Time travel is the fictional ability to "jump" from one point in time to another rather than simply progressing forward through time in the normal manner. The ability to move backward through time, visiting a previous point in history, is the most blatant expression of this phenomenon, although the ability to jump into the future is also common.
Time travel introduces many potential plot holes into a story. For instance, it opens the question of whether a time-traveler can alter history. The classic example of the problem is the "Grandfather Paradox": if you travel back in time and kill your grandfather before he conceives your father, thus preventing your own birth, how can you exist to go back in time?
While some time travel stories treat history as immutable -- the time-traveler failing to make the desired change in history or even becoming the cause of the event s/he wanted to prevent -- others allow alterations in history, sometimes resulting in parallel universes.
Time Travel in Star Trek
Quite possibly the most overused plot device in Star Trek, time travel can be accomplished by a variety of exotic means, but the one method that is consistently available to the Federation is the so-called "slingshot effect" (discovered in TOS "The Naked Time"). To accomplish this feat, a starship accelerates toward a star (or possibly a planet) at warp speed, using the star's gravity to provide acceleration greater than the ship's engines could normally provide. The ship "cracks the whip" at the closest point to the star in its course, briefly exceeding Warp 10 and initiating a time-travel effect. With precise knowledge of the ship's mass and careful control of the ship's trajectory, it is possible to direct the ship into the past or future by a specific amount of time.
Trekkie Time-Travel Arguments
Trekkie versus debaters sometimes claim that the Federation would use time travel to defeat the Galactic Empire by travelling back through time to assassinate the Emperor or otherwise prevent the Empire from becoming a threat. Such arguments typically ignore the parallel universe nature of time-travel in Star Trek: even if the Federation could successfully accomplish the objective in the past, they would merely create an alternate timeline -- nothing would change in the timeline from which the Federation launched the mission. Also, use of this argument is basically an admission that Star Trek factions could not take on Star Wars factions in a straight fight.
- See Mike Wong's article about this Trekkie argument on Stardestroyer.Net.
Time Travel in Doctor Who
The Time Lords of Gallifrey developed vehicles capable of travelling through time and space by passing through a space-time vortex. Travellers in such devices apparently can alter history, although there are limits to what changes they can safely make (some events are "fixed points in time"), and how they avoid paradox problems isn't clear. Several other species have devised methods of time travel as well, including the Daleks.
In the Great Time War, the Time Lords and Daleks both used time travel extensively to try to reverse defeats and prevent losses. The result was an increasingly convoluted chain of cause and effect that eventually caused the entire war to be come "time locked", preventing all access to its events via time travel.
Other Science Fiction with Time travel
- In the Terminator Universe, Skynet has developed a system capable of displacing objects through time. The degree to which time travelers can alter history is ambiguous.
- The entire premise of the Back to the Future Trilogy is entirely about time travel. The movie features a time machine built into a Delorean Sports Car. The car needs 1.21 Gigiwatts in order to power the Flux capaciter, which is the device that allows the car to travel through time. The car also needs to reach 88 MPH in order to move from one time period to another. Originally the car was powered using a small nuclear reactor but later was fitted with Mr. Fusion. The car was fitted with a flying rig during the second movie. It was built by Doctor Emmett "Doc" Brown. Most of the time the car is out of commission as the plot often revolves around finding a way to either get the power needed to run the time circuits or to get it up to required 88 MPH to jump through time. The end of the third movie showed a steam-punk flying steam engine version of the time machine.