No stories set in the Marvel Universe features actual time travel, since that's not possible in the Marvel Universe. It's explained in many stories (Earth X, Exiles, others) that any occasion where time travel is believed to occur, what is actually happening is either traveling to an alternate reality/dimension or creating a new alternate reality. In DC Comics it is quite more common as DC has comics from many more time frames than does Marvel (for instance the Legion of Super Heroes.) Even in DC though it is not as common. One character, Rip Hunter, was once portrayed as unable to travel through time in the same manner more than once.
Time travel can be used humorously as well as more scientifically with the "Grandfather Paradox" commonly incorporated as a point of interest.
Characters and writers have many motivations for time travel such as:
- Preventing a future disaster by traveling backwards.
- Traveling forwards to gain technology not yet available.
- Accidentally being trapped in more or less idyllic settings.
- Rebooting a character or bringing them up-to-date.
Also there are many plot devices to achieve time travel.
- Cryogenic freezing and Stasis to allow passage of time.
- Time machines.
- Transcendental Meditation
Types of Time Travel
A predestination paradox is also called a closed loop. It exists when a time traveler is caught in a loop of events that forces him or her to travel back in time. Because of the possibility of influencing the past while time traveling, one way of explaining why history does not change is by saying that whatever has happened was meant to happen.
A time traveler attempting to alter the past in this situation, intends to fulfill the role in the possible future. For example, Cable was destined to kill Apocalypse. So he was forced to travel forward to the future and save mankind from the immortal mutant. He also had to go back in time and stop Apocalypse and prevent other certain events in the past.
There has also been a series of different events that have been undone or postponed due to interference from the time travelers. For example, Charles Xavier's son Legion has traveled back in time to kill Magneto. Instead, he created a Time Paradox when accidentally killing his own father. Legion was ceased to exist and a divergent timeline occurred as a result.
This divergent timeline came to be known as Age of Apocalypse where the immortal mutant, Apocalypse once ruled. Some other events also seem to be a predestination paradox of sorts, where time travel is required to maintain a timeline rather than altering it.
There are events capable of influencing the others simply by the fact that every additional time travel event into the past or future establishes yet another set of possibilities. Multiple reactions to these timelines have been depicted, ranging from the seamless integration of separate future participants in certain situations, to spontaneous reality shifts, erasing events and consequences from the face of the multiverse in the blink of an eye.
For example, the Days of Future Past storyline of the X-Men series occurred as a result of the assassination of Senator Kelly who used to hate mutants until he learned that there can be peace. However, that peace has been affected by the Sentinels. Any action that has been made in the past would lead to a drastic future. Another example would be the creation of Skynet lead to the destruction of humanity in the Terminator franchise.
For this reason, certain time travelers had to jump back into the past to prevent the creation of Skynet. But several attempts have been failed due to the wrong information such as how to find the creator of Skynet or how and when to destroy it.
The parallel universes are a multitude of universes which exists in the same physical space but have a different dimensional structure from one another. A parallel universe can also occur when a time traveler attempts to alter the past. Therefore, a separate timeline or a parallel universe has been created. For example, a divergent timeline called the Age of Apocalypse has been created due to the result of Legion's actions in the past.
However, there are altered timelines which are not necessarily considered as parallel universes. The House of M storyline would be considered as an altered universe rather than a separate one. There are also parallel universes where time travel has not been used to alter the course of events.
An alternate history happens when a reality has already been depicted by an author's point of view. Alternate histories were also considered as parallel histories which are similar to our history. An alternate history can also occur when a time traveler changes something in the past. That past will also lead to unknown consequences in a possible future.
Known time travelers