So... given that Bestiary 4 officially confirmed that Time Travel is in fact possible in Golarion (Time Dragons - more powerful than a Wish spell), I've noticed that there is a distinct lack of rules related to traveling through time, or for that matter, much to do with interplanar travel at all either. I've been coming up with a sort of awkward set of houserules to make things consistent should things consistent should this all come up in any of my games, but I think I'd like some feedback. The rules are as follows:
The universe can be seen as having 8 dimensions
Three dimensions correspond to actual space. We can recognize them as the width, height, and length of the universe. In terms of geometry, we might call them the x, y, and z axes
Another two dimensions correspond to planar dimensions. We will call these axes w and v for now. Where one is on axis w might more or less correspond to which plane one is on. For simplicity's sake, w=0 will always be the prime material plane. But if one were to, for example, travel 20 kilometers in the positive w direction, one might end up on, say, the fire elemental plane. The v axis would also be a planar dimension, but would be used to represent planes that might intersect each other or be inside each other. If I had created a demiplane within the elemental plane of fire, I might be at w=20 km, v=5m, instead of w=20 km, v=0.
Altogether, the physical dimensions of the universe without respect to time might resemble a hyper-hypersphere... or something.
As for time, it should take 3 dimensions. One set of dimensions is concerned with causality. If you go back in time to change something, you are traveling along this axis. The next is concerned with alternate timelines. Strictly speaking, if a player goes back in time to change something, he should be placed in a different timeline so as to not create temporal causality loops. Otherwise a sufficiently bored CE player with a high enough charisma might just Dominate Monster a Great Wyrm Time Dragon, and force it to go back in time to kill its own grandfather, and destroy the universe from the resulting paradox.
In any case, when a player travels in the negative direction on the causality axis, they will also travel along the timeline axis until they reach a destination where no timeline exists. This timeline will be a copy of the timeline he just came from, with the exception that he is now in a different spot. It also stands to reason that since the desired goal of time travel is to change the future (or relative present), that a character who then decides to travel forward in time to see the results of what he has done will remain in the timeline he just came from. If other players were waiting for them to finish what they were doing after time traveling, one can assume the game will then pick up where they left off in the new timeline. The players in the other universe... well let's just say they'll never see their friend again. A time traveler can never return to their own native timeline.
This leaves one last axis unmentioned. This axis is the time traveler's own relative time axis, in which one cannot ever travel backwards in. A time traveler must age (unless they don't), and time stops exist, so time must be passing for someone, even if it isn't for others. The character will continue to progress along this time axis for eternity, even when they're a corpse.
In any case, this would be my house rules for interplanar/interdimensional travel. Thoughts? And also, why doesn't Paizo make an "Ultimate Planes" or something, considering the relative lack of resources we have on it. Maybe I want to run a campaign in floating rocks in the Elemental Plane of Air, or caverns built inside the Elemental Plane of Earth. Maybe I want to send the players to the ends of the Earth to get the spells to be able to safely travel to the Positive Energy plane and obtain some artifact that the King lost to a celestial in a drunken bet. These things could definitely use more depth, don't ya think?
