A few places in the narrative of the Shine Cycle, my planned series of fantasy novels, the ordinary linear flow of time is not quite sufficient. And so, for the purposes of the Shine Cycle, I posit the occasional existence of time travel.
I assume that if a physical, mechanical, chemical, or electromagnetic time machine is possible, it is prohibitively complicated and expensive by several orders of magnitude. And, while I reserve the right to have A Better Idea, I’m writing under the working assumption that such a machine is impossible. Instead of using a machine, time travel in the Shine Cycle is always achieved through applied metaphysics, with several limitations.
First, except in a handful of very rare periods of time or in very exceptional circumstances by divine intervention/authorial fiat, a person cannot be in two times at once. This generally prevents most contrived paradoxes, and most cases of influencing one’s own past. I had originally planned on using the first exception, for one specific period of history, heavily, but now conclude that this would be far too heavy-handed when the themes I’d be aiming for require a lighter touch. In a couple of places in one character’s history I use the second exception at great length, but that’s the only place I use it so far.
Second, time travel is extremely metaphysically expensive. When it’s needed, this does not pose a problem for the true mages, since as stewards of divine power they recognize that the Power is not theirs to begin with, but to reduce wasted power every time-travel working in common use (if there is such a thing as “common use” of something so rare) includes an explicit check of whether the trip is necessary, and refuses to proceed if that test returns a clear contraindication. Other applied-metaphysicians (magicians, wizards, witches, warlocks, and so on) travel through time even more rarely because they have to scrape together the power each time.
Now, all of this is assuming a quasi-static model of time. (That is, going into the past doesn’t change anything because any change has already been made.) But there is also one point in the series where we see what appears to be a quite different kind of time travel (in the form of an unarguable miracle, mind), which pictures going back in time as more like rewinding a tape than jumping from one place to another. On the other hand, I’m deliberately ambiguous as to whether the “previous timeline” actually happened or was merely an extended and particularly vivid vision. (I wrote at more length about this later.)