Sunday, March 11, 2007

What is gravity?



Gravity, the odd force out when it comes to small particles and the energy that holds them together. When Einstein improved on Newton's theory, he extended the concept of gravity by taking into account both extremely large gravitational fields and objects moving at velocities close to the speed of light. These extensions lead to the famous concepts of relativity and space-time. But Einstein's theories do not pay any attention to quantum mechanics, the realm of the extremely small, because gravitational forces are negligible at small scales, and discrete packets of gravity, unlike discrete packets of energy that hold atoms together, have never been experimentally observed.

Nonetheless, there are extreme conditions in nature in which gravity is compelled to get up close and personal with the small stuff. For example, near the heart of a black hole, where huge amounts of matter are squeezed into quantum spaces, gravitational forces become very powerful at tiny distances. The same must have been true in the dense primordial universe around the time of the Big Bang.

Physicist Stephen Hawking identified a specific problem about black holes that requires a bridging of quantum mechanics and gravity before we can have a unified theory of anything. According to Hawking, the assertion that nothing, even light, can escape from a black hole is not strictly true. Weak thermal energy does radiate from around black holes. Hawking theorized that this energy is born when particle-antiparticle pairs materialize from the vacuum in the vicinity of a black hole. Before the matter-antimatter particles can recombine and annihilate each other, one that may be slightly closer to the black hole will be sucked in, while the other that is slightly farther away escapes as heat. This release does not connect in any obvious way to the states of matter and energy that were earlier sucked into that black hole and therefore violates a law of quantum physics stipulating that all events must be traceable to previous events. New theories may be needed to explain this problem.

DISCOVER, <http://discovermagazine.com/2002/feb/cover>

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