So we can ask two questions. Daily Newsletter. But they do serve to illustrate the sorts of consideration that necessarily come into play when trying to spell out the physics of time travel in all detail. If the particle were a clock, we could not predict what time it would indicate when exiting the region. Learn more about science fiction and time travel. But, this rotation to the right will have to be abruptly discontinued as the announcements grow larger and I eventually approach 12 again, since I achieved 12 by not rotating the dial at all. Horwich, P. In this case the answer to the first question is yes and to the second is no : there are no constraints on the data which can be put on S , but those data are always consistent with an infinitude of different global solutions.