592
DOCS.
565,
566
JUNE
1918
which
is
critical
now
to the
relative
quantities of
both
previously equal
bodies.
I
know
very
well
that
Weyl
does
not
recognize
this.
He
would
say,
clocks
and
measuring
rods would have to
appear merely
as
solutions; they
do not
occur
in
the
foundations of
the
theory.[7]
I
find, though:
if
ds,
measured
with
a
clock
(or
a
measuring
rod), is
an
entity
independent
of
the
clock’s
prehistory, design,
and
material
composition,
then
this invariant must
as
such also
play
a
very
funda-
mental
role in
the
theory.
But
if real
natural
processes
were
not like
this,
there
would be
no
spectral
lines and
no
well-defined chemical elements.
Moreover,
for
Weyl
there
is
one
world line of
a
preferred
nature,
the
geodesic.
On
the other
hand,
in nature
there
is
one
type
of
preferred
world
line,
the
trajec-
tories of
electrically uncharged
point
masses.
It
is hardly
conceivable otherwise
than that both
are
identical. The
^’s
now
enter
the
equation
of
this
geodesic
line
as
moving forces,
and this in
a very
dubious
way.[8]
You
can figure
this
out
yourself.
All in
all: I
am
fully
and
firmly
convinced
that this
theory
does not
correspond
to
reality. Formally
as well,
it
appears
unsatisfactory
to
me
that the
angle
be
regarded
as
invariant to
displacement,
but
not
the
unit of
length.-[9]
I
was
very pleased
that
you
are no
longer
so
militarily
minded.[10] I
hope you
will
advance
well
in
your
career.
With
your downright
talent and
your
love
of
the
subject, I
should
think
so.
Accept my
warm
regards, yours,
Einstein.
In
any
case,
I
share
Weyl’s
conviction that it
must
be
possible
to
link
gravitation
and
electricity
to
a
unity;
I just
think that the
correct
link has
not
yet
been
found.[11]
566. From
Felix
Klein
Göttingen, 16
June
1918
Esteemed
Colleague,
I
discussed
your May
communication
now
variously
with
Hilbert
and
partic-
ularly
with
Runge[1]
but,
much
as
I
understand
your general
train of
thought, I
cannot
persuade
myself
of
its correctness. In
particular, I
do
not
see
how in
the
formula
Al
=
E
dx'4
dxß
dxn
dx,
Aa
Aß
you
can
replace
Aßa
with
Jß
.
Axa.[2]
So
for
the
time
being
probably nothing
will
come
of
a
lecture
by
me on
the
subject, especially
since in
the
coming
weeks
I