256
THE
RELATIVITY
PRINCIPLE
However,
the
totality
of
these clock
readings
does not
yet give
us
the
"time"
as
we
need
it for
physical
purposes.
For
this
we
also need
a
rule
according to
which
these clocks
will be set
relative
to
each
other.
We now
assume
that
the
clocks
can
be
adjusted in such
a
way
that
the
propagation
velocity
of
every
light
ray
in
vacuum-measured
by
means
of
these
clocks-becomes
everywhere equal
to
a
universal
constant
c,
provided
that the
coordinate
system
is
not
accelerated.
If
A
and
B
are
two
points
at rest
relative
to
the coordinate
system, which
are
equipped
with clocks
and
are
separated
by
a
distance
r,
while
tA
is the
reading
of
the clock
in
A
at
the
moment when
a ray
of
light
propagating through
the
vacuum
in the direction
AB
reaches
the
point
A,
and
tB
is the
reading
of the clock
at
B
at
the
moment
the
ray
reaches
B,
then
we
should
always
have
*B
' lA
whatever the
motion
of the light
source
emitting
the light
ray
or
the
motion
of other bodies
may
be.
It is
by no
means
self-evident that the
assumption made
here, which
we
will call "the
principle of
the
constancy
of the velocity of
light,"
is
actually realized
in
nature,
but-at least for
a
coordinate
system
in
a
certain
state
of motion-it is
made
plausible
by
the confirmation
of
the
Lorentz
theory1,
which
is
based
on
the
assumption
of
an
ether that is
absolutely at
rest, through
experiment2.
The aggregate
of
the
readings
of all clocks
synchronized
according
to
the
above,
which
may
be
imagined
as
being arranged
in the individual points
of
space
at rest
with
respect
to
the
coordinate
system,
we
call the
time
belong-
ing to
the coordinate
system
used,
or,
in
short, the time of
that
system.
The
coordinate
system
used, together
with the unit
measuring
rod and the
clocks
that
serve
for the determination
of the time of the
system,
we
call
"reference
system
S."
Suppose
that the
physical
laws
are
ascertained with
[17]
1H.
A.
Lorentz, Versuch einer
Theorie der elektrischen
und optischen
Erscheinungen
in bewegten Körpern [Attempt
at
a
theory
of
electrical
and
optical
phenomena
in
moving
bodies].
Leiden,
1895.
[18]
2It is
of special
relevance that this
theory
furnished the
drag
coefficient
(Fizeau's
experiment)
in accordance
with
experience.