics

of

the

reader's

hoice.

Among

these,

quantum

me-

chanics s

unique

becauseof

its

profoundly

adical

quality.Quantum

mechanics

orced

physicists

o

reshape

heir deas of

reality,

o rethink

he nature f

things

at the

deepest

evel,

and to revise heir

concepts

f

position

nd

peed,

as well as

theirnotions f cause

andeffect.

Although uantum

mechanicswas created

o describe n abstract tomicworld

arremoved rom

dailyexperience,

ts

impact

on our

daily

ives could

hardly

be

greater.

he

spectacular

dvances

n

chemistry, iology,

and medicine-and in

essentially very

other

science-could

not haveoccurredwithouthe tools thatquantummechanicsmadepossible.

Without

uantum

mechanicsherewouldbe no

globaleconomy

o

speak

of,

because

he electronics evolutionhat

brought

s the

computer

ge

is

a childof

quantum

mechanics. o is the

photonics

evolutionhat

brought

s the Information

ge.

The creation f

quantum hysics

has

transformedur

world,

bringing

with it all the benefits-and

therisks-of a scientific

evolution.'

Unlike

general elativity,

hich

grew

out of a brilliant

nsight

into the connection etween

gravity

and

geometry,

r the deci-

phering

f

DNA,

whichunveiled new worldof

biology,quan-

tummechanics

id not

spring

roma

singlestep.

Rather,

t was

created

n

one

of thoserareconcentrationsf

genius

hatoccur

from ime to

time in

history.

For 20

years

after heir ntroduc-

tion,

quantum

deaswere so

confused

hat herewas littlebasis

for

progress;

hen a small

group

of

physicists

reated

quantum

mechanics in three tumul-

tuous

years.

These scientists

o

"Quantum

were troubled

by

what

they

|:

were

doing

and n somecasesdis-

theory

is the

tressed

y

what

hey

haddone.

The

unique

ituation

f this

most

precisely

crucial

et

elusive

heory

s

per-

Papa

Quanta.

In

1900,

Max

t

haps

best summarized

y

the Planck tarted the

quantum-

|

ested and

most

following

observation:

Quan-

mechanicalnowball.

tum

heory

s themost

precisely

successful

tested ndmost uccessful

heory

n the

history

f science.Never-

theory

in

the

theless,

not

only

was

quantum

mechanics

eeplydisturbing

o its

theory

in

te

founders,

oday-75 years

after he

theory

was

essentially

ast

n

historyof

its currentorm-some of the luminariesf scienceremaindis-

satisfiedwith ts

foundationsnd ts

interpretation,

ven

as

they


3/7

PATHWAYS OF DISCOVERY

tially every

othermeasurable

roperty

f

matter,

uch

as

viscosity,elasticity,

lectrical nd hermal

onductivity,

o-

efficients of

expansion,

ndices of

refraction,

nd thermo-

elasticcoefficients.

Spurred y

the

energy

of

the

Victorian

work

ethic

and the

development

f

ever

more

ingenious

experimental

methods,

knowledge

accumulated t

a

prodigious

ate.

What is most

striking

to the contemporary ye,

however,

s

that the

com-

pendious descriptions

of

the

properties

of matter

were

essentially mpirical.

Thousands of

pages

of

spectral

data isted

precise

_ -

H

U

values for the

wavelengths

of the

elements,

but no-

_B

body

knew

why spectral _. :1

lines

occurred,

much less

what nformation

hey

con-

_

veyed.

Thermal ndelectri-

cal conductivitieswere in-

terpreted by suggestive

modelsthat fittedroughly

Superatom.

These

olorful

data

half of the

facts.

There measurementsf rubidiumtom

were numerous

empirical

ed Bose-Einsteinondensate.

laws,

but

hey

werenot sat-

isfying.

For

nstance,

he

Dulong-Petit

awestablished sim-

ple

relation etween

pecific

heatand he atomic

weight

of a

material.Muchof the time it

worked;

ometimes t didn't.

