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How To Spot a Rotating Black Hole

A newly discovered effect of general relativity means that rotating black holes should be visible to the current generation of telescopes, say astronomers

Black holes do strange things to the fabric of space time, particularly if they are rotating. One well known effect is that a rotating black hole drags this fabric with it, intermixing space and time in nearby regions.

Today, Fabrizio Tamburini at the University of Padova in Italy and a few pals say this ought to have a significant effect on light that gets caught up in this process and is then emitted from the disc of accreting matter around a rotating black hole. They say the rotation ought to distort the wave front and phase of this light, while imparting orbital angular momentum to the beam.

That’s an effect of general relativity that appears to be entirely new. What’s more, it should make rotating black holes visible. “It should be possible to detect and measure this twisted light,” say Tamburini and co.

So where might we find such a black hole? One candidate is the supermassive black hole at the centre of the Milky Way. Tamburini and pals have simulated the way this object ought to twist the light it emits as it rotates and say it could be detected today by the world’s best telescopes.

In principle, this kind of observation should be possible tonight, provided the scopes are kitted out with right kind of gear. And since the first observation of a rotating black hole would be a useful thing to have on an astronomical CV, it may not be long before we see the results of just this kind of observation.
Ref: arxiv.org/abs/1104.3099: Twisting Of Light Around Rotating Black Holes

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