EINSTEIN'S EARLY WORK ON THE

QUANTUM

HYPOTHESIS

I

From 1905

through

1909 Einstein

published

five

major

papers

on

the

hypothesis

of

energy

quanta,

its theoretical

implications,

and its

use

in the

explanation

of

various

phenomena:

Einstein

1905i

(Doc. 14),

Einstein 1906d

(Doc. 34),

Einstein

1907a

(Doc. 38)

with

a

correction

in Einstein 1907d

(Doc.

42), Einstein 1909b

(Doc.

56), and

Einstein

1909c

(Doc.

60).

In

addition,

he reviewed

Planck's

lectures

on

thermal radiation[1] in

Einstein

1906f

(Doc.

37),

and with Walter Ritz summarized their differences

on

the

radiation

prob-

lem in Ritz

and

Einstein 1909

(Doc.

57).

In

describing

four

of

his 1905

papers,

Einstein characterized

only

the

one on

the

quan-

tum

hypothesis

as revolutionary.[2]

It

is

now regarded as revolutionary

in

challenging

the

unlimited

validity

of Maxwell's

theory

of

light

and

suggesting

the existence

of

light

quanta.

The

paper

shows

that,

at

a sufficiently high frequency,

the

entropy

of

equilibrium

thermal

(or

"black-body")[3]

radiation behaves

as

if

the radiation consists

of

a

gas

of

independent

"quanta

of

light

energy" ("Lichtenergiequanten,"

or simply

"light quanta,"

"Lichtquanten"),

each with

energy proportional

to the

frequency.

Einstein showed how

to

explain

several otherwise

puzzling phenomena by assuming

that the interaction

of

light

with matter

consists

of

the emission

or absorption

of

such

energy quanta.

In

subsequent papers,

Einstein examined

the tacit

assumptions underlying

Planck's

der-

ivation

of

the

energy

distribution law for

black-body

radiation,[4] explored

several further

implications

of

the law

itself,

and showed

that the

concept

of

energy

quantization is

appli-

cable not

only

to radiation but also to material

oscillators. Einstein

1906d

(Doc. 34)

dem-

onstrates that

Planck's

derivation

presupposes quantization

of

the

energy

of

charged

oscil-

lators in

interaction with thermal radiation. Einstein 1907a

(Doc. 38) explains

the

anomalous

behavior of

specific

heats with

decreasing temperature by treating

the atoms

or

ions in

a

solid

as a

lattice

of

quantized

oscillators.[5] Einstein 1909b

(Doc. 56)

shows

that

Planck's

law

gives

rise to fluctuations in the

energy

and

pressure

of

black-body

radiation

that

appear

to arise from

two

independent causes: light waves

and

light

quanta.

In

a

masterful address to the

Salzburg meeting

of

the

Gesellschaft Deutscher Natur-

forscher und

Ärzte,

Einstein

1909c

(Doc. 60),

Einstein summarized his views

on

radia–

[1]

Planck

1906c.

[2]

See Einstein to Conrad

Habicht,

18

May-8

June 1905. The

paper on

the

quantum

hypothe-

sis is Einstein 1905i

(Doc. 14);

the other three

papers

mentioned

are on: relativity theory,

Ein-

stein

1905r

(Doc. 23);

molecular

dimensions,

Einstein

1905j

(Doc. 15);

and Brownian

motion,

Einstein

1905k

(Doc. 16).

[3]

The term

"black

body" ("schwarzer

Kör-

per")

was

in

general

use

by

the

turn

of

the

cen-

tury

(see,

e.g.,

Rayleigh

1900,

p. 539,

and

Planck

1900d,

p.

764).

Einstein first used the

term in

Einstein

1904

(Doc.

5), p.

354,

but

re-

ferred

to

the radiation

itself

as

"temperature

ra-

diation"

("Temperaturstrahlung"),

p.

361.

Einstein

1905i

(Doc.

14)

refers to it

as

"black

radiation"

("schwarze Strahlung").

[4]

Planck

presented

this law in

Planck

1900c.

He

gave

derivations

of

it

in

Planck

1900e,

1901a, and 1906c.

[5]

Einstein

1907d

(Doc. 42)

considers both

charged

and

uncharged

lattice oscillators.