xviii INTRODUCTION TO VOLUME

3

Zurich in the

summer

of

1912

when Einstein

accepted

a

call to

a

professorship

at

the

Swiss

Federal

Polytechnical

School

(ETH).[13]

III

When Einstein moved from the

Patent

Office to

the

university

in

October

1909,

the

problems

he had been

pondering

moved with him.

Foremost

among

these

was

surely

the

problem

of

radiation,

which

had

already

given

him

no

peace

for

several

years.

Only a

few weeks

before

moving

he

had

spoken on

this

subject

at

Salzburg

in

his first

major

address

at

a

scientific

meeting, arguing

that

a

new

theory

of radiation

was

needed.[14]

The

wave

theory

of light-and

Maxwell's

electromagnetic theory

which

provided

its

foundation-could

not

adequately

account

for

some phenomena

that

could, however, be

readily

understood

if

light

behaved

like

a

collection of

particles

of

energy.

A

particle

or

emission

theory

of

light,

on

the

other

hand,

failed to

account

for the

famil-

iar

phenomena

of

interference, diffraction,

and

so

forth

that

were explained

so

beautifully

by

the

wave

theory.

Einstein

expected

that

"the

next

phase

of

the

development

of theoretical

physics"

would

produce

"a

theory

of

light

that

can

be

interpreted as a

kind of fusion of the

wave

and emission

theories."[15]

He

based this

expectation

on

his

analysis

of the statistical fluctuations of the

properties

of

black-body

radiation

whose

average

behavior

is

described

by

the

experimentally

confirmed Planck distribution

law.

Einstein had carried

out two

independent

calculations of

such fluctuations,

one

using

Boltzmann's

principle

and the other

using

the

approach

of

his

own

theory

of Brownian motion. Both had

led to

fluctuations

demonstrating

the

presence

of

a

particlelike

structure

as

well

as a

wavelike

structure

in radiation.

These results had convinced Einstein that Maxwell's

electromagnetic theory,

which

predicted only

the wavelike

term

in the

fluctuations,

would have

to

be

modified

in

some

fundamental

way.

The fluctuation

arguments

had

per-

suaded him

that

the

quantum structure

in radiation

was a

necessary

conse-

quence

of Planck's distribution law and

not

just

an

assumption-"apparently

horrendous"[16] and

perhaps

avoidable-that

was

sufficient

for

deriving

this

distribution

law.

[13]See

Robert Gnehm

to Einstein, 23

January 1912,

and Einstein

to

Ludwig Forrer,

2

February

1912.

[14]Einstein

1909c

(Vol.

2,

Doc.

60).

[15]"...

daß die nächste

Phase der

Entwickelung

der

theoretischen

Physik

uns

eine Theorie

des Lichtes

bringen wird,

welche sich

als

eine

Art

Verschmelzung von

Undulations-

und Emis-

sionstheorie

des Lichtes auffassen läßt." Einstein

1909c

(Vol.

2,

Doc.

60),

pp.

482-483.

[16]"ungeheuerlich

erscheinenden Annahme."

Einstein

1909c

(Vol. 2,

Doc.

60), p.

495.