DOC. 42 SPECIAL
AND GENERAL RELATIVITY 321
78
Relativity
transmits this motion
to
the
body
attached
to
it.
The
tension
of
the
rope
is
just large enough to
effect
the
acceleration of the
body.
That
which determines the
magnitude
of the tension
of
the
rope
is
the inertial
mass
of the
body."
Guided
by
this
example,
we
see
that
our
extension of
the
principle
of rela-
tivity implies
the
necessity
of
the
law
of
the
equality
of
inertial
and
gravitational
mass.
Thus
we
have obtained
a
physical
interpretation
of this
law.
From
our
consideration
of
the accelerated chest
we see
that
a
general theory
of
relativity must yield important
results
on
the
laws
of
gravitation.
In
point
of
fact,
the
systematic pursuit
of the
general
idea
of
relativity
has
supplied
the
laws
satisfied
by
the
gravitational
field.
Before
proceeding
farther, however,
I
must
warn
the
reader
against
a
misconception
suggested
by
these considerations. A
gravitational
field
exists for
the
man
in
the
chest,
despite
the fact that
there
was no
such
field for
the
co-ordinate
system
first
chosen. Now
we
might easily suppose
that the
existence
of
a
gravitational
field
is
always
only
an
apparent
one.
We
might
also
think
that,
regardless
of the kind
of
gravitational
field which
may
be
present, we
could
always
choose another
reference-body
such that
no
gravitational
field
exists with reference
to
it.
This
is by
no means
true
for all
gravitational
fields,
but
only
for those
of
quite special
form.
It
is,
for instance,
impossible to
choose
a
body
of
reference such
that,
as
judged
from
it,
the
gravitational
field of the earth
(in
its
entirety)
vanishes.
We
can now
appreciate why
that
argument
is
not
convinc-
ing,
which
we
brought
forward
against
the
general principle
of
relativity at
the
end of Section
18.
It
is
certainly true
that the