判断题
We define time travel to mean departure from a certain place and time followed (from the traveler’s point of view) by arrival at the same place at an earlier time (from the sedentary observer’s point of view). Time travel paradoxes arise from the fact that departure occurs after arrival according to one observer and before arrival according to another. In the terminology of special relativity time travel implies that the time-like ordering of events is not invariant. This violates our intuitive notions of cause-effect relation. However, intuition is not an infallible guide, so we must be careful. Is time travel really impossible, or is it merely another phenomenon where "impossible" means "nature is more mysterious than we think" The answer is more interesting than you might think.
The B-movie image of the brave chrononaut climbing into his time machine and watching the clock outside spin backwards while those outside the time machine watch the "him" revert to babyhood is, according to current theory, impossible. In current theory, the arrow of time flows in only one direction at any particular place. If this were not true, then one could not impose a 4-dimensional co-ordinate system on space-time, and many nasty consequences would result. Nevertheless, there is a setting that is not ruled out by present knowledge. This usually requires an unusual space-time topology (due to wormholes or strings in general relativity) which has not yet seen, but which may be possible. In this setting the universe is well-behaved in every local region; only by exploring the global properties can one discover time travel.
It is sometimes argued that time travel violates conservation laws. For example, sending mass back in time increases the amount of energy that exists at that time. Doesn’t this violate conservation of energy This argument uses the concept of a global conservation law, whereas relativistically invariant formulations of the equations of physics only imply local conservation. A local conservation law tells us that the amount of stuff inside a small volume changes only when stuff flows in or out through the surface. A global conservation law is derived from this by integrating over all space and assuming that there is no flow in or out at infinity. If this integral cannot be performed, the global conservation does not follow. So, sending mass back in time might be all right, but it implies that something strange is happening. Why shouldn’t we be able to do the integral
The possibility of time travel in General Relativity has been known at least since 1949. The General Relativity space-time found by Godel has what are now called "closed time-like curves"(CTCs). A CTC is a world line that a particle or a person can follow which ends at the same space-time point (the same position and time) as it started. A solution to General Relativity, which contains CTCs, cannot have a space-like embedding — space must have "holes" (as in donut holes, not holes punched in a sheet of paper). A would-be time traveler must go around or through the holes in a clever way.