332 DOC. 42 SPECIAL AND
GENERAL RELATIVITY
Behaviour
of
Clocks
and
Rods
89
in
its
own plane
about
its
centre.
An
observer who
is
sitting
eccentrically on
the
disc
K'
is
sensible of
a
force
which
acts
outwards
in
a
radial direction,
and which would
be
interpreted
as
an
effect of inertia
(centrifugal
force)
by
an
observer who
was
at rest
with
respect to
the
original reference-body K.
But
the observer
on
the disc
may regard
his disc
as a
reference-
body
which
is
"at
rest";
on
the
basis
of the
general principle
of
relativity
he
is justified
in
doing
this.
The
force
acting
on
himself,
and in fact
on
all
other bodies which
are
at rest
rel-
ative
to
the
disc,
he
regards
as
the effect
of
a
gravitational
field.
Nevertheless,
the
space-distribution
of this
gravitational
field
is
of
a
kind that would
not
be
possible
on
Newton's
theory
of
gravitation.1
But since the observer believes in the
general theory
of
relativity,
this
does
not
disturb
him;
he
is
quite
in the
right
when he believes that
a
general
law
of
gravitation
can
be
formulated-a
law
which
not only explains
the
motion of the
stars correctly,
but
also
the
field
of
force
experienced by
himself.
The
observer
performs experiments
on
his circular
disc
with
clocks and
measuring-rods.
In
doing
so,
it
is
his intention
to
arrive
at exact
definitions
for
the
signification
of
time- and
space-data
with reference
to
the circular disc
K',
these defi-
nitions
being
based
on
his observations. What
will
be
his
ex-
perience
in
this
enterprise?
To
start with,
he
places one
of
two
identically
constructed
clocks
at
the
centre
of the
circular
disc,
and the other
on
the
1
The field
disappears at
the
centre
of the
disc
and
increases
proportionally to
the
distance
from
the
centre
as we
proceed
outwards.