The Grandfather Paradox is my biggest pet peeve about sci-fi. The Grandfather Paradox is so named because of the logical problem that if one goes back in time and kills their grandfather, than they never would have been born, and thus never would have killed their grandfather. Yet it doesn't seem possible that there is some kind of physical world that would allow time travel yet not allow a time traveler to kill their grandfather. The problem with time travel in science fiction has always been that the main reason why you'd want to travel to the past is to change something, but if you actually succeeded, then why would you have gone back in time to change something that never happened because you prevented it? It's a lazy plot device that doesn't make sense.
However, there are some ways time travel can be implemented in a story without causing a grandfather paradox. One of them was famously done by the TV show Lost, and I'm told the show Continuum does something similar. In Lost, there's an entire character, a physicist, who's only purpose in the show appears to be to constantly remind everyone that time travel is possible but the past cannot be changed. Several of the main characters travel to the past and of course they try to prevent something the show calls "the incident", but end up causing the very thing they try to prevent, thus avoiding the paradox.
Another way to avoid the paradox is to travel back in time to obtain something that you need but cannot get in the present. Star Trek IV: The Voyage Home, one of my personal favorites, has the crew of the Enterprise traveling back in time to pick up a couple humpback whales and bring them BACK TO THE FUTURE where humpback whales are extinct and unfortunately there is an alien probe trying to communicate with them whose signal is destroying the world. Gee, I hope they didn't talk very long the first time the probe came huh? An excellent example of grown-up yet still unreformed hippies making films. The Grandfather Paradox is avoided because they don't really change anything in the past. Humpback whales still go extinct, so presumably even though they interacted with some people and stole a couple of whales that were released into the ocean, saving them from being killed by whalers, nothing they did in the past changed anything that would have eliminated their reason for traveling back in time in the first place, so the Grandfather Paradox is avoided. However, they couldn't just leave it at that. In one famous scene Scotty, the engineer, explains to a 1980s glass manufacturer how to make "transparent aluminum" because they need some help modifying their ship to transport the whales. At the end of the classic scene, Scotty, confronted with the fact that he just altered the timeline by doing this, says "How do we know he didn't invent the thing?" In fact, he did invent the thing, and he always invented it, because if this is in fact actually the past corresponding to the future, then this is the way it always happened and they didn't actually change anything. Ofc, Scotty meant that maybe he invented it even without their intervention, but the point is that anything which constitutes an actual change in the past doesn't make sense, because then it would no longer be the same past or the same future.
Some sci-fi gets around this by claiming every time something is changed in the past a new timeline is created, branching off from the old one, essentially creating another universe. This is the approach of the new Star Trek films, conveniently allowing a reboot of the original series where they can do anything they like, and of shows like the new Flash series, which ends up with multiple characters from multiple universes, including an evil scientist from their timeline replaced by a happy-go-lucky, nincompoop author from what they call Earth-2, and of course Supergirl from another show who hails from Earth-38. I used to like The Flash, but's it's just gotten so ridiculously embroiled in various versions of the multiverse solution to the GP that I just can't handle it any more. Once you use time travel to solve a problem there's no going back. Time travel is a deus ex machina that can solve any problem, so now in order to keep the story plausible you must keep the stakes low enough that time travel is either not necessary or not worth it (boring), or invent an explanation why you can only use it in rare instances. In the end, it always creates infinitely self-replicating plot holes. There's an older show called Quantum Leap where the main characters are constantly traveling back in time to change something but every time they go BACK TO THE FUTURE something else they didn't intend happens which results in their future world not being the same as they remembered it (how could they remember it if it never was?). So they just keep going back changing things trying to make everything the way it's supposed to be in the future. Ultimately this trope only makes sense if you posit multiple universes. Some even argue that the multiverse already exists where everything that could happen does happen, so traveling back in time to change the past only means that when you travel BACK TO THE FUTURE, you are simply going to a different branch of the already existing multiverse where...you traveled back in time from a different universe and changed history in the new one, the one you now inhabit...in the future. I won't go through all the logical problems here but essentially this explanation doesn't make logical sense either in the end.
In the end, the Lost approach is the only way to go. The past has already happened and cannot be changed by someone from the future. The main problem with this is that it limits time travel as a plot device. It only works in Lost because the time travelers are missing all sorts of key information about what actually happened in the past. They don't know they were the ones who caused the incident they tried to prevent. If they did know, then obviously they wouldn't have gone back, and the thing wouldn't have happened, and you have a sort of reverse Grandfather Paradox even though no time travel occurs. So for time travel to make sense as a plot device it has to work the same way a prophecy does. You don't actually know what it means until it happens. The solution is limited knowledge, and this only makes sense in certain story contexts like Lost where nobody knows what's going on, including the writers.
