I recently read a very intriguing book about the paradoxes of time travel, "How to Build a Time Machine" by Paul Davies. It's a small book and an easy read, even for the non-physicist. I keep meaning to read "Time Travel in Einstein's Universe" by Richard Gott, which goes a little deeper into relativistic concepts.
Actual time travel seems unlikely, unless some form of massive and unlimited power was developed.
However, with the use of computers, we may be able to simulate the past. If computers continue to advance at the rate they currently are, they will be so powerful, that some sort of future supernation will be able to simulate the past exactly.
...I would love to lecture you all, but I have an exam in an hour...
If a river has no water in it, is it still a river? Or does it just become a canyon or something such as that?