‘Endgame was great, but there’s one little problem’
I thoroughly enjoyed Avengers: Endgame, but this article is not intended as a critical review. Instead, it’s simply intended to teach the reader a little philosophy by pointing out a philosophical oversight—a plot-hole, if you will—that easily could have been rectified.
Endgame pleasantly surprised me by not being a story about trying to get the Infinity Stones and Gauntlet back from Thanos. This, it seems, would have just been more of the same. Instead, it’s a time travel story; the (remaining) Avengers go back in time to steal the stones from a time before Thanos acquired each of them, so they can be used to “undo” what Thanos did.
But time travel stories are difficult to tell—at least in a way that is logically consistent. They often fall prey to something philosophers call “the grandfather paradox.” They tell a story that involves a time traveler changing the past in way that would undo the fact that they ever traveled back—say, by killing their own grandfather so that they are never born. Such a story is logically inconsistent and thus impossible (and thus disappointing).
This happens, for example, in Back to the Future where Marty McFly prevents his parents from meeting and falling in love. And dealing with the paradox by simply having Marty and his siblings slowly “fade out of existence” does not solve the problem. Either he exists, or he doesn’t. If Marty kept his parents from falling in love and thus doesn’t exist, then he didn’t exist to keep his parents from falling in love, and thus he does exists. Paradox! In the end, as I make clear in Lecture 7 of my Great Courses course Sci-Phi: Science Fiction as Philosophy, Back to the Future is not a logically consistent story. Scott Lang (Ant Man) put it well: “Back to the future is bullshit!” (I’m paraphrasing.)
Interestingly, Endgame somewhat unintentionally avoids invoking the grandfather paradox—at least initially. Because Tony Stark (Iron Man) loves his daughter (who was born after the Thanos culling), he insists that (if he is to help the Avengers) their use of the stones can’t undo what Thanos did directly, by making the world such that it never happened. That would mean his daughter was never born. Instead, he insists that they only use the stones to bring back to life—to resurrect (if you will) in the present moment—those Thanos culled. This avoids the paradox because, if the Avengers had done the former, they would have erased the five years of suffering that occurred because of the culling, and thus the entire motivation for developing the method of time travel and going back to get the stones in the first place!
But, as I explain in Lecture 8 of Sci-Phi: Science Fiction as Philosophy, there are two ways to describe or conceive of time travel that avoid invoking the grandfather paradox entirely. The first is endorsed by philosophers Nuel Belnap and David Deutsch: you can suggest that changing the past creates a “new timeline” that would have a different future. Such a timeline would contain similar but numerically distinct persons and thus you couldn’t do something like kill your own grandfather. You could kill someone that looks like your grandfather, thus preventing the existence of someone that looks like you from ever existing in that timeline. But you couldn’t kill YOUR grandfather. Indeed, you could do nothing to “undo” your own existence or the event of your time travel. Both are safe and sound in your original timeline.