But things are not so simple - we will question each of these.
We will not discuss these issues.
Here are some tenseful thoughts:
These thoughts are time-sensitive (i.e. tokens at different times might have different truth values).
Here are some tenseless thoughts:
These thoughts are time-insensitive (i.e. utterances at different times have the same truth value).
Reply: Why is spatial heterogeneity not a kind of change?
Reply: We need to be more careful about this last claim. What is it for an object, o, to move back and forth in space? It is for o to be at some point p1 at some time t1, at some distinct point p2 at some later time t2, and at point p1 again at some more later time t3. What would it be for o to move back and forth in time? It would be for o to be at some time t1 at some point p1, at some distinct time t2 at some further point p2, and at time t1 again at some more further point p3. But this kind of thing does indeed happen: a pair of hands clapping. So things can move back and forth in time.
Reply: Granted, we do not observe backwards causation; but why couldn't there be such a thing? If time travel into the past is possible (more about this below), then that's a good reason to think that backwards causation is possible. (And if backwards causation is possible then that's a good reason to think that time travel is possible: we'd be able to cause ourselves to be present in the past.)
But what are these times t1, t2, and t3?
But what if Zac did not move spatially when he traveled back - then we would have two temporal parts of Zac wholly located in the same place at the same time, and that is not metaphysically possible.
Lowe says there is an easy answer: Zac must move in space at the same time; he must accelerate very rapidly, eventually going faster than light, then start to go backwards in time.