From the Field Notes of Green Taylor Simms: One theory of time travel resolves the Grandfather Paradox by speculating that, at the moment one changes history, that change splinters the single flow of reality inot parallel branches. For example, after you've killed your ancestor, reality would fork into two parallel paths: one reality in which you continued to be born and your ancestor did not die, and one branch in which your ancestor died and you would never be conceived. Each revision one made in the past, the subsequent new reality it created, theorists refer to as a "bifurcation."