General relativity theoretically allows the existence of Closed Timelike Curves. This spacetime curving, if successfully created, seems to permit travelling 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.
The grandfather paradox is probably the most famous problem associated with the possibility of traveling into the past. The paradox, first suggested by the science fiction writer René Barjavel in “The Imprudent Traveler”, published in 1943, describes a situation where a man travels back in time and kills his grandfather before his grandfather meets his grandmother. With his grandparents not having the chance to meet, one of the man’s parents would have never been born and by extension, the same it true about the man himself. However, if this man would have never been born, he couldn’t possibly go back in time and kill his grandfather – hence the paradox.
One of the options suggested in philosophical literature to solve this paradox is known as the parallel universes theory, where killing the grandfather actually creates a new parallel universe living the original timeline intact in a sense. Although philosophers continue to argue about backwards time travel and its problems, it is work such as that done by Professor Ori which may finally allow us to realize the possibility of time travel (if indeed it is possible).