Timeline branching treats time-travel-induced changes as creating new branches of the timeline, thereby avoiding contradictions like the grandfather paradox. Each branch constitutes a new universe, and populated branches replicate individuals who existed at the moment of branching. In the example, Sally’s murder creates a branch where she does not exist while Ted exists in both branches, producing multiple Teds. The replication of persons across branches generates puzzles about personal identity, continuity, and how to account for multiple coexisting instances of the same individual when branching occurs.
One approach to time travel is to embrace timeline branching: when someone travels in time and changes something (which is inevitable), then a branch grows from the timeline. This, as was discussed in the previous essay, allows a possible solution to the grandfather paradox. But it also gives rise to various problems and questions, such as the need to account for the creation of a new universe for each timeline branch. The fact that these universes are populated also creates a problem,
Suppose that Sally and Ted travel back in time and Sally kills her grandfather Sam. Ted does not murder his grandfather. Assuming the timeline branching solution to the grandfather problem, Sally creates a new timeline branch in which Sam is killed. While Sally does not exist (one assumes) in the new timeline, Ted does. There would be at least two Teds now: one that is in the original timeline and the Ted in the new timeline.
To keep it simple, let us suppose that Sally and Ted are the first time travelers, so there have been no time branches. When they travel back, they create the first branch. While Sally will not exist in the new branch (and her act of murder might result in other people not existing or even new people existing), Ted will exist in both. So how would this work?
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