Time travel remains restricted by the geometry of spacetime, according to a mathematical proof presented by Hongsheng Zhang of Jinan University on October 24, 2026, on the arXiv scientific archive, as reported by Vietnam.vn.
Einstein’s general theory of relativity describes gravity as the curvature of spacetime, allowing paths that return to the past, known as closed timelike curves, or CTCs, according to XenoSpectrum.
“The laws of physics do not allow the appearance of closed timelike curves.” Hawking proposed his chronology protection conjecture in 1992 to determine whether backward time travel was possible, stating that the laws of physics do not allow closed timelike curves to appear, as ScienceAlert reported.
To examine whether gravity itself prevents these loops, physicists Moustafa Ismail and David Mattingly of the University of New Hampshire investigated an alternative framework called metric-affine gravity, ScienceAlert reported. This framework loosens the relationship between spacetime geometry and the mathematical connection, allowing researchers to construct a model with a built-in direction of time, according to Around Prague. They named this model Metric-Affine Gravity Imposing Chronology, or MAGIC.
The Scalar Mode Instability
While MAGIC successfully prevents time loops locally, the model introduces a new fluctuation called the scalar mode, ScienceAlert reported. On the cosmological background studied by the researchers, this scalar mode suffers from a gradient instability where disturbances grow exponentially, causing shorter wavelengths to grow faster, according to Around Prague. Because of this instability, the framework cannot yet provide a stable description of the universe in its current form.
Smaller obstacles also affect the model. Ismail and Mattingly have not yet shown that time loop prevention is permanently baked into the framework itself, requiring an additional assumption about the overall structure of spacetime, ScienceAlert reported. The safeguard designed to prevent the metric from breaking down does not function in every circumstance.
Global Null Foliation Proofs
Separate theoretical work posted on arXiv on October 24, 2026, approaches the problem through light propagation, XenoSpectrum reported. In that study, the researcher uses the null eikonal equation to prove that timelike paths cannot form closed loops as long as a global null foliation exists across the entire spacetime.
This proof assumes a scalar function whose gradient never vanishes and is everywhere null, meaning the squared norm of the vector computed with the metric is zero.
Outside the event horizon of a Schwarzschild black hole, the retarded null coordinate can be defined globally, making the theorem consistent with the established result that no CTCs exist in that region, XenoSpectrum noted. However, if the global null coordinate fails, the question of whether time loops can form must be examined separately through alternative topological structures.
Next Steps in Physics Research
Neither theoretical framework serves as a finished solution for the universe, but both represent steps in ongoing research directions, according to Around Prague. “Stabilizing the scalar without losing the timelike, closed projective mode is the natural next step toward a phenomenological completion,” they said.
For the MAGIC model, the researchers said that stabilizing the scalar mode without losing the timelike, closed projective mode is the natural next step toward a phenomenological completion, ScienceAlert reported. Their findings were published in Physical Review D.
Common Questions About Time Travel Physics
What are closed timelike curves?
Closed timelike curves are theoretical paths through spacetime that mathematically lead a traveler back to their own past, as described by Einstein’s general theory of relativity.
What did Stephen Hawking’s conjecture propose?
Stephen Hawking proposed his chronology protection conjecture in 1992 to state that the laws of physics do not allow closed timelike curves to appear.
What is metric-affine gravity?
Metric-affine gravity is an alternative framework for describing gravity that loosens the relationship between the metric and the mathematical connection, allowing them to operate more independently.
Where was the MAGIC model published?
The study detailing the MAGIC model was published in Physical Review D after being investigated by physicists Moustafa Ismail and David Mattingly.
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