Twisted Light Could Enable Black Hole Detection

Rotating black holes should put a spin on the light passing by them, potentially allowing astronomers a new way to gauge their properties
Twisted light

Courtesy Miles Padgett, University of Glasgow

Black holes, as their name suggests, are dark. Perfectly dark. A black hole's gravity is so intense that beyond a certain boundary in its vicinity, known as the event horizon, nothing can escape. Not a rocket with its boosters on full blast nor a photon of light. Nothing.

Despite the fact that astronomers cannot peer at what goes on inside the event horizon, a black hole's gravitational effects on its neighborhood allow for a number of indirect observations. Swirls of infalling gas heat up and give off radiation to illuminate a black hole's vicinity, and the orbits of stars around a black hole allow astronomers to estimate its mass. Now researchers have proposed a new optical technique to observe and study black holes by measuring the imprint they should leave on the light that passes near an event horizon.

A black hole's gravitational pull is so strong that it warps the spacetime around it. And if a black hole rotates, as would be the case for a hole that forms from the collapse of a spinning star, it drags spacetime along with it, a phenomenon known as frame dragging. (Less massive bodies also cause frame dragging on a smaller scale; NASA's Gravity Probe B launched in 2004 to measure the frame-dragging effects of Earth's rotation with sensitive gyroscopes.) According to a new analysis, the frame dragging of a black hole should put a detectable twist on nearby photons by imparting a trait known as orbital angular momentum. A light beam with orbital angular momentum looks a bit like a helix or coil when its component waves are mapped out. Whether any point along the beam is a wave peak, a trough or something in between depends on where that point lies with respect to the helix's central axis.

"It is a strange, rotating type of light," says Bo Thidé of the Swedish Institute of Space Physics in Uppsala. "We call it twisted light, spiraling light—there's no good name for it." The orbital angular momentum is distinct from polarization, which relates to the orientation of a light wave. Thidé and his colleagues from the University of Padua in Italy, Macquarie University in Australia and the Institute of Photonic Sciences in Spain reported their finding in a paper published online February 13 in Nature Physics. (Scientific American is part of Nature Publishing Group.)

Twisted light has not been exploited much for astronomy; it was not until relatively recently that physicists in the lab developed the ability to create and detect it. "Even for experimental physicists it takes some time to understand what it's doing," Thidé says. But in a 2003 paper, astronomer Martin Harwit noted that observing the orbital angular momentum from astrophysical sources could have numerous useful applications, including detecting and characterizing black holes.

Thidé and his colleagues have now calculated that a black hole's dragging of spacetime should indeed impart a twist to photons flying out from the vicinity of an event horizon. And what is more, the current generation of world-class telescopes might be able to detect and measure that twisted light. "The trick is not that it's difficult to observe, but you must look for different things than you have done," Thidé says. What is needed is a special instrument called a holographic detector, he notes, which would distort the phase structure of an incoming light beam to weed out light without the proper twist. "It's very analogous to polarized glasses," he adds. Thidé says the group is in discussions with "major telescopes" to explore the possibility of studying black holes by the new method.

Picking out twisted photons from a black hole would provide new information about the objects themselves and provide important tests of general relativity, says Martin Bojowald, a theoretical physicist at Pennsylvania State University who wrote a commentary on Thidé and his colleagues' work for Nature Physics. "I think it's very promising," he says. "Thus far we haven't gotten a lot of information about black holes."

"For astrophysics itself it gives us a new means to measure the spins and see how they are distributed," Bojowald says. But the bigger-picture implications may come from gaining more information of how matter and light behave in extremely powerful gravitational fields. Some modifications to relativity, Bojowald says, might even be ruled out by measurements of twisted light from black holes. At the very least, he notes, it is worth a try, since black holes are such important physical objects and yet so frustratingly difficult to observe. "It hasn't been done yet, so it's not clear how strongly one can constrain the parameters, but it's at least something you can try," he says. "And there's not much else you can do."

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