62 DOC.

47

JANUARY

1915

analysis.

If

I

require

that

a

coordinate

system

(p, q)

on

the

plane

be

selected in

such

a

way

that

ds2

=

dp2

+

dq2,

I therefore

assume

that then

the surface

can

be unfolded

on

to

a

[Euclidean]

plane.

Were

I

only

to

demand, however,

that the

coordinates be chosen in such

a

way

that

ds2

=

A(p,

q)dp2

+

B(p,

q)dq2,

i.e.,

that the

coordinates be

orthogonal,

then

I

am

assuming nothing

about the

nature

of the

surface;

this

can

be

obtained

on

any

surface.

You

say

that

regarding

coriolis and

centrifugal

forces

as

“real” field

compo-

nents is

unsatisfactory

because

we

cannot

attribute

any physical

cause

to

their

occurrence.

I would like to

respond

to this with

the

supposition

that

we never can

see

the

stars.

According

to

my

understanding,

these

force fields

are

determined

exclusively by

the

boundary

conditions and

the

field

equations,

if

the

influence

of

the

masses

belonging

to

the

system

under

study

can

be

disregarded

here.

It

is

admittedly

awkward

that the

boundary

conditions

must

be

picked

out suitably

instead

of

being

able

to

assume

that

all

boundary

conditions vanish

into

infinity.

But

are

you

so

sure

that

you

will

manage

with such

simple

boundary

conditions

in

you

view of the world?

Furthermore,

it

must

be considered that,

according

to

my view,

the

multifariousness

of

permissible

coordinate

systems

is

immense,

thus

also

the

multifariousness

of the attached

boundary conditions;

therefore,

if

these

boundary

conditions

appear

artificial in

the

individual

case,

this

does not

lie

in

the substance of

the

theory

but in the fact that

although justified,

the coordinate

system

is not suitably

chosen for

the

description

of

the

case

under

examination.

As

you know,

circumstances

are

similar

even

in normal mechanics. Let

the

world

to

be described

be

the

solar

system,

for

inst.;

then

it

is

clearly

useful

always

to

place

the

origin

of

the

coordinates

at

its

center

of

gravity,

but the

equations

are

obviously

also

valid

for

coordinate

systems

relative

to

which

this

center of

gravity

is

moving uniformly

and

in

a

straight

line. Here

also,

the

choice of

coordinates

is

prescribed

not

by

the

laws

of

nature

but

only

by

the

need for

the

simplest

possible description

of

the

case

at

hand.

Now

once

again

to

the

question

of whether

an

unrelativistic

physics

violates

the

postulate

of sufficient

cause.

You say

sufficient

cause (for preferring

K

over

K',

etc.)

can

be found in

that both

systems

move

in different

ways

relative to

the

ether. I

understand

“cause”

in

this

connection

as an

observable

fact

which

distinguishes

K

from

K'

not

as a

merely conceptual

characteristic.

I

ask

you please

to make allowances for

my

statements contained in

Kultur

der

Gegenwart.”[9]

Although

I

had

3

years

of time to

compose

it,

I

had

completely

forgotten

and

was

reminded of

my

commitment

by

Warburg

one

week

before

the