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Imagine two groups of people exploring the same location, but at different times. For convenience, think of the group exploring first as the "ancestors" and the group exploring later as the "descendants." It is easy to invent ways for the ancestors to affect the descendants. For example, they leave treasure buried and a map to find the spot in an urn. How can the exploration decisions the descendants make "matter" for the ancestors? This relevance could be figurative, narrative, or even silly. Not necessarily physical cause-and-effect. I would prefer to avoid traditional sci-fi time travel, with its commensurate split time-line paradoxes.

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    $\begingroup$ If you want to avoid time travel, you should untag time travel. $\endgroup$ – kingledion Dec 7 '16 at 2:43
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    $\begingroup$ This would have to involve some form of retrocausality, while this necessarily involve time travel, it can share some of its problems. An interesting concept though. $\endgroup$ – a4android Dec 7 '16 at 3:22
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    $\begingroup$ This question looks familiar, but I can’t find a duplicate. Anybody know where this was discussed before? $\endgroup$ – JDługosz Dec 7 '16 at 5:37
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    $\begingroup$ @AlexanderKosubek my comment that I think this is a duplicate? I don't follow. $\endgroup$ – JDługosz Dec 7 '16 at 10:57
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    $\begingroup$ @JDługosz It's a duplicate to a question that will be posted next year. $\endgroup$ – Sigma Ori Dec 7 '16 at 12:19

18 Answers 18

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Nothing impossible may happen.

In the C and C++ programming languages, there's such a thing called undefined behavior, where basically if the program performs certain illegal actions, it goes in an invalid state. Notice I said if, not when. The program is invalid after, during and even before the action that invoked undefined behavior. In other words, if your program performs a certain illegal action at some point, it's not invalid after that action, it's invalid for its entire execution. A compiler is free to assume that undefined behavior never happens and it can transform the implementation of the entire program, including parts that come before the illegal action, based on that assumption. If undefined behavior is actually invoked, the program may observe paradoxes.

This is a very real effect, happening right now in the world, with current technology. For example, an action on a specific variable (called a "pointer") is invalid if the variable contains a specific value ("null"). Consider this short snippet:

if (ptr == NULL) {
    some_action();
} else { 
    some_other_action();
}

action_that_assumes_ptr_is_not_null(ptr);

In this example, some_action() may never be included in the program at all, even if ptr can sometimes be equal to NULL. Why? Because afterwards, an action is executed unconditionally and that action will put the entire program in an invalid state if ptr is NULL. In other words, as far as the compiler is concerned, ptr cannot be NULL in a valid program, so only some_other_action() can exist in a valid program. The above program could be transformed by the compiler into this:

some_other_action();
action_that_assumes_ptr_is_not_null(ptr);

As you can see, an action that is supposed to take place later will disable a choice (the if construct) that was supposed to take place sooner. Of course, all of this is possible because there is an entity (the compiler) that analyzes the program statically, i.e. outside that program's execution. In other words, this analysis is performed outside the program's concept of "before" and "after".

If your story has metaphysical underpinnings, you could say that just as there are laws of physics that govern what can happen in a given universe, there are meta laws of physics that govern what a universe may or may not do. Much like the laws of a programming language govern what a program may or may not do.

Then, come up with an action (or a sequence of actions) which doesn't violate any specific physical law of our universe but, when performed under certain circumstances, it would put the universe in an inconsistent/invalid state that is illegal as far as the meta laws are concerned. If a universe contains, in its timeline, such actions that would bring it to an illegal state, that entire universe's existence could be rejected and that rejection would happen at a level outside the universe's execution and therefore its concept of time.

Now if the descendants perform an action that is possible but would be illegal ("apparently impossible") provided the ancestors had performed another action, then you're set. The ancestors are not allowed to have performed the second action, because if they had, their universe would have been in an inconsistent state and its existence would have been rejected. Such a universe could not have existed to begin with.

