June 6, 2014
Turbulent Gravity Around Black Holes
Brett Smith for redOrbit.com - Your Universe Online
According to conventional wisdom among physicists, gravity cannot become turbulent. However, new research from the Perimeter Institute for Theoretical Physics has found that gravity can become turbulent, specifically around a black hole, because it behaves like a fluid.
“There’s a conjecture in physics – the holographic conjecture – which says gravity can be described as a field theory,” said study author Luis Lehner, a researcher at the institute. “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 idea of gravity acting like a fluid has been developing in recent years, but physicists have yet to embrace gravity becoming turbulent, like any fluid.
“For many years, the folklore among physicists was that gravity could not be turbulent,” Lehner said.
Because the equations describing the forces of gravity are so different from fluid dynamics equations – popular belief held that gravity couldn’t be turbulent.
“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,” Lehner explained.
First, the study team considered an improbable scenario in which a black hole was confined in an anti-de Sitter space.
“There might be turbulence if you confine gravity in a box, essentially,” Lehner explained. “The deeper question is whether this can happen in a realistic situation.”
Next, the researchers looked into fast-spinning black holes to see if the gravity surrounding them is less viscous than the gravity around other types of black holes. Low viscosity raises the possibility of turbulence – in the same way than water could be more turbulent than motor oil.
The team then undertook the highly-complex, non-linear perturbation analysis of these black holes, with the understanding that turbulence is fundamentally non-linear. A series of complex equations revealed that gravity could indeed be turbulent.
Huan Yang, postdoctoral researcher at Perimeter, said physicists had never come to this conclusion because the analysis needed to go to non-linear orders.
“People didn’t have enough motivation to do a non-linear study. But, this time, we knew what we were looking for. It gave us the motivation to do a more in-depth study. We had a target and we hit it,” he said.
The study team noted that their theoretical work could be confirmed in the near future by Caltech’s Laser Interferometer Gravitational-Wave Observatory (LIGO) or NASA’s Laser Interferometer Space Antenna (LISA).
“We’ve been stuck for over 500 years on achieving a full understanding of turbulence,” Lehner said. “This gravity/fluid correspondence tells us that there is a way to use gravitational tools and gravitational intuition to take a fresh look at turbulence. We may end up as stuck as we are in our standard approach, or we may end up shedding completely new light that helps the field go forward. It’s very exciting.”
Read the original paper on arXiv.