DOCS. 108-110 AUGUST
1915 121
Independently
of
this,
I
understand
how
you
define
a
special
coordinate
system
within
a
two-dimensional manifold
by
the
curves
of constant
curvature and those
of
maximal curvature
gradient.
It
is
suspicious,
though,
that
on
structures
with
a
constant
degree
of
curvature,
the
curves (or planes)
of constant curvature
move
infinitely
far
away
from
one
another. The fundamental
dissimilarity
between
both introduced
coordinates
is
also
suspicious.
You
could
try
out, nonetheless,
whether
such
a
thing
can
be done in
a
four-dimensional manifold.
I
did not
understand the
suggestion
for
setting
up
a
gravitation
law,
because
I
cannot read
your writing
on
page 5.
As
you know,
in
my paper
I
showed
that
a
useful
gravitation
law cannot be
generally
covariant.[5] Do
you
not
agree
with
this
consideration?
So
once
again:
I
have
not
the
slightest
intention
of
requiring
“that the
world
be able
to
unwind
upon
itself” and do
not
grasp why you
would
expect
me
to
do
such
an
atrocious
thing.
In
my meaning
there
surely
is
an enormous
variety
of
adapted
systems
that,
however,
do not conform at the
boundary.
With best
regards
to
you, your
wife,
and
your already astonishingly
affable
and avid
writer
of
a son,[6]
and
eagerly awaiting
more
reports,
I
remain
yours,
A.
Einstein.
109. To
Pieter Zeeman
Berlin, 15 August
1915
Highly
esteemed
Colleague,[1]
Your two articles
on
the Fizeau
experiment delighted
me.[2] They
fill
a
previ-
ously
unpleasantly
perceptible gap.
I
can
imagine
how
difficult it
was
to
attain
such
a high degree
of
precision!
Now
the
validity
of Lorentz’s formula has
surely
finally
been
secured.[3]
With
many
thanks,
yours,
A.
Einstein.
110. To Wander
and Geertruida
de
Haas
[Berlin,]
Monday.
[16
August
1915][1]
Dear
Friends,
The furniture
has
now
left
(luckily
and
thank
God).[2]
I
ask
that
you
meet
the
same
in
Deventer,[3] so
that
no
storage
fees
accrue
for
you.
The entire business