708 DOCUMENT 496 MARCH 1918

[13]Herglotz

1916.

[14]tiK is Einstein’s

gravitational energy-momentum pseudo-tensor.

[15]See

Kottler

1918,

p.

431,

fn.

1.

[16]Kottler’s

assumption

is

correct.

See Lorentz 1916d and Levi-Civita

1917b;

see

Doc.

368,

note

6,

for

further discussion.

[17]As

was

shown,

e.g.,

in

Schrödinger

1918a

(see

Doc.

393, note 5,

for further

discussion).

[18]In

Einstein 1918a

(Vol.

7,

Doc.

1), p.

167,

it

is

pointed

out that it would be

compatible

with

a

law

of

energy-momentum

conservation

of

the form

GiK

+ kTik

=

0 that

a

material

system

would

simply cease

to

exist

(see

Doc. 368

for

a

similar

argument).

[19]See

Kottler 1916b.

[20]In

Einstein

1916p (Vol. 6,

Doc.

40),

Einstein’s

reply

to

Kottler

1916b,

Kottler’s

statement

that

gravitation

has

a

kinematical

character

is

interpreted as meaning

that

any gravitational

field

can

be

transformed

away

by

a

suitable coordinate transformation. In Kottler

1918,

p.

407,

fn.

1,

the

author

denies that this is what he meant.

[21]The

argument

in this and in the

following paragraphs

is

developed

in

more

detail in

secs.

23-

26

of

Kottler

1918,

in which the exterior and interior fields

of

an

isolated

homogeneous sphere are

calculated.

[22]In sec.

25

of

his

paper,

Kottler makes the

assumption

that the

energy density

of

an

isolated ho-

mogeneous sphere

vanishes and shows that it leads

to

an

interior line element

for

which the relation

g11g44

=

c2

holds.

(This

is the

generalization

for

polar

coordinates

of

the condition

g

=

-c2.)

The

fact that this relation is

satisfied,

Kottler

argues,

makes his solution

preferable

to

other

ones (such as

Schwarzschild’s).

[23]According

to Kottler,

his

assumption

of

vanishing energy density

is

a

"known

postulate" ("be-

kanntes

Postulat,"

Kottler

1918,

p.

439)

of

relativity theory,

because

an

isolated

sphere experiences

no

forces,

so

that its inertia and thus its

mass

vanish.

[24]Stability

of

the

ether

is discussed in

sec.

26

of

the

paper.

[25]In sec.

31

of

the

paper,

Kottler

gives a lengthy quotation

from

Poincaré

1902,

in which Henri

Poincaré discusses

a

situation in which stars

are

invisible and Earth’s inhabitants

assume

that the

Earth is at

rest.

Poincaré

proceeds

to

describe the difficulties that the

description

of

the

dynamical

ef-

fects

of

rotation would

cause-for

instance,

asymmetry

of

space

between left and right-and

predicts

that this will

eventually

lead

to

the conclusion that the earth

rotates.

See also Einstein’s

response

in

Doc. 493

to

a

similar

argument by

Gustav Mie.

[26]Kottler

and Einstein had

met at

the 85th annual

meeting

of

the Gesellschaft Deutscher Natur-

forscher

und Ärzte in

September

1913.

[27]Presumably

the

apparatus

for

producing

and

using

artificial seismic

waves,

developed by

the

geophysicist Ludger Mintrop

(1880-1956).

496. From Romeo Wankmuller

Berlin,

den 30. III. 1918.

Herrn Prof. Einstein

Berlin,

Haberlandstrasse

5.

Sehr verehrter

Herr

Professor!

Einliegend

überreiche ich Ihnen ein

Programm

der ordentlichen

Mitglieder-Ver-

sammlung

der

wissenschaftlichen

Gesellschaft,

für

Luftfahrt,

in

Hamburg, vom

16.-18.

April cr. zur gefl.

Kenntnisnahme.[1]

Herr

Maschke[2]

und ich haben die Absicht den

Vorträgen

dort

beizuwohnen,

und erlaube ich

mir

die

Anfrage,

ob Ihr

Gesundheitszustand

es

erlaubt,

dass Sie

ebenfalls den

Vorträgen

beiwohnen. Selbstverständlich würde meine Firma die

Ko—