So finally, to Avengers: Endgame! I'm not a huge fan of all these comic book movies, and I've never read comic books at all, but like everybody else I go see them. They are fun and make good movies, but I never take them too seriously because they just aren't worth it. After I saw Infinity War I figured, like pretty much everybody else, that all the characters who died from Thanos' snap were either not dead or weren't going to stay dead. I also figured that they would use time travel to fix it. I assumed they would somehow regain control of the time stone, in some way which Dr. Strange had anticipated or perhaps had even set up using the time stone before he gave it to Thanos. Then they would go back in time and change the events of Infinity War. They go back in time, fight Thanos again only this time they win because they have knowledge, or perhaps even help, from the future versions of themselves. It would have been appropriately epic, appropriately clever and a pretty satisfying ending. But it also would have been a Grandfather Paradox. Why go back in time to save the world if it never got blowed up in the first place?
SPOILER ALERT!
Instead, the Endgame writers tried to avoid the dreaded GP using the Star Trek IV method: they go back in time to obtain possession of the Infinity stones, travel BACK TO THE FUTURE with them, use them to magically make everyone who died not dead, they were just gone for five years, then go back to the past and put the Infinity stones back right where they found them.
Conceptually this could have worked, but the way it was implemented fails to avoid changing the past. I haven't done an extensive check of this, but it seems like they changed a dizzying number of scenes we've already seen in previous films. They for sure interact with characters in the past in a way that could not fail to have affected the way those characters behave and act in the future, including for instance a scene where Captain America fights himself. They also gloss over the part where Captain America travels back in time by himself and puts all six Infinity stones back where they found them. Major, major plot holes abound here, simply because a large portion of the movie involves the Avengers' intricate plan to obtain the Infinity stones in the past. It requires no less than three separate teams working, in present terms, all at the same time. This is hastily justified by some technobabble preventing them from making as many trips as they want with as many people as they want. Of course the real reason is they wanted to showcase all the Avengers' various talents in a scenario where they are all actually needed and not on screen just because. This is especially true of Hawkeye and Black Widow, the two members who are just plain old humans who are just really good at archery and martial arts, respectively. Why send them if you could send the Hulk instead? Their fight scene makes this evident because...well...they end up just fighting each other over who gets sacrificed to obtain the soul stone. Really? You are going to send the two regular humans, without Iron Man style suits, on the same mission with no superpowers to help out? Com' on, even the Hulk got an Iron Man suit once. So how come they need to do this, and it's super difficult, and all sorts of things go wrong, but suddenly when it's time to put the Infinity stones back where they found them in the past Captain America can do it all by himself? In a single trip? To multiple different times? Pure nonsense.
What concerns us here though is the dreaded GP. Do they actually avoid it? Obviously not. In the first part of the movie Thanos destroys the Infinity stones and is then found and killed by the Avengers, who find out from him the stones have been destroyed and can't be used to undo the snap. Conceptually, they could have just gone back in time, figured out a way to steal all the stones and put them back without being seen or altering the timeline. But like Star Trek IV, the writers just couldn't help themselves. Even if you take away Cap's fight with himself, Nebula killing her past self, and all the other things that happen which make great movie scenes but not great logical sense, there's still the big baddie. They couldn't just leave Thanos alone. Thanos from the past figures out what the Avengers from the future are doing and tries to stop them, even following them BACK TO THE FUTURE resulting in a huge battle between Thanos and all his minions and all the resurrected heroes over the Infinity stones. They just could not resist doing this. Thanos is KILLED. The past Thanos is killed in the future. Obviously if past Thanos is killed in the future he never would have gotten the Infinity stones in the first place and the snap never would have happened.
So why did the Endgame writers take such pains to avoid a GP with the Infinity stones but totally disregard it with respect to Thanos? Why did Cap even bother returning the stones when killing past Thanos already created the same kind of GP he was trying to avoid? In fact, the Avengers at the beginning of the film even discussed traveling back in time and killing Thanos, but reject it on the grounds that it would create a GP. But then they end up killing adult Thanos before the snap? They killed what is supposed to be the same Thanos twice in the same movie. Ugh.
Stuff like this is why sci-fi writers should just avoid time travel altogether. There's just too many ways to use it as a clever plot device that inevitably results in a Grandfather Paradox. Time travel is to a sci-fi writer what the apple was to Eve. It's just too big of a temptation to do the wrong thing. And as Avengers: Endgame shows, actually avoiding a GP would probably make your movie kind of suck. Suppose their plan succeeded without any GPs. All that would have happened was they come BACK TO THE FUTURE with the stones, do the reverse snap, and then put them back in the past. Game over. No epic final battle with a past Thanos and all his minions. No Tony Stark heart-to-heart with his father. No Nebula killing her past self in a thinly veiled commentary on the way her character has changed. No future anti-establishment Captain America fighting his past self defending world order. Just we win, yay, game over. In order to make the kind of comic book movie they wanted, they needed the epicness factor, and to do that they fell into the GP trap and destroyed the justification for the entire plot of Endgame, avoiding one GP just to create a dozen more.
