General relativity theoretically allows the existence of Closed Timelike Curves. This space time curving, if successfully created, seems to permit traveling back in time. The question remains if this unique curving is actually allowed according to the laws of nature in our universe, and if it is possible to create such curving artificially – thereby "building" a time machine.
One of the claims against the possibility of building a time machine is that a negative density material was needed for the creation of the machine. However, Professor Ori proposed a model in which such material is not needed. The time machine is actually only made of vacuum and dust! (One should keep in made that the terminology "machine" might be misleading, as there is no machinery involved in this model. It is actually a theoretical configuration of space-time that could allow travel back in time).
According to the model, traveling back in time may only be accomplished to a point in time which is later than the construction of the machine. In other words, if we were able to prepare such a space-time curvature mechanism today, our future selves wouldn't be able to go back and visit the dinosaurs, kill Hitler or stop 9/11. However, even this restriction doesn't prevent many of the philosophical difficulties associated with time travel.