Wolfgang Schwarz

Semantics for Time-Travelers

I'm somewhat stuck with the parts/counterparts paper. One of the problems is to find an acceptable semantics for time travel situations.

Part of the problem is that I'm often unsure what to say about these cases. I guess if time travel were more common, we would need some new linguistic conventions. Anyway, here are some sentences that seem true to me in the following scenario: Tina decides in 2025 to meet her younger self back in 2005. So at some time t in 2005, the younger Tina is in the living room and weighs 60 kg while the older Tina is in the kitchen and weighs 70 kg. Now, these all seem true to me:

1. At t, Tina has two arms.
2. At t, Tina does not have four arms.
3. Not: at t, Tina has four arms.
4. At t, Tina is in the living room.
5. At t, Tina is in the kitchen.
6. At t, Tina is both in the kitchen and in the living room.
7. At t, Tina weighs 60 kg.
8. At t, Tina weighs 70 kg.
9. At t, Tina does not weigh 130 kg.
10. Not: at t, Tina weighs 130 kg.
11. At t, the person in the kitchen is the same as the person in the living room.

Among the sentences which I'm uncertain about are:

1. At t, Tina is not in the kitchen.
2. At t, Tina does not weigh 60 kg.
3. At t, Tina weighs both 60 kg and 70 kg.
4. There are two people in the flat.

These are all pre-theoretic intuitions (because I don't have a theory yet), and none of them are very strong. So I would follow a good theory even if it doesn't exactly correspond to my intuitions. But not any theory is acceptable. In particular, it is not acceptable to treat time travel like fission/fusion. If Tina fissions into Tina1 and Tina2 at t, then the right thing to say is that "Tina" is ambiguous or indeterminate between something that includes Tina1 and something that includes Tina2. It is then also indeterminate whether at t, Tina is in the kitchen.

Time travel is not like this. In the time travel story, "Tina" is not indeterminate. Even if we settle Tina's spatial, temporal and modal boundaries with absolute precision, she is still doubly present at t.

I would like to say that Tina has two stages or temporal parts at t. This would be good because then I could apply something like the ordinary perdurantist semantics: Tina weighs 60 kg at t because one of her stages (temporal parts) at t weighs 60 kg. (Notice the strange existential, rather than universal, quantifier.)

But isn't a stage a timeslice, something like an intersection of a person and a time? The present timeslice of my liver is only a part of my present temporal part. But then the younger Tina and the older Tina at t are also only parts of a single Tina stage. So why does Tina weigh 60 kg at t if her temporal part at t weighs 130 kg? (The answer can't be that some part of her t-stage weighs 60 kg: other parts of that stage weigh 1 kg.)

I guess one needs to define stages in such a way that Tina has two stages at t: Tina's temporal part at t clearly has two parts that look a lot like ordinary person stages, whereas their fusion doesn't look like that at all. So we could say that a stage of a person is some part of a temporal part of a person that looks like an ordinary person stage (whatever that exactly amounts to).

Things get even more complicated if we focus not only on a single point in time. Suppose we want to talk about Tina's change of shape during the whole of 2005, using 4D shape predicates. Suppose the younger Tina has 4D shape A in 2005, the older Tina shape B, and their fusion shape C. Then

1. in 2005, Tina has shape A;
2. in 2005, Tina has shape B;
3. in 2005, Tina has shape C;

all seem true to me. So I want to say that "in 2005, Tina has shape x" is true iff at least one relevant temporal part of Tina has shape x. But what are the 'relevant' parts? Not any fusion of Tina-stages counts. (Probably some restriction on the psychological connectedness of the stage parts is needed here.)

Finally, suppose the younger Tina (in the living room) says, at t:

1. I am in the kitchen;
2. I am now in the kitchen;
3. at the present time, I am in the kitchen;
4. at t, I am in the kitchen;
5. at some time, I am in the kitchen;

At least in an ordinary context, the first seems definitely false and the last definitely true. Where do we draw the line, and why?