Been thinking a bunch about the mechanics of time travel. As far as I can tell, there are two types of possible time travel.
One is deterministic, where going back in time will only allow you to do things that already happened in history, and you cannot in any meaningful way change the past. This type requires a kind of self-consistency force in the universe that enforces things somehow. It also doesn’t preclude the idea of time travellers going back to gather information, for instance to scan people’s minds so as to be able to upload them into the future. Nevertheless, this form of time travel is not very interesting.
The second type involves branching timelines, the ostensible multiverse theory model. This is the form that I have previously assumed in my stories so far. It allows you to arbitrarily change the past how ever you want, but this only affects the second timeline and not the first, original timeline.
The issue with this type is that it seems to allow breaking the second law of thermodynamics, or at least enabling infinite energy or lifespan. A way of mitigating the capacity to mine new timelines for energy is to force wormholes to the original timeline to require energy to move anything through it, such that you can never break even. You also may want some way to prevent people from just jumping timelines over and over again to use the energy in each timeline and effectively live forever. A way to do this would be to require energy to stay in the new timeline, having some kind of force that pushes you back to the original timeline. But I’m not sure that really makes sense either.
In any case, the first type of time travel is uninteresting. You cannot change anything, so there’s really no motivation to use it, other than to ensure that things happen as they do, which only the most paranoid or risk averse will probably bother attempting. The second type of time travel is more interesting, albeit, because you can’t change the original timeline, only to those who want to create and expand new timelines. It also runs into a different problem of how does the new timeline even form. Like, where does the energy come from? Even if you can’t practically use it, this form of time travel is essentially a way to generate infinite energy.
One possibility is that the energy from the original universe is also split somehow in the process. Like, maybe each parallel timeline inherits half the zero-point energy of the original timeline, such that too many branches weakens the universe.
This suggests the idea that the universe might have a kind of weak self-consistency force that would try to prevent branches, or even try to push branches back together so that the downstream universe doesn’t change. This could explain why we don’t just see a massive butterfly effect that makes any time travel create a completely different and unfamiliar future, which would also probably make time travel less useful.
So, I have the thought that maybe you can merge type one and type two time travel by allowing the notions of convergence and divergence. Some time travel is convergent. It functions like type one and doesn’t change anything or much. However, sometimes time travel is divergent, and splits the timeline due to significant changes that can’t converge. Perhaps knowing which form you are undertaking is unknowable in advance.
Perhaps the reason we don’t see time traveller colonies into the past is because the universe tries to converge and acts to make significant divergences increasingly difficult to achieve. In practice, divergent timelines tend only differ to a small degree, and eventually tend to merge back together, until they all merge at the heat death of the universe.