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.