The grandfather paradox poses a contradiction: traveling to the past and killing one's grandfather would prevent the traveler's existence, making the killing impossible. A branching timeline solution treats time like a river where changes create new branches. A time traveler who kills a grandfather produces a new timeline where that grandfather dies, while the traveler's original timeline remains unchanged. Repeated trips produce many branches, each containing variants of events and possibly of the traveler. Each additional branch appears to require the creation of an entire new universe or at least a substantial ontological commitment, raising metaphysical concerns about the origin and proliferation of branches.
The grandfather problem is a classic time travel problem. Oversimplified, the problem is as follows. If time travel is possible, then a person should be able to go back in time and kill their grandfather before they have any children. But if they do, then the killer would never exist and would not be able to go back in time and kill their grandfather.
So, imagine that Sally goes back in time to kill her grandfather. She succeeds and thus creates a new timeline in which he dies. Presumably, she returns to her own timeline and finds that her original grandfather was never killed by her. She might keep trying and from her perspective she would kill him over and over, only to return to find that she never succeeds. But with each trip to the past, she creates another new timeline.
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