Time has been defined as a basic component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects. It is also described as the continuum of experience in which events pass from the past through the present to the future. The standard method to move through time is living one's existence, starting with a point in time at birth and then aging at a normal rate as time passes until the moment of death.
Methods of moving forward or backward in time have been developed by a number of means, including magic, technology, and personal or nontechnologically generated energies. Magical means include performing of spells by sorcerers such as Dr. Stephen Strange or the powerful mystic being Dormammu, or perhaps by novices having discovered a spell's text; and the use of magic items, such as Thor's hammer, Mjolnir (though most of its temporal properties have been stripped), or King Solomon's Brass Frogs. Technological means generally includes machinery specifically designed for time travel, such as that of Victor von Doom (a replica of which is possessed by Reed Richards) and Kang the Conqueror. Certain beings of power, such as the Silver Surfer (Norrin Radd) or the Phoenix Force, can generate sufficient energies to pierce the time barrier. All methods of time travel apparently involve generating "chronal displacement inertia" freeing one's chronological position in the timestream (just as escape velocity frees one from Earth's gravitation), possibly skimming through the extratemporal realm outside the timestream (Limbo), and re-entering the timestream at another chronological position.
Limbo is a realm that exists outside time and accesses all time periods simultaneously. Time does not pass in a linear fashion within Limbo, and it is possible for a being within Limbo to encounter his or her own "past" or "future" selves. While there are a number of timeless realms referred to as limbo (such as the realm in which the Galadorian Spaceknight Rom imprisoned the Dire Wraiths via his neutralizer, or the dimension Otherplace, often referred to as Belasco's limbo), the true Limbo is a specific realm, typically under the domain of the timelord Immortus (the capital "L" generally denotes true Limbo, with other such realms referred to as the lower-cased limbo); there is some evidence that the lesser limbos are linked to true Limbo. Some theories speculate that all methods of time travel do so via passing through Limbo. Because no time exists outside the timestream, the perceived duration of the passage through Limbo may be anything from nonexistent to an eternity. In some cases one can return to one's own time at the exact moment of one's departure, while in other cases one is gone for a time period corresponding to the amount of time spent in the past or future.
Divergent realities are identical up until a certain "What if" point, in which they diverge from each other and follow different paths. Alternate realties have similar cosmic power structures to each other but different histories and inhabitants, with no apparent single point of divergence. The multiverse is a collection of alternate and divergent realities, while the Omniverse refers to all realities, including multiverses with different cosmic power structures.
In many cases, efforts to travel through time actually result in transport to an alternate reality. As future events have yet to occur, most future time periods seen are referred to as alternate or potential futures, with the true path of the future yet to be decided; the true future of any given time period may or may not be identical to any previously glimpsed future reality. In some situations, a certain destiny has been described for a being, group, planet, etc. It would seem that despite various choices, in some situations certain events seem to be virtually inescapable, though in most cases more extreme changes can clearly avoid a specific outcome. In an infinite multiverse, it is exceedingly unlikely that no future exists where a certain outcome has or has not occurred.
Travel into the past seems to vary with the method of transport, and for every "rule" there is one, if not many exceptions. Trends can be reported based on the most commonly observed patterns. In many circumstances, efforts to change the past result in a divergence that yields two timelines, one where events proceeded as they "originally" occurred, and one where the change occurred and the future was guided by these changes. It is believed that the "second" such timeline was not actually created by the change, but that it was pre-existing and became linked to or briefly overlapped with or replicated within the "first" timeline. This is evidenced where certain beings "remember" the changes that occurred under some conditions, despite their pasts having been seemingly changed. Most beings, however, are unaware of any change, as, to their perspective, time flowed normally over a series of events, with no discernible disruption of continuity. Sometimes the overlapping of the timelines can be seen in a "ripple" effect, where changes seen to happen to people and objects, moving in an expanding spatial and/or temporal wave from the point of divergence.
