I read Homestuck for the first time around the same time I rewatched Terminator (1 and 2), and it got me thinking about using travel through immutable time as a game concept. The upshot of immutable time is that it lets you send the players throughout time without having to figure out how the minor and major changes they make would alter the future (which, in any kind of non-predestined time, would be a lot). The drawback is that you instead have to figure out how to keep them from changing anything. The following is an overview of how this kind of time travel would work (largely patterned on how it works in Homestuck, so minor spoilers for that if you plan to read it), and includes several elements to try to keep the scope of attempting to change the past manageable within a game framework. (This is presented as in-character advice from a time traveler.)
1) Time is internally consistent
Time travel doesn’t actually let you change anything. Circumstances will conspire to negate whatever change you’re attempting to make. Killed your grandfather before your mother was conceived? Turns out your grandmother never mentioned her husband’s twin and his untimely murder. Attempt to travel back and tell yourself something on a day you know you didn’t meet yourself in your own timeline? You’ll never actually get to the meeting due to seemingly random accidents, or something will happen to erase your memory of the meeting in the past, or some other bullshit.
Your best bet, when time traveling, is to know as little as possible about any events you’re trying to interact with. Your ignorance doesn’t mean that things aren’t objectively true (you still can’t actually change anything, from the point of view of someone who does know what happened). But at least you won’t know for sure whether what you’re trying to do is futile.
2) Information ignores paradox
Sometimes information-based time loops become stable with no true cause. Brought yourself back the same formula for cold fusion that you gave yourself when you were young, and told yourself to pass on once you discovered time travel? No problem. Pulled a time-travel Oedipus and became your own father, creating a Y chromosome from nowhere? Gross, but not a paradox.
Maybe the information comes from the zeitgeist. Maybe it comes from the gods of time fucking with us. Nobody knows for sure. Maybe someone will pass the secret of how it’s possible through a self-generating time loop at some point.
3) Matter, however, can’t get stuck in a time loop
Matter is much easier to degrade than information. Reality seems to be fine with the secrets to a better watch coming from nowhere, but the watch itself needs a provenance. If a time traveler gives you an object, you cannot give that same time traveler the same object to take back to you. You can take it apart and make an identical duplicate, as long as the matter for the duplicate came from somewhere real (or you can just find where the item originally came from, send that one back, and keep your version). It’s all down to a loop being infinite: a piece of finite matter stuck into it would gradually erode.
Don’t even try to mess with this, it can get really wonky, really quickly. Note that a really obnoxious exception is that immortal, ageless living things can actually come from nowhere, since they regenerate their bodies with new matter from the environment; they can just expect memory loss somewhere along the line before they start the loop over again.
4) You can fork a timeline, but you absolutely shouldn’t
In extreme circumstances, it is sometimes possible to change something. Despite the implications in rule one, there isn’t actually some kind of omniscient deity keeping you from changing things. Sometimes events seem weirdly contrived to try to keep you from making alterations, but if you had a god’s view of the situation it would all make sense as a series of interlocking causes. Reality’s defenses against changing the past are passive, and if you really, really set your mind to changing something in a way you know for a fact is a change, you often can.
Don’t do that. You are no longer a resident of the prime timeline if you do. Back in reality, you made a different decision, and things proceeded as they always had. You and the entire reality you forked off are now doomed; the universe doesn’t have enough energy to keep forks going for very long. If you stay there, you’ll start to notice that entropy is on overdrive, and everything quickly begins to fall apart, including your own cells. You can time travel back, usually as the special paradoxical exception that got you to reconsider making a change in the prime timeline. Then you’ll likely die. Gruesomely.
5) Because reality is sturdy, it’s worth it to try things
You will not step on a bug in prehistory and totally change the course of evolution (unless you already did). Your modern flu will not start a new plague that wipes out Europe (thought it might have started a documented one). You will not accidentally give Genghis Khan the future tech he needs to start the industrial revolution centuries early and conquer the globe (though he might have you burned as a witch). Really, unless you’re trying to fork a timeline by enacting something you know is a change, the worst things that can happen are that attempting to change things gets you hurt or killed with no alterations, or you find out that you were responsible for something that already happened anyway.
Want to save a loved one that died in an accident? Just be ready to leave a convincing replacement corpse and it might all work out. Need to find out a piece of long lost information? Wander around asking questions in your future clothes and it’ll be fine, in the grand scheme of things. Timeline predestination is freeing, in a way: if it’s possible to succeed, then you always did, if it’s impossible, then you’ll find out pretty quickly, and if you fuck up grandly along the way, the future was always a product of that catastrophe.
Just keep the rules in mind, try not to learn anything that makes you certain you’re going to fail because you already did, and experiment. A time traveler with guts and cunning can accomplish a lot of marvelous things… that were always destined to have happened.