DOCS.
179,
180
JANUARY
1916 173
179. To Paul Ehrenfest
[Berlin,]
3 January 1916
Dear
Ehrenfest,
An
addition
is
still needed to
my replies
to
your
comments.[1]
Namely,
it
is
conceivable
that
your
four relations
(IV)
contradict the
field
equations,
that
is,
do
not
agree
with them.
That
is
why
it
is of
interest to
see
that
your
four
equations
be satisfied
identically.
This
can
be
most
easily
understood with
reference to
the
calculations
developed
in
my
papers as
follows.
(In
the
citations,
the
paper
“On
the
gen.
theory
of
rel.” is
denoted
as A,
and the
pap[er]
“The
field
eqs.
of
gravitation” as
B.)[2]
If
instead
of
(7)B,
the
equation[3]
Edt/dx
=
EK^+U,
(U.
* 0)...
(7a)
is
taken
as a
basis,
then
(9)B
is
valid
regardless
of this.[4]
On
the
other
hand,
from
the method indicated
in
the
papers,[5]
d
dxß
'
ß2gaß
^
dxadxß

k(T
I1)
2kU"
results
in
contradiction
to
(9)B.
Therefore
(7a)B
is
a
necessary consequence
of
(6)B.[6]
That
is
why
nothing
can
be inferred from
the
combination of
both
these
systems
that
is not already
contained in
(6)B.
From
this
I
conclude
that
your equation (IV)
must
be satisfied
identically;
for in
any
other
way
it
is
not conceivable
that
(IV)
contains
no new
conditions to
(6)B.[7]
With best
regards, yours,
Einstein.
180. To
Paul Ehrenfest
[Berlin,]
Wednesday
[5
January
1916][1]
Dear
Ehrenfest,
The
great
interest
you[2]
are showing
in
my
theory
delights me.
Were
the
trip not
so
difficult
now,
I
would
come
immediately, despite
lecture
course[3]
and
Academy.
But it
seems
that the
border
is
closed
a
lot, causing people
to
turn back