There are different types of time travel (they have different rules and constraints) throughout literature, so I was wondering if you could list some of them here, and which ones are best for driving forward narration without creating plot holes. I would like a list, but I don't think it's good for a Q&A website, so I will just ask for the best or preferred way to use time travel in narration without plot holes, and which types of time travel are best or you personally like to use for achieving that.
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There are basically three kinds of time travel paradoxes in fiction:
- Grandfather paradoxes alter history in a way that would prevent the time travel into the past that made such an alteration happen, e.g. because the original timeline motivated the journey to achieve such an alteration, or because the traveller's existence is prevented. The name derives from the latter, since if you killed your grandfather when he was very young you'd never exist, which means you wouldn't kill him, which means... well, you get the idea. Marty McFly faces such a threat in Back to the Future, and a subtler variant - preventing his return to 1985, and hence eventual second visit to 1955 - is a danger in the first sequel. The motive-deleting version is a plot point in a very unfaithful-to-the-book 2002 film adaptation of The Time Machine. The protagonist eventually learns the reason he couldn't prevent his lover's death is that then he wouldn't have gone back to do it.
- Predestination paradoxes (in which an event happens because someone travelled in response to it, e.g. accidentally starting a fire while trying to discover how it happened) aren't really paradoxes, and you could argue they're not plot holes either. But they sure bother people. The 2010 Doctor Who episode The Big Bang relies heavily on predestination, as can be seen if you watch the episode and imagine it from the Doctor's perspective instead of Amy's or Rory's. Rory lets him out of the Pandorica and tells him everything he'll do for the first two thirds or so of the episode, and all because the Doctor ends up time travelling in ways that told him what it would involve. I remember at the time this was what reviewers minded, not the grandfather alternative. Neither paradox type we've discussed so far need contradict the time travel rules you establish, but Back to the Future can't keep its story straight on this one: Marty changes history from what he remembered in some ways, but to what he remembered in others (e.g. encouraging a man to run for mayor).
- The last one, which people don't discuss as much, is when a loop technically doesn't make sense if you keep track of the age or entropy of objects. Futurama: The Game gives an example. The crew find their ship damaged and repair it, then later go back in time in a way that damages their ship. They land next to the ship when it was younger and take the undamaged ship, causing their younger selves to find the newly wrecked one minutes later. If you think about it, this means the ship they find has already been around the loop an indeterminable number of times. I guarantee your readers/audience won't pick up on things like this.
Plot holes aren't necessarily a bad thing; but if you're worried about them, set clear rules on whether you can change history to or from what you remembered, then stick to them. The most profitable time-travel film series, BttF, got away with it, but that doesn't make it bad advice.
The easiest way to time-travel without paradoxes is the rewind universe.
Think of the time-machine as a bubble that preserves your body and mind. The entire universe around this bubble reverts to 1963. When you get out, the future is gone, it hasn't happened, and from now on, the universe will evolve again, and may evolve differently even if you do absolutely nothing to affect it.
The reason for this is (putting on a physicist's hat) due to quantum wavefunction collapse. When the wavefunction collapses (say, spontaneously for some reason), there are several super-positions of possible particle configurations, and the "collapse" means exactly ONE of these is selected entirely at random as "reality", all the others are discarded.
But if by some means your time-machine finds the magic switch that reverts the universe back to as it was in 1963, all the wavefunctions of that moment are restored, and as they collapse, they again select one eigenstate (the technical name for the super-positions) entirely at random as "reality".
So, although most things happen exactly as they did before, not everything will, this "fundamental randomness" will not be repeated the same eigenstate selections from the first nanosecond after the rewind.
Then you have the interference of yourself. You were preserved inside the bubble, all your memories and flesh were preserved. So in a sense you know the most-likely future, the bigger things are the more likely they are to repeat. If you already exist in 1963, then two of you exist; you are just another human 56 years old (from 2019). You could go kill your younger self, your parents, if you go far back all your ancestors.
It doesn't make a difference in this new world that you wouldn't be born or wouldn't survive, because you don't have to invent or use a time machine to get back there.
Only your brain contains the memories of the future that evolved before, with your parents or siblings. But memories aren't real, they are entirely encoded in the configuration of neurons in your brain, and you brought that with you.
As far as the universe is concerned, you just randomly materialized in 1963 and the universe continues from there. There is no future after the rewind, so there is no communication or cause-and-effect with the future.
