DOCS. 499-501 APRIL
1918
523
your
invariant
of
“weight” zero
also
is
very
striking.[4]
Except
for
the
agreement
with
reality,
it
is
in
any
case a
grand
intellectual achievement.
With
cordial
regards,
yours,
Einstein.
500.
To
Felix Klein
[Berlin,
10 April
1918]
Esteemed
Colleague,
I
have received
your
wonderful
expositions[1]
and
am
studying
them
diligently.
When
I
am
finished
with
them,
I shall
give
them to Sommerfeld. Levi-Civita
and
Weyl
have offered
a
new, very
intuitive
interpretation of
the
Riemann curvature
tensor,[2]
Weyl
additionally
a
very ingenious theory through
which he
attempts
to
merge
gravitation
and
electromagnetism
into
a
unified
whole.[3] I
am presenting
his
paper[4]
tomorrow to
the
Academy.
With
respectful greetings, yours truly,
A.
Einstein.
501. From Willem de Sitter
Leyden, 10 April
1918
Dear
Einstein,
I just
received
your
paper
“Critical Remarks
on a
Solution Offered
by
de
S[itter].”[1]
You demand
“that
equations
(1)
be valid
for
all
points
within the
finite
realm.”[2]
Formulated
this
way,
it
is
a
philosophical requirement.
To make
it
into
a physical
one,
one
must
say
“all
physically
accessible
points.”
The surface
r
=
1/2nR,
however,
is
physically inaccessible,
as
I
have shown in
M. N.
LXXVIII,
pages
17-18,[3]
and
my
solution therefore satisfies
the
physical requirement,
but
not
the
philosophical one.[4]
You
naturally
have
the
right
to
lay
down
the
philosophical
condition and
thus
to
reject my
solution.
I, on
the
other
hand,
also have
the
right
to
reject
the
philosophical requirement,
but
not the
physical
one.
Whether
my
solution
really
can
be
interpreted
as a
world in
which
all matter
is
concentrated
on