Ph103: Discussion Questions 2

Short Answer Questions

1. Explain why Arntzenius says that the Lewis definition of time travel (in terms of personal and external time) "ain't quite right." How does Arntzenius recommend we correct the definition?
2. Consider a time travel spacetime that describes a perfectly boring ordinary region of the Universe up until a certain point in time, after which a wormhole appears that allows for time travel. Explain how this provides an example of "indeterminism," i.e. a situation in which it the past state of the world does not determine the future.
3. What is Arntzenius' "simple argument" that time travel is unlikely to occur in our universe?

For Further Discussion

• Plausibility of time travel universes. Critically evaluate Arntzenius' argument from Question 3 above. Do you think it succeeds as an argument against time travel? Do you think that it fails? Why?
• Defining Time Travel. Consider the following simple example of a universe that has been "rolled up" along the time access. The red line indicates the path of a time traveler over time. Use the technique of drawing a "local region R" as in the reading to show that this is a time travel universe according to the Arntzenius definition.
• Motion in time travel universes. Using the above time travel universe as a model, draw an image of a body that...
  
  1. ... sits in one place without moving for its entire lifetime.
  2. ... moves with constant velocity to the right. (How many copies of this ball are there at any given moment in time?)
  3. ... moves with constant velocity to the right, collides with its future self and comes to rest, then collides with its past self sending it into motion to the right again.
• Indeterminism. Your drawings b. and c. in the previous previous example illustrate yet another sense in which time travel universes can be indeterministic. Explain how.