A physicist at the University of Jinan in China has published an argument that, in a wide range of theoretical universes, a journey into one's own past is ruled out by the geometry of spacetime itself. Hongsheng Zhang's paper, which has been submitted for peer review, reduces the question to three lines of proof, according to Euronews.com. The work does not claim that time travel is impossible; it asks when a given spacetime allows a loop and when its structure forbids one.
Why it matters
The idea of travelling backwards has a long history in physics, largely because Einstein's equations permit it in a few special settings. Zhang's contribution is a general test for when they do and when they don't, and it narrows the surviving escape routes to two. Euronews.com reports that the result undercuts a common plot device in science fiction while leaving the physics of forward travel untouched.
The staircase argument
Zhang's reasoning rests on a single property of light. As he put it to Euronews Next, light can never point sideways relative to a traveller's direction in time.
Picture spacetime divided into sheets that light itself can travel along. Anything moving more slowly than light has to cross those sheets in one direction only, the way a person on a staircase can climb but never descend. If every step must be taken upward, there is no way to return to the step where the journey began.
Physicists describe a trip into the past as a closed timelike curve: a route through spacetime that returns to its own starting point in both place and moment. Zhang offered a concrete illustration to Euronews Next. To shake hands with your younger self, he said, you would need a path that loops back on itself without ever going faster than light.
Einstein's equations do allow such paths in a small number of situations, among them the region around the core of a rotating black hole. Zhang's paper sets out the conditions under which the geometry blocks them and the conditions under which it does not.
Two ways around the staircase
The research also identifies exactly how a time machine could evade the argument, and Euronews.com reports there are only two routes.
The first is twist. If light rays spiral around one another like the strands of a rope rather than running parallel, the clean sheets of light cannot form, and the staircase that keeps a traveller moving one way disappears. This is what happens near the centre of a spinning black hole, where Einstein's theory says the rotating mass drags space and time along with it so strongly that moving forward in time bends into a circle. A traveller on that circle would experience their clock running normally while arriving back at the moment of departure. The physicist Brandon Carter demonstrated in 1968 that such loops exist inside rotating black holes.
Zhang is careful to say that twist by itself does not produce a closed curve. It only blocks his argument, he told Euronews Next, adding that twist is present throughout the universe and, as far as is known, harmless.
The second route is the overall shape of the universe. The staircase could be built perfectly and still lead around the outside of a building, so that the top step connects straight back to the bottom, in the manner of an Escher drawing. A traveller would keep climbing and still finish where they started. Most time machines physicists have sketched on paper, including those assembled from wormholes, depend on this trick, which researchers call topology.
Zhang considers topology the loophole worth watching. He told Euronews Next that it is the one that could matter and also the one that looks least likely to arise without deliberate engineering.
Forward travel is already routine
Movement toward the future is not in dispute. Einstein established that travelling very fast or sitting in strong gravity slows a person's clock relative to everyone else's. According to the European Space Agency, astronauts who spend six months aboard the International Space Station come home roughly five thousandths of a second younger than they would otherwise have been.
Zhang was explicit about the boundaries of his conclusion, telling Euronews Next that the paper is not a claim that time travel is impossible. The question it addresses is narrower: which spacetimes permit a loop and which forbid one through their geometry alone.
The paper now awaits peer review, and the debate it is likely to provoke centres on topology — whether any universe could curve back on itself without someone building it that way.
