228

DOC.

18

DISCUSSION OF DOC.

16

to

about

10-11

so

that,

for

example,

in measurements

with

a pendulum one

would

have to

measure

the

length

of

the

pendulum

with

an

accuracy

of

up

to

a

fraction of

the diameter of

an

atom

in order

to

find it.

My theory

can

therefore

not

be refuted

on

the basis of Eötvös's

or

similar

experiments.

Einstein: But that

was

also

not

my

intention. But the fact that the

identity

of

the

inertial and the

gravitational mass

has

proved

correct

with such

a

remarkable

accuracy seems

to

me

to

be

one

of the

most

important pointers

for the

development

of the

theory.

The need

to

find

a more profound

explanation

for that

identity,

and

besides that also the view

concerning

the

relativity

of inertia advanced

by

Mach,

was

actually

the motive that

compelled

me

to

devote

myself

to

the

problem

of

gravitation.

It is therefore

understandable that theories that do not

correspond

to

my starting

premises

are

far from

my

mind. But

by

no

means

do

I

claim that these theories must

be

rejected

on

account

of the

current state

of

our

empirical knowledge.

[18]

Reißner:

Mr.

Einstein has

spoken

about the

deflecting

effects of the

gravitational

field

on

the

oscillation

energy

of

the

light

ray.

May

I

now

ask Mr. Einstein

to

express

his view about

a more

elementary question, namely

the effect

of

the

gravitational

field

on

its

own

static field

energy.

As Mr.

Einstein has

shown,

one

term

in his nonlinear

equation

for the

potential,

i.e.,

the extended La Place

equation,

can

be

interpreted

as

the

gravitational

effect of

the static field

energy.

How

can

it

then still be made

plausible,

i.e.,

how does it

come

out

mathematically,

that

even

though

the static

energy

of

the

pure gravitational

field

possesses

inertia and

gravitation,

it

does

not

possess

the further attributes of

ponderable mass, i.e.,

it

does

not

manifest

ponderomotive

forces and motions?

Or,

in other

words,

how does it

come

out

that the field remains

a

static

one even

though

the field

energy

of

empty space

is

subjected

to

gravitation?

How should

one

characterize the

special

kind of

energy

that,

in

contrast to

other forms of

energy,

is

specific

to

the

ponderable

mass?

Einstein

(subsequently improved answer):

In

the absence

of

a

gravitational

field,

the

stress

components

of

an

electrostatic field remain in

equilibrium.

This

equilibrium

will be

somewhat

modified,

but

not abolished,

by

the existence of

a

gravitational

field.

Comparison:

The

parts

of

a gas

enclosed

in

a

container

are kept

in

equilibrium

through

the

gas pressure.

If the effect

of

a

gravitational

field is

added,

this

[19] equilibrium

will be modified but will

not

be abolished.1

1Mr.

Reißner

subsequently

remarked

concerning

Mr. Einstein's

answer:

Mr. Einstein

responded

to

my question as

if

it

expressed

doubt

about

whether

or

not,

in his

theory,

the

energy

of

empty space

reached

a

state

of

equilibrium.

Such

a

doubt

was

the farthest

thing

from

my

mind. But since he otherwise did

not

deal with

my question,

permit me

to

attempt, once again,

to

pose

the

question more clearly

and to ask

him

for

an

answer.

In

doing

so, I

will start from Mr. Einstein's

comparison

with the

equilibrium

of

a