[krapp]

>Out of all the branches of the space-time continuum, what's the chance that we are on the original one?

It could be the case that time resembles a directed acyclic graph. Traveling "back in time" merely creates a new branch at a point on the graph in the past where that event (the presence of the time traveler) occurs, marking a divergence from the "original" past, in which where there was no time traveler. But you can't ever actually revisit your own timeline, forming a loop.

And, since every quantum interaction would create a new node in which all possible outcomes of that interaction are new branches (many worlds theory,) the concept of an "original" timeline wouldn't even make sense.