The theory goes, if going back in time was possible, there isn't any possible way you could change the present from where you came from. That would cause a paradox. Imagine if you went back in time to stop your father from ever meeting your mother. You can never completely stop your father from ever meeting your mother, because if you had been successful at doing so, how would you have been able to go back in the first place if you had prevented the entire meeting from occurring at all? You see it cannot work. The theory is that nature would correct itself from allowing the paradox to occur. You may delay the meeting, but in the end, your father will always meet your mother, thus creating your existence. Once an event in time occurs, it can never be changed. The only exception to this is if you were able to go back and upon doing so, you created an entirely different universe/ separate time stream. If this is how it works, it would kind of suck since you would never be able to return to your original time/universe since you merely arriving in the past changes/creates the new universe. Now it, is possible to travel to the future. If you were to travel just under the speed of light to say.....the next star closest to ours, and let's say it took you like a week to get there. Because you are traveling so fast, time around you speeds up which means your trip there and back for you, would only be about a week or whatever length, but here on Earth, 50 + years would have passed.