INTRODUCTION

TO VOLUME

8

xlv

papers

on

quantum

theory

from

1916 and

1917.[12]

Was this because

they

did

not

stir

up controversy, as

did the

papers

on

relativity?

It is true that the basic

ideas and

techniques

of

the

two

papers

from

1916[13]

go

back

to earlier work

by

Einstein,

such

as

his

work

on

fluctuations and the

papers

he

wrote

together

with

Ludwig

Hopf,

but

their

results

were striking

nevertheless:

a new

and

simple

derivation

of

Planck’s

law,

characterized

by

Einstein

as

“the

derivation”

(Doc. 250),

and the conclusion

that

radiation emitted

or

absorbed

by

atoms

has

a

directed character.

Perhaps

Ein-

stein’s

readers could not

help

but be

swayed by

the

conceptually simple

and

per-

suasive

arguments

that

made these conclusions

inescapable.

As to the other

important

paper

on

quantum

theory,

Einstein 1917d

(Vol. 6,

Doc.

45),

the

only

information

this volume

provides

is that

Einstein

had turned

to

the

mathematician

Constantin

Carathéodory (Docs.

255 and

285)

and

to

Planck

(Doc. 295)

for

clari-

fication

of

Hamilton-Jacobi

theory,

one

of

the tools used in

the

paper.

In

addition,

a

fair

amount

of

other

topics

that

occur

in the

correspondence

have

no

or

hardly any

connection

with Einstein’s

published

papers. Examples

are

dis-

cussions

on quantization

with Ehrenfest in

May

1914 and

an exchange

with

Edgar

Meyer

and Lise

Meitner

in the fall

of

1918

on experiments

to

determine the nature

of

the

absorption process

of

y-rays.

Another

important

topic

which merits

special

attention is the fundamental

matter

of

the

validity

of

Walther Nernst’s

heat

theorem

and the related issue

of

the

entropy

constant in the framework

of

quantum theory.

In

one

of

the

first

papers

published during

his Berlin

years,

Einstein 1914n

(Vol.

6,

Doc.

5),

Einstein

challenges

his readers to

provide

a strictly

thermodynamic

proof

of Nernst’s heat

theorem,

promising

that

he will refute all

of

them. Einstein

had been concerned with the heat theorem

for

several

years already, as

becomes

clear,

for

instance,

from

a

discussion

with

Nernst

during

the First

Solvay Congress

in

1911

(Vol. 3,

Doc.

25, sec. 5),

and from

a paper

criticizing

Nest,

submitted

to

Physikalische Zeitschrift,

but

eventually

retracted.[14]

In

the fall

of

1913,

at the Sec-

ond

Solvay Congress,

Einstein

again spoke

out,

this time

against

a

recent thermo-

dynamic

proof

by

Nernst

(Vol.

4,

Doc.

22,

sec.

4).

The

paper

of

1914

can

be

seen

as a logical

next

step

in the

development

of

the

views

put

forward

by

Einstein at the

Solvay meetings.

For

Einstein,

as

he

explains

in

the

paper, quantum theory was

needed

as an

essential

ingredient

in

any

derivation

of

the heat theorem. The

same

point

is made in

a

manuscript

from 1916

on

the

theory

of

Otto Sackur and

Hugo

Tetrode

for

the

entropy

constant

(Vol.

6,

Doc.

26),

in which Einstein reexamines

the issue

of

the value

of

the

entropy

at

absolute

zero,

and reaffirms

a

conclusion

already

drawn in Einstein 1914n

(Vol. 6,

Doc.

5):

the heat

theorem,

in the formu-

lation that at absolute

zero

the

entropy vanishes,

is valid for

pure

crystals

only,

not

for mixed

crystals.