Physicist Outlines Conditions Under Which Time Travel to the Past Is Impossible
Hongsheng Zhang, a researcher at the University of Jinan in China, has proposed a mathematical criterion that rules out travel into one's own past in certain models of the universe, reports Euronews.
According to his calculations, if spacetime can be divided into special surfaces associated with the propagation of light, a person or another object can cross them in only one temporal direction. Returning to the original moment is then impossible—just as one cannot descend a staircase that only leads upward.
However, the theory has exceptions. With a specific geometry of spacetime, such as near rotating black holes, or with unusual topology, such 'time loops' are mathematically permitted. This does not prove the existence of a working time machine.
Travel to the future through time dilation is physically possible: when moving at enormous speeds or in a strong gravitational field, time passes more slowly for the traveler than for those around them.
