Returning to this thread after a few months... I see Eliezer has responded in a way which kills my theory stone cold. (Though it was dead anyway if the 6 light-hour separation by Pioneer wasn't reached in 1992.) But basically what I was thinking was this. Consider any two space-time points x and y. Either they have a time-like separation, or a space-like separation or a null separation. If they have a space-like separation then there is a particular inertial reference-frame in which they are only separated in space, not in time. If the spatial separation in that frame is > 6 light-hours, then information cannot travel from x to y. (Or, if you want to think of it in terms of a causal graph, and Pearl's intervention calculus, then every intervention to the graph at x will leave events at y unaltered.) Incidentally, this formulation implies the rule that "information can't go back in time more than 6 hours" and implies it in any inertial reference frame. For if information could travel from x to a point z, more than 6 hours in the past of x (but at the same place) in some reference frame, then it could be sent further along a future-pointing null vector from z to y (by an ordinary light-beam), where y is > 6 light-hours from x in the same reference frame. So the restriction of "no spatial jumps > 6 light-hours" neatly implies "no temporal jumps back > 6 hours". Basically, this looks something like the Minkowski interval formulation: there is no privileged reference frame, just a new constant of nature (i.e. whatever 6 light-hours translates to in Planck lengths).

Why the heck is this being voted down? It's a perfectly valid question! You could have some Minkowskian interval that Time Turners can't go further back than, and it would make sense in terms of Special Relativity, but there's no obvious analogy for a maximum spacelike separation being built into the laws of magic. I may be willing to put Time Turners in my fic - I may even be willing to swallow the single-world interpretation of QM which that necessarily implies - but even I'm not going to give magic a privileged reference frame, or talk like "hours" are an intrinsically meaningful measure. Special Relativity is... I mean... it's over the local properties of the variables on which everything else is built, it's the stuff that the fabric of reality is locally made of. It's like having Harry not be made of atoms.