Time travel is a method used for transporting one's self or someone else to another point in history, or in the future. Time itself is referred to as the Fourth dimension. When that dimensional barrier is broken, time travel is possible.
There are a variety of ways to travel through time, one is by the use of a time machine, which the Legion of Doom use to transport the Hall of Doom back or forward in time. Lex Luthor once invented a time machine that was capable of transmitting messages into the past, which he used to contact Merlin. Another way to travel through time is to travel faster than the speed of light, a speed which is fast enough to penetrate the time barrier. Magic can also allow someone to travel through time. Black holes are also known to send people through time.
Because of time travel, often times the past can be changed in such a drastic manner that it could be quite noticeable to the time traveler upon returning to the future. Once Superman fell through a black hole and arrived in the past at a point in time prior to Krypton's destruction, he decided that this time he would save Krypton, but in the process, a new dark timeline was created, due to his tampering with history. All of his fellow JLAers were dead except for Robin, and the Earth was enslaved by the Legion of Doom. Naturally he had to go back and make sure Krypton was destroyed so that he could preserve his own timeline.
In another instance, in the post-apocalyptic future, a group of aliens learn how Earth was destroyed by a solar flare, so they travel back into time to stop the Legion of Doom from firing a missile at the sun, which created an alternate timeline.
There was also another instance when the Legion of Doom went back to prevent Kal-El, Hal Jordan and Princess Diana from becoming the superheroes they were destined to become. This made it easier for them to take over the world. Nevertheless, when the other heroes found records of Superman, Green Lantern and Wonder Woman in the Hall of Doom, they pieced it all together and set history straight again.
Predestination paradoxes are time travel paradoxes that exist when one is traveling through time. They are not very common, but they do pop up from time to time. Basically, in a nut shell, a predestination paradox is a time traveling phenomenon that occurs before time travel even occurs. In other words, the result of the trip through time could be seen by a time traveler before he even travels through time. For example, on Krypton, Argo City is saved by Zor-El, Jor-El's brother, who encased the city in a dome.
That is common history. But the whole reason that was made possible was because Superman had traveled back in time and told him that Krypton was about to explode, and so we have a predestination paradox.
Some characters have time manipulation as a super power. This doesn't necessarily mean that they can travel through time, although in most if not all cases it does, but it does mean that a person with such a power could have the ability to control certain linear mechanics. As an example, one might have the ability to use his time manipulating powers to cause someone to age rapidly, by speeding up time but only for him alone, and not the rest of the universe. One might also be able to cause time to stop, but only allow time to move for a few people. A character with such power could also create time loops. With characters such as the Time Trapper, there's basically no limit.
- ↑ Although it is a common theory that Time is the Fourth dimension, it was mentioned in Super Friends # 8.
- ↑ As seen in Secret Origins of the Superfriends.
- ↑ As seen in The New Adventures of Superman episode Merlin's Magic Marbles.
- ↑ Superman, Flash and Black Vulcan are all fast enough to travel through time.
- ↑ Merlin used his magic to transport J. Quiggly Bent back to the 27th century
- ↑ As seen in The Krypton Syndrome.
- ↑ As an example of what this is, you should watch two very popular science-fiction films that deal with this subject: Groundhog Day and 12:01. A more popular example might be the Star Trek: The Next Generation episode Cause and Effect.