If you travel forward in time, faster than one-second-per-second like we all do, then you will just see the slightly different evolution of this rewinded universe, and many things at the macro level will be nearly the same, and many things at the micro-level may be different.
Particularly, perhaps, the effects of spontaneous decisions that had little preamble to push them in a particular direction. Impulse buying, or impulse sex that resulted in a pregnancy, or impulse violence that changes the course of a life.
Other things you might count on, and ultimately change the course of the future. You could stop the JFK assassination, perhaps. You could be there (as your future self) to save a childhood friend of yours from drowning, or dying in a car accident.
Things like that do not change the future you lived through, but change the future of the reset world. You might find your 56-years younger self, because your friend lived, did not experience the shock or grief you did, and as a result chooses a slightly different path in life, and never invents a time machine (e.g. if you jump forward a year at a time to check).
There are no paradoxes, once you travel back, or rewind the universe, nothing of the future exists anymore, except for what was in the protected bubble: You, your brain, your memories, your notes, computers or whatever tech or anything else you brought with you. The people, the history, everything after the point in 1963 when you arrive has been obliterated, just as if it never existed. And the world starts again, with you in it. But even that is not a paradox, you and your machine are just energy (E=MC^2) in a particular configuration of particles. Nothing in quantum mechanics prevents that particular configuration from spontaneously appearing. (It is only astronomically unlikely, but not impossible.)
Time travel is cheating. You are already cheating. Even if you come up with a worldbuild-y excuse how your cheat works or what limits it, you are already cheating, so just accept it.
Plot holes are not created by time travel, they are created by poor writing – like inventing "rules" for something that is already breaking the rules, and then forgetting to follow your own rules.
Is an old professor going to pop up at the beginning of the story and tell the reader the rules? How are the rules known, did someone try to cheat on their cheat and then discovered they couldn't? Did it kill them, or did they keep trying over and over until they gave up? Did everyone else agree that they would honor the rules and never attempt to prove or disprove the "theory"?
What controls this rule (the author)? Is it pretend physics (the author)? Destiny and fate (the author)? Or maybe some doodad not been invented yet (sequel)? Maybe a daredevil with sex appeal and charisma will come along and break the rule (just a little) – is that a broken story with a plot hole, or is he a truly the chosen one who cheats better than the other cheaters in every story that involves (magic) cheating?
You are the author, not some rulebook that says what is the correct way to cheat (in this story). If the story has plot holes, it's because the author put them there. Maybe it's not an accident because this old professor's theory of how cheating works is wrong, and it takes the right kind of hero to break the rules (of cheating).
This isn't worldbuilding, this is writing. We don't dictate made-up rules by consensus. The only master of consistency within your story is you. If you want to break the rules that you have set up within your own story, you might have a good reason. It might fit your theme about taking a leap of faith, or trusting the unknown, or heck, just risking it all on one last gamble. There are narrative reasons to break your own rules, even for individual characters – because you want to say life is unfair, or a higher power can intervene, or maybe the whole point is that the rules are breaking down, or "Crap! We just broke the rules! and now we are doomed".
It's your universe. We can help you with mistakes or bad plot choices, but there is no system of rules that makes that easier or foolproof. Not cheating physics is one way to stay "safe" but that is the whole point of the story, isn't it?.
Time travel v. 1: Back to the Future model. In this model people can go back in time and change the past; however, they must avoid the grandfather paradox or else a "bad thing" will happen like disappearing, which resolves it by force, but may also destroy the universe. This brings a second "dimension" of time into the mix, wherein regardless of how Marty moves in the first dimension he's always going to disappear unless he fixes the loop that will allow him to go back in time.
The problem with this form of time travel is when the "bad thing" does happen. The act of the bad thing happening causes many changes in of it self. Imagine if somebody noticed a Marty disappearing that would set in motion a series of events that would probably have prevented him from ever being able to disappear, which causes more problems. What BttF fails to define is what would happen if Marty disappears, would all of causality keep going in loop where he cannot exist but disappears and thus can exist, or would time in each case go on without him, creating what are essentially two universes. Or would somebody notice Marty disappearing and/or the universes changing into one another thus causing a cascading effect where eventually, everything this side of the event horizon will be in a state of complete uncertainty. Either way the "bad thing" seems to be an overly convoluted way to bring about a simpler result, and leaves a whole lot to be explained. Finally, ignoring the "bad thing" entirely, this form of time travel is hard to keep straight in your head.
