104 DOC. 25 WHAT IS THE THEORY OF RELATIVITY
WHAT
IS THE
THEORY
OF
RELATIVITY?
231
by
the
same
constant (equality
of
inertial and
gravitational
mass).
Imagine a
coordinate
system
which
is rotating uniformly
with
respect
to
an
inertial
system
in
the Newtonian
manner.
The
centrifugal
forces which manifest themselves
in relation
to
this
system
must,
according to
Newton’s
teaching,
be
regarded
as
effects
of
inertia.
But these
centrifugal
forces
are,
exactly
like
[6]
the forces of
gravity,
proportional
to
the
masses
of the bodies.
Ought
it
not to
be
possible
in this
case
to
regard
the coordinate
system
as
stationary
and
the
centrifugal
forces
as
gravitational
forces?
This
seems
the obvious
view,
but
classical mechanics
forbid it.
This
hasty
consideration
suggests
that
a
general theory
of
relativity
must
supply
the
laws
of
gravitation,
and
the
con-
sistent
following
up
of the idea has
justified our hopes.
But the
path
was
thornier
than
one
might
suppose,
because
it
demanded the
abandonment of
Euclidean
geometry.
This
is
to
say,
the
laws
according
to
which solid bodies
may
be
arranged
in
space
do
not completely
accord
with
the
spatial
laws
attrib-
uted
to
bodies
by
Euclidean
geometry.
This
is
what
we mean
when
we
talk
of the
“curvature of
space.”
The
fundamental
concepts
of the
“straight line,”
the
“plane,”
etc.,
thereby
lose
their
precise significance
in
physics.
In the
general theory
of
relativity
the
doctrine
of
space
and
time,
or
kinematics,
no
longer figures
as a
fundamental inde-
pendent
of the
rest
of
physics.
The
geometrical
behavior
of
bodies and the motion of clocks
rather
depend
on
gravitational
fields,
which
in their
turn
are
produced
by
matter.
The
new theory
of
gravitation diverges considerably,
as re-
gards principles,
from Newton’s
theory.
But its
practical
results
agree so
nearly
with those of Newton’s
theory
that it
is
difficult
to
find
criteria
for
distinguishing
them
which
are
accessible
to
experience.
Such have been discovered
so
far:
1.
In the revolution of the
ellipses
of the
planetary
orbits
round the
sun (confirmed
in the
case
of
Mercury).
2.
In
the
curving
of
light
rays by
the
action of
gravitational
fields
(confirmed
by
the
English photographs
of
eclipses).