Physicist Hongsheng Zhang Proves Time Travel to the Past Is Impossible in Most Universes
Travelling backward in time remains a staple of science fiction, but new mathematical research suggests the universe may have built-in traffic laws that make such journeys impossible. Hongsheng Zhang, a researcher at the University of Jinan in China, has shown in a preprint study posted to arXiv that travelling into one’s own past is barred in a broad class of theoretical universes. ScienceAlert and Universe Space Tech reported that the entire argument rests on a simple geometric rule regarding the direction of light in general relativity.
The proof runs to just three lines and relies on a single fundamental fact. In relativity, a light-like direction can never sit at right angles, or orthogonal, to an object’s path through time. Spacetime can be imagined as layers that light travels along, forcing anything moving slower or faster than light to cross these surfaces in the same direction. Much like climbing an endless one-way staircase, a traveller can only ever move upward, making it impossible to arrive back at the step where the journey began.
Three Lines of Math and the One-Way Street of Spacetime
The mathematical model uses the long-established null eikonal equation, which physicists normally use to describe the propagation of light waves. Universe Space Tech noted that Zhang used this equation to create a concise numerical barrier without needing complex calculations involving quantum effects, infinite energy, or gravitational singularities. Because the light-like direction cannot be perpendicular to a traveller’s timelike path, a specific mathematical quantity must continuously increase along the journey. As a result, no object can ever loop back to its starting point in its own past.
This new numerical prohibition adds a fourth family of criteria to theoretical physics, supplementing three existing criteria that already restrict closed timelike curves. The first three families include temporal criteria requiring a universal cosmic clock, energy criteria ruling out exotic matter with negative mass, and topological criteria describing the shape and connectedness of spacetime itself. Zhang’s work provides physicists with a rapid way to test mathematical models of the universe for time travel without solving the intricate equations of general relativity.
The Two Theoretical Loopholes That Could Permit Time Loops
While the theorem rules out time loops in many universes, Einstein’s equations still allow for closed timelike curves in specific scenarios. Zhang’s research highlights exactly two theoretical ways a time machine could bypass the prohibition. The first escape route involves local twisting. Near the centre of a rapidly rotating black hole, extreme gravity drags space and time around with it in an effect NASA’s Gravity Probe B satellite measured around Earth in 2011. Physicist Brandon Carter showed in 1968 that loops can exist inside these spinning black holes, where the forward tick of a clock is bent into a circle.
The second loophole relies on the global shape and topology of the universe itself. In this scenario, the spacetime staircase might lead around a structure so that the top step connects directly back to the bottom, much like an impossible Escher drawing. Most theoretical time machines designed on paper, including wormholes, rely on this topological trick. However, whether nature permits either local twisting or global looping in our actual universe remains entirely unknown.
Future Travel Stays Open While Yesterday Remains Out of Reach
While trips to the past face severe mathematical roadblocks, journeys into the future are well established by physics. Einstein proved that moving at high speeds or sitting in strong gravity slows a clock relative to everyone else. According to the European Space Agency, astronauts aboard the International Space Station return to Earth a few thousandths of a second younger than they would otherwise have been. Zhang acknowledged to ScienceAlert that his work does not settle whether our own universe definitively bans yesterday, leaving the ultimate answer open for future research.
Frequently Asked Questions About the New Time Travel Theorem
What exact equation did physicist Hongsheng Zhang use for this study?
Zhang used the null eikonal equation, a long-established formula normally employed to describe the propagation of light waves, to create a three-line mathematical proof.
Where has the research been published?
The research is currently awaiting peer review and has been posted to the preprint repository arXiv.
What prior space mission measured the dragging of spacetime around Earth?
NASA’s Gravity Probe B satellite measured the frame-dragging effect around Earth in 2011, confirming predictions made by Einstein’s theory of general relativity.
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