BY:SpaceEyeNews.
Time travel to the past faces a new mathematical restriction, but the result comes with an essential condition. A physicist has identified a structure of spacetime that prevents travellers from returning to their own past. Wherever that structure exists, a journey cannot close into a time loop.
The finding offers a sharper way to examine theoretical time machines. However, it does not establish that every possible universe obeys the same restriction. Its most interesting feature lies in that boundary: the proof identifies what must fail before its prohibition stops applying.
What the Proof Claims
Hongsheng Zhang, a physicist at the University of Jinan in China, posted the research on arXiv on October 24, 2026. Euronews subsequently explored the argument in an interview with him. At the time of that coverage, the work awaited peer review.
The study examines closed timelike curves. These paths return to their starting event while a traveller’s own clock continues forward. Zhang asks which geometric conditions exclude such paths.
His theorem requires a globally defined function with a nonzero, lightlike gradient everywhere. In accessible terms, spacetime must admit a consistent arrangement of lightlike surfaces with labels that work throughout the region under examination.
If that condition holds, no closed timelike curve can exist there. This is a conditional mathematical statement. It neither establishes that our entire universe meets the requirement nor rules out every other spacetime geometry.
That distinction matters because a universal ban would require a much broader argument. Claims about time travel to the past therefore need to name the conditions behind them. Otherwise, a result about certain models can sound like a final verdict on all of physics.
Light’s Barrier to Backward Time Travel
No return to the starting label
Picture the relevant surfaces as layers carrying ordered labels. A traveller moving below light speed must cross them with the label changing consistently in one direction.
Returning to the original event would require returning to its original label. Yet a quantity that changes strictly in one direction throughout the journey cannot do that. The proposed loop therefore contradicts the required geometry.
These layers are mathematical surfaces, rather than physical walls made from light. They describe spacetime’s structure without requiring anyone to construct a barrier.
Why the global condition matters
A useful arrangement in one small neighbourhood does not automatically work across an entire journey. The labels must remain consistent everywhere the proof applies.
This requirement explains why the headline needs qualification. A local description may appear perfectly orderly while the larger geometry prevents a single global labelling system.
The central argument is brief, but its assumptions do substantial work. Understanding those assumptions reveals both the theorem’s value and the limits of its reach.
Two Time Travel Loopholes
Twist can obstruct the required surfaces
The first possibility involves twist in a family of lightlike directions. Instead of fitting together into the required surfaces, those directions can wind around one another.
Zhang discusses rotating black hole geometry as a theoretical setting relevant to this issue. However, twist alone does not create a closed timelike curve. It can obstruct the particular construction without supplying a route into yesterday.
Nor does ordinary rotation automatically turn an object into a time machine. The discussion concerns specific mathematical properties, not a general claim that spinning matter enables backward travel.
A failure to apply the theorem leaves a question open. It does not answer that question in favour of time travel.
Topology concerns the larger connections
The second possibility involves topology: how spacetime connects on a global scale. Local layers may exist even when their labels cannot remain single valued everywhere.
The staircase comparison helps here. Each nearby step may lead upward, yet unusual global connections could defeat a consistent numbering of the whole structure.
Euronews places theoretical wormhole time machines within this discussion. Still, a model that allows unusual connections does not establish that nature supplies them.
Both loopholes therefore require careful wording. They describe ways around the argument’s assumptions, not two confirmed methods for building a working time machine.
Future Time Travel Has Experimental Support
Travel into the future involves a different question. Relativity allows two clocks to accumulate different amounts of elapsed time before comparison. Experiments have repeatedly confirmed that difference.
In February 2022, NIST announced that JILA researchers had measured gravitational time dilation across a millimeter-scale cloud of atoms. Different heights within the sample produced measurably different ticking rates.
The measurement provides a concrete example of gravity’s effect on time. It does not test Zhang’s theorem or demonstrate a journey into the past.
Likewise, a traveller who ages less than people on Earth can reach a later Earth date after experiencing less personal time. Their clock still advances normally along their journey.
This distinction keeps established measurements separate from hypothetical loops through spacetime. Evidence for unequal aging does not become evidence for revisiting earlier events.
What Remains Unanswered
Mathematics versus physical reality
The study offers a way to exclude certain time loops before considering how anyone might attempt such a journey. That makes its contribution useful even without a practical application.
However, three questions remain distinct. Does a mathematical model permit a loop? Can physical conditions produce that model? Could any traveller actually follow the proposed route?
An answer to the first question cannot settle the other two. Equally, showing that this particular prohibition fails cannot establish that a loop exists.
Independent scrutiny must also assess the paper’s claimed novelty and the scope of its applications. Its preprint status should remain visible in coverage.
The question our universe must answer
Ultimately, the relevant issue concerns the geometry nature permits. The research does not demonstrate that our universe contains usable backward paths or that anyone could engineer them.
Its immediate contribution is a clearer boundary between mathematical circumstances that exclude loops and circumstances requiring further investigation.
Time Travel to the Past: A Sharper Boundary
Time travel to the past remains an unresolved physical possibility, not an established capability. Zhang’s argument identifies a precise condition that forbids closed timelike journeys wherever it holds.
The two proposed escape routes explain why the result stops short of a universal prohibition. Meanwhile, measured time dilation continues to support unequal elapsed time without providing a return ticket to earlier events.
For now, the research sharpens the question rather than closing it. The next challenge is determining which mathematical possibilities survive closer scrutiny and describe conditions that nature can actually support.
Main sources:
Euronews — No going back: physicist’s proof claims time travel can only take us to the future
https://www.euronews.com/2026/11/09/where-would-you-go-in-a-time-machine-physicist-says-time-travel-only-goes-forward
Hongsheng Zhang, arXiv — A no-go theorem for closed timelike curves in globally null-foliated spacetimes
https://arxiv.org/abs/2610.33706
NIST — JILA Atomic Clocks Measure Einstein’s General Relativity at Millimeter Scale
https://www.nist.gov/news-events/news/2022/02/jila-atomic-clocks-measure-einsteins-general-relativity-millimeter-scale
NASA — Is Time Travel Possible?
https://spaceplace.nasa.gov/time-travel/en/
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