276 DOC. 24 RESPONSE TO

QUESTION

BY REIßNER

Doc. 24

Supplementary Response

to

a

Question

by

Mr. Reißner

by

A. Einstein

[Physikalische Zeitschrift

15 (1914):

108-110]

Inexplicably,

I

totally

misunderstood and

incorrectly

answered

a

question

that Mr.

[2]

Reißner1

addressed

to

me

in

the discussion

following my gravitation

lecture,

even

though

the

question

was

clearly posed.

First

I

shall

repeat

the

question:2

"Mr.

Einstein

spoke

about the

deflecting

influences

of

the

gravitational

field

on

[4]

the oscillation

energy

of the

light

ray.

May

I

now

ask Mr. Einstein to

say

also

something

about

...

the effect of the

gravitational

field

on

its

own

static field

energy?

As

Mr. Einstein has

shown,

in his nonlinear Einstein

equation

for the

potential,

the extension

of

Laplace's equation,

one

term

can

be

interpreted as

the

gravitational

effect of the static field

energy.

How

can

it be made

plausible,

or

how does it

come

out

mathematically,

that

even though

the static

energy

of

the

purely gravitational

field

possesses

inertia and

gravity,

it does

not

possess

the further attributes

of

ponderable

mass,

those

of

displaying ponderomotive

forces?

Or,

in other

words,

how does it

come

to

be that the field

remains

a

static

one,

even

though

a

field

energy

of

empty

space

underlies the

gravitation?

How would

one

characterize the

special

kind of

energy

that is

peculiar

to

ponderable mass,

in

contrast to

other forms

of

energy?"

First of

all,

let

me

recall that it

absolutely

must

be demanded that

matter

and

energy

taken

together satisfy

the conservation laws of

momentum

and

energy.

This

amounts to

demanding

the existence

of

an

equation

of

the

form

(9b),

i.e.,

of the form

[5]

£^-($ov+tov)

=

0

(9b)

oxv

For if

one assumes

that

a

system

is

finitely

extended

as regards

matter

and the

gravitational

field,

then

one

obtains from

(9b),

if

one

integrates

over

the entire

(three–

dimensional) space

occupied by

the

system,

the four

equations

d

{J($o4

+

to4)dV\

=

0

dt

i.e., equations having

the

customary

form of the conservation laws. There

can

hardly

[1]

1This

Jour.

14

(1913):

1265.

[3]

2I

omitted the footnote because the

question

is

sufficiently clearly

fixed

by

what

is

repeated

here.