Avengers: Endgame — How time travel works

Marvel Studios’ AVENGERS: ENDGAME..L to R: Hawkeye/Clint Barton (Jeremy Renner), War Machine/James Rhodey (Don Cheadle), Iron Man/Tony Stark (Robert Downey Jr.), Captain America/Steve Rogers (Chris Evans), Nebula (Karen Gillan), Rocket (voiced by Bradley Cooper), Ant-Man/Scott Lang (Paul Rudd) and Black Widow/Natasha Romanoff (Scarlett Johansson)..Photo: Film Frame..©Marvel Studios 2019

Why there are still time travel paradoxes

Even if you’ve managed to grasp the concept of time travel in Avengers: Endgame, you probably still have some questions or two. For instance, you might be asking, “But if I travel back in time and the past becomes my new present, and my former present becomes my new past, how can go still back to my former present?” That’s a good question, because theoretically you shouldn’t be able to. Instead, you could only travel into the future of the new timeline you’re now in, not the one from which you originally came from.

But even if you could travel back to your own timeline, you shouldn’t also be able to travel back into a timeline you’ve previously visited and changed, either. Again, according to how time travel works in Endgame, when you travel into the past, you create a brand new timeline that diverges from yours every single time.  In other words, you cannot revisit the same timeline twice. This also means Cap shouldn’t be able to return the Infinity Stones to the timelines from which they came from either.

The way Endgame gets around this, however, is two-fold. First, they have Stark’s GPS devices to navigate through the corridors of the Quantum Realm. Combined with how the time machine in 2023 acts like a tunnel, the Avengers can find their way back-and-forth across different timelines without any fuss. Presumably, this is how Cap can return the Infinity Stones to their proper timelines, even though he technically shouldn’t be able to. Also, since you still cannot return to the precise moment from which you left, Endgame shows the Avengers returning just a few seconds before they left.

That said, how did the 2014 Nebula bring her Thanos and his entire fleet into 2023? As Endgame itself establishes, you need both Pym Particles and protective environmental suits to travel through the Quantum Realm and into different timelines. Granted, 2014 Nebula gave 2014 Thanos 2023 Nebula’s Pym Particles so perhaps 2014 Ebony Maw or what have you replicated them. Also, since their ships and space whales can travel through the ravages of space, maybe they’re shielded from the Quantum Realm’s effects, too.

“But wait,” you might be asking. “Since Thanos’ ship destroyed the original time machine in 2023, doesn’t that also mean the connections to those previously visited timelines have also been severed?” Yep, which means, once again, Cap shouldn’t be able to return the Infinity Stones back to their points of origin. If you think that’s a plot hole, it’s nothing compared to the ending.

When Steve returns the Infinity Stones back to their original times, he decides to stay in the past and live a normal life of wedded bliss with his beloved Peggy Carter (Hayley Atwell). The next time we see Steve back in 2023, he’s an old man. As sweet and poignant as this ending is, it completely contradicts Endgame’s time travel rules. After all, if Cap travels to 1945, he also travels into a different timeline. Therefore, he shouldn’t have reappeared as an old man–with a fully restored shield, to boot–in 2023.

Unless, of course, Endgame‘s ending is an alternate timeline. In other words, there’s one MCU without an old Steve Rogers, and another timeline with an older Steve Rogers from the original MCU timeline–the one that we’ve seen play out over the course of twenty-two movies and elven television shows. Perhaps this was Endgame’s real twist all along.