Black holes take light for a spin

Newly discovered effect could aid observations of Milky Way's monster

Given how weird black holes are, it’s only fitting that researchers have found a screwy way to detect the rotation of these gravitational monsters. Existing telescopes could be equipped with special detectors to record the twist imprinted on light waves that pass near a rapidly spinning black hole, Bo Thidé of the Swedish Institute of Space Physics in Uppsala and his colleagues report online February 13 in Nature Physics.

The newly discovered effect that spinning black holes have on light waves is a consequence of Einstein’s theory of relativity and based on numerical simulations performed by his team, says Thidé. Researchers had already predicted and found some evidence that rotating black holes and neutron stars stir the fabric of surrounding space and time like pancake batter, an effect known as frame dragging (SN: 9/2/00, p. 150).

But researchers hadn’t explored in detail the possibility that rotating black holes could also take light for a spin, imparting angular momentum to the radiation, comments Martin Bojowald of Pennsylvania State University in University Park. “The black hole influences spacetime in such a way that light with net orbital angular momentum is automatically produced,” he says.

Light waves are made up of crests and troughs. Those light waves that travel in unison and unimpeded through space have wave fronts — the imaginary surface over which the crest of one wave lines up with the crest of another — that are planes. In contrast, when light passes near a black hole, each photon acquires a twist that alters the wave surface from a plane to a spiral staircase centered around the direction of travel of the light beam.

“What is new and exciting is the proposal that the effect is actually measurable for the black hole at the center of our galaxy,” says Saul Teukolsky of Cornell University.

Thidé says his team will review radio telescope observations of the Milky Way’s supermassive black hole to see if the twisted light effect has already shown up. Bojowald says the technique “will not be an immediate tool for actual observations of black holes, but it looks promising enough to suggest upgrading telescopes” so they can search for it.

In the meantime, he says, twisted light “gives us a new means to test general relativity and spacetime.”

TURN SIGNAL Photons emitted near a rotating black hole get a twist, in the form of orbital angular momentum, represented by the spiral-staircase shape. Detecting that pattern, researchers say, will allow astronomers to directly measure a black hole’s spin. Nature Physics © 2007

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