University of New Hampshire researchers propose gravity model to prevent closed timelike curves

Moustafa Ismail and David Mattingly, researchers at the University of New Hampshire, published work in Physical Review D that puts forward MAGIC, short for Metric-Affine Gravity Imposing Chronology, a metric-affine gravity model designed so spacetime has a built-in direction of time that prevents closed timelike curves.

Closed timelike curves are routes through spacetime that can bring a traveler back to their own past. In a closed timelike curve, the traveler’s route through both space and time loops back to its starting point, so the traveler could encounter their earlier self. General relativity remains the best gravity description we have, but it allows these strange spacetime arrangements, leaving whether time loops actually exist an open question.

The grandfather paradox makes the problem concrete: if a traveler killed their own grandfather before he had children, the traveler would never exist to take the trip. This classic paradox shows why time loops threaten cause-and-effect consistency, the issue Hawking’s conjecture tried to resolve.

In 1992 Stephen Hawking proposed the chronology protection conjecture, arguing that physics forbids closed timelike curves, though researchers have not confirmed that the universe enforces this rule. Hawking suspected quantum effects would prevent closed timelike curves, but no mechanism had been conclusively pinned down before Ismail and Mattingly’s proposal. That gap motivated Ismail and Mattingly to ask whether gravity alone could supply the answer.

Ismail and Mattingly’s answer is metric-affine gravity, a framework in which a spacetime’s metric and its connection are not locked together as they are in general relativity. According to the sources, general relativity is just one gravity framework among others, and metric-affine gravity gives the metric and connection more freedom than standard general relativity does. In plain terms, the metric defines distance and time intervals, while the connection governs how objects and vectors move from one point to another.

Using that freedom, the researchers made MAGIC so that time runs forward along every path a traveler might take, making a loop back into one’s own past impossible. In other words, the model claims that spacetime itself carries a forward direction of time, which blocks closed timelike curves without any quantum effects.

The mathematics of the proposal hinges on how gravity fluctuates. In general relativity, gravity comes in two fluctuations, which match the way gravitational waves stretch and squeeze space. Beyond those two fluctuations, MAGIC adds a third one called the scalar mode, and that extra mode is what creates the instability on the cosmological background they studied.

On that background, the scalar mode suffers a gradient instability: disturbances grow exponentially, and the shortest wavelengths grow fastest. The disturbances amplify without limit, particularly at short wavelengths, so the instability prevents a stable description of the universe.

The MAGIC model does not provide a stable account of the universe. That makes the gradient instability the main obstacle to a viable gravity theory.

The model’s chronology protection works only on a local scale. Protecting time everywhere would require an extra assumption about the structure of spacetime as a whole. Because that assumption does not follow from MAGIC itself, protection against time loops is not yet built into the theory.

The model contains a safeguard intended to keep the metric from collapsing in ways that would make spacetime impossible to describe, yet that safeguard fails under some conditions. The researchers have not demonstrated that such breakdowns actually occur in MAGIC, but they have not ruled them out either.

The way forward, according to the researchers, is to keep the scalar mode stable while preserving the timelike, closed projective mode. The authors say that move would push the model toward a fuller physical description.

"Stabilizing the scalar without losing the timelike, closed projective mode is the natural next step toward a phenomenological completion,"

Moustafa Ismail and David Mattingly, ScienceAlert

MAGIC is not intended as the final answer. They call it an intriguing demonstration of building chronology protection into gravity, describing it as a step in the research process rather than a finished solution. The work does not settle whether time travel is possible, and everyday life continues unaffected.

The contrast with Hawking is telling. Where Hawking looked to quantum effects to enforce chronology protection, MAGIC tries to build the ban on closed timelike curves directly into spacetime’s geometry through a modified theory of gravity. So far, neither approach has yielded a confirmed mechanism for keeping time loops out of the universe.

What remains open is whether the scalar mode can be stabilized without sacrificing the timelike, closed projective mode, and whether the extra assumption behind universe-wide chronology protection can be derived from MAGIC rather than imposed on it. Until that work is done, the model’s promise of protection against time loops stays conditional.