INTRODUCTION TO VOLUME

3

xix

The

problem was

to construct

a

theory

of radiation

that

would

account

for

both

its

particle

and

wave

structures,

but

how

was

one

to

do that? "The

difficulty,"

Einstein

wrote, "lies

mainly

in the fact

that

the fluctuation

proper-

ties

of radiation

...

offer few

formal

clues

on

which

to

build

a theory."[17]

He

struggled

with this

problem

for

years,

but

his efforts met

with

no

suc-

cess.

In his

paper

of

January

1909

and in

a

letter

to H. A.

Lorentz written in

May

of

the

same

year,

Einstein

wrote

of

his

attempts

to find

a

new system

of

differential

equations

that

might

allow

one

to construct

both the

light quan-

tum

and the

natural

unit of electric

charge.[18]

Neither the nonlinear

equa-

tions referred

to

in the

paper

nor

the linear

ones

discussed in his letter

to

Lorentz served his

purpose.

At the

Salzburg meeting

Einstein had

suggested

that

the

electromagnetic

fields

corresponding

to

radiation

might always

be

associated with

singulari-

ties,

in much the

same

way

that

electrostatic

fields

are

in Lorentz's

theory

of

electrons. The entire

energy

of

the field

might

then

perhaps

be

localized in

these

singularities just as

it had been in the old

theory

of action

at

a

distance.

This idea

stayed

with

Einstein,

and he

thought

about

it

a

great

deal

during

his

first months in Zurich.

By

the end of December

1909

he had become

aware

that

one

could indicate

an

infinite number of

ways

in which the electro-

magnetic

fields

could determine the

spatial

distribution of

energy,

all

of them

compatible

with Maxwell's

equations.[19]

"Perhaps,"

he wrote to

Besso,

"the

solution

to

the

quantum question

is to

be found here." Einstein

saw

this

freedom

to

choose the

equation

specifying

the

spatial

distribution of electro-

magnetic energy

in

terms

of the

fields

as

a

necessary accompaniment

of the

theory

of

relativity.

"Without

ether,

the continuous distribution of

energy

in

space seems

an

absurdity

to

me,"

he

wrote Planck.[20]

He

saw a

similarity

between the

current status

of the

theory

of radiation

and the situation

that

had

prevailed

before the formulation of the

theory

of

relativity.

The

preconception

that

energy

had

to

be distributed in

space

according to

the

standard

Maxwellian formula

might

be

standing

in

the

way

[17]"Die

Schwierigkeit liegt hauptsächlich

darin,

daß die

Schwankungseigenschaften

der

Strahlung

...

wenig

formale

Anhaltspunkte

für die

Aufstellung

einer Theorie bieten." Einstein

1909c

(Vol. 2,

Doc.

60), p.

499.

[18]Einstein 1909b

(Vol. 2,

Doc.

56)

and Einstein to

H. A. Lorentz, 23

May

1909. See

also

McCormmach

1970.

[19]"Gefunden

habe ich nicht

viel.

Das Interessanteste

ist,

dass

man

eine unendliche

Mannig-

faltigkeit

von

Energieverteilungen angeben kann,

die mit Maxwells

Gleichungen

verein-

bar sind. Vielleicht

liegt

hierin die

Lösung

der

Quantenfrage."

Einstein

to

Michele

Besso,

31

December

1909.

[20]"Ohne

Aether erscheint mir continuierlich

im

Raume verteilte

Energie

ein

Unding."

"Re-

sponse

to

Manuscript

of Planck

1910a"

(Doc.

3);

see

also

Einstein 1910b (Doc.

5).