Now that's whack.
However, there are some ways time travel can be implemented in a story without causing a grandfather paradox. One of them was famously done by the TV show Lost, and I'm told the show Continuum does something similar. In Lost, there's an entire character, a physicist, who's only purpose in the show appears to be to constantly remind everyone that time travel is possible but the past cannot be changed. Several of the main characters travel to the past and of course they try to prevent something the show calls "the incident", but end up causing the very thing they try to prevent, thus avoiding the paradox.
Another way to avoid the paradox is to travel back in time to obtain something that you need but cannot get in the present. Star Trek IV: The Voyage Home, one of my personal favorites, has the crew of the Enterprise traveling back in time to pick up a couple humpback whales and bring them BACK TO THE FUTURE where humpback whales are extinct and unfortunately there is an alien probe trying to communicate with them whose signal is destroying the world. Gee, I hope they didn't talk very long the first time the probe came huh? An excellent example of grown-up yet still unreformed hippies making films. The Grandfather Paradox is avoided because they don't really change anything in the past. Humpback whales still go extinct, so presumably even though they interacted with some people and stole a couple of whales that were released into the ocean, saving them from being killed by whalers, nothing they did in the past changed anything that would have eliminated their reason for traveling back in time in the first place, so the Grandfather Paradox is avoided. However, they couldn't just leave it at that. In one famous scene Scotty, the engineer, explains to a 1980s glass manufacturer how to make "transparent aluminum" because they need some help modifying their ship to transport the whales. At the end of the classic scene, Scotty, confronted with the fact that he just altered the timeline by doing this, says "How do we know he didn't invent the thing?" In fact, he did invent the thing, and he always invented it, because if this is in fact actually the past corresponding to the future, then this is the way it always happened and they didn't actually change anything. Ofc, Scotty meant that maybe he invented it even without their intervention, but the point is that anything which constitutes an actual change in the past doesn't make sense, because then it would no longer be the same past or the same future.
Some sci-fi gets around this by claiming every time something is changed in the past a new timeline is created, branching off from the old one, essentially creating another universe. This is the approach of the new Star Trek films, conveniently allowing a reboot of the original series where they can do anything they like, and of shows like the new Flash series, which ends up with multiple characters from multiple universes, including an evil scientist from their timeline replaced by a happy-go-lucky, nincompoop author from what they call Earth-2, and of course Supergirl from another show who hails from Earth-38. I used to like The Flash, but's it's just gotten so ridiculously embroiled in various versions of the multiverse solution to the GP that I just can't handle it any more. Once you use time travel to solve a problem there's no going back. Time travel is a deus ex machina that can solve any problem, so now in order to keep the story plausible you must keep the stakes low enough that time travel is either not necessary or not worth it (boring), or invent an explanation why you can only use it in rare instances. In the end, it always creates infinitely self-replicating plot holes. There's an older show called Quantum Leap where the main characters are constantly traveling back in time to change something but every time they go BACK TO THE FUTURE something else they didn't intend happens which results in their future world not being the same as they remembered it (how could they remember it if it never was?). So they just keep going back changing things trying to make everything the way it's supposed to be in the future. Ultimately this trope only makes sense if you posit multiple universes. Some even argue that the multiverse already exists where everything that could happen does happen, so traveling back in time to change the past only means that when you travel BACK TO THE FUTURE, you are simply going to a different branch of the already existing multiverse where...you traveled back in time from a different universe and changed history in the new one, the one you now inhabit...in the future. I won't go through all the logical problems here but essentially this explanation doesn't make logical sense either in the end.
In the end, the Lost approach is the only way to go. The past has already happened and cannot be changed by someone from the future. The main problem with this is that it limits time travel as a plot device. It only works in Lost because the time travelers are missing all sorts of key information about what actually happened in the past. They don't know they were the ones who caused the incident they tried to prevent. If they did know, then obviously they wouldn't have gone back, and the thing wouldn't have happened, and you have a sort of reverse Grandfather Paradox even though no time travel occurs. So for time travel to make sense as a plot device it has to work the same way a prophecy does. You don't actually know what it means until it happens. The solution is limited knowledge, and this only makes sense in certain story contexts like Lost where nobody knows what's going on, including the writers.
