A proposed time-travel machine using a wormhole would (hypothetically) work something like this. A wormhole is created somehow. One end of the wormhole is accelerated to nearly the speed of light, as with an advanced spaceship, and then brought back to the point of origin. Now one end of the wormhole has experienced less subjective time compared to the stationary end. An object that goes into the stationary end would come out of the other end sometime in the past. One significant limitation of such a time machine is that it is only possible to go as far back in time as the creation of the machine.
Creating a wormhole of useful size, keeping it stable, and moving one end if it around would require significant energy, many orders of magnitude more than the sun can produce in it's lifetime. Construction of a wormhole would also require the existence of a substance known as '[exotic matter]?', which, while its existence is not impossible, there is not evidence that it does in fact exist. Therefore it is unlikely such a device will be ever constructed, even with highly advanced technology.
Einstein's special theory of relativity allows travel forward in time by means of time dilation (simply accelerate close to the speed of light, or spend time in a deep gravity well); but once one has gone forward in time one cannot go back.
Where can I even start to describe time travel themes in Science Fiction Literature?
Basically you can group the science fiction treatment of time travel (based on effect - methods are extremely varied and numerous) into two types, each of which is further subdivided.
Time Travel in a type 1 universe does not allow any paradoxes, although in 1.2, events can appear to be paradoxical.
In 1.1, The Novikov Self-consistency Principle asserts that the existence of a method of time travel constrains events to remain self-consistent (i.e. no paradoxes). This will cause any attempt to violate such consistency to fail, even if extremely improbable events are required.
Example: You have a device that can send a single bit of information back to itself at a precise moment in time. You receive a bit at 10:00:00 PM, then no bits for thirty seconds after that. If you send a bit back to 10:00:00 PM, everything works fine. However, if you try to send a bit to 10:00:15 PM (a time at which no bit was received), your transmitter will mysteriously fail. Or your dog will distract you for fifteen seconds. Or your transmitter will appear to work, but as it turns out your receiver failed at exactly 10:00:15 PM. Etc, etc. An excellent example of this kind of universe is found in Timemaster, a novel by [Dr. Robert L. Forward]?.
In a universe that allows time travel but no paradoxes, any present moment is the past for a future observer, thus all history/events are fixed and there is no true free will (however illusions of free will do exist). History can be thought of as a filmstrip where everything is already fixed.
In 1.2, any event that appears to have caused a paradox has instead created a new time line. The old time line remains unchanged, with the time traveler or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated, unless the mechanics of time travel required that mass-energy be exchanged in precise balance between past and future at the moment of travel.
Time Travel in a type 2 universe is much more difficult to explain. The biggest problem is how to explain changes in the past. One method of explanation is that once the past changes so do all memories of all observers. This would mean that no observer would ever observe the changing of the past (because they will not remember changing the past.) This would make it hard to tell whether you are in a type 1 universe or a type 2 universe. However, you could infer that you were by knowing that a) communication with the past was possible and b) it appeared that the time line had *never* been changed as a result of an action someone remembers taking, although evidence exists that other people are changing their time lines fairly often. An example of this kind of universe is presented in Thrice Upon a Time, a novel by [James P Hogan]?. Larry Niven suggests that in a type 2.1 universe, the most efficient way for the universe to "correct" a change is for time travel to never be discovered, and that in a type 2.2 universe, the very large (or infinite) number time travelers from the endless future will cause the timeline to change wildly until it reaches a history in which time travel is never discovered.
In many Science Fiction books about Time Travel, there is a physical machine for transporting people through time but there is a minority which involve Time Travel through mental disciple. [Jack Finney]?'s Time And Again is one such book.
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