So how would that look? Anything that would prevent the ancestors from performing the action would suffice. However, it would probably be much more interesting to include a person or an entity that can recognize the inevitability of their actions in that meta context and be able to articulate why the ancestors are bound by the actions of their descendants. Without such an agent in the story, the entire explanation could be lost on the audience.

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    $\begingroup$ Oh I like this idea. The universe as the result of our actions being compiled by some compiler ... this opens up a lot of interesting possibilities. $\endgroup$ – Daniel Jour Dec 7 '16 at 11:10
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    $\begingroup$ I’m afraid that somebody unfamiliar with UB may not understand the point you’re getting at: namely, that the compiler is free to call some_other_action unconditionally, even if ptr happens to be 0 in an actual program execution. That is, even though the code specifically tests for ptr != 0, some_other_action still ends up being executed, merely due to the presence of the subsequent action_that_assumes_ptr_is_not_null call. It’s that part of UB which seems to reverse cause and effect (but doesn’t actually). $\endgroup$ – Konrad Rudolph Dec 7 '16 at 11:48
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    $\begingroup$ Yes, I think it’s more understandable now. Anyway, this is a very funky, and (at least to me) unheard-of premise. I’d love for this to be used as a plot device. $\endgroup$ – Konrad Rudolph Dec 7 '16 at 14:25
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    $\begingroup$ Amazing! The universe as gcc with -O3 on! $\endgroup$ – someben Dec 7 '16 at 17:05
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    $\begingroup$ IIRC quantum mechanics sort-of allows this. Under the multiverse interpretation, it translates to saying that universes where impossible things happen cease to exist (or never existed - from a mathematical perspective, they amount to the same thing, but that's less interesting as a story). I think this is how the grandfather paradox is usually resolved in (theoretical) work on quantum time travel. $\endgroup$ – James_pic Dec 8 '16 at 14:34
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That is simply not possible. Cause always precedes effect. The only way to change that would be to time travel (which OP excluded)

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    $\begingroup$ simple and to the point. Time travel of information is as impossible as time travel of matter. And for the very same reasons. $\endgroup$ – Mołot Dec 7 '16 at 7:41
  • $\begingroup$ This is possible without time travel depending on which version of relativity proves correct. See the premonition and subjective time answers for a simple example. $\endgroup$ – nijineko Dec 7 '16 at 17:02
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One classic alternative to time travel is premonition.

One of the ancestors is a seer, a person who can peer into the future.

Given enough cooperation, the seer can replace the typical 'push' request (aka travel back in time) with a 'pull' request: the seer peers into the future and follows advice of the descendant.

How does the descendant know to give the advice? Because the ancestor left a note with a request to do so.

There is no hard time paradox here (killed my grandfather) - merely the soft kind (sent lottery numbers to past self to fund time machine research). The seer gets guidance from a descendant despite the descendant never actually seeing the seer; but upon receiving the much needed guidance, the seer leaves a note for the descendant, asking to provide the advice, asking to speak, show, lecture - into thin air - at the right time and place - the exact time and place the premonition had taken the seer. This stabilizes the time loop, leaving no unsolved paradoxes.