Time Travel v. 2: The Terminator way. These rules are pretty simple, when somebody goes back in time there is no need to remember the "previous" timeline at all, you can kill your own grandfather or do whatever you want because it doesn't matter, a whole new separate timeline has been created.
There are a couple problems here, the first one is that if all of the world lines in space time can change then what is the point of time anyway? 4D space time is what allows time to be possible. And if it is not necessary wouldn't it be simpler for conscious beings to just live in a 4D reality that changes?
Although the only thing this paradox defies is Occum's Razor, and universe design best practices, it is a pretty clear problem, while a "bad thing" will always be a little more foggy as to whether or not it may be logically consistent; however, many people will still prefer the Terminator method over something as messy as disappearing.
Time Travel v. 3: The Lost method. In Harry Potter and Lost, the future is destiny. Anything that a time traveler thinks they might be changing, is actually just something they never knew happened, and any attempts to change history will ultimately fail. Spacetime is a completely static object, albeit yet highly responsive to our actions, constantly thwarting that which might change things.
The problem with this form of time travel is probably the easiest to spot. So maybe the first time one tries to kill their grandfather it turns out, that he wasn't actually the father of your father. And maybe your gun jams on the second attempt, and on your third attempt he already had sex with your grandma by the time you got him. On your 8th try you might even learn that your dad didn't even do the thing you are trying to prevent him from being born for, but it's principal at this point. Eventually; however, your going to have witnessed nearly every inch of every second surrounding the time your grandparents bang, and the universe will have run out of space to bring up some wacky antic to stop you.
As logically inconsistent as this seems there can ever be any proof that time travel actually works via a self fulfilling destiny. Both Harry Potter and Lost might actually follow the Back to the Future model, we are only seeing things from the point of view after a stable sequence of events have been set in place. In BttF we only see when Marty first goes on his adventure; however, as his t2 goes on his memories of hearing about how his mother and father met by being hit by a car will eventually disappear with the image on the picture. We don't see this (as it is kind of sad), but they will be replaced by the knowledge that he was named after the person who brought his rich parents together after being hit by a car. From his new perspective his adventure will never have changed anything, he was simply fulfilling what had to be done. Likewise, when Harry saves himself he might actually just practicing responsible time travel ship by keeping the timeline consistent. Even though he knows that he would some how survive the death eaters, trying to wait for another savior could easily put him in an endless loop of getting half his soul sucked out, saving himself, only to have himself not save himself, and so forth, or wind up causing a "bad thing".
There are also reasons why each of the other types of time travel might appear to be this one; however, I don't want to spend all day on this.
Time travel v. 4: The last one. It might be the case that whenever somebody might use a time machine to cause a Grandfather paradox, or some other kind of problem, the time machine just doesn't work. I cannot think of an example of this one, but I would bet it would lead to tension between characters who cannot ever get the time machine to work, and the characters who have the resolve required to stop themselves from doing whatever these things are.
I cannot think of any paradoxes with this form of time travel, at least none that are unique to it. Nor does the failure of the time machine introduce a hundred more questions like a "bad thing" does; however, it does not give you quite as much creative freedom as the Terminator method does, which could possibly be why I haven't seen it implemented, sacrificing quality for consistency defeats the whole point of consistency. However, I would argue that there is something to be said about originality.
To avoid plot holes use the end point consistency, or self-correcting, time travel model where, by definition, paradoxes cannot occur and then be careful to keep track of the final timeline. End point consistency basically means that no matter what a traveler might do in the past the end result, in terms of global events and conditions, is the same. This doesn't necessarily mean that the past cannot be changed at all but it does mean that small changes cannot lead to large ones. For example if someone follows the cliche and goes back in time to kill Hitler at birth then you simply get another leader with the same background and ambitions but a different name leading the Nazi party into World War II, the only change to world history is the name in the books. This is the simplest, in my estimation at least, model of time travel to write because you don't have plot holes or causality paradoxes, you can be "grandfathered" out of existence but there's no point because someone else, who's exactly the same as you while not being you, will take your place so deliberate grandfathering is a waste of time and energy. Most importantly as an author you don't have to explain the exact events that get time back on track you simply have to assert that it happens and make sure you stay consistent about your event series and their ultimate outcome.