318 DOC.

328

APRIL

1917

328. To

Felix Klein

Berlin,

21 April

1917

Highly

esteemed

Colleague,

If

presently

I

weren’t

the

slave of

a

book taken out of

the

library,

I

would

have

already

looked at

the

papers you

mentioned,

not

only your

own

but the

other

two

as

well.[1]

Thus,

however,

I must

allow

another

couple

of

weeks

to

pass.

Grossmann

(I believe)

had

the little

book

by Wright

when

we were working

to-

gether

on

relativity

4

years

ago.[2]

I have

already

had Batman’s

paper

in

hand but

must confess

that

I

cannot

quite

imagine

it

possible

that

any physical meaning

be

afforded to

substitutions

of

reciprocal

radii.[3]

Not

only

does the

Lorentz trans-

formation

leave

the

velocity

of

light invariant,

but

the coordinates of coordinate

systems

related

in such

a

way

have

a

simple physical meaning

as

measurement

results

from

measuring

rods and

clocks.

This latter

quality

is certainly

lost in

Bateman’s transformations.[4]

I

am

very

much

looking

forward to

your lectures,

which

you

have

promised

me.[5]

I

shall

pass

them

on

to Sommerfeld

as soon as

I

have received and

studied

them.

Probably-knowing

me-nothing

will

come

of

my

article for

the

encyclo-

pedia.

I

was

cautious

enough,

if

I

recall

correctly,

not

yet

to

give a

positive

answer

to

our

colleague

Sommerfeld.[6]

Hilbert’s second

relativistic

paper

interested

me

very

much.[7]

Just the

geodet-

ic reference

system

introduced

there,

which

is characterized

by

g14 = g24 = g34

=

0

and

g44

=

1,

does not

seem

acceptable

to

me.

For

it

can

be

demonstrated

through

examples

that

this

system

does

not

preserve

the

uniqueness

of the

assignment

of

coordinates

to

space-time points.

This

stems

from

the

temporal

geodetic

lines

intersecting

one

another.

However,

it would not be inconceivable

that

otherwise

definable coordinates

existed,

which would

simplify

the

relations.

Although

the

coordinate

choice

I

made

in

some

papers according

to

the

condition

g

=

-1

is

natural

in

some

cases,

it limits

the

choice

just

a

little

(only

one

limiting

condition

rather than

four).[8]

With best

regards,

I

am

yours truly,

A. Einstein.