According to the Theory of Relativity, a time machine should be possible thanks to Wormholes.
An advanced civilization would start with two parallel plates, separated by an extremely small gap. These parallel plates would then be reshaped into a sphere, so the sphere consists of an inner and outer shell. Then they would make two such spheres and somehow string a wormhole between them, so a tunnel in space connects both spheres. Each sphere now encloses a mouth of the wormhole.
Normally, time beats in synchronization for both spheres. But if we now put one sphere into a rocket ship that is sent speeding near the speed of light, time slows down for that rocket ship, so that the two spheres are no longer synchronized in time. The clock on the rocket ship beats much slower than the clock on Earth. Then if one jumps into the sphere on Earth, one may be sucked through the wormhole connecting them and wind up in the other rocket ship, sometime in the past. (This time machine, however, cannot take you back before the creation of the machine itself)
Status:: #wiki/notes/mature
Plantations:: Quantum Physics
References:: Parallel Worlds