DIVERGENCE THEORY
Some time travel theories speculate that a divergent reality is created by every choice or variation. In these theories, one root timeline splits into an infinite number of divergent timelines. One can diverge any number of different timelines by one's significant actions. Whether one will be able to return to the present of the timeline where one did no reality-tampering divergences or one which diverged as a consequence of one's past actions is a function of the means of time travel. Altering an incident in the past will indeed affect the future reality of the timeline diverged by the time traveler's presence. Returning to one's present also diverges another timeline. If one has been gone any length of time, one may find differences have accumulated in accordance with the length of time one was away. In divergence theory, there are no such things as time paradoxes. If one were to accidentally kill one's mother while time traveling, then two timelines have diverged: one where the mother died, and one where she did not. Nothing happens to the traveler - the "original" timeline still exists, and so does the traveler. When/if the traveler returns to his or her own time, the mother is still there as normal. If, for whatever reason, the traveler moves up the timeline the mother died in instead, still nothing happens to the traveler - he or she is now simply alive and well in a universe that has no record of him or her ever having existed. Beings or whole realities have seemingly been shown to cease to exist via critical alterations of their pasts, but generally there will be divergent realities in which those beings still exist.
According to divergence theories, selective alterations in the present as a consequence of the time traveler's actions in the past do not occur. An alteration in the past will diverge an entirely new timeline with events proceeding smoothly from the point of divergence. To the denizens of that timeline's "present," the past is a continuous series of events that always happened as they happened. In some cases, a time storm, or time wave, has been witnessed, as was the case when the New Warriors fought Advent or when Iron Lad and the Young Avengers battled Kang. These distortions may be caused by the gradual or progressive overlapping/merger of realities caused by efforts to alter the past. Alternatively, as most modes of time travel involve accessing, in some way, the dimension of Limbo, time travel methods which allow Limbo's energies to leak into other dimensions can create reality distortions. These distortions almost always fade once access to Limbo is closed (usually when the time travelers have been restored to their own times). Unusual "time storms" or other reality-altering events are due to effects from Limbo or other factors (such as hallucinations), and not necessarily to time travel itself. Occasions where memories of time travel fade unnaturally (or where individuals remember two separate timelines) are also likely due to the overlapping realities, Limbo's effects, or other factors.
More extreme theories state that any temporal arrival in a reality actually creates a divergence and that it is not possible to visit one's own past; these theories further state that it is impossible to return to a timeline one has previously visited, and that attempts to do so result in further divergences. Numerous examples would seem to contradict these extremes, but it is virtually impossible to determine the truth.
CONSERVATION OF CAUSALITY
Other theories, including Kang's Third Law of Time, aka the Conservation of Causality, state that the creation of a divergent timelines requires a massive amount of energy and therefore only significant changes result in a divergence, with minor changes being absorbed into the existing timestream/reality. Kang's Law specifically notes that one major or four minor changes are necessary to produce a divergence. However, as different methods and energies are utilized in time travel, it would seem most likely that these may also play some factor in the likelihood of divergence as opposed to modification of the existing reality. Regardless, it would likely not be apparent to the inhabitants of the involved realties whether a divergence occurs or not. Similarly, inhabitants would have little way of knowing whether divergent realities were physically created or whether the two realities pre-existed.
In some instances, traveling into the past and encountering one's past self results in a massive energy disruption. In other instances, a traveler has not been able to physically appear in a timeline in which one's past or future self lives; sometimes the travel is simply not possible, and in other circumstances the traveler can only appear in wraith-like form, only able to observe events with minimal ability to affect the physical world. It is not clear what circumstances define such interactions, but the coexistence of multiple counterparts of the same being in one timeline should not cause time paradoxes, which are believed to be possible only in single timeline universes.
ALTERED REALITY
A reality is the combination of spatial and temporal events and the substance of a universe (and those dimensions associated with said universe). A being, object, etc. may be changed in the present without affecting reality by such means as physical mixture, chemical interaction, molecular rearrangement, and/or elemental transmutation. Generally when larger regions or groups of people have changes in their appearance, behavior, memories, abilities, etc., there are alterations in reality. Such changes may affect a relatively small location or be city-, nation-, planet-, or universe-wide. In some cases, only the present is altered, but in many cases there are memories and/or physical evidence of a different past. It can be difficult to determine whether the changes are just to the present with memory alteration or whether the past has been changed, and a divergent reality either created from or merged with the pre-existing reality. In many of these cases, memories of their previous existence may begin to surface in certain beings within the altered reality. In some cases it has been shown that a divergence has been created in the past via time travel, but in other cases it is believed that the changes were made by energies reaching back in time; it is often not clear whether there was a clear divergent point or not. The reality of Earth-616 (the mainstream Marvel reality) can be altered and restored to status quo, the inhabitants of Earth-616 can be involved in altered and/or overlapped realities, and objects and beings can be affected by the different history of the altered reality due to its temporary overlapping, but the events themselves are only retained within the reality in which they occurred.
