We all know the rules. If, for any reason, by any means, you travel back in time, you must not find or kill your own grandfather before he has had a chance to procreate. Doing so would break the Universe, because if your grandfather never has offspring, you would not be born and you would not be able to travel back in time to kill him. It is not clear how the Universe would solve this puzzle. Maybe the laws of physics would simply prevent this from happening in the first place. Stephen Hawking said it. But today we have good news. Physicists have found a mathematical way to avoid this crisis. There’s just one small problem: Your solution introduces an instability that could cause small perturbations in the fabric of spacetime to grow exponentially. Oh my god. Hawking proposed his chronological protection conjecture in 1992 to explore the question of whether it was possible to travel back in time. The conjecture says: “The laws of physics do not allow the appearance of closed time curves.” The problem is that, although Hawking’s calculations supported his conjecture, physicists have yet to establish that the Universe actually imposes it. Einstein’s general theory of relativity, our best description of gravity, allows for some decidedly peculiar arrangements of spacetime. Among them are time-shaped sharp curves: paths through space-time that, at least mathematically, can take a traveler to their own past. Whether such paths could actually exist in the physical Universe is another question entirely. Hawking suspected that quantum effects would prevent their formation. But no one has been able to conclusively identify what that mechanism might be. Physicists Moustafa Ismail and David Mattingly of the University of New Hampshire wondered if gravity itself might hold the answer. After all, if the mathematics of gravity allows time loops to exist, perhaps a different formulation of gravity could prevent them. Now, there is something you need to understand about gravity. Although general relativity is the best framework we currently have to describe it, it is not the only framework we have to describe it. In general relativity, spacetime has a geometry described by something called a metric, which determines distances and time intervals, and a mathematical connection that describes how directions change as you move through that geometry. The two are linked in a particular way. An alternative framework for gravity called metric affine gravity loosens that relationship, allowing the metric and connection to function more independently. This is where Ismail and Mattingly found some wiggle room. They used metric affine gravity to build a model in which spacetime could have a built-in direction of time. Under the right conditions, time would always advance along any path a traveler could take through spacetime, making it impossible to return to one’s own past. They called this model Imposing Chronology of Metric Affine Gravity. That’s how it is. MAGIC.So far so good, right? Well. In general relativity, gravity can fluctuate in two different ways, corresponding to the two ways that gravitational waves can stretch and compress space. MAGIC introduces a third way that gravity fluctuates: the scalar mode. And scalar mode doesn’t play particularly well with reality as we know it. In the cosmological context the researchers studied, it suffers from something called gradient instability. Instead of propagating as stable waves normally do, perturbations in MAGIC’s scalar mode can grow exponentially, with shorter wavelengths growing faster. This means that, in its current form, MAGIC cannot provide a stable description of the Universe. That is the biggest obstacle to making MAGIC a viable theory of gravity. There are also some smaller ones. First of all, MAGIC only sets the conditions for chronological protection locally. To ensure that time cannot retrace itself anywhere in the Universe, researchers have to make an additional assumption about the overall structure of space-time that they have not shown to follow from the model itself. So while they can get MAGIC to prevent time loops, they haven’t yet shown that time loop prevention is built into MAGIC. Second, MAGIC is designed to prevent the metric from decomposing in ways that would make spacetime impossible to describe adequately, but this safeguard does not work in all circumstances. We don’t know if these glitches actually occur in MAGIC (the researchers haven’t proven that they do), but they haven’t ruled them out either. So we have a framework that is designed to do specific things that the specific things don’t always do. But that’s okay. Because MAGIC is not supposed to be the final answer. It’s an intriguing demonstration of how chronological protection could be incorporated into gravity: a step in a research direction, but not yet a working solution. And once that step is taken, researchers have a clearer idea of where to go next. “Stabilizing the scalar without losing the closed and temporal projective mode is the next natural step toward a phenomenological completion,” they write. So tell your grandfather to stay calm. Physicists might still find a way to save their skin from their time-traveling, rule-breaking grandchildren. The research has been published in Physical Review D. This article was verified by Fiona MacDonald and edited by Fiona MacDonald. While we take pride in our process, we are human. If you spot an error, please let us know.
