362

DOC. 484 NOVEMBER

1913

484. To

Paul Ehrenfest

[Zurich,

second

half

of

November

1913][1]

Dear

Ehrenfest,

Forgive

me

my ugly

silence.

First of

all, accept my

thanks for

your

congratulations

on

my

Berlinerization.[2]

I accepted

this

odd sinecure because

giving

lectures

gets

on

my

nerves

in such

an

odd

way

&

there

I

do

not

have to

lecture

on

anything.

And

now

to

business.

As

things

stand

now

with

my

appointment,

I

am

not

entitled

to

an

assistant.[3]

But

since I love to work in

collaboration

with others, I will

request

an

assistant. If

I

am

given

one,

I

will

gladly

take either of the

two

gentlemen;

whether Droste

or Fokker,[4]

I

don't

know.

Fokker

is

very

much

to

my liking,

and he

is quite

capable.[5]

One

sees

in

him,

most

of

all,

Lorentz's

excellent

training.

You

assure me

that the

same

is true

of Droste.

So I

don't

know

whom I should

prefer. But,

as

you

see,

the

thing

can

wait,

since it

is not

yet

sure

whether the

position

in

question

is

going

to

materialize.

With

Fokker

I have

already

discovered

something

that

is

as

interesting

as

it

is

curious,

namely

that

mechanics, electrodynamics-applied

to

the

rotating

dipole-and

Jeans's radiation

law

are

not

compatible

with

each

other; rather,

the

quasi-monochro-

matic

Planck oscillator

occupies

a

special

position here.[6]

The calculation that

we

employed

is

absolutely

flawless. This

conflicts with H. A.

Lorentz's

general result.[7] I

wonder

if

we might

not

yet

find

a

handle

for

the

modification

of

the

theory

with

the

help

of

our

methods.

The

questions regarding

the

gravitation

theory

that

were

still

unsettled

in

the

summer

have in

the

meantime been

clarified.[8]

An

unambiguous

determination of the

g^v

from

the

T^v

is

only possible

if

special

coordinate

systems

are

chosen

(can

be

rigorously

proved).[9]

Momentum-energy

conservation

permits

only

linear substitutions. This makes

the

theory completely

satisfactory.[10]

Only one

point

has

not

been

clarified:

Are there

straight

lines that

are

purely temporal (or purely spatial)

along

their entire

extension?

What the

answer

is, yes

or

no,

must follow from

the

equations

for

gravitation.

Unfortunately,

answering

this

question is

difficult from

a

mathematical

point

of

view.

But if

the

answer

turns

out

satisfactorily,

then the

theory

will be

perfectly

secure

from

a

logical

point

of

view.

With best

regards

to

you,

your

wife,

and the

little

ones,

also from

my family,

your

A.

Einstein