DOC.
21
THEORY OF RELATIVITY
253
automatically repeats
the
same process.
The number of
processes
of this kind that
have
already
taken
place, counting
from
an
arbitrarily
chosen
one,
is
the
temporal
determination of the clock. The
temporal
determination of the clock that
is
simultaneous
with
an
event is
called the time of the
event
as
measured
by
the clock.
Suppose
that
a
clock
U0
is set
up
at
the
origin
of
our
coordinate
system (x
=
y
=
z
=
0),
and that
some
event
takes
place
at
a
location
immediately
adjacent
to
this
origin.
Then-as
everybody
will
admit-experience
teaches
us
that
we are
in
a
position
to
specify
the clock time that
is
simultaneous with the
event,
i.e.,
the time of the
event (referred to
our
clock).
But if the location of the
event is
far
away
from the location
at
which the clock
is set
up,
then
we
cannot
ascertain
directly
the
clock-reading
that is simultaneous with the
event.
For
an
observer
standing
next
to
the clock
cannot
perceive
the
event
directly,
but
only
via
some
intermediary
process (signal)
induced
by
the
event
and
propagated
to
the observer
(e.g.,
via
light
rays).
The observer determines
only
the time of arrival of the
signal
but
not
the time
of the
event.
He could ascertain the latter
only
if he knew the
length
of time
during
which the
signal was en
route.
But it
is
impossible
in
principle
to
ascertain this
length
of time
by
means
of the clock
U0
set
up
at
the
origin
of
K. Only
the times of
events
occurring
in the immediate
vicinity
of
the
clock
can
be
ascertained
directly by
means
of the clock.
If
we
also had
a
clock
(U1)
at
the location
at
which the
event
took place-we
will
assume
right away
that this clock
is
of
exactly
the
same
constitution
as
the other
one-and
if
an
observer stood there who determined the time of
the
event
from this
clock,
even
this would
not
yet
do
us
any good.
For
we are
at
present
unable
to
determine the
temporal reading
of
the
clock
U0
that
is
simultaneous with the
temporal
determination read off the clock
U1.
From this
we see
that for
a
definition of time
a
physical definition
of
simultaneity
is
also needed. If the latter
is
given,
then the
physical
definition of time that
we are
seeking
is
complete.
In other
words,
we
also need
a
rule
by
which
to
regulate
the clock
U1
according
to
the clock
U0.
We
give
this rule
in
the
folowing manner.
Let
some means
be
given
for
sending signals
from the
origin
O
of the
system
K
to
the location E
of
U1
and,
vice
versa,
from E
to
O,
such that the
signal
O-E and the
signal
E-O
are
completely
equivalent
physical processes.
Then
we can
and
will demand that the clocks
U0
and
U1
be
so
regulated
that the
two
signals-measured
by
these
clocks-take
the
same
time. If
t0
is
the
U0
-
time of emission of the
signal
O
-
E
t1
" "
U1 -
"
"
arrival
" " " O
-
E
t1'
" "
U1 -
" "
emission
" "
"
E -
O
t0'
" "
U0
-
" "
arrival
" " "
E
-
O,
then clock
U1
should be
adjusted
such that the condition