Fluid Turbulence in Gravitational Fields around Black Holes

This artist's concept depicts a supermassive black hole at the center of a galaxy. 

The blue colour here represents radiation pouring out from material very close to the black hole. 

The grayish structure surrounding the black hole, called a torus, is made up of gas and dust. 

Credit: NASA/JPL-Caltech

Fasten your seatbelts, gravity is about to get bumpy. Of course, if you're flying in the vicinity of a black hole, a bit of extra bumpiness is the least of your worries. But it's still surprising.

The accepted wisdom among gravitational researchers has been that spacetime cannot become turbulent. New research from Perimeter, though, shows that the accepted wisdom might be wrong.

The researchers followed this line of thought: Gravity, it's thought, can behave as a fluid. One of the characteristic behaviours of fluids is turbulence, that is, under certain conditions, they don't move smoothly, but eddy and swirl. Can gravity do that too?

Perimeter Faculty member Luis Lehner explains why it might make sense to treat gravity as a fluid. "There's a conjecture in physics, the holographic conjecture, which says gravity can be described as a field theory," he says.

"And we also know that at high energies, field theories can be described with the mathematical tools we use to describe fluids."

"So it's a two-step dance: gravity equals field theory, and field theory equals fluids, so gravity equals fields equals fluids. That's called the gravity/fluids duality."

The gravity/fluids duality is not new work, it's been developing over the past six years but hidden at the heart of it is a tension. If gravity can be treated as a fluid, then what about turbulence?

"For many years, the folklore among physicists was that gravity could not be turbulent," notes Lehner.

The belief was that gravity is described by a set of equations that are sufficiently different from fluid dynamics equations, such that there would not be turbulence under any circumstances.

Lehner highlights the emerging paradox: "Either there was a problem with the duality and gravity really can't be fully captured by a fluid description, or there was a new phenomenon in gravity and turbulent gravity really can exist."

A team of researchers; Lehner, Huan Yang (Perimeter and the Institute for Quantum Computing), and Aaron Zimmerman (Canadian Institute for Theoretical Astrophysics), set out to find out which.

They had hints about what directions to go. Previous simulations at Perimeter, and independent work out of MIT, had hinted that there could be turbulence around the non-realistic case of black holes confined in anti-de Sitter space.

"There might be turbulence if you confine gravity in a box, essentially," says Lehner. "The deeper question is whether this can happen in a realistic situation."

More information: Read the original paper on arXiv: arxiv.org/abs/1402.4859

Saturn Moon Titan: Cyclones May Swirl on Icy Moon

An artist's conception of a lake on Titan. 

Cyclones could form above the Saturn's moon seas if they are mostly made of methane, new research indicates. 

CREDIT: NASA/JPL

Titan, an ocean-covered moon around Saturn that's usually so cold methane falls as rain, actually warms up enough in the summertime for high-speed cyclones to whip across its seas, according to new research.

Sea evaporation could create enough energy to produce winds as high as 44 miles per hour (70 km/h) on Titan, which is the largest of Saturn's dozens of moons.

But whether cyclones form at all depends very much on what Titan's seas are made of. If more than half of an ocean is composed of methane, the chemical recipe would be perfect for a storm.

The next step is getting Cassini, a NASA spacecraft orbiting Saturn and its moons, to look for one.

"In the next few years, we will approach summer in the [northern] polar region and we might have the chance to see a cyclone, if the condition is favorable," said Tetsuya Tokano, a researcher with the Institute for Geophysics and Meteorology at the University of Cologne.

Tokano's research is appearing in the April 2013 issue of the journal Icarus.



Cyclones on Earth happen principally in two ways. The first, which cannot happen on Titan because the temperature range is too small, occurs when cold fronts and warm fronts run into each other. Warm and cold air bend around each other and generate high-speed winds.

The second happens when heat from Earth's water warms the air and makes it rise, creating an energy cycle that produces high-speed winds. As the cycle continues, it fuels a spinning storm. This is what could happen on Titan.

Such winds could occur on Titan only above its mid-latitude seas, where there is the right combination of moisture and temperature to create the rising air. Tokano said the difficulty is that we don't yet know the exact chemical composition of Titan's seas.

"There is big uncertainty, and many possible types of hydrocarbons," he said. However, if the seas are mostly methane, they could transfer enough energy from the surface of the sea into the atmosphere to create cyclones. Methane is the only liquid on Titan that can condense like water vapour on Earth.

"This potentially would be large enough to make a cyclone in favorable conditions," Tokano said.