Two physicists have built a mathematical model in which gravity itself stops time from looping back on itself. The catch is that their model wobbles. In its current form, it cannot describe a stable universe.
Moustafa Ismail and David Mattingly of the University of New Hampshire published the work in Physical Review D. They started from a question Stephen Hawking posed in 1992: if the equations of general relativity allow paths that lead a traveller into their own past, why does the universe never seem to allow it?
Their answer uses an alternative framework called metric-affine gravity. In it, spacetime carries a built-in direction of time, so every path a traveller could take moves forward. They named the model Metric-Affine Gravity Imposing Chronology. The initials spell MAGIC.
The model introduces a third way for gravity to fluctuate, a scalar mode. On the background the researchers studied, that mode suffers from a gradient instability. Small disturbances in spacetime grow exponentially instead of settling down, with shorter wavelengths growing fastest.
The researchers say that instability is the biggest obstacle to making MAGIC a workable theory of gravity. The model also protects against time loops only locally. Extending the ban across the whole universe would need an extra assumption about the shape of spacetime that the model does not yet supply.
Ismail and Mattingly present MAGIC as a demonstration of how chronology protection could be built into gravity. Stabilising the scalar mode is the next problem to solve.
Source: ScienceAlert
