I will not go through the physical difficulties of time travel, or anything to do with how one could travel through time. I'm going to go through the theories, paradoxes, and why they can/'t be used. 1. Seeing the future and changing it. This will nullify itself, since if you see the future, and then that future does not come to pass, you did not see the future. 2. Going to the past to change the 'now'. This would open up the possibility of preventing you from going to the past in the first place, by, for an example, killing your parent. Simply because that could happen, traveling back can not. 3. Seeing the past would be possible, there's nothing to stop that other than technology. 4. Going to the future. You could, but you could not return. The then-future would be the now, and thus the then-now would be come the past. 5. Seeing the future and changing it, or going to the past to change the now, pt.2. Another possibility is the 'Predestination paradox'. In your effort to change what will come later, you cause it to happen. This is rather weak, though, for you can't really cause your own birth by burning down your grandparents' house. Although Ka would mess up your plan, and your mother/father would survive it, and meet your other parent through the event. 6. Technology/information transfer between times. A scientist is working on a time-machine, when his future self comes to him, and tells him how to build his own time machine. The scientist goes ahead, and makes his machine. To test it, he goes back in time and teaches himself how to make the machine. You might ask where the information came from in the first place, but I'd say that's incorrect. It didn't. His future self got it from his future self's future self, and so on. It didn't come from anywhere, exactly. It's similar to the machines built by/for Skynet, in the Terminator series. Skynet was created using bits of a Terminator Skynet had itself built years later. Remember children, what will happen will happen, and will not happen in any other way. What has happened happened, and would not have happened in any other way. What you do, what you have done, what you will do, was decided at the Big Bang. Cause and effect. Brains are made of the same particles as everything else, and they follow the same laws. If you know the brain, and you know the chemical, you know what will happen when that chemical is released into the brain. Free will is an illusion, your emotions can be measured, created, stopped; manipulated. Your senses can be, too.