Avengers: Endgame — How time travel works

Marvel Studios’ AVENGERS: ENDGAME..L to R: Nebula (Karen Gillan) and Captain America/Steve Rogers (Chris Evans)..Photo: Film Frame..©Marvel Studios 2019

Why you cannot change the past

I’m sure many of you saw time travel the same way Scott Lang, Rhodey Rhodes (Don Cheadle), and Clint Barton (Jeremy Renner) did. After all, every single time travel movie out there “proves” that changing the past changes the future. So why not, as Rhodey suggests, couldn’t they just kill Thanos before he takes the Infinity Stones? Also, doesn’t taking the Infinity Stones and bringing them into the future before Thanos gets a hold of them also prevent him from erasing half the universe?

Not so, according to Professor Hulk (Mark Ruffalo). As it turns out, whatever you do in the past has no effect on the future whatsoever. In fact, according to Professor Hulk, that method of time travel is scientifically impossible.

Let’s suppose the Avengers did prevent the snap from happening by taking the Infinity Stones before Thanos gets them. This would mean that if the snap never happened, the Avengers wouldn’t also go back in time because the reason for doing so no longer happened. But if they didn’t go back in time, then it means the past remains the same and the snap still happens. In other words, you’d have two things which both happened and didn’t happen simultaneously. And if this sounds a lot like Schodineger’s Cat, that’s precisely what this paradox this would create.

Moreover, everyone, including time travellers, experiences time as a linear, forward progression which cannot be changed. From a time traveller’s perspective, then, travelling back in time is something they’ve already done, experienced, and cannot change. Whatever future actions they make in the past also, from their perspective, hasn’t happened yet. Or, as Professor Hulk puts it, “If you travel into the past, that past becomes your future, and your former present becomes the past which now can’t be changed by your new future. (i.e. the past).”

Thus Endgame shows that when the Avengers are travelling through time, what they’re actually doing is travelling into a parallel universe to a different time in that universe. This concept of time travel has basis on what’s known in quantum mechanics as the many-worlds interpretation. As postulated by physicist Hugh Everett III, the idea is that there’s an infinite number of possible universes, each one created by different outcomes. For example, if you flip a coin and it ends up heads, there’s another universe where it came up tails.

What this basically means is that the very act of you travelling into the past creates a new reality, one which branches off from the point where you first arrived. Whatever changes you decide to make in this new reality can only effect that new reality. Meanwhile, the reality which you came from remains unchained. This also explains why Nebula (Karen Gillan) doesn’t cease to exist when she kills her past self from 2014; she actually killed a version of herself from a different reality.

But before you think all of this is a nice (and convenient) way of avoiding all sorts of paradoxes and time loops, this also opens a whole new paradoxical can of worms.