Problems with time travel

Problems with Time Travel
If we are ever able to develop a workable theory for time travel, we would open up the ability to create very complicated problems called paradoxes. A paradox is defined as something that contradicts itself. Here are two common examples:
Let's say, for the sake of argument, that you could travel back to a time before you were born. The mere fact that you could exist in a time before you were born creates a paradox. If you were born in 1960, how could you exist in 1955?

Possibly the most famous paradox is the grandfather paradox. What would happen if a time traveler went back and killed one of his or her ancestors before the traveler was born? If the person killed his or her grandfather, then how could that person be alive to go back and kill his or her grandfather? If we could change the past, it would create an infinite number of paradoxes.
Another theory regarding time travel brings up the idea of parallel universes, or alternative histories. Let's say that you do travel back to meet your grandfather when he was a boy. In the theory of parallel universes, you may have traveled to another universe, one that is similar to ours, but has a different succession of events. For instance, if you were to travel back in time and kill one of your ancestors, you've only killed that person in one universe, which is no longer the universe that you exist in. And if you then try to travel back to your own time, you may end up in another parallel universe and never be able to get back to the universe you started in.
The idea here is that every action causes the creation of a new universe, and that there are an infinite number of universes that exist. When you killed your ancestor, you created a new universe, a universe that was identical to your own up until the time you changed the original succession of events.

Confused yet? Welcome to the world of time travel. Just imagine how complicated the ticket prices will be.

Datos copiados de http://science.howstuffworks.com/time-travel6.htm