DOCS.
363,
364
JULY
1917 353
363. To
Willem
de Sitter
Arosa, Saturday.
[22
July
1917]
Dear
Colleague,
Your
interesting
letter
reached
me
with
some delay,
because
I
was traveling
around
in Switzerland before
your
letter
could be forwarded
to
me
from Lucerne.
Your
new
expression
for
the
line
element[1]
ds2
=
-dr2
-
R2
sin2
\dip2
+
sin2
¡/aM2]
+
cos2
j;dt2
is
very
instructive.[2]
Spatially,
all
the
points
in
your
world
are
equivalent,[3]
but
the
rate of
a
standard
clock at rest
equals
ds
r
dt=C°SR’
hence, depends
on
position
and reaches
the
value
zero
at
an “equator.”[4]
This
is
a
position
of minimal
gravitational potential
&
of
vanishing light
vel.
Such
a
singularity
(g44
=
0)
in
a
finite world
obviously
is,
in
my
opinion,
to
be
discounted
as physically
out of the
question.
Masses would have
the
tendency
to
aggregate
at
the
“equator.”[5]
The
total
energy
of
g44
dx4/ds
of
a
mass-point
would
vanish at
the
“equator”!
Don’t
you
also have
the
feeling
that,
in
reality,
such
cases
do not
come
into
consideration?
Nevertheless, I
cannot
give a
precise, general
formulation
of
what,
in
my opinion,
must
be viewed
as
ruled
out.-[6]
My sojourn
in
Switzerland
is
physically
and
mentally
very
reinvigorating.
I
am
staying
until
mid-August.
If
you
inform
me,
at 16A
Bramberg
St., Lucerne,
of
how
you
now
consider
the
matter, I
would be
very pleased.
With
cordial
regards,
I
am
yours,
A.
Einstein.
364. From Franz
Selety[1]
Vienna
I,
11
Zedlitz
Alley,
23 July 1917
Esteemed
Professor,
It
may
not
seem
of importance to
you
if
I
inform
you
herewith
that
I
had
already corresponded
with Professor
Philipp
Frank
three
years
ago
in
the
spring