The boundedness mechanic is a solution to a common time travel story shortfall: it’s too easy, or too hard, to take advantage of time travel.
In a story that follows the Novichov self consistency principle, the only way a character could, say, stop the death of a loved one would be if he was mistaken about the death in the fist place(as, he wasn’t dead, because his future self stopped that from happening). No real change can be made, and paradoxes are impossible.
In the other extreme, most commonly the “multiverse theory”, paradoxes are resolved by the fact that the time travel event itself is not actually causally related to any of its consequences(As, the consequences occur in a different universe, identical to the one traveled from at the given time), and any act is permitted. The usual result of this is that instead of focusing on the time travel itself and the difficulty in making a change, the focus is on the unintended consequences of actions from the butterfly effect.
These both are predictable and common tropes of time travel fiction, and they’re both frustratingly nihilistic in their outcome. In the one case, whatever you do doesn’t really matter because either it’s impossible or predetermined, and in the other case it’s double, as you never effect ‘your universe’, but also that you cannot make a cause you wish except as a trade for absurd and extreme consequences.
For this reason, a lot of time travel fiction has attempted to come up with a resolution, combining the solidity of self consistency with the flexibility of multiverse travel. Doctor who does this by introducing vaguely-defined “fixed points” which cannot be changed, while steins;gate introduced world-lines, not changed every jump but changed by extreme circumstances.
The criteria
I have come up with a mechanic of my own that I think does a good job mediating between the two. I have some criteria in mind when considering this:
1. Realism
A common resolution often involves the “fixed points” or “world jumping criteria” or something along those lines being highly related to humans and their machinations with little explanation. This is not very convincing, as time travel is a very scientific and physical kind of thing. Gravity doesn’t go up no matter how hard we hope, so why should causality bend based on our personal relationships to it, instead of some scientific explanation? And yet the reasoning for it is understandable as a narrative choice, and it is a challenge to have it otherwise.
those problem also have another issue, somewhat less egregious in “world jumping” than “fixed points”, which is the violation of a very common pattern in the universe: Continuousness. While not everything of the world is continuous, a lot is. Dimensional space seems to be continuous, so the idea that time is discrete, and something is either ‘fixed’ or ‘flexible’ with no in-between seems unusual.
2. Narrative balance
The trials and tribulations in time travel as it is are not inherently bad. Their greatest fault is their staleness, and the difficulty involved in making more than a few kinds of stories around it. It’s important that the ramifications and difficulties involved in time travel are not completely erased, but a story should be able to have time travel without it being exclusively about those struggles.
Another balancing issue is this: If you were to imagine how you might take over the world with a time machine, become akin to a god, does that seem particularly hard? It doesn’t to me. Money, power, fame all seem readily accessible, and nobody short of another time traveler would have any chance at stopping you. This may be another reason time travel is often kept behind ‘gates’. The story of the all powerful time traveler can be quite bland, even if they’re a good guy, without some kind of limitation.
The solution
With these things in mind, my idea for a time travel mechanic is as follows:
Time acts like matter in some ways. temporal links are not sometimes ‘fixed’, neither are they always ‘flexible’, nor always ‘fixed’, but that they are continuously different degrees of “fixed” or “flexible”, and can themselves be interacted with in ways that alter their state — as a liquid can transition to a solid, and a solid can still change, (Chemically and physically as in position and shape) but it’s harder.
the material is more like the actual event taking place, and the form is like the structure of causes that led to the consequences — of course, this is inexorably linked, and everything is related to everything else, as in physical matter, but they are also distinct properties from one another, again, as in physical matter.
An event that has bigger consequences, as in the effects of it are tightly linked to it(It is hard to get done another way — or it has lots of things relying on it — or it is bound up in itself, as in causal ‘loops’) are harder to change
In terms of the ‘bounded-ness’ an event has based on it’s consequences, it’s measured logarithmically with relation to the time after the event took place. Otherwise, every event would be similarly ‘bounded’, since the butterfly effect means any small change eventually leads to a wildly different universe, as with the pendulum effect. With the logarithmic scaling, a quick change in the world and chains of causality is ‘harder to make’ than a smaller change.
You cant change iron into gold by repeatedly beating it with a hammer — but that doesnt mean you can never make the transition. You have to have intermediate materials, you have to have specially prepared chemicals, etc. The same way, to make a difficult(But doable) change in this system, you have to have ‘new causes for its effects’, specially prepared chains of events, etc.
In the cases that the event is ‘less bounded’, it may be more willing to diverge from it’s normal causality. Say, a car is sold off a lot and lives it’s life under an owner, eventually rusting away. Changing the wheels so that it skids off the road soon after it’s sale, killing the owner causing a company to go down changing the course of the entire economic structure ‘does’ a lot, but changing the color of the car from one shade of blue to another may ‘do’ a lot less, at least in the same timeframe. Therefore, going back in time, you may find there are more ‘final destination coincidences’ to apply the self-consistency principle to the wheel change (You may change the wheel, to find that someone decided to change them back, or you may have a strangely difficult time getting to the wheels, or the owner could miraculously survive, etc), whereas the change in paint might be much easier to make changes out of(Except, they’d be further forward in time).