
Gravity Basics
Gravity
is an alwaysattractive force that acts between particles
of matter. (It can also act between quantities of energy!)
More technically, it has an infinite range through space,
and weakens as the inversesquare of the distance between
bodies.[MORE]
All theoretical and observational studies are completely consistent
with the idea that it travels no faster than the speed of
light  and no slower. [MORE]
Many physicists are firmly convinced that gravity is a cousin
to the other forces in Nature that we know about including
electromagnetism. The exact, mathematical, way to show this
unity  called Superstring Theory  remains experimentally
untested. According to some skeptical physicists, this theory
may be permanently untestable. [MORE]
It
doesn't take a rocket scientist to remind you that humans
have always known about it! Its first mathematical description
as a 'universal' force was by Sir Isaac Newton in 1666. Newton's
description remained unchanged until Albert Einstein published
his General Theory of Relativity in 1915. Ninety years later,
physicists, such as Edward Witten, Steven Hawkings, Brian
Greene and Lee Smolin among others, are finding ways to improve
our description of it to accommodate the strange rules of
quantum mechanics. Ironically, although gravity is produced
by matter, General Relativity does not really describe matter
in any detail  certainly not with the detail of the modern
quantum theory of atomic structure.
Perhaps
the most unusual thing about gravity we know about is that,
unlike the other forces of nature, gravity is intimately related
to space and time. In fact, space and time are viewed by physicists,
and the mathematics of relativity theory, as qualities of
the gravitational field of the cosmos that have no independent
existence. Gravity does not exist like the frosting on a cake,
embedded in some larger arena of space and time. Instead,
the 'frosting' is everything, and matter is embedded and intimately
and indivisibly connected to it. If you could turn off gravity,
it is mathematically predicted that space and time would also
vanish![MORE]
Einstein's
theory of General Relativity, published in 1915, is our most
detailed mathematical theory for how gravity work,
With it, astronomers and physicists have explored the
origin and evolution of the universe, its future destiny,
and the mysterious landscape of black holes and neutron stars.
General Relativity has survived many different tests, and
it has made many predictions which have been confirmed. So
far, after 90 years of detailed study, no error has yet been
discovered in Einstein's original, simple theory. Currently,
physicists are exploring two of its most fundamental predictions:
The first is that gravity waves exist and behave as the theory
predicts [See LISA
and LIGO]; the
second is that a phenomenon called 'framedragging' exists
around rotating massive objects [see Gravity
ProbeB]. Other tests focus on the laboratoryscale measurements
of the force of gravity to look for signs of 'extra dimensions'.
Data from any of these experimental studies will greatly improve
our understanding of gravity, and will show us how to go beyond
the mathematics of General Relativity to create an evenbetter
theory.
Theoretically,
gravity waves must exist in order for Einstein's theory to
be correct. They are distortions in the curvature of spacetime
caused by accelerating bodies, just as radio waves are produced
by accelerating charged particles. Gravity waves carry energy
and travel at lightspeed. Observationally, they have not
been directly detected yet. But indirectly,
astronomical bodies such as orbiting pulsars have been discovered
that are losing energy by gravity waves at exactly the predicted
rates. [MORE]
Astronomers
have also detected the 'framedragging' phenomenon in Xray
studies of distant black holes. As a black hole (or any other
body) rotates, it actually 'drags' space around with it. This
means that you cannot have stable orbits around a rotating
body, which is something totally unexpected in Newton's theory
of gravity.[MORE].
