Gravity is an always-attractive 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 inverse-square 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 'frame-dragging' exists around rotating massive objects [see Gravity Probe-B]. Other tests focus on the laboratory-scale 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 even-better 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 light-speed. 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 'frame-dragging' phenomenon in X-ray 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.
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