Source of Gravitational Attraction

## Question:

What actually causes the force of gravity?
## Answer:

Well... The short answer is that I do not exactly know. In the
absence of knowledge I have sometimes found it convenient to
create a little mental image that helps me think about effects
without going too far into causes. Let me make up a story that is
almost certainly not completely true but may contain some true
elements and which may help you think about the nature of
gravity.
The universe we inhabit has three observable dimensions.
Let's call them the left-right, front-back and up-down
dimensions. There is a fourth dimension which is perpendicular to
all three of these called the time dimension. The observable
dimensions have no absolute reference point. We can only measure
relative distances along these. Along the time dimension we seem
to be drifting at a fixed rate measured relative an event that
happened about 16 billion years ago, the so-called big bang.

Now to compensate for our lack of ability to visualize more
than three dimensions, lets imagine a universe in which there is
only one observable dimension, say left-right and the time
dimension running perpendicular to it. The inhabitants of this
dimensionally challenged universe would be aware of the distance
between them but not of their uniform motion along the time
dimension. If this whole two-dimensional space were flat then the
motion along the time dimension would not have any effect on the
distances between objects. Things would just drift along in
parallel lines unless some force was applied to move them closer
together or farther apart.

Suppose however that the whole two-dimensional
universe lay on the surface of a three dimensional sphere rather
than on a plane. Then the curvature of the space would cause the
lines that were parallel when the space was flat, to look like
lines of longitude on the earth. Now the drift along the time
dimension would have an effect on the distance between objects.
The inhabitants would be forced to propose some sort of force
between objects tending to bring them all together.

If then in our universe the four-dimensional space composed of
three observable dimensions and unobservable time dimension were
curved, lying on the five-dimensional equivalent of a sphere,
might we not be tempted to make up a force to account for the
tendency of things to be drawn together as time passes. Perhaps
we could call it gravity.