DOC.
23
143
1.
If the clock in
B
is
synchronous
with the clock
in
A,
then the clock
in
A
is
synchronous
with the clock in
B.
2.
If the clock in
A
is
synchronous
with the clock
in
B as
well
as
with
the clock in
C,
then the clocks in
B
and
C
are
also
synchronous
relative
to
each
other.
With
the
help
of
some
physical
(thought) experiments,
we
have
thus laid
down
what
is
to be
understood
by
synchronous
clocks
at rest
that
are
situated
at
different
places, and have
obviously
obtained
thereby
a
definition
of
"synchronous" and
of
"time."
The
"time"
of
an
event
is the
reading
obtained
simultaneously
with
the
event from
a
clock
at rest
that is located
at
the
place of
the
event and
that for all time determinations is in
synchrony
with
a
specified
clock
at rest.
Based
on
experience,
we
also
postulate
that the
quantity
2TB
r/
pi5'
is
a
universal
constant
(the
velocity
of light in
empty
space).
It
is essential
that
we
have
defined time
by means
of
clocks
at rest
in
a
system
at rest;
because it
belongs
to
the
system
at rest,
we
designate
the
time just defined
as
"the time of the
system
at
rest."
§2.
On
the
relativity
of
lengths
and
times
The
considerations that follow
are
based
on
the principle
of
relativity
and
the principle of the
constancy
of the
velocity of
light,
two
principles
that
we
define
as
follows: [11]
1. The
laws
governing
the
changes
of
the
state
of
any
physical
system
do
not
depend
on
which
one
of
two
coordinate
systems
in
uniform translational
motion
relative
to
each
other these
changes
of the
state
are
referred
to.
[12]
2. Each
ray
of
light
moves
in the coordinate
system
"at rest" with
the
definite
velocity
V
independent
of whether
this
ray
of light is emitted
by a
body
at rest
or a body
in
motion.
Here,
[13]