From a link on Art Bell's website (to Wikipedia) http://en.wikipedia.org/wiki/Time_travel :Cil said:
Time travel
From Wikipedia, the free encyclopedia.
Time travel is the concept of travelling forward and backward to different points in time, much as we do through space. It also includes the concept of traveling sideways between parallel realities or universes.
Unsolved problems in physics:
Is time travel theoretically possible? Is it practically possible? If so, what are we to make of the time travel paradoxes, such as going back in time and killing one's own grandfather, etc.?Humans are in fact always travelling in time in a linear fashion, from the present to the immediate future, inexorably, until death. Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or certain types of motion in space, may allow time travel into the past and future if these geometries or motions are possible. It has been confirmed that the effects of relativistic and gravitational time dilation can cause a traveller who starts at and returns to a point of origin that remains stationary, to arrive at a time farther in the future in that reference frame than their subjective elapsed time would indicate (a constrained form of time travel into the future) [see, time travel interactive link in previous post]
Often it is a plot device used in science fiction and many movies and television shows to set a character in a particular time not their own, and explore the character's interaction with the people and technology of that timeas a kind of culture shock. Other ramifications explored are change and reactions to it, parallel universes, and alternative history where some little event took place or did not take place, but causes large changes in the future.
Famous fictional time machines include the TARDIS from the long-running BBC science fiction television series Doctor Who and the titular time machine of H. G. Wells's novel. On film there were the modified Delorean of the Back to the Future trilogy, the telephone booth of Bill and Ted's Excellent Adventure, the space-time portal of Army of Darkness and the first three Planet of the Apes movies.
Other books, films and series which feature time travel are A Connecticut Yankee in King Arthur's Court by Mark Twain, Timequake by Kurt Vonnegut, Star Trek IV: The Voyage Home, Star Trek: The Next Generation, Star Trek: Deep Space Nine, Star Trek: Voyager, Star Trek: Generations, Star Trek: First Contact, Star Trek: Enterprise, the Terminator series, La Jete, 12 Monkeys, Primer, Futurama, Family Guy, The Simpsons, Quantum Leap, Logan's Run and Sabrina The Teenage Witch.
Fictional time travel even exists in the medium of video games, such as Time Pilot, Chrono Trigger, The Journeyman Project, Blinx, Viewtiful Joe, Prince of Persia and Timesplitters.
In physics, the concept of time travel has been often used to examine the consequences of physical theories such as special relativity, general relativity and quantum mechanics. There is no experimental evidence of time travel, and it is not even well understood whether (let alone how) the current physical theories permit any kind of time travel. However, theories do exist about the possibility of folding time to hop from one point to another.
MD
Post-post: I might have to check out Timequake by Kurt Vonnegut. He's a trip to read!
Einstein, special relativity (1905) and general relativity (1915)?!? 
It's clear I'm no Einstein...