Physicists Find a Math Fix for Time-Travel Paradoxes, With a Catch

Two physicists at the University of New Hampshire have built a gravity model in which time can only run forward, which would rule out looping back into your own past. The catch, according to ScienceAlert, is that the model is unstable in its current form, so it is a research step rather than a working theory.

The study was published in Physical Review D.

What problem is the model trying to solve?

Einstein's general theory of relativity allows closed timelike curves, which are paths through spacetime that can, at least mathematically, lead a traveler back to their own past. That raises the classic grandfather paradox: if you went back and prevented your grandfather from having children, you could never have been born to make the trip.

Stephen Hawking proposed his chronology protection conjecture in 1992, which says the laws of physics do not allow closed timelike curves to appear. ScienceAlert reported that physicists have not yet shown that the universe actually enforces this, and no one has conclusively identified the mechanism, though Hawking suspected quantum effects.

What did the researchers do?

Moustafa Ismail and David Mattingly asked whether gravity itself might supply the answer. General relativity is the best framework for gravity, but not the only one. In it, spacetime has a geometry, described by a metric, and a mathematical connection that describes how directions change along that geometry, and the two are tied together in a particular way.

An alternative called metric-affine gravity loosens that link. Using it, the pair built a model in which spacetime has a built-in direction of time. Under the right conditions, time moves forward along every path a traveler could take, so no loop back is possible. They named it Metric-Affine Gravity Imposing Chronology, or MAGIC.

What is the catch?

In general relativity, gravity can fluctuate in two ways, matching the two ways gravitational waves stretch and squeeze space. MAGIC adds a third, called the scalar mode. On the cosmological background the researchers studied, that mode suffers from a gradient instability: disturbances can grow exponentially, with shorter wavelengths growing faster. As a result, MAGIC cannot yet describe the universe stably.

There are smaller problems too. The model establishes protection against time loops only locally, and guaranteeing it everywhere requires an extra assumption about the overall structure of spacetime that the researchers have not shown follows from the model. A safeguard meant to keep the metric from breaking down also does not work in every circumstance. The researchers have not shown such breakdowns occur, but have not ruled them out.

What does this mean for ordinary readers?

Nothing changes in daily life. This is theoretical work, and ScienceAlert described it as a demonstration of how chronology protection might be built into gravity, not a finished solution. It does not show that time travel is possible or impossible.

What happens next?

The researchers write that stabilizing the scalar mode without losing the model's timelike, closed projective mode is the natural next step. That would move the idea toward a more complete description of the physics, according to the report.

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Around Prague, based on reporting by ScienceAlert