We all know the Grandfather Paradox. You go back in time kill your grandfather so that you would no longer exist ( not that anyone would actually try to do that ). However taking in account of the World Line Theory this can easily be proven otherwise.
So let say you can back in time the very fact that you are there means you have created a world line different then your own. So lets title them World A is your home world and, World B is the same world only you have gone back into time causing a new world line to be created. Now we kill your grandfather as a result we have a final world line which is World C as a result the only 'you' that died was the original 'you' in World C.
As simple as that the grandfather paradox is no longer possible.