If a time machine is ever invented, its first destination may turn out to be not yesterday, but tomorrow. Physicist Hongsheng Zhang of Jinan University in China presents a theorem according to which, in a broad class of theoretical universes, travel into one’s own past is impossible. The study has not yet undergone peer review.
The argument is related to the way light shapes the structure of spacetime. In the theory of relativity, the direction of light cannot be at a right angle to the direction in which an object moves through time. This makes it possible to represent spacetime as a sequence of “layers” through which objects pass in only one direction.
Zhang compares it to a staircase that can be climbed upward, but on which it is impossible to descend back to the same step. According to him, the proof does not require exotic matter, quantum effects or assumptions about what fills the Universe. “The entire content boils down to keeping track of directions,” the physicist told ScienceAlert.
The Two Possible Breakthroughs
In physics, travel to the past is described through so-called closed timelike curves — trajectories through spacetime that return to the place and moment from which they began. Einstein’s equations allow for such solutions in a limited number of scenarios.
Zhang’s theorem points to two ways in which the restriction could be circumvented. The first involves twisting light trajectories. Near the center of a rotating black hole, the rotation of matter drags spacetime along with it. If the effect is strong enough, forward motion through time can mathematically bend into a circle.
In 1968, physicist Brandon Carter showed that such closed curves exist in the mathematical models of rotating black holes. This does not mean, however, that a person could enter such a region and return unharmed — the theory describes a mathematical possibility, not an existing technology.
The second way is related to the Universe’s global shape. If spacetime has a specific topology, moving only forward could ultimately lead back to the starting point — much like traveling around a closed surface. Some models of hypothetical time machines, including those involving passages through wormholes, rely precisely on such geometry.
The Future Is a Different Physical Possibility
The study does not resolve whether our Universe permits travel to the past. Zhang himself emphasizes: “What the theorem does not say is whether nature allows any of these phenomena. That remains an open question.”
Travel to the future, however, is a confirmed consequence of the theory of relativity. When moving at very high speed or in a strong gravitational field, the traveler’s clock falls behind those of other observers. Astronauts on the International Space Station return to Earth a few thousandths of a second younger than they would have been without this effect, the European Space Agency points out.
For now, mathematics allows for various scenarios, but the experimentally confirmed direction remains one: forward.