If you really want an answer to your question you need to first look up more information on the arrows of time. Here are some beginning articles. http://en.wikipedia.org/wiki/Arrow_of_timehttp://preposterousuniverse.com/eternitytohere/faq.html Some things can move back and forth with no problem, like chemical reactions. Take water for example. You can split water to make its component gasses and then you can combine the gasses to make water again. Its reversible. But there are some things that are not reversible, they are one way reactions and cannot be reversed. With this being the case then it becomes impossible for anyone to reverse these arrows of time and thereby "time travel". If it was possible to reverse time there are other complications as well. Everything is moving in the universe relative to everything else. To truly reverse time you would have to also travel back to the exact same space that the event occurred. If it was an event on earth then the earth would have to return to the same place in the galaxy that the event occurred. So along with the earth you would also have to move the sun and all its accompanying planets and space debris as well. Then each planet would have to be reversed to be in the exact same position including rotation to where they were. Remember its time and SPACE, you can't separate one from the other. Then you have to include all the chemical reactions that occurred on the earth and the other planets and the nuclear reactions in the sun. All the energy that the sun radiated away would have to be brought back to it and those reactions reversed. Then there are the reactions of the other stars and the light coming from them. In effect you would need to reverse every reaction in the entire universe to get it all back to the same time and PLACE where and when the event occurred. No event takes place in isolation, everything is relative and everything effects everything else. It would take more energy than exists in the universe to reverse all the processes of the universe, and by the simple act of trying to do that you would alter the conditions anyway. Read about the Heisenberg's uncertainty principle. You can measure the energy of a particle, or you can determine its location, but you cannot do both at the same time. The process of measurement alters what you are measuring. Science fiction is fun, science fact is what the universe runs on.