That is quite a question! Gravity is one of the most familiar forces of
nature, but also one of the most resistant to a proper, complete, and
satisfying explanation. Everyone knew all about gravity on Earth for as
long as people knew about anything, but it took Newton to realize that
this same force is what keeps the Moon in orbit around the Earth and
the planets around the Sun, and was responsible for the tides. Newton's
tremendous achievement, helped by the precise astronomical data and
Kepler's inferences, describes gravity's behavior for most practical
purposes, but fails to answer why there is gravity in the first place;
it merely says that the strength of gravity depends on how much mass
there is and how far away you are from this mass.
Later work on gravity was done by Einstein. Einstein's description
works in more cases than Newton's. For example, a collection of
energetic particles in a box will have a gravitational effect that
depends not only on the mass of the particles but also their energies.
And his model, the "general theory of relativity" (which is a theory of
gravity, space and time) predicts that light will bend in a
gravitational field. Einstein's theory predicts the existence of black
holes (that bend light so much it cannot get out), and is used in
descriptions of the entire universe (cosmology), where Newton's model
would be inadequate. Einstein's theory also predicts that energy can
propagate as waves in the gravitational field, and evidence for this
has been observed from a pair of compact objects rotating around each
other, losing energy at the rate predicted by the loss due to
gravitational waves. Experiments of ever-increasing sensitivity are
being carried out to detect these waves, but no gravitational waves
have yet been observed directly.
The interpretation in Einstein's viewpoint is that there is no
"force" of gravity at all, but rather that space and time are bent in
such a way that a particle moving freely with no other forces on it
will follow a path that bends along with the local geometry of space
and time, and follow the paths that are described by Newtonian gravity
in the limits in which Newton's model applies.
Einstein's (and Newton's) theory have as central features that
mass (and energy in Einstein's model) create the effects of gravity,
but do not explain (as far as I know) why this must be the case, and
why there are not other sources of gravity. For instance, the other
forces depend on "charges" -- like electrical charge, to generate the
fields and react to them. Gravity's "charge" is matter and energy.
Once one supposes that it is in fact the case, plus some other
assumptions, such as the laws of physics must be the same in inertial
frames, and that gravitation is locally indistinguishable from
acceleration, then the description of gravitation is very constrained
to the models we have now.
One problem with all of this is that it is hard to incorporate
gravity into our quantum-mechanical models of the other forces and
unify them. Electricity and magnetism, the strong, and the weak nuclear
forces all seem to have similar quantum structures and understanding
one of them helps us to understand the others. This hasn't been true
with gravity. Quantum theories of gravity predict the existence of
"gravitons" which carry gravity just as photons carry the
electromagnetic force, but no direct evidence for gravitons has been
found. Simple quantum theories of gravity predict nonsense, however.
String theory, a more complicated model, attempts a unified explanation
of gravity along with the other forces, but it needs to be tested --
some prediction of it which is not a prediction of other models must be
verified with an experiment.
All that having been said, we still don't know "why" matter and
energy has gravity, only that it does. Given that as a starting point,
we know lots about gravity already but still not as much as we would
like to, and it continues to be an active area of research.
(published on 10/22/2007)