The grandfather paradox is a genuine paradox if we confine physics to a single dimension of time, a line, because there can be only one line unless we bring additional universes into it with each trip back in time (which feels like a BS solution, because it is). Imo, traveling back in time doesn't create a new universe anymore than each quantum event creates an infinite array of alternative universes, as we've seen with the MWI.
So if you only have the one line to move around on, then if you travel backwards on that line, then you're re-writing your own history because you (the older you) weren't there during the first go-around. And how can you retain the memory of a timeline that you've changed? In the TV show 12 Monkeys, your brain gets re-wired when you change your own history, so you're unaware that it's changed. But that seems silly to me. Because it doesn't solve the grandfather paradox - if you travel back in time and prevent your own birth, then how did you ever exist to prevent your own birth in the first place? A linear timeline runs into a brick wall with that simple paradox, and that's why most physicists decided to just sweep all of that shit under the carpet and stipulate that the positive energy theorem bars the possibility of time machines. But now we know that the positive energy theorem doesn't apply to our universe, so that evasive maneuver of rationalization is finished.
The three attempted solutions to this paradox, which we went over a couple of posts back, all seem either wildly contrived, or grossly uneconomical in the case of the MWI cop-out.
The two-time dimension solution is the only one that appears to solve the riddle with logical consistency and economy: you can travel back in time without changing your own history, but only by incurring a displacement in the extra time dimension that we humans aren't even conscious of, so you're never actually ever collocated with yourself in 6D spacetime...but we puny Earthlings can't discern the displacement in the Ty time dimension. If we could see aboard each craft, though, we might notice that each one's clock is different, and that the younger ones don't seem to be aware of the older ones, and thereby surmise that they're displaced from one another in the Ty dimension even though they're all at the same Tx position that we are.
