Time travel hasn't been invented yet. Which sucks:< Who wouldn't want to change their past mistakes. What if it has been invented in the future and people do travel back. However, every time they change something, however minute, an alternate reality is created and the one that we exist in isn't changed. Could this be why, we haven't been visited by time travelers from the future?
P.S. I am asking this because of this Article here. And yes, I do know that by the rules of physics, time travel backwards isn't theoretically possible.