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    $\begingroup$ I like this answer... but if we are being picky, this would be a time travel of sorts. Just instead of materia time travel, this is information time travel. $\endgroup$ – Pedro Gabriel Dec 7 '16 at 10:31
  • $\begingroup$ @PedroGabriel: For point X in spacetime to affect point Y, the influence needs to move from point X to point Y somehow. Transition through space is simple. Transition through time, into the future - at standard one second per second. But for transition back in time, time travel related shenanigans are necessary. Fred's answer is correct - cause and effect need to be bound somehow and if they are separated by time, that time needs to be covered somehow. Be it universe's time looping on itself, time behaving in unpredictable ways, wormholes or alternate timelines; that's all time travel. $\endgroup$ – SF. Dec 7 '16 at 10:52
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    $\begingroup$ There is one way to bypass that, but that's a way the readers will hate. One signature of a true god - not a mere godling - is not being bound by rules of logic. Can an omnipotent god create a rock so heavy even he can't lift it? And then is he omnipotent if he can't lift that rock? Yes, he can, and yes he is. "But that's wrong! That's illogical!" - Yes! But these are mortal's concerns. The god is not bound by such petty limitations! The fact your limited, mortal mind cannot comprehend it, is merely a problem with your mind! That way any paradox, fallacy and plot hole becomes moot. $\endgroup$ – SF. Dec 7 '16 at 11:00
  • $\begingroup$ What if the descendant asks the seer to kill his parents? Then he would never exist and wouldn't have been able to send that request. That looks like a paradox to me. $\endgroup$ – Fred Dec 8 '16 at 0:31
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    $\begingroup$ @Fred He could ask, but as long as the ancestor has the common sense to not comply everything will be fine. On a related note, the descendant revealing the identity of some antagonist as one of his parents could yield some interesting plot points, if the ancestor has the chance to kill him/her (thereby preventing multiple mondial tragedies) but would have to risk creating this paradox to do this. $\endgroup$ – 11684 Dec 8 '16 at 8:09
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Don't do what we did

I don't know what your technology or magic level is going to be, but the classic example would be that the ancestors wish the decedents to avoid the mistakes they made that lead to turning their farmland into a desert, releasing the Dark One, or destroying their homeworld.

There are many mechanisms for this to happen, from the familiar (climate change) to the sci-fi (shouldn't have sent those SETI signals towards the Zerg Overmind) to the fantasy (don't break the seals).

Robert Jordan's Wheel of Time and the in-brain conversations of Rand al'Thor and the Dragon are the best example I can think of in fiction. That brings us to another point. If history or time is cyclical in your world, then it may matter a lot to the ancestors what choices their decedents make.

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Time May Be Subjective Only

According to physics, the linear flow of past to present to future is created by human awareness:

PAST. PRESENT. FUTURE.
In physics, they are all the same thing. But to you, me, and everyone else, time moves in one direction: from expectation, through experience, and into memory. This linearity is called the arrow of time, and some physicists believe it only progresses that way because humans, and other beings with similar neurological wiring, exist to observe its passing.

Time moves as it does because humans are biologically, neurologically, philosophically hardwired to experience it that way. It’s like a macro-scale version of Schrödinger’s cat. A faraway corner of the universe might be moving future to past. But the moment humans point a telescope in that direction, time conforms to the past-future flow. “In his papers on relativity, Einstein showed that time was relative to the observer,” says Lanza. “Our paper takes this one step further, arguing that the observer actually creates it.”

This line of theorizing is still developing, but perhaps there is some possibility for unobserved current events of descendents lives to coexist in time with unobserved events in ancester's lives. During that coexistence, perhaps events could influence each other and have the effects last even once observation begins.

Perhaps "unobserved" events could be when people are asleep, or when people simply are not looking in that direction.

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    $\begingroup$ just to throw an even crazier theory into the mix of that physics... maybe all of the times exist simultaneously and it only conforms to a particular 3D slice when we observe it. $\endgroup$ – The Great Duck Dec 7 '16 at 6:16
  • $\begingroup$ I seriously suspect that after some time, science will discover a way to interact with all of spacetime as if it were all superpositioned. Given the fact that quantum mechanics might seem absolutely unbelievable to someone in the Middle Ages (yet nonetheless still be true), similarly, I think that as science evolves, equally unbelievable truths will emerge. That seems to be the very nature of scientific development--the unthinkable and the unbelievable become reality. $\endgroup$ – Thom Blair III Dec 7 '16 at 11:05
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You don't want time travel in the conventional sense, but to get what you want, you need to mess with time. More specifically, you need to mess with causality. You would have to come up with an entirely new physics in which causality points both ways. The implications of that are staggering and thinking it through is no small feat. All the questions of causality, like free will, re-appear with a vengeance.

You also get the same paradox effects as time travel stories get, just several orders of magnitude worse. In a time travel story, you need to worry about future knowledge affecting past events and what ripples it causes through the timeline that is already established. But with retro causality, everything in your world is in time-loops, and not just one, but millions. If you thought the Many Worlds Theory is mind boggling, the Many Timelines Theory that you have to invent for this to work makes it look like 1st degree math.

