304
THE
RELATIVITY PRINCIPLE
to E, and
if
the
clocks
and
measuring
rods
we use
for
measuring
the time
and
length
are
identical
with those
used
for the
measurement of
time
and
space
in
nonaccelerated
systems. Thus
the principle
of constancy
of
the
velocity of
light
can
be used
here
too
to
define simultaneity
if
one
restricts
oneself
to
very
short light
paths.
We now
imagine
that the clocks of
E are
adjusted, in the
way
described,
at
that time
t
=
0
of
S at
which
E
is
instantaneously
at rest
relative
to S.
The
totality
of
readings
of the clocks
of
E
adjusted
in
[96]
this
way
is called the "local time"
a
of
the
system
E.
It
is
immediately
evident
that the
physical
meaning
of
the local time
a
is
as
follows. If
one
uses
the local
time
a
for the
temporal
evaluation
of
processes
occurring
in
the individual
space
elements of
E,
then the
laws
obeyed
by
these
processes
cannot depend
on
the position
of
these
space
elements,
i.e.,
on
their
coordi-
nates,
if
not only
the
clocks,
but also the other
measuring
tools
used in
the
[97]
various
space
elements
are
identical.
However,
we
must not
simply
refer
to
the local
time
a
as
the
"time"
of
E, because according
to
the definition
given
above, two point
events
occurring
at
different points of
E are
not
simultaneous
when
their local times
a are
equal.
For
if
at
time
t
=
0
two
clocks
of
E
are
synchronous
with
respect
to
S
and
are
subjected
to
the
same
motions, then
they
remain
forever
synchronous
with respect
to S. However,
for this
reason,
in accordance with
§4,
they do
not
run
synchronously
with
respect to
a
reference
system
S'
instantaneously at rest
relative
to
E
but in
motion relative
to
S,
and
hence
according to
our
definition
they
do
not
run
synchronously
with respect
to
E
either.
We now
define the "time"
r
of
the
system
E as
the
totality
of
those
readings
of the
clock situated
at
the coordinate
origin of
E
which
are,
according to
the
above
definition,
simultaneous with the
events which
are
to
be
temporally
evaluated.1
We
shall
now
determines the relation
between
the time
r
and
the local
time
(j
of
a
point
event.
It follows
from
the first of
equations
(1)
that
1Thus
the
symbol
"r" is
used
here
in
a
different
sense
than
above.