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, specifically one with personal identity. Since I used the grandfather paradox as the context previously, I will continue to do so.
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. In fact, anyone who was alive for the creation of the new timeline would presumably also in the new timeline—the entire population of the universe would be replicated. This raises some obvious questions about how this would work and issues within the context of personal identity. For the sake of simplicity, I will focus on Ted as my example—but this would also apply to anyone who ends up being branched.
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?
One option is to take the time splitting metaphor literally: the universe and the people in it are split into two. Think of this as being like an amoeba dividing. This does raise the obvious question of whether whatever makes a person the person they are can be split. For example, Leibniz took a person to be a monad and his monads are metaphysically simple: they cannot be split. But if personal identity rests on something that can be split up, then this would be possible. For example, Hume (sort of) advances a bundle theory of personal identity. On his view, the self is not a simple, indivisible entity. It is a collection or bundle of perceptions. To use a metaphor, just as a bundle of marbles could be divided up, this bundle could also be divided between timelines. This would lead to questions about trans timeline identity: would there be one divided person or two people who arose from a past person? In the case of Ted, there would be one Ted in each timeline, and they might (or might not) be the same person.
Another option is that the people in the new timeline are identical duplicates. This would require that the basis of personal identity be something that can be copied. Locke, for example, makes consciousness (memory) the basis of personal identity and even considers a case in which a person’s consciousness (memory) is duplicated. With a basis of personal identity that can be copied, the problem is solved: each new timeline person is a copy of the original. This also leads to the question of whether they are a trans timeline person or multiple different people who happen to have originated from the same person. One obvious consideration is that the basis of personal identity is supposed to be what makes a person distinct from all other things and this suggests that there should only be one of each person. But this view can be countered by arguing that it is philosophically fine to have multiples of the same person. This could be reigned in a bit by limiting it to one person per timeline—the challenge would be justifying and explaining this restriction. On this view, there would be Ted in the original timeline and a Ted copy in the new timeline—who might or might not be the same person.
A third option is that the new timeline is a completely new creation that just resembles the original. The people do exactly resemble each other, but this is analogous to having two unrelated people that just happen to look exactly alike: while the appear to be the same, this is not due to any connection between them. This is essentially an alternative reality view in which the reality begins with a timeline branch. While this does have some appeal, if the branch is not connected to the main timeline, then one must explain how it connects to time travel. One way to do this is to take the view that what seems to be time travel just creates an alternative reality and there is, in fact, no travelling. In this case, there would be a new Ted-like person who just happens to be exactly like Ted but has no metaphysical connection to the original Ted.
This matter becomes even more complex is one starts to consider theological and moral matters. For example, if God exists and people are souls that are sent to an appropriate afterlife, then God would need to sort out who is responsible for what. This should be super easy for God, barely an inconvenience. But without God, the ethics become more challenging: if a person is split into two people, which one is accountable for the past deeds? Perhaps they both are—like an amoeba who has split into two. If a new alternative reality is created and all the people are new, they should not be accountable for any past deeds—because they have no past at that moment of creation. Time travel is, of course, an even bigger mess than one would imagine.