Light passing near to the spinning black holes thought to reside at the centre of many galaxies becomes twisted, possibly offering a new way to test Einstein's general theory of relativity. That is the conclusion of an international team of physicists, who say the phenomenon could be seen with existing telescopes.

The general theory of relativity (GR), put forward by Einstein more than 90 years ago, predicts few phenomena that can be easily tested. One example is gravitational lensing – that the gravity of stars and black holes can warp space–time enough to bend the passage of light. Another is time dilation, which makes clocks sitting in regions of lower gravity – say, at high altitudes – tick faster. Scientists are still trying to directly detect yet another general-relativity phenomenon called gravitational waves. These are ripples in space–time thought to be generated when large masses accelerate.

In 2003 Martin Harwit of Cornell University hinted that there might be one more testable effect to add to this GR toolbox. He was discussing a property of photons called orbital angular momentum (OAM). This is distinct from the more familiar intrinsic spin angular momentum of the photon, which is related to the circular polarization of light.

Interesting effects

A straightforward way to detect the OAM of photons in the lab had just been discovered in 2002 and, according to Harwit, there could be several astrophysical applications of OAM, including the investigation of spinning black holes. "A full theoretical investigation of such effects would be of interest," he concluded. Now, in a paper published today in Nature Physics, a group of physicists led by Bo Thidé of the Swedish Institute of Space Physics in Uppsala have done just that.

The team performed numerical calculations of light passing spinning black holes, which are thought to account for most of the black holes in the cosmos. Around these ultra-dense objects, space–time becomes twisted in an effect called frame dragging. When light enters this region, say Thidé and colleagues, its normally flat wavefronts become twisted too, taking on a corkscrew shape and a change in OAM. The faster the black hole spins, the greater the change in OAM, say the researchers.

"This is a nice, and seemingly sound, piece of theoretical analysis couched in the framework of modern optical theory," said Gary Gibbons, a theorist specializing in general relativity at Cambridge University. Marcus Werner at Duke University commented "This could be rather significant, since it would open up an entirely new observational method."

Tantalizing possibility

To test the researchers' prediction, astrophysicists would need to examine the phase of photons using radio telescopes such as the Very Long Baseline Array at Socorro in New Mexico, US. If the prediction is borne out in measurements, general relativity would be further reinforced as a theory. If it isn't – a remote yet tantalizing possibility – there is the chance general relativity is not telling the whole story about space–time.

Martin Bojowald, at Pennsylvania State University, suggests the possibility that the OAM prediction could allow the direct detection of spinning black holes – a feat never accomplished despite widespread acceptance of their existence. Although nominally "black", black holes are thought to emit a haze of photons called Hawking radiation, but this is so faint it is currently impossible to see over the universe's background radiation. However, Bojowald believes the change in OAM might one day be just enough of a signature to filter out Hawking radiation for observation.

"New calculations of the quantum processes that generate Hawking radiation are required, but before one can address that, the twisting of light already opens the way to exciting new possibilities in black-hole physics," he says.

The work is reported in Nature Physics DOI:10.1038/NPHYS1907.