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If a character - or a bunch of them - travel back in time to undo a chain of events that would have led to their conception and births, is it within reason that I have them die or 'fade from existence' within the time period they've traveled back to? Would it be feasible for them to continue living, or would the more logical thing would be to have them erased altogether?

Also, while we're on this topic, would readers call this a cop-out, since said characters are going to be MCs in their own right and this happens at the very end of a series? How can I do this - have them fade or die as it were - in a way that won't anger my readers?

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    $\begingroup$ Welcome to Worldbuilding, Jack Rabbit, there two questions. The time travel about undoing their pasts whether they die or fade from existence is a worldbuilding question. It still has to run the gauntlet of being acceptable or not. Whether readers will like this or not is really a question for Writers SE and should be asked there. $\endgroup$ – a4android Sep 10 '17 at 9:25
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    $\begingroup$ I will point out you don't need the grandfather paradox (which is easily solved if, perhaps, your grandfather is not really your grandfather...). Permanently and irrevocably lock yourself in a room with a time machine. Wait six hours. Get in the time machine and go back FIVE hours, appearing in the same room. Can you exit the time machine and kill your earlier self? No tricks, the person that traveled back in time IS the earlier self, the one that waited six hours alone in this room and unable to leave. Not the time traveled version of himself, not a twin. Paradox! $\endgroup$ – Amadeus Sep 10 '17 at 19:01
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Time travel is a dicey one and there's essentially three ways of doing it (+ variations) and you just have to pick some rules and stick to them.

  1. Preserved Chronology (Prisoner of Azkaban)

    You don't prevent your own birth because you were born. You can try, but you'll end up causing your birth. This is the one Stephen Hawking likes.

  2. Malleable Chronology (Back To The Future)

    There are many variations on this one but essentially you CAN change the past but it will either "Catch Up To You" (Marty fading out) or you just find yourself in a world where you shouldn't exist. (In Doctor Who they have creatures called Reapers who come in and remove paradox people from the timeline.) If you want a more "Fantasy" view of time, this is the one to go with.

  3. Many Worlds Chronology (Flash?)

    I have never actually seen Flash but to my knowledge, every time they time travel in that show it creates a new timeline, and the original timeline never sees you again. This also has many variations depending on how much handwaving you like. You might find another copy of yourself when you get to the new timeline or not, for instance.

The thing about science fiction is that it's all about consistency. Readers want to be swept away on impossible adventures but they want those adventures to stick to the rules they establish.

On the "cop-out" part of your question, I think if you framed it as a self-sacrificial mission then it's fine as long as they "die". If they don't die then you'll need a damn good explanation to prevent breaking the suspension of disbelief.

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    $\begingroup$ Credit [en.wikipedia.org/wiki/Flash_(comics)](Flash) if you like, but as I understand it, the Many worlds interpretation actually has scientific basis, widely recognized as being worthy of serious academic study. However, there's a huge difference between quantum physicists conjecturing about infinite possibilities each actually being real, and us having actually figured out how to practically jump between the time lines, hence why we don't regularly do any time traveling. $\endgroup$ – TOOGAM Sep 10 '17 at 15:08
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    $\begingroup$ The Flash TV show has the least coherent theory of time travel of any fiction I have ever seen. The TV show Continuum mostly follows a Many Worlds interpretation; at one point a character who has traveled back in time decides to (essentially) kill their younger self for a compatible organ transplant so that their older self can continue to live. $\endgroup$ – Charles Staats Sep 10 '17 at 15:34
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    $\begingroup$ @TOOGAM Just noting that the "many worlds interpretation", which you linked to the Wikipedia article for, addresses a philosophical question about the meaning of quantum mechanics. As far as I know, its relevance to QM is not considered to make it any more viable as a paradigm of time travel. $\endgroup$ – David Z Sep 10 '17 at 17:19
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    $\begingroup$ The QM thing has nothing to do with time travel. You can employ it in your stories, but what it's more like is saying that infinitely many worlds exist on top of each other and interact, creating quantum wibbliness. $\endgroup$ – Douglas Sep 10 '17 at 17:58
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    $\begingroup$ The time travel many worlds is more about changing macroscopic events. (There is a website that gives you a yes or no answer to a question like a magic 8 ball but it's based off a quantum event so in another universe you make a different choice.) There's nothing stopping you from saying that by travelling back in time you "reset the random of the universe" and QM events can turn out differently. The main point in my answer was that it's your choice. There's loads of websites documenting various fictional categories of time travel and some really cool examples. You just need to keep consistent. $\endgroup$ – Douglas Sep 10 '17 at 18:01
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Would it be feasible for them to continue living,

This is actually the most feasible outcome! It requires the least amount of magical effects.

There is a mode of time travel that isn't discussed above: The future is obliterated. Imagine the time machine resets the entire universe to a previous state, with the only difference being the machine itself, and Bill inside it, are protected from this reset. (for discussion, call this TT-Bill, for Time Traveled Bill).

