Avengers: Endgame’s Philosophical Oversight

Endgame was great, but there’s one little problem.

Posted Apr 27, 2019

SPOILER ALERT: This article assumes the reader has already seen Avengers: Endgame. Major spoilers ahead.


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 PhilosophyBack 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.

Fortunately, Endgame does not describe the Avenger’s ability to time travel in this way—and I say “fortunately” because the ability to time travel in this way is pretty useless…at least if “saving the people you love” or “undoing what Thanos did” is your endgoal. Why? Well, with such an ability, you could create a timeline where (someone who looks like Thanos) never got the stones, and people who look like your loved ones are not blinked out of existence. But with such an ability you could never undo what YOUR Thanos did in YOUR timeline to YOUR loved ones. Indeed, once you create this new timeline, you could never return to your own. (If you travel to the future, you will travel to the future of that new timeline. If you travel back again, you will create yet another timeline.) 

The other way of conceiving of time travel that avoids the grandfather paradox comes from philosopher David Lewis. He suggests that, if one were to travel to the past, one would find it impossible to change. It may seem to the time traveler that they could do whatever they want, but in reality they could do only that which is consistent with the future they knew. Indeed, although they could cause past events, they could only cause the past events that it was already true that they would cause. On this view, the timeline exists as a whole—past, present, and future. And the past already contains the event of the traveler’s activities, even before the time traveler pushes the button on their time machine to travel back. On this view, if the Avengers went back to prevent Thanos from snapping his fingers and culling half of all sentient life, they would inevitably fail. The fact that Thanos succeeded in doing so is already a past fact that cannot be undone.  

It is this conception of time travel that Endgame initially seems to endorse. At one point Professor Hulk (or "Hipster Hulk" as he is apparently now known) says something along the line of "If you travel back to the past, the present becomes your past, and the past becomes your future.") More clarifying is Banner’s conversation with The Ancient One (Tilda Swinton), where she raises the concern that his taking of the stone would bifurcate the timeline, leaving the one she would find herself in vulnerable. She shows him a “air diagram” of splitting timelines. But Banner promises to return the stone—all the stones, in fact—to the place and time from which they were taken. As he demonstrates this idea, the “air diagram” returns to a single line. If he returns them, the stones will still be where they “need to be” for Thanos to find them and thus succeed in his initial “culling” efforts. Indeed, on this view of time travel, if they are taken, the stones must be returned; nothing else could happen, because you can’t undo the past and Thanos finding them where and when he did is already a past fact. In fact, their being stolen and returned by the Avengers (from the future) must be something that happened to the stones, during the first movies, off screen.

At this point in the movie, I was really excited. They had already avoided invoking the grandfather paradox by refusing to try to change the past event of Thanos snapping his fingers and culling the herd; now they were going to preserve the past by stealing the stones from the past, using them to resurrect the victims of Thanos’ culling, and then returning them.  And when past Thanos learned about the location of all the stones from Neubla’s memory of the future Avengers reviewing where he had found them, I realized that his knowledge of the location of all the stones was (what philosophers calls) a Jinnee—a self-created object that results from a causal loop. (For more on Jinnee, see lectures 7 and 8 from my Sci-Phi course.) Thanos learns the whereabouts of the stone from the Avengers, and the Avengers learn about it from Thanos. Jinn (that’s the plural of Jinnee) are common in stories that invoke Lewis’ conception of time travel. It really seemed the writers had done their homework!

So once I realize that past Thanos was going to show up in the future, I said to myself: “Well, I know where this is going, and it’s awesome! They have to stop Thanos, but they can’t do so by killing him. If they do, they’ll create a paradox. Thanos has to survive, so that he can go back, find the stones, and do exactly what he did before.” This, I thought, was the perfect way to explain why Captain Marvel wouldn’t just sweep in and put Thanos down like a dog. And I was particularly looking forward to the scene were Thor had the chance to kill Thanos again, but had to hold back; he’d have to not kill Thanos specifically so that Thanos could go on to eliminate half of all sentient life. And if the Avengers had to do all that BEFORE they had the chance to use the stones—well, that would have generated some real tension! Will the Avengers be able to resurrect those that Thanos had culled, or would Thanos use the stones to destroy the entire universe and start over?

Of course, the Avengers ended up doing the former before Thanos arrived and had a chance to stop them—and I was okay with that—but when Future Nebula shot Past Nebula, I got worried. Granted, the wound wasn’t necessarily fatal…but the scene strongly implied that it was. And if future Nebula killed past Nebula, we’ve got a problem. Past Nebula can’t go back and turn into Future Nebula if she’s dead.

When Stark stole the stones from Thanos, and thus had the wish granting powers of the gauntlet, I thought “Ok, if they do this right, Stark will wish Thanos and his army back to the past—thus preventing him from destroying the universe, but allowing him to do what he had done before, thus preserving the past.” But instead, Stark killed past Thanos and his entire army, making them all disappear just as half of all sentient life had done before. Paradox! If past Thanos is dead, he can’t gather the stones and cull the universe; and without that, the Avenger’s would have no reason to travel back to the past to gather the stones in the first place. (*Sad Trombone*) My heart sunk.

So, in the end, Avengers: Endgame was this close—THIS CLOSE—to being a real rarity, something special, a real marvel (forgiven the pun): a time travel movie with a logically consistent plot-hole free story. Instead, it’s just a fun romp, with a paradox inducing time travel device that allows the main characters to re-revisit the past movies and end with all the heroes involved in one giant final battle. I’m not saying it’s bad; I’m just saying, it could have been better.

ENDNOTE: I found this article that tries to explain away the paradox/plothole by suggesting that the Avenger's time travel created a new timeline, and that it was the Thanos from that new timeline that traveled back to the original timeline (and thus killing him does not undo the past of the original timeline). But in branching time travel, travel back to the original timeline from a branching timeline cannot happen. Without this constraint, grandfather paradoxes are still possible. This theory is also inconsistent with Banner's explanation of why he must return the stones (he says it is to prevent bifurcation) and with the appearance of Old-Captain American at the end of the film. If he decided to stay in the past after returning the stones, he would still be in that other timeline--not the original. If something like this is what the writers had in mind, they should have used all the "Quantum Mechanics Talk" to generate the concept of alternate realities. Instead of traveling to the past, the Avengers could have been stealing infinity stones from another universe and then returning them. Indeed, this would not only have avoided invoking temporal paradoxes, but would have been more scientifically valid. As I explain in Lecture 9 of the Sci-Phi course, there are "multiverse interpretations" of Quantum Mechanics that are very plausible.  

Copyright 2019, David Kyle Johnson