Time travel is something that has been in the imagination of humans for years, and is something i have a great interest in. What does everone think about time travel? What about paradoxes? There are a few main theories for time travel which make some sense. 1. The time line is consistent and can never be changed. 1.1 One does not have full control of the time travel. One example of this is in Michael Moorcock's The Dancers at the End of Time in which time has tendency to reject time travelers who travel to past to change it by pulling them back to point from when they came from. 1.2 The Novikov self-consistency principle applies (named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University). The principle states that the timeline is totally fixed, and any actions taken by a time traveler were part of history all along, so it is impossible for the time traveler to "change" history in any way. The time traveler's actions may be the cause of events in their own past though, which leads to the potential for circular causation and the predestination paradox; for examples of circular causation, see Robert A. Heinlein's story By His Bootstraps. 1.3 Any event that appears to have changed a time line has instead created a new one. It has been suggested that travel to the past would create an entire new parallel universe where the traveler would be free from paradoxes since he/she is not from that universe.[49] 1.3.1 Such an event can be the life line existence of a human (or other intelligence) such that manipulation of history ends up with there being more than one of the same individual, sometimes called time clones. 1.3.2 The new time line might be a copy of the old one with changes caused by the time traveler. For example there is the Accumulative Audience Paradox where multitudes of time traveler tourists wish to attend some event in the life of Jesus or some other historical figure, where history tells us there were no such multitudes. Each tourist arrives in a reality that is a copy of the original with the added people, and no way for the tourist to travel back to the original time line. 2. The time line is flexible and is subject to change. 2.1 The time line is extremely change resistant and requires great effort to change it. Small changes will only alter the immediate future and events will conspire to maintain constant events in the far future; only large changes will alter events in the distant future. (Example: The Saga of Darren Shan, where major events in the past cannot be changed, but minor events can be affected. This is explained as if you went back in time and killed Hitler, another Nazi would take his place and commit his same actions.) 2.2 The time line is easily changed. (Example: Doctor Who, where the time line is fluid and changes often naturally; even changes to the traveler's own timeline are possible, though it is suggested such an act would destroy most of the universe.) 3. The time line is consistent, but only insofar as its consistency can be verified. 3.1 The Novikov self-consistency principle applies, but if and only if it is verified to apply. Attempts to travel into the past to change events are possible, but provided that: -They do not interfere with the occurrence of such an attempt in the present (as would be the case in the Grandfather Paradox), and -The change is never ultimately verified to occur by the traveler (e.g. there is no possibility of returning to the present to witness the change). These were pulled from Wikipedia as starting points...who else has a theory or would like to talk about one of the above? Sources: http://en.wikipedia.org/wiki/Time_travel#Ideas_from_fiction