Time travel has become a pretty common concept in the MCU. However, there are some rules that have been established by Marvel. Avengers: Endgame explored quite a few of these rules. Later on, the TVA in Loki season one also partook in a lot of time travel. So here are all the rules about time travel that you need to be aware of to not get confused in the Marvel multiverse.
This is an absolute beginner rule for time travel. You cannot travel back in your own timeline as the moment you reach that reality it will be a slightly different one. This would essentially create a diverging branch. In other words, a new timeline. If you travel back, the reality changes right at the moment of your arrival. Thus, by this logic, the Avengers in the MCU created 4 realities during the Time Heist in search of the infinity stones. Steve Rogers too created a new reality when he went back in time to Peggy Carter in the MCU.
The past simply cannot be changed. They involve events that have already happened and are basically set in stone. You can create different realities by changing the course of time but you cannot change the past no matter what you do.
The new reality that is created by the branching can follow the exact events of the original timeline. However, if the time traveler changes the course of events then the timeline may completely change into a new reality. For example, the timeline with Loki’s escape was a completely different one than the original MCU timeline.
Time travelling is none other than travelling through the fourth dimension. This is essentially the space-time continuum. Time in itself is a dimension.
The Quantum Realm allows you to travel to alternate realities. It isn’t just travelling backward or forward. This is basically dimension travel. For example in the MCU when Steve Rogers went to return the Infinity Stones, he did not travel back in time. Instead he travelled back to the adjacent reality that the Avengers created when they time travelled in the first place.
So the Time Stone allows temporal manipulations. Doctor Strange used the Time Stone in the MCU during the first movie to change the state of an apple. He also used it to see the missing pages from the Book of Cagliostro that were torn by Kaecilius. After Hong Kong was destroyed by Kaecilius and the other followers of Dormammu, Strange once again used the time stone to revert things back to how they were. The Time Stone was also used to create time loops by Strange to trap Dormammu and eventually defeat him in the MCU.
Contrary to popular belief, the future has already been decided and set. Nothing can change the future just like nothing can change the past. In fact, when people think they are changing the future, they are actually doing the exact things that they are meant to be doing. This was explained by He Who Remain aka Kang during the final episode of Loki in the MCU. Kang tells Sylvie that he knows exactly what will happen right till the end of time and can predict every move.
The infinity stones are extremely important to every reality. Whenever a stone is removed, a branched timeline is created and that reality is prone to attacks for dimensional beings such as Dormammu as said by the Ancient One in the MCU. However, if the Infinity Stones are returned at that exact point in time then the new reality that was created will simply be erased. No harm no foul.
We mentioned before, that travelling in time creates a different reality at the very moment of your arrival. However, this does not apply when a time traveler simply returns to the present. You would pretty much return at the exact same moment and would not have caused any changes.
There are three methods of time travel in the MCU. They are the Time Di’Alla, CTCs and Quantum Realm. They must be used carefully as otherwise it can have devastating consequences.