330
DOC. 42
SPECIAL AND GENERAL RELATIVITY
A
Few
Inferences
from the
General
Principle of
Relativity
87
ativity,
the
general
law
of the
gravitational
field
must
be
sat-
isfied for
all
gravitational
fields obtainable in
this
way.
Even
though by
no means
all
gravitational
fields
can
be
produced
in
this
way,
yet
we
may
entertain the
hope
that
the
general
law
of
gravitation
will
be derivable
from
such
gravitational
fields
of
a
special
kind.
This
hope
has
been realised
in
the
most
beau-
tiful
manner.
But between the clear
vision
of
this
goal
and its
actual realisation
it
was
necessary to surmount
a
serious diffi-
culty,
and
as
this
lies
deep at
the
root
of
things,
I
dare
not
withhold it
from
the reader. We
require to
extend
our
ideas of
the
space-time
continuum
still
farther.