Light curls up into corkscrew patterns when it passes near black holes, offering a powerful new way to probe the distorted space around them.
In an ordinary light beam observed far from its source, successive peaks of light waves form essentially flat wave fronts. Not so for light with so-called orbital angular momentum, which has long been produced in the lab. Its peaks spiral around to form a corkscrew pattern.
According to general relativity, spinning black holes drag the fabric of the surrounding space around with them. Fabrizio Tamburini of the University of Padua in Italy, and colleagues, calculated how light rays emitted by matter spiralling into a black hole are distorted by this effect, called frame dragging. They calculated that it transforms ordinary light into the corkscrew type that possesses orbital angular momentum.
In future, telescopes could be equipped with detectors to measure this light, says Martin Bojowald of Pennsylvania State University in University Park.
Physicists have measured frame dragging around black holes before by observing the rotation of discs of matter around them. But hydrodynamical processes also affect this rotation, muddying the frame dragging signal. "The new results will allow stricter tests of general relativity thanks to their higher precision," says Bojowald.
Such light could also be used to measure a black hole's spin more accurately, he adds. Currently, astronomers infer the spin by measuring the distance between the black hole and the nearest matter around it, a technique that requires high-resolution observations. Using twisted light would require less spatial resolution and therefore "should make it possible to measure the spin of black holes farther away", Bojowald says.
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