Warning: Contains SPOILERS for What If...? episode 4.

Thanks to Marvel's What If...?, the MCU has just changed its time travel rules - again. Time travel is a popular concept in science-fiction, fantasy, and superhero stories, but it's actually quite a problematic concept. The core issue is that time travel is, of course, purely theoretical; as such, writers can never resist tweaking the "rules" in order to fit the requirements of the plot. As a result, there are precious few franchises that handle time travel in anywhere near a consistent manner.

The MCU is, unfortunately, no exception. Time travel became an established part of the MCU in Avengers: Endgame, a film that spawned 14 million debates about whether Steve Rogers had created a branching timeline when he traveled back to be with his beloved Peggy Carter, or whether he'd lived in the shadows of the MCU all along. Avengers: Endgame's writers and directors publicly disagreed on this point, and gradually it became clear Endgame's time travel didn't make sense because it changed during production - with Tilda Swinton recalled to reshoot key lines of dialogue as the Ancient One. Since then, Loki attempted to iron out the problems and set up the Multiverse, but even now Marvel don't seem to have committed to hard and fast rules. It's telling that Marvel only held a meeting about the Multiverse's rules in July 2021, after they'd made a slew of Multiversal adventures - with production complete on even Doctor Strange in the Multiverse of Madness. The studio would perhaps have been wise to have that meeting a couple of years ago.

Related: The MCU Is Still Making The Same Mistake With Every Release

Still, that context does mean it's no surprise to see the MCU continue to contradict itself in terms of its temporal mechanics. Marvel's What If...? episode 4, a superb and emotional adventure in which a grief-stricken Doctor Strange unwittingly destroys an entire universe, is a classic case in point.

How The MCU's Alternate Timelines Worked Before

Why What If doesnt call branching realities Nexus events

Loki revealed that time in the MCU is not linear in nature, but rather is chaotic. An entertaining animated introduction in Loki episode 1 saw Miss Minutes explain any moment can lead to the creation of a branching timeline, one where history plays out differently; the juncture between these two diverging timelines is known as a "nexus." It's important to remember these don't have to be major events, because a nexus can be caused just as easily by someone being late for work one day as by an individual starting a civil war that, in the Time Variance Authority's view, should never have happened. But these nexus events can also be caused by a time traveler jumping back in time, exerting an influence on history they should never have had; Sylvie was only able to evade the TVA's detection by jumping to notable disasters in history, living in the shadow of catastrophes that would wipe out all trace of her presence, meaning any changes she made were swiftly erased.

According to Loki, this is the basis of the MCU's Multiverse; the series ended with the TVA giving up on their sacred task to police the Multiverse, instead choosing to allow all these branched realities to emerge once again. The result will be a Multiversal war, because sadly a number of these timelines will ultimately lead to the emergence of variants of Kang the Conqueror. Some will be benign, but enough will be warlike, with inevitable consequences as the rulers of different realities go to war on one another.

MCU's Nexus Events & What If...?'s Fixed Points In Time Differ

Doctor Strange What If episode 4

Marvel's What If...? episode 4 introduces another time travel rule - an absolute point in time. The idea is lifted straight from Doctor Strange, where Wong and Mordo cautioned using the Time Stone threatened to break the entire spacetime continuum. "Temporal manipulations can create branches in time," they told Strange. "Unstable dimensional openings. Spacious paradoxes! Time loops... You weren’t manipulating the spacetime continuum, you were wrecking it." In the timeline seen in What If...? episode 4, a grief-stricken Stephen Strange attempted to use the Time Stone to resurrect his lost love Christine Palmer, only to learn this moment was not so easy to change. Because Christine's death had set him upon the path to becoming Sorcerer Supreme, changing it would create a paradox that destroyed the entire universe. Strange proceeded to attempt to change time regardless, absorbing the magical power of countless mystical entities in order to do so, but the result was catastrophic. This weaves a new idea into the MCU's temporal mechanics. Going from Loki, viewers would have expected Strange's interference in his past to simply create a nexus, generating a branched timeline, so this is all rather contradictory.

Related: The MCU Just Proved How Weak The Ancient One Really Was

It's possible the reason for the inconsistency lies in the fact Doctor Strange was using the Time Stone, coupled with his own sorcery, to time travel. Marvel Comics has explained that magical time travel works differently to its science-based equivalent, for "unlike fully scientific time travel, it has the power to alter history." Significantly, this was the time travel sorcery mastered by Cagliostro, the mystic Doctor Strange sought out in Marvel's What If...? episode 4. It's possible these limits - the inability to create paradoxes - apply only to magical time travel.

The Ancient One's Mirror Timelines In One Universe Explained

Doctor Strange Duels

Marvel's What If...? episode 4 throws in another surprising twist, though, when it reveals the Ancient One possesses magic that allows her to manipulate the fabric of the Multiverse itself. She is able to weave a spell upon Doctor Strange that essentially takes advantage of a nexus point, a moment of decision where Strange could either choose to pursue his insane attempts to save Christine Palmer or abandon them. The Ancient One was actually able to ensure these two timelines coexisted, meaning two variants of Doctor Strange inhabited the same timeline. No doubt this was only possible because she sensed the rogue Strange would pursue an isolated life for centuries, his life extended by dark magic, meaning these two versions would only collide when the time was right.

The Masters of the Mystic Arts have always been noted to possess the power to alter reality on a fundamental level, but this is a remarkable demonstration of magical might - achieved by tapping into the power of the Dark Dimension. Given this state of affairs was wholly unnatural, it doesn't really contradict the Multiversal rules established in Loki, but it certainly adds new depth to them - and raises untold possibilities. In the main MCU timeline, for example, Mordo is currently hoarding power by absorbing the abilities of other sorcerers; using their combined might, he may well be able to merge timelines in the same way as the Ancient One. The outcome of Doctor Strange 2's "Multiverse of Madness" could change the MCU forever by folding in multiple timelines.

All in all, then, Marvel's What If...? episode 4 throws in a lot of new ideas about how time travel and the Multiverse work in the MCU. They don't all fit terribly well, but frankly that's to be expected given Marvel is only just figuring out the basic "rules" of its temporal mechanics. No doubt there will be further revelations as the animated series continues, more wrinkles in the conceptual framework underpinning the MCU.

More: Every Marvel Phase 4 Movie & Show That Includes The Multiverse

Marvel's What If...? releases new episodes Wednesdays on Disney+.

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