660
DOCS.
628,
629
SEPTEMBER
1918
system
K' that
is
rigidly
connected to U'.
Only
if
systems
K
and
K'
were
both
justified systems
in
the
sense
of
the
special principle
of
relativity
would there be
a
contradiction
with
the
(special) principle
of
relativity;
they
are
not
both
justified,
however,
because
K'
is
not
a
Galilean
(acceleration-free) reference system. Only
when
general
relativity is
taken
as a
basis
are
both
frames of reference
equivalent.
In
this
case,
the
difference in
the
rates of
the
clocks is
explained by
the
combined
effect of
the
influence of
the
velocity
and
the
gravitational potential.
Nowhere
is
a
contradiction
evident
(Berg
and
Petzoldt
are
thus
mistaken).[6]
Finally,
to
the
galvanometer
experiment.[7]
Here
your
bias for
absolute
time
or
for
the instantaneous
signal
is
exposing
itself. Under
no
circumstance
will
a
current
flow
if
the
contacts made
are so
brief
that their duration
is
small
against
the
propagation
time of
light
or
of
an
electrical
wave
between
the
two
contact
points.
The
negligence
of
the
continuously
distributed
capacity,
which determines
the
finite
propagation
velocity
of
the
electrical
waves
along
the
rods,
must
not
be
disregarded here,
in
principle. However, your example
can
be
replaced
with
a
similar
one.
Two rods of
equal length
when observed at rest have
an
orthogonally
projecting
arm
at
both
ends which
are
each
rotatable around
the
rod’s axis. At
the
middle of each rod
is
the
push-button
of
a
signaling
device
that
can
make
the two
arms
rotate in such
a way
that, from
the
rod’s
point
of
view,
both
arms
are
simultaneously
rotatable.
The
paradox
then
occurs
that
“at
the instant
of
meeting”
each
rod
can
prove, by
means
of
its
arms,
that it
is
longer
than the
other. The
paradox disappears
immediately
when
one
considers
that-judging
from
the
point
of
view of
the
other
rod-the
motions
of
a
rod’s
two
arms
are
not
simultaneous.
With
cordial
regards, yours,
Einstein.
Author’s
marginal note: “I
am
keeping
the
manuscript for
the time
being,
so
that
you
can
refer
to
it
again.”
629.
To
Friedrich Adler
[Berlin,
30 September
1918][1]
Dear
Friend,
The
editor
of the
Naturwissenschaften[2]
is
pestering
me
for lack
of
manuscripts,
saying
I
should
give
him
something
to print
in his
journal.
So I
have in mind to
write
an
article in
dialogue
form
on
paradoxes
in
the
theory
of
relativity.[3]
You
have
provided
a
particularly
suggestive
form for
one
paradox
with
your example