Friday, June 6, 2014

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:

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