In the original Star Trek series, Dr. McCoy falls through a time portal in a city "on the edge of forever," and changes the past in a way that erases the Enterprise and her crew, with the exception of Captain Kirk and Mr. Spock, who must return to the past to fix what McCoy has undone. Time travel is a well-worn staple of science fiction writers, but not only does it violate numerous physical laws, there are fundamental problems of consistency and causality. The most prominent is the "grandfather paradox," in which you travel back in time and kill your grandfather before you were born, which means you could not have been born to then travel back in time to kill your grandfather. In Back to the Future, Marty McFly faces a related but opposite dilemma, in which he must arrange for his mother to date his father in order to ensure his conception.
One way around such paradoxes can be found in extremely sophisticated virtual-reality machines (think of a holodeck), programmed to replicate a past time and place in such detail that it is indistinguishable from a real past (which one can never know in full in any case). Another option involves a multiple-universes model of cosmology in which you travel back in time to a different but closely parallel universe to our own, as portrayed in Michael Crichton's novel Timeline, where the characters journey to another universe's medieval Europe without worry of mucking up our own chronology.
Your Past or Someone Else's?
The fundamental shortcoming for both of these time-travel scenarios is that it isn't really your past. A virtual-reality time machine is simply a museum writ large, and transporting to some other universe's past would be like going back and meeting someone like your mother, who marries someone like your father, producing someone like, but not, you--surely a less appealing trip than one in your own time-line.
To make that trip you need the time machine of Caltech's Kip Thorne, who had his interest piqued in time travel when he received a phone call one day from Carl Sagan. Sagan was looking for a way to get the heroine of his novel Contact--Eleanor Arroway (played by Jody Foster in the film version)--to the star Vega, 26 light-years away. The problem Sagan faced, as all science fiction writers do in such situations, is that at the speed of, say, the Voyager spacecraft (the fastest human-made object), it would take about 490,000 years to get to Vega. That's a long time to sit, even if you are in first class with your seat back and tray table down. Thorne's solution, adopted by Sagan, was to send Ellie through a wormhole--a hypothetical space warp similar to a black hole in which you enter the mouth, fall through a short tube in hyperspace that leads to an exit hole somewhere else in the universe. (Think of a tube running through the middle of a basketball--instead of going all the way around the surface of the ball to get to the other side, you tunnel through the middle.) Since, as Einstein showed, space and time are intimately entangled, Thorne theorized that by warping space one might also be warping time, and that by falling through a wormhole in one direction it might be possible to travel backward in time.
Thorne's initial calculations showed that it was theoretically possible for Ellie to travel just one kilometer down the wormhole tunnel and emerge near Vega moments later--not even time for a bag of peanuts. After he published his theory in a technical physics journal in 1988, the media got a hold of the story and branded Thorne as "The Man Who Invented Time Travel." Not one to encourage such sensationalism, Thorne continued his research and by the early 1990s began growing skeptical of his own thesis.
Trouble with Time Machines
Whether it is possible to actually travel through a wormhole without being crushed out of existence, Thorne reasoned, depends on the laws of quantum gravity, which are not fully understood at this point. What he and his colleagues ultimately discovered is that, as Kip told me, "all time machines are likely to self-destruct the moment they are activated." Thorne's colleague Stephen Hawking agreed, only half sardonically calling this conclusion the "chronology protection conjecture," in which "the laws of physics do not allow time machines," thus keeping "the world safe for historians." Besides, Hawking wondered, if time travel were possible, where are all the time tourists from the future?