But I see this as the only solution to your question. Causality in your world has no direction. That means time has no direction. So in your world time actually is an illusion (some scientists and philosophers discuss this in respect to our world, but so far the evidence is strong that time actually does have a direction).

If causality has no direction, you have the mother of all feedback loops. The butterfly effect with self-reinforcement.

Frankly speaking, the longer I think about it, the more I'm sure your world, if it came into existence, would instantly explode. :-)


Some reading material:

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You can come up with novel ways to describe time travel besides the obvious “time machine”, but retro-causality as you described is explicitly the very meaning of time travel. So no, you can not have retro-causality without time travel.

In the spirit of what you’re getting at, you might look into delayed choice in quantum mechanics. But what you come up with won’t “matter” to the ancestors in their own experience, only show something that can be determined after the fact.

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  • $\begingroup$ No, retro-causality and time travel are not the same thing. Time travel generally means jumping backwards in time, but the arrow of causality remains pointing forward, you just add a jump. Retro-causality would mean reversing the flow of causality, not just jumping over everything. $\endgroup$ – Tom Dec 7 '16 at 7:57
  • $\begingroup$ No, the jump allows you to cause things in your original past. Sending an operator or sending a beam of bosons—no difference. Any “effect” must be due to some fundimental partical interaction. $\endgroup$ – JDługosz Dec 7 '16 at 10:55
  • $\begingroup$ See my answer for a full exploration of the idea. The fundamental difference is that in common time travel, time (and causality) still flow in one direction, there is just a loophole, wormhole, whatever, that allows one specific deviation. $\endgroup$ – Tom Dec 7 '16 at 12:39
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Another perspective that is not being considered is this: some ancient mythologies (and even contemporary philosophers like Nietzsche) advocated for a circular timeline.

IOW the time would flow in cycles. At the end of each cycle, time would be reset and another similar cycle would begin. The ancestors and the descendants would explore the world again in a similar fashion, unaware that they had already done it.

Which means that the future of a timeline is the past of the subsequent timeline.

If the events are not completely reset at each cycle, the events of the future may indeed influence the past, even though in different timelines.

So the causality would go like this: ancestors from timeline 1 ==> descendants from timeline 1 ==> ancestors of timeline 2

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I see 2 possibilities:

  1. precognition
    This has already been mentioned. The ancestors somehow know about the descendants and their actions. This still involves some "magic" and possible paradoxes.

  2. guessing The ancestors may know (or at least guess), that some people will explore the same location in the future. So they may guess what the descendants will do and act accordingly. Since these are only guesses, there is no time-travel-paradox.

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  • $\begingroup$ Ah, the effect can be due to planning. His future grandchildren affect his choices (saving money for them instead of investing in a risky scheme) because he considers them. $\endgroup$ – JDługosz Dec 7 '16 at 17:01
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Technically, if the two groups were relatives of one another, since the term "Ancestor" is defined as anyone you are descended from usually more distant than a grandparent (but not necessarily dead even though it is implied), it is possible for the actions of the descendants to affect the ancestors without the application of time travel. Only if the ancestor is still living. For instance, if your great grandfather had done something to assure his last years on earth were to be peaceful and enjoyable (possibly by composing a treaty that would bring about world peace), and then you were to violate that treaty by building a nuclear weapon. Therefore, causing world peace to come to an end, causing war to break out all over the globe, and leaving your great grandfather to have to fight for his life or hide out and live in fear for his last years on the planet. Or, if he had set it up so that his remains were to become incorporated into the liquid metal used to pour a sculpture of himself that would live on the family estate and you had melted the statue down to make your own bullets or even simply moved the statue to another location off of the family estate. Then, the actions of the descendants could affect the ancestors because your great grandfather would now be unable to live his last years on earth in a way which would be peaceful and enjoyable as referred to in scenario #1 or be a statue living on the family estate as referred to in scenario #2 So, it really depends upon if the "ancestors" you are referring to in your original question had already passed away or not. If they were already dead, it would require time travel or a multi-directional timeline in order for the actions of the descendants to affect the ancestors.

