The Sleepwalkers (245 page)

It
is
equally
impossible
to
discover
when
and
under
what
precise
circumstances
the
cornerstone
of
the
theory
was
laid
–
the
Law
of
Gravity,
which
states
that
the
force
of
attraction
is
proportionate
to
the
attracting
masses,
and
diminishes
with
the
square
of
the
distance.
It
had
been
suggested,
but
without
concrete
proof,
as
far
back
as
1645
by
Boulliau.
Perhaps
it
was
derived
by
analogy
from
the
diffusion
of
light
which,
as
Kepler
knew,
also
diminishes
in
intensity
with
the
square
of
distance.
Another
suggestion
is
that
it
was
deduced
from
Kepler's
Third
Law;
Newton
himself
says
that
he
found
the
formula
by
calculating
the
force
required
to
counterbalance
the
moon's
centrifugal
force
–
but
it
does
not
sound
entirely
convincing.

The
details
are
obscure,
but
the
grand
outline
is
dazzlingly
clear.
With
true
sleepwalker's
assurance,
Newton
avoided
the
booby-traps
strewn
over
the
field:
magnetism,
circular
inertia,
Galileo's
tides,
Kepler's
sweeping
brooms,
Descartes'
vortices
–
and
at
the
same
time
knowingly
walked
into
what
looked
like
the
deadliest
trap
of
all:
action-at-a-distance,
ubiquitous,
pervading
the
entire
universe
like
the
presence
of
the
Holy
Ghost.
The
enormity
of
this
step
can
be
vividly
illustrated
by
the
fact
that
a
steel
cable
of
a
thickness
equalling
the
diameter
of
the
earth
would
not
be
strong
enough
to
hold
the
earth
in
its
orbit.
Yet
the
gravitational
force
which
holds
the
earth
in
its
orbit
is
transmitted
from
the
sun
across
93
million
miles
of
space
without
any
material
medium
to
carry
that
force.
^2a
The
paradox
is
further
illustrated
by
Newton's
own
words,
which
I
have
quoted
before,
but
which
bear
repeating:

"It
is
inconceivable,
that
inanimate
brute
matter
should,
without
the
mediation
of
something
else,
which
is
not
material,
operate
upon,
and
affect
other
matter
without
mutual
contact...
And
this
is
one
reason,
why
I
desired
you
would
not
ascribe
innate
gravity
to
me.
That
gravity
should
be
innate,
inherent,
and
essential
to
matter,
so
that
one
body
may
act
upon
another,
at
a
distance
through
a
vacuum,
without
the
mediation
of
anything
else,
by
and
through
which
their
action
and
force
may
be
conveyed
from
one
to
another,
is
to
me
so
great
an
absurdity,
that
I
believe
no
man
who
has
in
philosophical
matters
a
competent
faculty
of
thinking,
can
ever
fall
into
it.
Gravity
must
be
caused
by
an
agent
acting
constantly
according
to
certain
laws;
but
whether
this
agent
be
material
or
immaterial,
I
have
left
to
the
consideration
of
my
readers."

The
"agent"
to
which
he
refers
is
the
interstellar
ether,
which
was
supposed
somehow
to
transmit
the
force
of
gravity.
But
how
this
is
done
remained
unexplained;
and
whether
the
ether
was
something
material
or
not,
remained
an
open
question
–
not
only
in
the
reader's,
but
evidently
also
in
Newton's
mind.
He
sometimes
called
it
a
medium,
but
on
other
occasions
used
the
term
"spirit".
Thus
the
ambiguity
which
we
noted
in
Kepler's
use
of
the
term
"force"
as
a
half
animistic,
half
mechanistic
concept,
is
equally
present
(though
less
explicitly
stated),
in
Newton's
concept
of
gravity.

Another
appalling
difficulty
of
this
concept
was
that
a
universe
filled
with
gravity
ought
to
collapse,
i.e.
all
the
fixed
stars
should
rush
together
and
meet
in
a
kind
of
final,
cosmic
superexplosion.
^*
The
difficulty
was
indeed
unsurmountable,
and
Newton
found
no
other
solution
than
to
assign
to
God
the
function
of
counteracting
gravity
and
keeping
the
stars
in
their
places:

____________________

"And
though
the
matter
were
divided
at
first
into
several
systems,
and
every
system
by
a
divine
power
constituted
like
ours;
yet
would
the
outside
systems
descend
towards
the
middlemost;
so
that
this
frame
of
things
could
not
always
subsist
without
a
divine
power
to
conserve
it..."
³