So finally, to Avengers: Endgame! I'm not a huge fan of all these comic book movies, and I've never read comic books at all, but like everybody else I go see them. They are fun and make good movies, but I never take them too seriously because they just aren't worth it. After I saw Infinity War I figured, like pretty much everybody else, that all the characters who died from Thanos' snap were either not dead or weren't going to stay dead. I also figured that they would use time travel to fix it. I assumed they would somehow regain control of the time stone, in some way which Dr. Strange had anticipated or perhaps had even set up using the time stone before he gave it to Thanos. Then they would go back in time and change the events of Infinity War. They go back in time, fight Thanos again only this time they win because they have knowledge, or perhaps even help, from the future versions of themselves. It would have been appropriately epic, appropriately clever and a pretty satisfying ending. But it also would have been a Grandfather Paradox. Why go back in time to save the world if it never got blowed up in the first place?
SPOILER ALERT!
Instead, the Endgame writers tried to avoid the dreaded GP using the Star Trek IV method: they go back in time to obtain possession of the Infinity stones, travel BACK TO THE FUTURE with them, use them to magically make everyone who died not dead, they were just gone for five years, then go back to the past and put the Infinity stones back right where they found them.
Conceptually this could have worked, but the way it was implemented fails to avoid changing the past. I haven't done an extensive check of this, but it seems like they changed a dizzying number of scenes we've already seen in previous films. They for sure interact with characters in the past in a way that could not fail to have affected the way those characters behave and act in the future, including for instance a scene where Captain America fights himself. They also gloss over the part where Captain America travels back in time by himself and puts all six Infinity stones back where they found them. Major, major plot holes abound here, simply because a large portion of the movie involves the Avengers' intricate plan to obtain the Infinity stones in the past. It requires no less than three separate teams working, in present terms, all at the same time. This is hastily justified by some technobabble preventing them from making as many trips as they want with as many people as they want. Of course the real reason is they wanted to showcase all the Avengers' various talents in a scenario where they are all actually needed and not on screen just because. This is especially true of Hawkeye and Black Widow, the two members who are just plain old humans who are just really good at archery and martial arts, respectively. Why send them if you could send the Hulk instead? Their fight scene makes this evident because...well...they end up just fighting each other over who gets sacrificed to obtain the soul stone. Really? You are going to send the two regular humans, without Iron Man style suits, on the same mission with no superpowers to help out? Com' on, even the Hulk got an Iron Man suit once. So how come they need to do this, and it's super difficult, and all sorts of things go wrong, but suddenly when it's time to put the Infinity stones back where they found them in the past Captain America can do it all by himself? In a single trip? To multiple different times? Pure nonsense.
What concerns us here though is the dreaded GP. Do they actually avoid it? Obviously not. In the first part of the movie Thanos destroys the Infinity stones and is then found and killed by the Avengers, who find out from him the stones have been destroyed and can't be used to undo the snap. Conceptually, they could have just gone back in time, figured out a way to steal all the stones and put them back without being seen or altering the timeline. But like Star Trek IV, the writers just couldn't help themselves. Even if you take away Cap's fight with himself, Nebula killing her past self, and all the other things that happen which make great movie scenes but not great logical sense, there's still the big baddie. They couldn't just leave Thanos alone. Thanos from the past figures out what the Avengers from the future are doing and tries to stop them, even following them BACK TO THE FUTURE resulting in a huge battle between Thanos and all his minions and all the resurrected heroes over the Infinity stones. They just could not resist doing this. Thanos is KILLED. The past Thanos is killed in the future. Obviously if past Thanos is killed in the future he never would have gotten the Infinity stones in the first place and the snap never would have happened.
So why did the Endgame writers take such pains to avoid a GP with the Infinity stones but totally disregard it with respect to Thanos? Why did Cap even bother returning the stones when killing past Thanos already created the same kind of GP he was trying to avoid? In fact, the Avengers at the beginning of the film even discussed traveling back in time and killing Thanos, but reject it on the grounds that it would create a GP. But then they end up killing adult Thanos before the snap? They killed what is supposed to be the same Thanos twice in the same movie. Ugh.
Stuff like this is why sci-fi writers should just avoid time travel altogether. There's just too many ways to use it as a clever plot device that inevitably results in a Grandfather Paradox. Time travel is to a sci-fi writer what the apple was to Eve. It's just too big of a temptation to do the wrong thing. And as Avengers: Endgame shows, actually avoiding a GP would probably make your movie kind of suck. Suppose their plan succeeded without any GPs. All that would have happened was they come BACK TO THE FUTURE with the stones, do the reverse snap, and then put them back in the past. Game over. No epic final battle with a past Thanos and all his minions. No Tony Stark heart-to-heart with his father. No Nebula killing her past self in a thinly veiled commentary on the way her character has changed. No future anti-establishment Captain America fighting his past self defending world order. Just we win, yay, game over. In order to make the kind of comic book movie they wanted, they needed the epicness factor, and to do that they fell into the GP trap and destroyed the justification for the entire plot of Endgame, avoiding one GP just to create a dozen more.
Now that's whack.