Themassesof

equal

volumesof

gas

were n the ratiosof in-

tegers-mostly.

The Periodic

Table,

which

provided

key

organizing rinciple

orthe

flourishing

cienceof

chemistry,

had

absolutely

o theoretical asis.

Among

he

greatest

chievementsf therevolutions this:

Quantum

mechanicshas

provided quantitativeheory

of

matter.

We

now understand

ssentially

very

detailof atomic

structure;

hePeriodic able

as

a

simple

and

natural

xplana-

tion;

and hevast

arrays

f

spectral

ata it intoan

elegant

he-

oretical ramework.

uantumheory

permits

he

quantitative

understanding

f

molecules,

f solidsand

iquids,

ndof con-

ductors nd semiconductors.t

explains

bizarre

henomena

suchas

superconductivity

nd

superfluidity,

ndexotic orms

of matter uchas the stuffof neutron

tars

nd

Bose-Einstein

condensates,

n whichall the

atoms

n

a

gas

behave ikea sin-

gle superatom. uantummechanics rovides ssential ools

forall of the

sciences nd or

every

advanced

echnology.

Quantum hysicsactually ncompasses

wo entities.The

first s the

theory

of matter t the atomic

evel:

quantum

me-

chanics. t is

quantum

mechanics hat allows us to under-

standand

manipulate

he materialworld.The

second

s the

quantumheory

of fields.

Quantum

ield

theoryplays

a to-

tallydifferent ole

n

science, o whichwe shallreturnater.

Quantum

Mechanics

The clue that

triggered

he

quantum

evolution

ame

not

from

studiesof

matter

ut

froma

problem

n

radiation. he

specific challenge

was to understandhe

spectrum

f

light

emitted

by

hot bodies:

blackbody

radiation. The

phe-

nomenon s familiar o

anyone

who has stared t a fire. Hot

matter

lows,

and he hotter

t

becomes he

brighter

t

glows.

The

spectrum

f

the

light

is

broad,

with

a

peak

that shifts

fromred

to

yellow

and

finally

o blue

(although

we

cannot

see

that)

as the

temperature

s raised.

l,fr

IS

C

It shouldhavebeen

possible

o understandhe

shape

of

the

spectrum y combining oncepts

rom

hermodynamic

and

electromagneticheory,

utall

attempts

ailed.

However,

by assuming

hat he

energies

f the

vibrating

lectrons hat

radiate he

light

are

quantized,

lanckobtained n

expres-

sion that

agreed

beautiful-

ly

with experiment.

ut as

he recognized

ll too well,

the theorywas physically

absurd, an act of desper-

ation," as he later de-

scribed t.

Planck applied his

quantum ypothesiso the

.?~ ,?.~~~~?energy

of the vibratorsn

the walls of a radiating

body. Quantumphysics

mighthave ended here f

in 1905 a novice-Albert

Einstein-had not reluc-

tantlyconcluded hat f a

vibrator's nergy s quan-

tized, then the energy of

the electromagneticield

romNIST n 1995, emerged from

that it radiates-light-

:oalescing

nto he firstdocument- must also be quantized.

Einstein thus imbued

light

with

particlelike

e-

havior,

notwithstanding

hatJamesClerkMaxwell's

heory,

and over a

century

of definitive

experiments,

estified to

light's

wave nature.

Experiments

n the

photoelectric

ffect

in the

following

decade evealedhatwhen

ight

s absorbed

its

energyactually

rrives

n

discrete

bundles,

as

if carried

by

a

particle.

he dualnature f

light-particlelike

r wave-

like

depending

n whatone looks for-was the firstexam-

ple

of a

vexing

heme

hatwould

recur

hroughoutuantum

physics.

The

duality

onstituted theoreticalonundrumor

thenext20

years.

The first

step

toward

quantumheory

had been

precipi-

tated

by

a dilemmaaboutradiation.The second

step

was

precipitated

y

a dilemma boutmatter. t was known hat

atoms contain

positively

and

negatively hargedparticles.