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  • $\begingroup$ The ancestors being still alive in some sense is an important nuance. $\endgroup$ – someben Dec 7 '16 at 17:10
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Here's an idea:

What if, through magic or technology, there exists an object which can be anywhere in the x,y,z dimensions but which has infinite length in the time dimension. Therefore whatever you do to that object changes its state at all points in the past, present and future. It could be something they could write on and therefore send messages throughout the timeline. Or it could be a box into which an item can be placed and then collected at any other point in time.

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  • $\begingroup$ Welcome to Worldbuilding, Simon, that's an interestingly novel idea. You may need to provide extra detail about how this "magic" time thingummy works. However, I think you're going to have fun here. $\endgroup$ – a4android Dec 7 '16 at 11:00
  • $\begingroup$ Interesting, but difficult. If the object would change it's state in all points in time, then that means that your manipulation of it has already happened before you do it. To me, that means that you cannot actually manipulate the object. What's the point of writing a message on it if the message is already there when you want to write it? The whole concept of manipulating an object is that it has a different state at point t+n in time from the state at point t. $\endgroup$ – André Dec 8 '16 at 9:36
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Applying a bit of lateral thinking: are the actions of the ancestors defined by what truly happened, or by what we know about it?

You could set up the story of the ancestors using a narrative device, for example a diary written by one of the expedition members. You follow their actions day by day as written.

Then cut into the future in the story. The descendants find a piece of evidence that completely changes the interpretation of the diary. Perhaps the purpose of the expedition wasn't mere science, but a failed search for a lost person? The brave research efforts change into desperate search of their lost companion, whose destiny they have since edited out of the diary.

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By becoming notable in their exploration, the descendants make the ancestors notable, i.e., "But Anna Ancestress had discovered it first in 1967!"

Thus the ancestor's explorations, until then obscure, receive worldwide attention.

Option 2: The ancestors were already notable, but either hailed as heroes or mocked as charlatans. The descendants discover or clarify something, reversing the public image of the ancestors from one extreme to the other.

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Imagine if you can an entity that exists outside of time.

50 years ago your father fought a dragon and it took his eye. today you are fighting a dragon in the same place you father fought one 50 years ago.

It is the same dragon though as it exists outside of linear time.

From your perspective the strike you made that you thought missed/was to shallow opens up into a serious wound/ turns out to be a critical hit. In reality it was your father striking it 50 years ago in a similar location.

From your fathers perspective the dragon often leaves itself open/moves randomly. in reality it is dodging/parrying your attacks.

From the dragons perspective it is simply a 2 vs 1 fight and in a few of its minutes it will be a 3 vs 1 fight. In reality every 50 years the chosen one climbs the mountain to stop the dragons return.

As the dragon interprets all 3 times as a single liner event your actions are taken into account in your fathers time.

if you prescribe to the closed loop theory then you actions and and that of your sons were already accounted for when the dragon took your fathers eye, if you believe in free will then that parry you made could leave your fathers eye healed after your battle concludes in linear time - a miracle in action.

the perspectives offers a large scope for story telling. you are raised on the tales of a god of destruction that resurfaces every 50 years that must be stopped, the dragon is popping down to the shop for milk when 3 similar looking guys jump him in an alley.

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You listed narrative connection as an option. Use it.

It is fairly common for events that happen later to affect the interpretation the reader gives to earlier events. The trivial example would be the classic detective story. A classic detective story will first give the reader a description of the actual events that includes all the clues necessary to determine the murderer and then present the final solution of the detective that connects the clues and makes the reader reinterpret the prior events.

This is not of course really reverse causality, but you did give "narrative connection" as an option, and the reinterpretations caused by the later investigation can in theory be quite drastic. Stories where the narrator or even the detective turns out to be the murderer have been done. So you probably could get the effect you want with this method.

