INTRODUCTION TO VOLUME

3

xxiii

any

harm because the

most

intimate

contact

with

experiment

is

maintained.

Einstein's

"quantum

hypothesis"

is

probably

one

of the

most

re-

markable

ever

devised;... if

it

is

false, well,

then it

will

remain for all

time "a beautiful

memory".[36]

Nernst's

positive

reaction

to

Einstein and his work would

soon

have

impor-

tant

consequences

for Einstein's

career.

Einstein had

developed

his

theory

of the

specific

heats of

solids

in

1907

on

the basis of

a simple

model: the N

atoms

of

a

solid vibrate

independently

about

their

positions

of

equilibrium,

and

all

3N of these vibrations have

the

same

frequency

v.

The

average energy

of each such oscillation

was

then

assumed

to be

given by

the

same

formula Planck had used in his

theory

of

black-body

radiation. The

frequency

v

was

the

only quantity characterizing

a

given

solid,

and the

key question, therefore,

was

how

to

determine this

frequency

from

some

measurable

property

of

this

solid, a

property

other than

the

specific

heat.

For

those substances

that

absorb infrared

radiation,

Einstein

suggested

that

the

frequency

at

which this

absorption

is

a

maximum-the

frequency

of the residual

rays

(Reststrahlen)-should be

identified with the

frequency

of atomic vibrations.

Since not

all substances exhibit this

phenom-

enon,

there

was no independent general

method available for

finding

the

atomic vibration

frequency

of

Einstein's

theory.[37]

When Nernst's

experiments

confirmed

beyond

any

doubt that

Einstein's

theory captured

the essential features of the behavior of

specific

heats with

temperature,

Einstein returned

to

the

problem

of

determining

v,

the

frequency

of atomic

vibrations.[38] Since

the

same

forces

give

rise to

both

the atomic

vibrations and the elastic

properties

of the

solid,

it

is

not

surprising

that

Einstein

was

able

to

derive

an

equation expressing

v

in

terms

of the

com-

pressibility

of the solid

along

with its

density

and molecular

weight.

Einstein

was

not alone

in

making

this connection between

an

elastic

constant

and

an

optical property

(the

absorption

frequency)

of

a

solid.

The Australian

physicist

[36]"Wir können

im Hinblick auf

der

Entwicklung

der

Physik, wie

ich

glaube,

sehr froh

sein,

einen

jungen so originellen

Denker

zu

besitzen,

einen 'Boltzmann

redivivus';

dieselbe Intensität

u. Schnelligkeit

der

Auffassung-grosse

Kühnheit in der

Theorie,

die

aber

nichts schaden

kann,

weil

der

innigste

Contact mit dem

Experiment gewahrt wird.

Die

'Quantenhypothese'

Einsteins

gehört

wohl

zu

dem

Merkwürdigsten, was

erdacht

wurde;

...

ist

sie

falsch, nun so

wird

sie

für

alle

Zeiten 'eine

schöne

Erinnerung'

bleiben!" Walther

Nernst

to

Arthur

Schuster,

17

March

1910

(UkLRS, Sc.

130).

See

Barkan

1990,

which

first

drew

attention

to

this remarkable letter.

[37]See Einstein

1907d

(Vol. 2,

Doc.

42).

[38]Einstein

1911b

(Doc.

13).