"UNIVERSAL RESET"
One of the difficult to categorize time travel modes involves the universal "reset": after some major event occurs and is then "reversed," everybody in the universe is no longer aware that something happened. The nature of this event is sometimes hard to determine as there are three main possibilities: (1) Every being in the universe simply forgot (with reality altered as necessary to allow this); (2) Time itself wound back; (3) Realities merged and then separated. If the first case has occurred, then clearly this is not a time travel phenomena; there will be diverging realities relating to the event, but no time travel. If the second, then this is a time travel phenomena like any other, and various realities are diverged at both ends of the event. In the first diverged universe, something that had led up to the "event" is changed, and the event never happened. In the second diverged universe, the event happened and proceeded up to the point where it was to be reset, at which point it diverged again into one universe where the event continued and one where the event ended.
With reality overlapping and merging, which is particularly difficult to conceptualize, both (or all) realities exist prior to the event despite the seeming divergence. Memories of the previous existence sometimes appear during the warp and/or following the reversion, and the original reality may or may not retain some characteristics of the overlapping reality following the reversion of the warp to something closer to status quo. Events of this type include the Age of Apocalypse, the Infinity Gauntlet restoring reality, Thanos wielding the Heart of the Infinite to destroy and then recreate the universe, "Forever Yesterday," and the House of M.
In the "Age of Apocalypse," while it initially appeared that Reality-616 (the mainstream Marvel universe) had been altered by the premature death of Charles Xavier, it later became clear that this was a divergent reality, Reality-295, with a set point of divergence. Subsequent accounts have confirmed that this reality has a past before the "present" that was initially seen, and that this reality's timeline persisted well after the apparent reversal of the events that had seemingly created it. Reality-616 was overcome by a distortion wave, presumably via overlapping of the two realities or by leakage of Limbo's energies. Only Bishop "remembered" his previous existence, as he was the only one present in that reality that had been part of the -616 timeline.
After Thanos used the combined Infinity Gems, aka the Infinity Gauntlet, to eliminate half the universe's population and to conquer the universal powers, his alleged granddaughter, Nebula, usurped the Gauntlet and restored reality to the way it was before Thanos instituted its changes. This would appear to be pure reality manipulation, as events and memories of the events before the restoration persisted.
When Thanos usurped the power of the Heart of the Infinite, he traveled back in time to prevent the Pharoah Akhenaten from acquiring its power, diverging the reality in which Akhenaten conquered Earth to Reality-4321. When Thanos subsequently used the Heart's power to obliterate the universe and all of its cosmic powers, and then to restore the universe as it was before, this would similarly appear to be pure reality manipulation. This appears to be the case with most situations involving universal obliteration and restoration.
In "Forever Yesterday," the Sphinx (Meryet Karim) seemingly changed the past by having Moses slain by the Pharaoh's sorcerer, leading to Egypt becoming the dominant world power. However, certain beings within this new reality (notably the New Warriors) retained memories of their past lives, and the changes to the past were eventually reversed. Residents of the "Forever Yesterday" timeline were subsequently seen to still exist, indicating that this, too, is a divergent reality (Reality-9105) that persisted following the seeming reversal.
When used on a large scale, Cosmic Cubes have been shown to overlap realities with or without specific points of divergence in order to bring about such changes. Reality manipulation tends to follow this pattern, which can be confirmed when the reality is shown - usually in subsequent accounts - to exist prior to the warp or after the warp has been apparently reversed.
The Time-Keepers, powerful beings who originated at the end of time, have used equipment apparently able to terminate realities, though it remains to be seen if divergent realities persist.