Other Time Machines
There have been other time travel mechanisms that have been conceived that allow backward time travel. But in most cases there is a physical reason that these objects would be excluded from existence in our universe. For instance there are schemes where we have a spinning universe, known as Gödel's Universe, after Kurt Gödel who first conceived of it. In this scheme closed time-like curves would exist where you could travel in a circle in spacetime and return to a point in spacetime that you have already been, thereby meeting yourself. However our universe is not believed to be spinning so this situation is considered unphysical. There is a new time travel scheme put forward by J.R. Gott where one could build a time machine by moving two cosmic strings parallel and relative to one another. Cosmic strings are theoretical remnants of the Big Bang which would be extremely long and thin and have nearly infinite density. Almost like a line singularity. The relative motion of the two strings and the geometry of spacetime about them would allow for closed time-like curves. Unfortunately, Stephen Hawking showed that unless gigantic amounts of matter having negative energy were used to surround this system it would become a black hole. So our search for a time machine continues.