134

DOC.

8

REPLY TO

A

COMMENT BY

M.

ABRAHAM

to

the fact that he had

already given

in his

paper2

the

expression

mc

1-

q2

N

c2

for the

energy

of the

material

point

in

a

gravitational

field; I had, unfortunately,

overlooked this. In

any case,

this result

is

in conflict with the fundamental

equations

of

Abraham's

theory.

For

it

follows from this

expression

for the

energy

that the force

[17] acting

on a

material

point

at rest

in

a

gravitational

field

is

-m

grad c;

but in

[18]

contradiction

to

this, equations

(2)

and

(6)

of Abraham's

paper yield

the

expression

-m c

grad

c

for the

same

quantity.

Abraham further claims that

I

had used his

[19] expressions

for the

energy density

and the

stresses

in

a

gravitational

field. This is

not

so;

for

example,

the

energy density

in

a

static

gravitational

field is

c2/Y

grad2c

[20] according

to

Abraham and

1/2k

grad2c/c

according

to

my

theory.

The

way

in which

c

enters

in the

two theories

is

different.

(Received

on

4

July 1912)

[16]

2

M. Abraham, Physik.

Zeitschr.

13 (1912):

2.