494

DOCS.

479,

480 MARCH

1918

energy

=

const.

densely throughout,

that

is,

it

traverses

any

chosen

area

of

this

hypersurface

however small

it

happens

to be.

I

obviously

cannot

prove

at all

whether

ergodic systems

in

this

sense exist,[1]

but

I

assume

it

for the

moment

pending proof

to the

contrary,

and

I

believe

that the

statement in

the

above

equation is

not at variance with the

wave

theory

of

radiation. The

equation

cannot be

applied

to

black-body

radiation, because

black-body

radiation

is

not

ergodic.

But

now

I

must

give

you

a

sharp

rebuke, namely,

that

you

did not

impart to

me

that

your

doctor

had

prescribed you

not to

go

out for

longer

than

a

1/2

hour!

How

much

it

pains

me

to think that

I had led

you

to

commit

a

breach of

the

holiest of

your

laws. So

from

now

on,

I

shall

think

of

it

a

bit

more.

I

shall

keep

the

surviving beneficiary question

in mind

and

report

to

you

about

it

later.[2]

Cordial

regards, yours,

Planck.

480.

To

Felix Klein

[Berlin,]

13 March

1918

Highly

esteemed

Colleague,

It

was

with

great pleasure

that

I

read

your extremely

clear and

elegant

ex-

planations

on

Hilbert’s first

note.[1]

However,

I

do not find

your

remark about

my

formulation of the conservation

laws appropriate.

For

equation

(22)

is

by

no

means an

identity,

no more so

than

(23); only

(24)

is

an identity.

The

conditions

(23) are

the mixed form of

the

field

equations

of

gravitation.

(22)

follows

from

(23)

on

the

basis of

the

identity

(24).[2]

The relations here

are

exactly analogous

to those for nonrelativistic theories.

In

my view,

the formal

importance

of

the

tvo’s

consists

precisely

in

that

they

occur

next

to

the

Evo’s

in

equations (22),

which

are

valid

independently

of

the

choice of

coordinate

system.

Their

physical importance[3]

is not

only

that

they

give,

together

with

the

Tvo’s,

the

conservation

laws,

but

also

that

(23)

permits

an

interpretation

that

is

entirely analogous

to Gauss’s law

div(S

=

p or

ƒ

BndS

=

ƒ

pdr

in

electrostatics. In

the static

case, you

see,

the

number of

“lines

of

force” running

from

a

physical

system

into

infinity

is,

according

to

(23), only

dependent

on

the

3-dimensional

spatial

integrals

fCK

+

Odv,