But

oppositely harged

articles

ttract.

According

o

elec-

tromagnetic heory,

herefore,

they should spiral into each

Atoms_~~ i; other, adiating

ight n a

broad

1913,_

esor r

spectrum

ntil hey

collapse.

Once again, the door

to

I

progress

was opened

by a

o

of_n~

ao

anovice:

Niels Bohr. In

1913,

_ ohr proposed a

radical

hy-

pothesis:Electrons n an atom

proble

aoisb

exist only

in certain

tationary

states, ncluding ground tate.

Electronschange their energy

dictions,

by "jumping"

etween the

ta-

.p..---

..-he tionary

states, emitting light

Atoms

go quantum. In whose wavelength ependson

1913,

Niels Bohrushered the energydifference. y com-

quantum hysicsnto

world

biningknown awswithbizarre

of atoms. assumptionsboutquantum e-

X

havior, Bohr swept away the

o

problem

f atomic tability. ohr's heorywas full of contra-

dictions,but it provideda quantitative escription f the

F

spectrum f the hydrogen tom.He recognized oth he suc-

11 AUGUST

000

VOL

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4/7

PATHWAYS OF DISCOVERY

cess and he

shortcomings

f his model.With

uncanny

ore-

sight,

he rallied

hysicists

o create new

physics.

His vision

was

eventually

ulfilled,

lthough

t took 12

years

anda new

generation

f

youngphysicists.

At

first,

attempts

o advanceBohr's

quantum

deas-the

so-called ld

quantumheory-suffered

one defeatafteran-

other.Then a series of

developments otally changed

he

courseof

thinking.

In

1923 Louis de

Broglie,

n

his Ph.D.

hesis,

proposed

that heparticle ehavior f lightshould

have ts

counterpart

n the wave

behav-

ior of

particles.

He associated wave-

length

with he

momentum f a

particle:

The

higher

he momentum he

shorter

the

wavelength.

he deawas

ntriguing,

but no one knewwhata

particle's

ave

nature

might ignify

or how t related o

atomic structure.

Nevertheless,

de

Broglie'shypothesis

was an

important

precursor

orevents oon o

take

place.

In the summerof

1924,

there was

yet

another

precursor.Satyendra

N.

Bose

proposed totally

new

way

to ex-

plain

he Planck adiationaw.He treat-

ed lightas if it were a gas of massless

particles now

called

photons)

hat do

not

obey

the classical laws of Boltz-

Getting

weirder.

mann

tatistics utbehave

according

o

said hat f

wavelike

a new

type

of

statistics asedon

parti-

like

particles,

hen

cles'

indistinguishable

ature.

Einstein

have

ikewaves.

immediately pplied

Bose's

reasoning

to a real

gas

of massive

particles

ndobtained new law-

to become known as the Bose-Einsteindistribution-for

how

energy

s shared

y

the

particles

n

a

gas.

Undernormal

circumstances,owever,

he new and old theories

predicted

the samebehavior or

atoms

n

a

gas.

Einstein ook no fur-

ther

nterest,

nd the result

ay undeveloped

or more

thana decade.

Still,

ts

key

idea,

he

ndistinguishability

of

particles,

was about

o

become

criticallymportant.

Suddenly,

tumultuous eries of eventsoccurred,

culminating

n

a scientific evolution.n the

3-yearpe-

riod rom

January

925 o

January

928:

*

Wolfgang

Pauli

proposed

he exclusion

principle,

providing

theoretical asis or he Periodic able.

*

Werner

Heisenberg,

with Max

Born

and

Pascual

Jordan,

iscovered

matrix

mechanics,

he first version

of

quantum

mechanics.The historical

goal

of

under-

standing

lectron

motionwithinatomswas abandoned

in favorof a

systematic

method

or

organizing

bserv-

able

spectral

ines.

*

Erwin

Schrddinger

nventedwave

mechanics,

a

second ormof

quantum

mechanics n which he state

of a

system

s described

y

a wave

function,

he solu-

tion to

Schrodinger'squation.

