DOC.

111

AUGUST

1915 123

and tried

all

sorts

of

possibilities, e.g., required:

The

system

must be chosen

such

that the

equations

rtr=0'*=1-4)

are

satisfied

throughout.[4]

At least

it

seemed definite to

me a

priori

that

a

transformation

group

exceed-

ing

the

Lorentz

group

must

exist,

because those observations summed

up

in

the

words

“relativity principle”

and

“equivalency principle”

point to

it.

The coordinate limitation

that

was finally

introduced deserves particular trust

because it establishes

a

link between it and the

postulate

of the event’s

complete

determination.

A

theoretical

differential

geometric

interpretation of

preferred

systems

would

be of

great

value. The weakest

point

of

the

theory

as

it

stands

today

consists

precisely

in

this,

that the

group

of

justified

transformations

are

by

no means

closely

assessable.[5]

There

is not

even

any

exact

proof

that

arbitrary

motions

can

be

transformed

to motionlessness. This is because

the

difficulties connected

with the

dissimilarity

of

elliptic

&

hyperbolic types

of differential

equations

stand

in

the

way

of

a general

observation. The

equation

3V

ç?V

dx2

dy2

0’

can

be solved

with

arbitrarily

given

boundary

values for

(p

p

given

The

equation

d2p

d2p

dx2

dy2

by

contrast, cannot.

Now,

how does

it

look for the

complicated

transformation

conditions

of

the

gen.

theory

of rel.? I

am

stuck there

like

a

bewildered

ox.

Maybe

we

could

gain

an

overview

of

the

question

if

the

geometric

interpretation

you

are

looking

for

is

found.

Cordial

greetings

and

best

wishes for

progress

in

your

efforts!

Yours,

A.

Einstein.

I

am

leaving

for Switzerland for

about

3 weeks

(26

Aug.

until

about

15 September.

Address

there

for

any

letters: Prof.

H.

Zangger, Berg

St.,

Zurich).