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.