Matrixmechanicsand

wave

mechanics,

pparentlyncompatible,

ere shown

to be

equivalent.

Unl

*

Electronswereshown o

obey

a new

type

of statis-

arti

tical

law,

Fermi-Diractatistics. t was

recognized

hat

sorl

all

particles bey

eitherFermi-Dirac

tatistics r Bose-

ceri

Einstein

tatistics,

nd hat he two classeshavefunda-

mentally

ifferent

roperties.

*

Heisenberg

nunciatedhe

Uncertainty rinciple.

*

PaulA. M. Dirac

developed

a relativisticwave

equa-

tion for the electron hat

explained

lectron

pin

and

pre-

E

dicted

antimatter.

*

Dirac aid the foundations f

quantum

ield

theoryby

providing quantum escription

f the

electromagnetic

ield.

*

Bohr

announced he

complementarityprinciple,

a

philosophical rinciple

hat

helped

o resolve

apparent

ara-

doxesof

quantum

heory, articularly ave-particleuality.

The

principal layers

n the creation f

quantumheory

were

young.

In

1925,

Pauli was 25

years

old,

Heisenberg

and EnricoFermiwere

24,

and Diracand Jordanwere 23.

Schrodinger,

t

age

36,

was a late bloomer.

Born

and

Bohr

wereolder till,and t is significanthat heir

contributionswere

largely interpretative.

The

profoundly

adicalnatureof the intel-

lectual

achievement

s

revealed

y

Einstein's

reaction.

Having

nvented ome

of the

key

concepts

that

led to

quantum heory,

Ein-

stein

rejected

t. His

paper

on

Bose-Einstein

statisticswas his last contributiono

quan-

tum

physics

andhis last

significant

ontribu-

tion

to physics.

i~ '

^

~

Thata new

generation

f

physicists

was

needed to create

quantum

mechanics is

hardly surprising.

Lord Kelvin described

why in

a letter o Bohr

congratulating

im

on his 1913

paper

on

hydrogen.

He saidthat

therewas much ruth n Bohr's aper, uthe

would never understandt

himself. Kelvin

Louisde

Broglie

recognized

hat

radically

ew

physics

would

.

light

an

behave

need o come fromunfetteredminds.

particles

an

be-

In

1928,

the revolutionwas finished

and

the foundations f

quantum

mechanicswere

essentially

omplete.

The frenetic

pace

with

which it

occurred

s revealed

by

an anecdote ecounted

y

the late Abraham ais in Inward

Bound.

n

1925,

the con-

cept

of electron

spin

had been

proposed by

Samuel

Goudsmit nd

George

Uhlenbeck.

Bohrwas

deeplyskepti-

cal.

In

December,

e traveled o

Leiden,

he

Netherlands,

o

attend the jubilee of

Hendrik A. Lorentz's

d octrate. Pauli met

the traint

Hamburg,

Germany,

o

ind

out

Bohr's

pinion

a

bout

the

possibility

of

le

c-

tron pin.Bohrsaid

he

proposal was "very,

very interesting," his

well-known

put-down

phrase. Later at Lei-

den, Einstein

and Paul

Ehrenfest met Bohr's

train, also to discuss

spin. There, Bohr ex-

plained his

objection,

but Einstein showed a

way around t and con-

knowable eality.WernerHeisenberg vertedBohr nto a sup-

culated ne of the mostsocietally b- porter. On his

return

bed

deasof quantum hysics:he Un- journey,

Bohrmet with

tainty

rinciple.

yet more

discussants.

When the trainpassed

throughGdttingen,Germany,Heisenberg

nd Jordanwere

waitingat the station o ask his opinion.And at the

Berlin

station,Pauliwas waiting,having raveled specially rom

Hamburg.

ohr old them all that he

discovery

f electron

spin

was a

great

advance.

www.sciencemag.org

CIENCEVOL 89 11 AUGUST000

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2010_kleppner_one hundred years of quantum physics

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