That becomes the equivalent of a machine and person(s( from the future simply materializing, in an instant, in our time. TT-Bill's time and what led to TT-Bill no longer exists. The universe is reset from 2342 to 2017 with a simple edit: a cut-and-paste of a sphere with a ten foot radius. Everything inside that sphere is severed from cause and effect, TT-Bill's 2342 universe and all the events leading up to him, his mind and knowledge and equipment, cease to exist, but the product of all those events, the physical manifestation of TT-Bill and his machine, is preserved (and transported to wherever in the Universe Earth was in the past; since Earth is looping its way through space).

You can let TT-Bill travel into the future in a matter of seconds, as far as you want; this is just Einsteinian relativistic time dilation, with a little hand-waving to stay in place. Any effects TT-Bill had on the past just evolve naturally for however many years or centuries are desired. So anything TT-Bill did in 2017, including exploding nuclear bombs and killing dozens of his own ancestors (or all of his own ancestors) has its effect.

In the new version TT-Bill is never born, but it doesn't matter: That was true the moment TT-Bill appeared in 2017: the future was dissolved. Now, just like us in real life, every tiny action or decision TT-Bill takes has an effect on the new future. But what happens in that new future does not affect TT-Bill's body or mind back in 2017; there is no thread of causality running from the future to the past, just as what is going to happen to us, in real life, does not change our bodies or mind right now.

This version of time travel also means duplication of people. Let's have a different Time Traveler, TT-Chuck, he travels ten years from 2017 into the past (2007), the younger Chuck is there. TT-Chuck is there, as a separate and real person. They both evolve; could embrace each other, speak to each other, whatever any two real people could do. TT-Chuck could kill his earlier self, or vice versa, or TT-Chuck can leave his earlier self completely alone.

However they both evolve, ten years later they are both there. If TT-Chuck travels into the future, they are both there; although Chuck has had 10 different years of experience.

For example, say TT-Chuck does something to prevent the imminent accidental death of his (in 2007) 12 year old girl, successfully. That 2007 world is now different than the one TT-Chuck experienced. Let TT-Chuck jump 10 years into the future: Chuck is now the same age as TT-Chuck, but never experienced the death of his little girl: What happened is Chuck came out to drive her to school one day, and found a flat tire with a nail in it. TT-Chuck hammered it in at 3:30 AM, and made sure the tire was completely flat, but Chuck was just irritated: He spent 45 minutes changing the tire, drove his girl to school and made excuses, and that is it. Ten years later, he barely recalls the incident, and neither does his girl.

Chuck was never in the car accident that killed his girl. The rough-housing speeding teens that crashed into them were not killed either. TT-Chuck still remembers it all vividly, the accident, waking with his girl dead and staring beside him, his inability to revive her. The funeral. His emotional scars all remain intact. Nothing is faded.

Because TT-Chuck's brain and neural patterns and connections were transported from 2017 to 2007 intact, a physical system of synapses and neurons, that are not magically affected by what then transpires. New memories (or injuries) can be formed (or suffered) but that isn't magic, that is normal biological processes. Whereas, it would take some form of magic to undo such physical systems and replace them with some other system.

So if he likes, he can try to find a way to watch his little girl grow up. With some preparation from 2017, he can become nearly instantly wealthy in 2007. The changes he intends to make won't have much or any effect on the stock market or Vegas betting markets for sports games. In 2017 he can find convicted forgers that were active in 2007; and get a forged identity he can use in 2007. He can buy the house next door to Chuck and remark on the incredible resemblance between them, they must share some ancestry! TT-Chuck can become lifelong friends with Chuck (he obviously knows enough about Chuck to do that); so he can watch him raise their little girl. And if necessary, time travel again to save her again; creating yet-another Chuck. Or if the proliferation of time traveling Chucks becomes an issue, send a video from the future explaining everything to the first TT-Chuck, including how to prevent the problem: The TT-Chuck sending such a video dissolves; the TT-Chuck receiving the video checks his time-machine's messaging system every day for precisely this reason; and knows it is the real deal.

All other systems of time travel, characters fading out (like in Back to the Future), vanishing, or the universe preventing paradoxes, require some kind of unexplainable magic effect. So does this "reset" version of time-travel, but it is an instantaneous effect that is not operating in any continuous manner or causing any observable and unexplainable physics. One second there is air, in the next second the air is pushed aside and replaced by TT-Chuck, with his very high-tech Time Traveling device surgically implanted as a marble-sized object in his sinus cavity. Then the world, physics, biology, life and death all proceed normally.

TT-Chuck cannot help but have some effects, he is an obstacle to breeze and sunlight, he is consuming oxygen and weighs a few hundred pounds as he walks on the ground. But there is effectively zero chance such effects "butterfly" into any significant changes to the future in the next few years. Heck, 99.9% of normal human deaths have no measurable impact on the course of stock markets, business, politics, wars, sports, or anything else.

