: Now apply this in a macroscopic sense. To use the cliche grandfather paradox,
: If I travel back in time to kill my grandfather before my father was born
: will I cease to exist? "I" as it pertains to the consciousness
: that chooses to kill the grandfather will continue existence in the new
: reality, however, the "I" that exists in the previous reality
: (the one I traveled from) will cease. Why? Because now that I have altered
: the structure of reality (in general) the links between the universe I
: left and the universe i now exist in will be irrevocably altered.
: Multi-linear timelines CAN be undone, but only through destruction (which
: really changes nothing) because, lets say that there are 2 universes
: completely identical to this except in one im and inch taller and in the
: other im an inch shorter; if i leave this universe through time-travel and
: go and kill the other mes in the 2 universes then return to this one it
: would inherently destroy both universes because they would become
: identical. They would collapse upon themselves.
I agree with most of what you said, but I disagree that this action you describe would make the two universes identical, or collapse anything. Right now, lets say it is possible that a me from an alternate timeline will pop over and kill me . . . nope, didn't happen. But if it *was* in fact possible for that to have happened (which is a whole other question in itself), then there exists now an alternate timeline that was identical with this one up to the moment alter-me popped over and killed 'me'. So now we're talking about three timelines: this one I'm in, one where "I" was killed by a timeline-jumping alter-me, and some timeline where I have a timeline-jumping device and alter-suicidal tendencies. Of course there are infinitely many other minor variants between all those timelines, but the point is that none of the timelines where some 'me' was killed by an alter-me are identical to any of the timelines from which an alter-me departed to kill his alter-self, and there are still alternate timelines (like this one I'm typing in right now) where alter-me never appeared to kill me.
So now say I am that alter-me, I have this time thingy, and I am about depart for some timeline where "I" am a poor hobo living on the street, to put my alter-self out of his misery. Now, *this* timeline will have many possible futures after the moment where I depart, but they can be grouped into three important categories for the purposes of our discussion: futures where I do not return from my travels (I just vanish forever), futures where I return at a later point in time (I vanish for a bit, then return some time later), and futures where I return at that exact same moment (I don't vanish at all; I just teleport a little, and suddenly have memories and possibly scars from my adventures in the alternate timeline). And for the poor hobo alter-me in another timeline, there are many infinitely many possible futures as well, but they can be grouped into ones where either I appear and kill him, and thus there is now a dead alter-me lying around, or at least a missing person and slightly less mass on the Earth, if I abducted my alter-self's dead body; or, those where I don't appear, and alter-me keeps on living in hobo misery. Note that none of those possible futures of the hobo-fobo-verse overlap with the possible futures from this timeline. So, nothing has collapsed, and there is still some timeline out there where I didn't kill my alter-self. Time travel or timeline-hopping can only make more possible futures, not less. But, you do have a small point in that, if by some means two universes did happen to become completely identical (just by statistical mechanics or something), you would have effectively merged timelines; but then, they can diverge again after that. That's just to say that two random lines through a probability space can intersect all they want, and where two lines intersect, they are actually the same line. But that's not really interesting for purposes of our discussion.
Now excuse me, I have to go put my alter-self out of alter-my misery...
