374
DOCS.
381,
382
SEPTEMBER
1917
381. To Michele Besso
[Berlin,]
22 September
[1917]
Dear
Michele,
I
am
still
very
well. I
had
only one
little
attack,
which has
passed,
however.[1]
It
is
touching
how worried
you
both
are
about
me.
The
food
is
good,
and
I
am
resting
a
lot.
Today
I
applied
the
quantum theory to
(freely moving) rigid
bodies.
It
is
kind of
Anna
to
concern
herself
about
my wife.[2]
Tete
must return; I cannot
come
up
with the
money;
I have
already
written
to
Miza.[3] I
do
not
believe in
the
new
medical
magic
with
X-rays.
I
am
at
the
point
where
I
only
trust
post
mortem
diagnoses,
nothing else.
Dear
Michele,
now
you
have
to
visit
me
here
soon.
It
is
spacious
and
comfort-
able
enough
in
the
new apartment.[4]
Can’t
you
come
for
a
few weeks at
Christmas
or
Easter?
Get used to
the
idea
gradually!
Warm
regards, yours,
Albert.
382. From Gunnar Nordstrom
Leyden,
22 September 1917
Dear Prof.
Einstein,
Upon
taking up
my
scientific research
again
now,
as
a
happily
married
man,[1]
one
of
my
first tasks
is to
finish
writing
the letter
I
had
drafted
already
a
few
months
ago.[2]
The
matter
does
not
apply
at
all
to
the
big questions
in
your
last
gravitation
paper[3]
but
only
to
some
calculations with
your
formulas from
the
previous paper
(Berl.
Ber.
1916, p. 1114).[4]
When
I began
with
these
matters, I
asked
myself
initially
what
is
meant
by a
system’s
mass.
The
mass
must
naturally
be able to be
expressed by
an
integral
that
just
needs
to
be extended
over
the material
system
but
not
over
those
parts
of
the
space
where all
Tuv’s
are zero.
Without
treating
the
issue
very generally,
I
soon
restricted
myself
to
the
case
of
a
reference
system
in which
the
gravitational
field of
the
system
under
consideration of
bodies
is
stationary.[5]
With
reference to the
mentioned
paper
of
yours,
let
us
initially
assume
that
for
a
very general
field
we
have[6]
-©*
=
0
-
diva,
(1)
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