Published online 13 February 2011 | Nature | doi:10.1038/news.2011.90

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Twists in space-time caused by rotating black holes should be visible from Earth.

Black holes put a twist on light passing by. Fabrizio Tamburini

An international group of astronomers and physicists has found that rotating black holes leave an imprint on passing radiation that should be detectable using today's most sensitive radio telescopes. Observing this signature, they say, could tell us more about how galaxies evolve and provide a test of Albert Einstein's general theory of relativity.

General relativity says that very massive objects such as black holes warp space-time, bending the path of light that passes them — an effect known as gravitational lensing. The theory also predicts that a rotating black hole will drag space-time around with it, creating a vortex that constrains all nearby objects, including photons, to follow that rotation.

Astronomers already have indirect evidence that the supermassive black holes believed to lie at the core of many galaxies rotate. The rotation of the Milky Way's black hole, for example, is suggested by the velocity distribution of stars within the galaxy, but this provides only an inexact measurement, because it is not known exactly how much matter the galaxy contains. Some astronomers believe that the black hole is rotating very quickly, whereas others maintain that its rotation is slow.

In a paper published today by Nature Physics1, Fabrizio Tamburini, an astronomer at the University of Padua in Italy, and his colleagues show how the rotation can be detected more directly, by measuring changes to the light that passes close to a black hole.

The team says that a wavefront of radiation travelling in a plane perpendicular to the black hole's axis of spin will get twisted as it passes close to the black hole, because half of the wavefront will be moving in the direction of advancing space-time and the other half in the direction of receding space-time. This will give the phase of the radiation — that is, the precise position of the waves' peaks and troughs — a distinctive distribution in space. This will make it possible to determine the speed at which the black holes are spinning much more accurately.

The researchers used a computer simulation to model the phase distribution resulting from the rotation of the Milky Way's black hole, and found that the pattern ought to be visible from the ground. They say it could be measured by pointing an array of radio telescopes at the centre of the galaxy, using different telescopes to observe different segments of the approaching wavefront, and then superimposing these segments on each other to calculate their relative phase. This procedure would be repeated many times, with the telescopes pointing to a different section of the sky surrounding the black hole each time.

Galactic origins

Tamburini describes his group's findings as "fundamentally important", given that most massive objects in the universe rotate. In particular, he says, studying the rotation of black holes in active galactic nuclei will improve astronomers' understanding of these active galaxies, given that the rotation of these black holes would heat the galaxies considerably and so potentially alter their evolution .

The researchers say that, assuming they receive funding, they could carry out measurements of the phase distribution of photons around black holes within the next two years using an existing array of radio telescopes, such as the Very Long Baseline Array of ten radio telescopes in Socorro, New Mexico. The planned Square Kilometre Array, an international project consisting of thousands of antennae set to be 50 times as powerful as any radio instrument in use today and scheduled to be in operation from 2024, would be even more useful for the task.

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Richard Matzner, an astrophysicist at the University of Texas at Austin, agrees that the proposed measurements would give us a much better idea of what is happening near black holes. He also points out that observation of the phase-distribution pattern found by Tamburini and his colleagues would provide extra experimental proof of the theory of general relativity. Conversely, if the pattern isn't found, it might indicate that an alternative theory of gravity should be sought, or at least that previously unknown astrophysical processes are in play.

But Matzner does not believe that current radio telescopes are sensitive enough to make such demanding observations. The measurements involve not only imaging an extremely small portion of the sky, but also measuring the phase variation across it, which will tax the capabilities of the Very Long Baseline Array.

Matzner says that because the radiation emitted from the vicinity of a black hole tends to be brightest at high frequencies, such as those of X-rays or gamma-rays rather than light or radio waves, it would make more sense to use instruments operating at these frequencies. Matzner points out, that would mean launching new space-based observatories, given that X-rays and gamma-rays are absorbed by the atmosphere.