November 18

``Time branched,'' says Tom. ``You and the other Jim have the same past, but different futures.'' He paused and added soberly, ``That is, if the other Jim had a future.''

``On the other hand,'' he said brightening, ``we know now that the Law of the Excluded Middle is false. This law [see Chapter Five] states that for any proposition P, either P is true or P is false. But what if P is `On November 18, Jim will be forty-four years and five days old.'? Two days ago, we probably would have agreed with the Law of the Excluded Middle, since P is true, but now we know that in one branch (the branch we snatched Jim from), P is true and in another branch (the branch we are in now), P is false. Thus two days ago P was neither true nor false!''

CHALLENGE: Can you devise experiments with any imaginable device that would definitely answer any of the following questions?

Does anything cause anything?
Does time have a direction?
Are there alternative universes?
Does time branch?
Does time loop?

Write a time travel adventure involving the paradox of the surprise exam.