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  • $\begingroup$ This is brilliant! Almost all time travel fiction goes for the Timey-Wimey option; this lets you change the past, but without creating logical contradictions. $\endgroup$ – andrewf Sep 11 '17 at 13:14
  • $\begingroup$ @andrewf Some recent sci-fi series about time travel are almost like this, allowing changing of the past and the full ramifications of that when the characters return to the future. But they don't deal well with paradox; they either have magical effects (scratches and scars appearing out of nowhere, memories from multiple timelines, etc). Or the writers just avoid paradoxes with the time travelers themselves (one returns to find she never had the sister she remembers: But no character ever prevents their own birth). This approach could fix all that. $\endgroup$ – Amadeus Sep 11 '17 at 18:38
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This depends on how time travel works in your fictional world. Time travel comes in several different flavours.

If the past cannot be changed your characters are living in a deterministic universe and nothing your characters can do will change the past. The simplest way the universe can stop your characters would be to kill them either immediately before they try to change the past ("their time machine explodes in mid-flight") or during the act of changing it ("the gun used to stop grandfather explodes and kills the character").

If there are multiple timelines, your characters travel into an alternative past which they can readily change, but the present time they came from, their original timeline, remains unchanged. There will, however, be a new timeline they don't exist.

A non-deterministic universe they allow your characters to change their pasts, but they will soon discover the entire timeline will have changed too. They will exist as people who never previously existed, but who are in possession of a time machine (if they're lucky). The other alternative is that the moment they remove themselves from the past, they cease to exist. On the rather curious logical rationale that they never existed ("due to changing their pasts") and so they now don't exist from that point onwards.

Time travel where the characters fade away seems rather silly. There is no known plausible logical or scientific reason why this should remotely happen.

Stephen Hawking favours what he calls the Chronology Protection Conjecture. This is essentially a version of the deterministic universe. Either time travel is impossible or time travellers cannot arrive in their pasts or the past itself cannot be changed.

There is the Novikov self-consistency principle.

Novikov intended it to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general relativity that contain what are known as closed timelike curves. The principle asserts that if an event exists that would cause a paradox or any "change" to the past whatsoever, then the probability of that event is zero. It would thus be impossible to create time paradoxes.

This prevents time-paradoxes from occurring basically because there are an infinite numbers of pathways events can work themselves out and preventing those events from being changed. So no matter what your characters do to their pasts they will still exist and the past will effectively remained unchanged. Dying in the attempt seems the most likely way for this to happen. There are other rather yucky ways it can go too. Like becoming your own father or mother depending on your gender. It doesn't bear thinking about.

The take-home message is, unless you know time travel works in your fictional universe, then your characters are well advised not to tamper with their pasts. They might not like the results (assuming they survive).

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This is a version of the Grandfather Paradox. I found a very interesting solution to this which i have not seen incorporated into a story, so far. The solution goes something like this:

  1. You go back in time and prevent your birth.
  2. Because you were not born you can not travel back in time.
  3. Without going back in time you couldn't have prevented your birth.
  4. And therefore you were born all along.

This paradox is resolved by creating a time loop where you were born and were not born simultaneously. Bringing the affected time frame into a superposition. Weather or not such time loop are possible remains to be seen.

Here a diagram to visualize this:

time loop

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The variations are essentially covered above, but to address your first question:

If you travel back in time (from A to B) and succeed in creating a chain of events that prevent your birth, you cannot logically fade from existence in B because the sequence of events in B would not exist unless you did, and the timeline in B would also have to fade from existence, because the you that created it does not exist. (Back to the Future makes no sense).

To address your second question, assuming the above scenario - there is no way to get back to A from B because, from your perspective, you have effectively destroyed A as a viable destination.

Meanwhile, in A, nothing will have changed - your mission will have appeared to have failed.

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This is a paradox, a very basic one: if you go back in time to kill your father before you were conceived, you will not be conceived, and therefore you will not exist; you will not go back in time to kill your father, and your father will not die and will conceive you, so you will exist after all. Impossible!

The solution to this is for you to figure out how the laws of time work.

One solution that hasn't been mentioned yet is to have many possible futures for every point in time; that if you go back to an earlier time and change things, you will cause a different future based on the changes, and the "original" future (where you came from) will not change and will not cease to exist; but you can not go back there, you can only go back to the future you just now created. I.E. in the "original" future, you will be gone; you travelled back in time but you never come back.

If your question is, on the other hand, "I created this solution; would readers call this a cop-out?" then the question may be off topic.

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Quite frankly, the only way I have ever thought to resolve time travel (at least, without leaving glaring plot-holes) is "You were already there". If they go back to the past, they have already been in the past, and ergo the future they tried to stop would nevertheless happen anyway.

Of course, you could have the 'time travel' actually be multiverse travel, in which case, they would not in fact be erased. They would simply cause a deviation from the otherwise identical universe, as per the others who have brought up the many-worlds theories.

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