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