She started one day in a relative way, and returned on the previous night. The scientific premises suggesting a theoretical time travel mechanism are Einstein's Theory of Relativity and its successor, quantum mechanics. If we place a second ball on the surface, the new ball rolls toward the indentation made by the first, curved space telling matter how to move. If we place a bowling ball in the center of our flat spacetime, the indentation will be very deep, possibly tearing a hole in the fabric of our spacetime, a black hole. A brief review of some of the current concepts in physics reveals several likely candidates.
Black holes occur when stars of a certain size use up all of their nuclear fuel.
By choosing the right path around the black hole, such a journey, which might take a few hours according to the clocks on the falling spacecraft, could be made to take as long as you like according to the outside Universe. White holes are a similar concept, except that they are postulated to be the result of other universes' black holes, spilling matter and energy into our universe. Assuming that a civilization has the technical capability to orbit black holes and move wormhole mouths, there is still the question of the time traveler's journey into the past and his possible influence on his own present existence. Given the massive distortions of spacetime involved in time travel, a person would need to think very carefully about the possibility of returning to find all his friends long dead, his apartment rented, and his job nonexistent. Gardner, pp.102, 173.