Interesting .. Physorg's article provides more detail on how they're doing the detection. It is
here.
It appears the information in my previous post, which was sourced from the BBC referenced
AstroPhys published paper, dated 2003, was the original paper which documented the hypothesised method.
The Physorg article cites a revised mehtod of detection, by using modelled phase distributions of the light …
Quote:
In the latest work, Fabrizio Tamburini of the University of Padova in Italy and colleagues instead show how to detect the rotation by measuring changes to the light from a distant star or from the disk of accreted material surrounding a black hole.
They point out that a wavefront travelling in a plane perpendicular to the black hole’s axis of spin will get twisted as it passes close to the black hole, since half of the wave front will be moving in the direction of advancing space-time and the other half in the direction of receding space-time. In other words, the phase of the radiation emanating from close to a rotating black hole should have a distinctive distribution in space.
..
They say the way to measure it is to point an array of radio telescopes at the centre of the galaxy, using different telescopes to observe different segments of the approaching wave front, and then superimpose these segments to calculate their relative phase. This procedure would be repeated, each time the telescopes pointing to a different section of the tiny patch of sky surrounding the black hole.
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The principle behind the two descriptions is the same but interestingly, the more recent work describes the principle using the 'twisted wavefront' analogy (not the photon spin model as per the original concept paper) …
As a result of thinking about it all from the wavefront analogy, they developed a way of doing the observation using a phased array of radio telescopes .. far easier than using interferometry …
Fascinating .. always worthwhile thinking about other ways to 'skin a cat' (apologies to the cat lovers .. no offence intended).
Cheers