A word of warning: such classic detective stories do require some work to plot correctly because you have to keep track of all the relevant facts and be totally sure you are not missing something that would be relevant. It also takes skill to fool the reader without breaking the rules by hiding relevant clues. You wouldn't really be bound by rules of classic detective story, of curse, but those rules were agreed on for good reasons so you probably should try to follow them.

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"How can the exploration decisions the descendants make "matter" for the ancestors?"

Time Capsules work that way: ancestors are leaving items in a sealed box because they expect their descendants to find them (and somtimes be amused). If it wasn't for the descendants, clearly the ancestors would care less about such items.

Let's say your ancestors want a cave to be open, but not until a certain date. What is inside doesn't matter. It can be an elder god, a treasure, or anything you want. They can set up traps along the way, just to be sure no random person would open it before. At this point, the ancestors are taking all the decisions... but! since they know and WANT their descendants to find the good path and enter the cave, they have to think and set the traps in a different way than usual: in a conventional story, they would teach something to their children or grand-children to avoid these traps. But what if, instead, they counted on the natural skills of their descendants?

My grandfather made me crosswords when I was a child because I loved crosswords. But my sister loved archery, so he also made her a bow and a target in the garden. Would we have any other hobby, he would have made something else to please us. And this is how the descendants decisions matter for their ancestors... given the opportunity for the ancestors to have that information.

Also it is not uncommon, in real life, to have two or three generations of children in a row adopting the same way of life as their parents: your world could have entires families just knowing what their descendants will do, because that's what they do and there's little chance their behaviour in the future will be very different. With that knowledge, any decision taken by a descendant will mirror the "hope" one ancestor had when setting a particular trap (which could be solved by force, by answering a riddle, by a particular skill they know or hope the descendant will have, etc.).

If you take the fantasy course, you could have a prophecy about someone who will have a specific skill or knowledge in the future, so they can think ahead and plan the route for this "chosen one". Here again, the decisions of the descendant will force the ancestors to act a certain way to clear him the path in the future. You even can have just the idea of a chosen one, given that at any point in time, there eventually will be someone who fit the characteristics of said chosen one.

In other words, either the ancestors know how their descendants will behave, and react upon it, either they decide to trace a particular path, with required skills and knowledge, and wait and see until the good person show up.

...Or maybe they would just have a map to a certain treasure that reveal itself "when the right stars are aligned", centuries later. The ancestors may know how to get to the door, but only the descendants will have the key.

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The descendants already have by their very existence have had an impact on the ancestors.

Their existence implies that the ancestors did and all their descendants did not go extinct.

In short, self-consistency. Anything that the descendants experience must be consistent with what the ancestors left behind.

This is a strange kind of "causal channel", but narrative-wise it can work (with effort!)

Imagine the ancestors run into a large military force. Then a "cut" to another scene, followed by the descendants learning about the result of that encounter.

More generally, the ancestors can set a goal or encounter a challenge, then you can cut to the descendants discovering the result of the goal/challenge, then cut back to the ancestors having completed the goal/challenge.

The narrative causality doesn't line up with the linear time causality.

You can even go meta on the narrative. Set up this non-linear pseudo-causality, then throw in the fact that the pseudo-causality was a misunderstanding. The descendants aren't actually exploring the actions of the group of ancestors you are following! But instead a different people. Or maybe only possibly a different people.

Then you cut back to the ancestors, who now have to overcome some problem that the narrative had earlier indicated was predestined to be solved.

And there is always the cheesiest version of that switch -- the descendants are actually the ancestors and vice versa.

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If you leave a giant red flashing button, someone is sure to press it. If you don't want it to be used (action that may happen in future), you can't design it to look like that (action happening now, depending on actions by your ancestors).

We don't want future people to open bunkers with barrels full of radioactive waste, so we can say, we are forced to mark them with scary symbols.

If you are looking for a place to lock Ba'al The Souleater and DON'T want him to be released, ever, you can't put him at the last level of some long dungeon full of treasures, as your ancestors are sure to explore them.

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