jonesy
A Wicked Kendragon
In the many worlds interpretation, assuming time travel to the past is possible, you have already gone back and killed your grandfather. You have also, at the same time, not have gone back at all, and have gone back and not killed him. Many worlds accounts for all possibilities. But nothing would change for your grandfather in your original starting point, because that starting point exists in a timeline where you didn't go back to kill him. The timeline where you did go back to kill him isn't your original starting point, and the 'changes' there were technically already there and lead through a different crossroads.
It's like a choice between being Doctor Manhattan or Paul Atreides. Doc can't change anything because he experiences every point of change at the same time and he is already having been making every one of his choices at the same time in every single point. Paul on the other hand sees every possible and impossible choice at the same time, even the ones he has already passed (like recognizing the other potential Kwisatz Haderach who were not to be).
It's like a choice between being Doctor Manhattan or Paul Atreides. Doc can't change anything because he experiences every point of change at the same time and he is already having been making every one of his choices at the same time in every single point. Paul on the other hand sees every possible and impossible choice at the same time, even the ones he has already passed (like recognizing the other potential Kwisatz Haderach who were not to be).