Avengers: Endgame is the culmination of over 20 previous movies, it includes scores of varied superheroes, and it dips into outer space, technological wonders, and even magical powers. Suffice it to say: the movie can get pretty confusing sometimes, even for the biggest Marvel superfan. The most confusing element of all is that Endgame introduces time travel as a key component, which is always going to be a tricky concept. So, how does the Quantum Realm time travel work in Avengers: Endgame? Let's try to demystify the movie's most confusing moments.
Spoiler alert: This post contains spoilers from Avengers: Endgame. The Quantum Realm was first introduced in last year's Ant-Man and the Wasp, and fans rightly predicted that the subatomic dimension would play a major role in Avengers: Endgame as well. Hank Pym first discovered the Quantum Realm when he perfected his Pym Particles, which power the shrinking abilities in Ant-Man's suit. In Ant-Man and the Wasp, Scott Lang finds Pym's wife Janet van Dyne in the Quantum Realm, after she had gotten trapped there from shrinking too much decades ago. At the end of that movie, Thanos' snap dusts Hank Pym, Janet van Dyne, and Hope van Dyne while Ant-Man is still in the Quantum Realm collecting quantum energy, leaving him trapped.
Ant-Man is stuck in the Quantum Realm for five years until a rat finally scurries across the gate's controls in a storage facility in Avengers: Endgame. However, only five hours have passed for him, confirming that time works differently within the Quantum Realm. This gets Ant-Man thinking that maybe the time-defying properties of the Quantum Realm could be the key to going back in time to collect the Infinity Stones and prevent Thanos' snap.
Thanks to Tony Stark, the Avengers are able to enter the Quantum Realm and then exit it at the exact place and time of their choosing, which allows them to collect the Infinity Stones from before Thanos nabbed them.
But how does the Quantum Realm work exactly? Are there any rules at all? According to both Hank Pym and Marvel's actual science consultant who named the Quantum Realm, not really. Pym described the Quantum Realm as a dimension where time and space become irrelevant, and mathematical physicist Spyridon Michalakis further explained the properties of the Quantum Realm in an essay for TheWrap last year. Michalakis explained the Quantum Realm through a thought exercise, asking readers to imagine how space and time first came to be. Both concepts sprung from nothing, existing in a realm beyond what we call reality. Basically, it is this realm of non-reality, unrestricted by space, time, or any other concepts, that is the Quantum Realm.
So, when the Avengers enter the Quantum Realm, they enter a dimension where they become capability of pretty much anything, no longer bound to any sort of reality. The Avengers only use the Quantum Realm for time travel in Avengers: Endgame, but theoretically, the possibilities are infinite as to what they could achieve by spending time there. We will have to wait to see if and how the Avengers utilize the Quantum Realm in future Marvel movies.