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Ulviyya Poladova

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A Chinese physicist has proposed a theory suggesting that travelling back in time may be impossible.

According to research by Hongsheng Zhang, a researcher at China's Jinan University, travelling into one's own past is impossible in a broad class of theoretical universes, Euronews reported.

Zhang's argument is based on a principle of Einstein's theory of relativity: the direction in which light travels can never be perpendicular to the direction of an object's movement through time. This principle suggests that motion through spacetime is possible only in certain directions, making a return to an earlier point in time impossible.

However, in his study, which has not yet undergone peer review, Zhang identifies two theoretical exceptions that could potentially allow travel into the past.

The first involves rotating black holes. According to Einstein's theory, a rotating mass drags space and time along with it. Near the centre of a black hole, this effect could become so strong that the forward passage of time follows a circular path. A person travelling along such a path would see their own clock ticking normally but could eventually return to the moment they started.

The second possibility concerns the structure of the universe itself. Under this model, spacetime could be arranged in such a way that continuous movement in one direction eventually brings an object back to its starting point. Most theoretical time-machine models developed by physicists, including those based on wormholes, rely on this principle.

Zhang stressed, however, that his research does not establish whether nature actually allows such phenomena to occur.

Travelling into the future, by contrast, is already considered scientifically established.

According to the European Space Agency, astronauts aboard the International Space Station (ISS) are a few milliseconds younger when they return to Earth than they would have been had they spent the same period on the planet.