Rotating black holes could leave a twisty signature on light escaping their gravitational maws. If this screwy light can be detected from Earth, it would give astronomers a new way to detect exotic black holes and a new test of Einstein’s theory of general relativity, says a team of physicists.

“For relativity, it’s very important,” said physicist Martin Bojowald at Penn State University, who was not involved in the new work. “There are very few classic tests of relativity. It now seems that we are pretty close to actually using this.”

Black holes are greedy beasts. Not only do they attract matter so strongly that even light can get trapped in their great gravitational bellies, they also grab hold of the fabric of space-time in their vicinity. When a black hole spins – and astronomers expect that most do, although none have been definitively observed – it swirls its surrounding space-time around with it like water spiraling around a drain.

This phenomenon, called frame-dragging, has been proven to work even around bodies as small as Earth. Observations of two Earth-orbiting satellites over the last few decades show that the satellites drag by several feet per year as Earth’s spin tows the fabric of space and time in circles.

“If you can see it, such a tiny little effect from this minute mass that the Earth has compared to a black hole, how much easier would it be to see it around a black hole?” said space physicist Bo Thidé of the Swedish Institute of Space Physics, coauthor of a paper published online February 13 in Nature Physics. “That’s how we started.”

From other researchers’ experiments using lasers and lenses, Thidé and colleagues knew that light traveling in a straight line can be forced into a spiral if sent through the right kind of lens. The twisted beams come out looking like corkscrew-shaped fusilli pasta, Thidé says.

Frame-dragged space-time can produce twisted light in exactly the same way, the physicists argue. A photon fleeing the warped region near a black hole’s event horizon will pick up a wiggliness that could be visible to telescopes on Earth.

“If we have empty space but the space itself has this strange behavior, you don’t need a lens,” Thidé said. “The space itself is already twisted.”

The twist would show up in a property of light called orbital angular momentum, which describes how a light particle revolves around a fixed point, similar to the way the Earth revolves around the sun. Orbital angular momentum is invisible to human eyes, but it’s as fundamental as color, Thidé says. In principle, there’s no reason why an array of telescopes working together couldn’t see light do the twist.

“Light can have color, light can be polarized, and light can have twists,” he said. “There are many qualities of light that we are unfamiliar with because our eyes are so stupid.”

Thidé and colleagues generated simulation data describing light emitted from near the black hole at the center of the galaxy. They then combined traditional techniques for computing the paths light waves take near a black hole with new ways of determining the twisting.

They found that the amount of twisting depends on how fast the black hole is rotating, a result that could allow astronomers to directly measure the rotation rate of a black hole for the first time. Previous estimates of black holes’ spinning speeds were based on the way stars moved in the black holes’ vicinity, but they were not very precise.

“If we can see this twisting, it would be a much more sensitive way to detect the rotation and compare different black holes,” Bojowald said. “To me it was surprising, the sensitivity that can be achieved.”

Getting precise measurements of the spins of lots of black holes could help figure out how black holes form in the first place. The twisted-light signature could also help detect the faint glow black holes may emit as they evaporate, called Hawking radiation, which was predicted in 1974 but has yet to be observed in space.

But Thidé is most excited about the possibility of knocking over Einstein. His computer experiments were based on the predictions of Einstein’s theory of general relativity, which describes how gravity warps time and space. Since Einstein’s 1915 paper describing the theory, only about five real-world tests have been completed.

If a real telescope detects fusilli-shaped light, as Thidé and colleagues predict, it’s another feather in Einstein’s relativistic cap. But if not, space-time may be even more warped than Einstein thought.

“The nice thing is when you find there is a contradiction between existing theories and reality,” Thidé. “That is what everybody is hoping for, including myself.”

Image: 1) J. Bergeron/Sky & Telescope. 2) Tamburini et al, Nature Physics 2011.

*"Twisting of light around rotating black holes." Fabrizio Tamburini, Bo Thidé, Gabriel Molina-Terriza, Gabriele Anzolin. Nature Physics, Feb. 13, 2011. DOI: 10.1038/NPHYS1907

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