In science fiction there seems to be two theories on the nature of time that are used the most. First is the one where time is looked at like the flow of a mighty river. In this model, events that might happen are like a pebble being thrown into the water. There is a localized ripple effect, but eventually the river is restored to it's natural path.
Stories that embrace this idea often use the concept that 'time will find a way to right itself'. For example, if you went back in time to stop someone from being run over by a car, you might succeed only to have that person struck down by a bus the following day. In other words, if the person was meant to die, then they will die.
The other main theory is the infinite multiple futures concept. In this one every choice made by anyone causes a new alternate future to come into existance. Kind of like the branches of a tree sprouting off. Using this model, if someone went back in time to change something (like saving someone from dying) their actions would cause a new timeline to form and thus the future they came from would no longer exist. This also has the side effect of possibly affecting the time traveler. If the 'change' pre-dates his/her birth then the possibility that they were never born, or a person very different was born with their identity is likely. This could lead to a couple of consequences often used in time travel fiction. First, the traveler could just fade away because they never existed. Examples of this were seen in the episode 'Meet John Doe' where the peacekeeper Andrus disappears when Tempus' manipulations cause his character to have not existed. Also, for those who watch Charmed. The whole Chris saga was an example of this. He came back in time to prevent his brother from becoming evil, which he succeeds in doing. But that meant that his future no longer existed and so he disappeared. A new baby Chris was born, but it wasn't the same person as the one who came back in time to change things, that Chris no longer existed.
The other is that the travel would become stranded in time since they no longer have a place in the 'real' world.
Time travel stories can be fun, but no matter how you set up your premise, there will always seem to be some logic flaw in the execution. Such is the nature of trying to conceptualize something that contrary to the natural order.
Tank (who figures that CC is now sorry she asked)
