A Chinese physicist, Hongsheng Zhang of Jinan University, has presented a simple algebraic proof ruling out the possibility of travel to the past for a broad class of hypothetical universes, the website infohub.kz reports.
In a preprint posted to the arXiv repository, the author formulates a mathematical prohibition on the formation of closed timelike curves. The proof rests on applying the classical eikonal equation for lightlike surfaces, which describes the propagation features of light rays in spacetime.
As the scientist himself explained, the entire mathematical proof takes just three lines and relies on one fundamental geometric fact: a lightlike direction can under no circumstances be perpendicular to the direction of an object's motion in time. It inevitably follows that along the trajectory of any traveler a certain scalar function strictly increases. This rules out any chance of returning to the starting point in time: motion is possible only forward, like a drive along a street with continuous one-way traffic and no exits.
The proposed model became the fourth family of criteria prohibiting time loops, complementing the concepts of global time, energy conditions and topological barriers. According to the physicist, his theorem requires no quantum effects, negative mass or singularities. To circumvent this limitation, the fabric of spacetime would need strong twisting or space would have to be closed into a loop around a massive rotating black hole.