What is gravity?

StarChild Question of the Month for February 2001
Question:
Answer:
We don't really know. We can define what it is as a field of
influence, because we know how it operates in the universe. And some
scientists think that it is made up of particles called gravitons
which travel at the speed of light. However, if we are to be honest,
we do not know what gravity "is" in any fundamental way - we only know how it behaves.
Here is what we do know...
Gravity is a force of attraction that exists between any two masses, any
two bodies, any two particles. Gravity is not just the attraction between
objects and the Earth. It is an attraction that exists between all
objects, everywhere in the universe. Sir Isaac Newton (1642 -- 1727) discovered that a force is
required to change the speed or direction of movement of an object. He also
realized that the force called "gravity" must make an apple fall from a
tree, or humans and animals live on the surface of our spinning planet without being flung off.
Furthermore, he deduced that gravity forces exist between all
objects.
Newton's "law" of gravity is a mathematical description of the way bodies are
observed to attract one another, based on many scientific
experiments and observations. The gravitational equation says that the
force of gravity is proportional to the product of the two
masses (m1 and m2), and
inversely proportional to the square of the distance (r) between their centers
of mass. Mathematically speaking,
F=Gm1m2 / r2,
where G is called the Gravitational Constant. It has a value of
6.6726 x 10-11 m3 kg-1 s-2.
The effect of gravity extends from each object out into space in
all directions, and for an infinite distance. However, the strength of the gravitational
force reduces quickly
with distance. Humans are never aware of the Sun's gravity pulling them, because the pull is
so small at the distance between the Earth and Sun. Yet, it is the Sun's gravity that keeps the
Earth in its orbit!
Neither are we aware of the pull of lunar gravity on our bodies, but the Moon's gravity is
responsible for the ocean tides on Earth.
The story of Isaac Newton discovering the laws of gravity by watching
apples falling from a tree is
probably just a myth. He did do his work on gravity while at a farm, but
that is about as much as can be proven.