EARLY WORK ON QUANTUM

HYPOTHESIS

141

quanta,

Einstein

1906d

(Doc. 34).

He returned

to this

question

in

Einstein

1907a

(Doc.

38), showing

that,

if

the structure function

in

phase space

he had

introduced

earlier[48]

is

assumed

to

restrict the oscillators

to

orbits with

energies

that

are integral multiples

of

hv,

then the

average

oscillator

energy

in

a

canonical ensemble

yields

Planck's

law,

when sub-

stituted in

eq.

(1).

This

assumption

about the oscillator

energies

is

inconsistent

with

Planck's

derivation

of

eq. (1),

which

assumes

that the

energy

of

a radiating

oscillator

varies

continuously.

So Einstein also had

to

assume that,

in

spite

of

this

inconsistency,

Maxwell's

theory gives

the correct

average energy

of

an

oscillator

in

a

radiation field.

Except

for

his

paper on specific

heats,

Einstein 1907a

(Doc. 38),

submitted in Novem-

ber

1906,

Einstein did not write another

major paper on

the

quantum hypothesis

for

over

two

years. Although

his

publications

concentrate

on relativity theory during

this

period,

he did not

cease

to wrestle with the

problem

of

quanta.

He wrote Laub

in

April?

1909

about his intense

struggle

with the

problem, especially as

it

pertains

to

light:

I

am ceaselessly occupied

with the

problem

of

the constitution

of

radiation.

. . .

This

question

of

quanta

is

so extraordinarily important

and difficult that

every-

one

should be concerned with it.

Ich

beschäftige

mich

unablässig

mit der

Frage

der Konstitution der Strah-

lung.

. . .

Diese

Quantenfrage

ist

so ungemein wichtig

und

schwer,

dass sich

alle darum bemühen

sollten.[49]

Einstein

presented

the results

of

those

two

years

of

reflection

in

Einstein 1909b

(Doc.

56),

and

in

his

Salzburg

lecture,

Einstein 1909c

(Doc. 60),

which

are

discussed in sections

V

and VI below.

IV

In addition to their contributions

to

theory,

each

of Einstein's

first three

papers on

the

quantum hypothesis

also

provides ingenious explanations

of

observed

phenomena or pre-

dictions

of

new ones.

Einstein 1905i

(Doc.

14)

examines three interactions

of

light

with

matter,

treated

"as

if

light

consisted

of

such

energy quanta"

("wie

wenn

das

Licht

aus

derartigen Energiequanten bestünde"):[50]

Stokes's

rule for

fluorescence;

the ionization

of

gases by

ultraviolet

light;

and the

photoelectric

effect. In Einstein 1906d

(Doc. 34),

he

deduced

a relationship

between the electromotive force series for metals and

their

photo-

electric

sensitivity.

Einstein 1907a

(Doc. 38)

offers

an explanation

of

the anomalous de-

crease

of

specific

heats with

decreasing temperature.

Einstein's

explanations

of

the

photo-

electric effect and

of

the behavior

of

specific

heats

proved

to

be

especially significant.

Einstein 1905i

(Doc.

14) proposes

what later became known

as

Einstein's

photoelectric

equation,

[48] See,

in

particular,

Einstein 1904

(Doc.

5),

p.

359.

[49]

Einstein

to

Jakob

Laub, April?

1909.

[50]

Einstein

1905i

(Doc. 14),

p.

144.