DOC. 38 ETHER AND RELATIVITY 179
ETHER
AND
RELATIVITY
21
In this
case
the
universe
must of
necessity
be
spatially
unbounded and
of finite
magni-
tude,
its
magnitude
being
determined
by
the value
of
that
mean density.
If
we
consider
the
gravitational
field
and
the
electromagnetic
field from
the
stand-
point
of
the ether
hypothesis, we
find
a re-
markable
difference between
the
two. There
can
be
no
space
nor
any part
of
space
without
gravitational potentials;
for these confer
upon
space
its metrical
qualities,
without
which
it
cannot be
imagined
at all. The
existence
of
the
gravitational
field
is
inseparably
bound
up
with the
existence
of
space.
On
the
other hand
a
part of
space
may very
well
be
imagined
without
an
electromagnetic
field; thus in
contrast
with
the
gravitational
field,
the
electro-
magnetic
field
seems
to be
only secondarily
linked to
the
ether,
the
formal nature of
the
electromagnetic
field
being as
yet
in
no way
determined
by
that
of
gravitational
ether. From the
present
state
of
theory
it
looks
as
if
the
electromagnetic field,
as
opposed
to the
gravitational
field,
rests
upon
an
entirely
new
formal
motif, as
though