Gravity May Protect The Universe From Time Travel Paradoxes, But There's a Catch

We all know the rules.

If, for whatever reason, by whatever means, you travel back in time, you absolutely must not find and 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 could not travel back in time to kill him.

Quite how the Universe would resolve this conundrum is unclear. Maybe the laws of physics would simply prevent it from happening in the first place. Stephen Hawking said so.

But today, we have good news. Physicists have found a mathematical way to avert this crisis. There's just one little problem: their solution introduces an instability that could cause tiny disturbances in the fabric of spacetime to grow exponentially.

Oh goody.

Hawking proposed his chronology protection conjecture in 1992 to explore the question of whether backward time travel was even possible. The conjecture states, "The laws of physics do not allow the appearance of closed timelike curves."

The problem is that, although Hawking's calculations supported his conjecture, physicists have yet to establish that the Universe actually enforces it.

Einstein's general theory of relativity, our best description of gravity, allows for some decidedly peculiar arrangements of spacetime. Among them are closed timelike curves – paths through spacetime that, at least mathematically, can lead a traveler back to their own past.

Whether such paths could actually exist in the physical Universe is another matter entirely.

Hawking suspected that quantum effects would prevent them from forming. But nobody has managed to conclusively identify what that mechanism might be.

Physicists Moustafa Ismail and David Mattingly of the University of New Hampshire wondered whether 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's something you need to understand about gravity. Although general relativity is the best framework we currently have for describing it, it's not the only framework we have for describing it.

In general relativity, spacetime has a geometry described by something called a metric, which determines distances and intervals of time, 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 the connection to operate more independently.

This is where Ismail and Mattingly found some wiggle room.

They used metric-affine gravity to construct a model in which spacetime could have a built-in direction of time. Under the right conditions, time would always move forward along any path a traveler could take through spacetime, making it impossible to loop back into their own past.

They named this model Metric-Affine Gravity Imposing Chronology. That's right. MAGIC.

So far so good, right?

Well.

In general relativity, gravity can fluctuate in two different ways, corresponding to the two ways gravitational waves can stretch and squeeze space.

MAGIC introduces a third way for gravity to fluctuate – the scalar mode. And the scalar mode doesn't play particularly nicely with reality as we know it.

On the cosmological background the researchers studied, it suffers from something called a gradient instability. Rather than propagating as stable waves normally do, disturbances 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's the biggest obstacle to making MAGIC a viable theory of gravity. There are some smaller ones, too.

First, MAGIC only establishes the conditions for chronology protection locally.

To guarantee that time can't loop back on itself anywhere in the Universe, the researchers have to make an additional assumption about the overall structure of spacetime that they haven't shown follows from the model itself.

So while they can make MAGIC prevent time loops, they have not yet shown that time loop prevention is baked into MAGIC itself.

Second, MAGIC is designed to prevent the metric from breaking down in ways that would make spacetime impossible to describe properly – but this safeguard doesn't work in every circumstance.

We don't know that these breakdowns do actually occur in MAGIC – the researchers haven't demonstrated that they do – but they haven't ruled them out, either.

So, we have a framework that is designed to do specific things that may not always do the specific things.

But that's OK. Because MAGIC isn't supposed to be the final answer. It's an intriguing demonstration of how chronology protection might be built into gravity – a step in a research direction, not a working solution yet.

And, having taken that step, the researchers have a clearer idea of where to go next.

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

So tell your grandpa to sit tight. Physicists might find a way to save his bacon from time-traveling, rule-breaking grandchildren yet.

The research has been published in Physical Review D.