DOC. 18 DISCUSSION
OF
DOC. 16
225
immediately
evident. Einstein denotes
by k
the
gravitational
constant that
I
named
y;
the
quantity
that
I
write
wuv
he calls
guv/2k;
the
quantities
(Kuv,
iwuv)
enter
in
his
equations
as
linear functions of the
(guv, iuuv),
the coefficient of which
depend
on wuv
(i.e.,
on
the
guv).
huv
is
a
tensor
representing
the
density
of the
gravitational
mass.
If
the
quantities
huv were
identical with the
components
of the
stress-energy
tensor,
then the inertial
mass
and the
gravitational
mass
would
really
be identical in
principle.
But that is
by no means
the
case;
the
huv are
different from the
compo-
nents
of the
stress-energy
tensor,
and therefore the
gravitational
mass
will
not
be
equal
to
the
energy
content
of the
body,
but, instead,
the
ratio
of
the two
masses
depends
also
on
other
quantities, e.g., velocity,
and
temperature.
Einstein also
mentions that the ratio
of the
gravitational
to
the inertial
mass
depends
on
the
gravitational potential.
Since the individual
atoms
of
a body are
surrounded
by
gravitational
fields,
the ratio
of
the two
masses
of
a
body
must
depend
on
its
density
as
well.
Einstein: The
reason why
I
did
not
speak
about the
theory
of
Mr.
Mie
is
because
the
equivalence
of
the inertial and the
gravitational mass
has
not
been
rigorously
carried
through
in
it.
It would have
been
illogical
of
me
to start
out from certain
[6]
postulates
and then
not to
adhere
to
them.
I
admit that
I
have
not
read Mie's
theory
as
attentively
as perhaps
would have been
good,
but
I
did
not
have the
slightest
intention
of
disparaging
Mie's
theory by
not
mentioning
it
in
this
context.
As far
as
Nordström's
theory
is concerned,
I
cannot
say
that Abraham
was
the first
to
adopt
the
course
that has been
adopted by
Nordström. For Abraham's
theory
is
based
on
the
premise
that the
velocity
of
light
is
variable,
that it is
to
some
extent
a measure
of
the
gravitational potential.
He
uses
the form
of
the
ordinary theory
of
relativity
in
spite
of
this,
so
that he finds himself in
a
contradictory, hybrid position.
This
objection
is
so
weighty
that the whole
theory
looks
to
me
totally
untenable.
[7]
Mie: I
consider these
objections justified;
but if
one
has the
equations
of
Abraham's
theory
it is
not
so
hard
to
arrive
at
Nordström's
theory;
as
far
as
I
know,
Nordström started
out
directly
from Abraham's
equations.
Einstein:
Yes,
psychologically
it
is
really so,
but
not
logically;
because
Nordström's
theory
differs
fundamentally
from that
of
Abraham.
Mie: I
will
shortly publish
a
paper
in which
I
show that Einstein's
theory
also
does
not
satisfy
the
postulate
of
the
equality
of
the
inertial and the
gravitational
mass
absolutely exactly.
[8]
Now
I
would like to
bring up
a
second
objection,
and
I
believe
that,
in
doing
so,
I
am speaking
in behalf of the entire audience. In his
work,
Mr. Einstein has
postulated
the
very interesting,
or so
it
seems
to
me, principle
of
a
general relativity.
To be
sure,
this
principle
is not
yet
satisfied in the
present theory,
but it should be
of interest all the
same
to discuss it
per
se
for
once.
It has
not
become clear
to
me