Total phase variation of light generated in a region of 100 times 100 Schwarzschild radii (Rs) in the equatorial plane of a quasi-‐extremal rotating black hole (a = 0.99) as seen by an asymptotic observer located at infinity.
A compilation of blog and archive entries on Physics arXiv.org:
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.
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Kerr black holes are among the most intriguing predictions of Einstein's general relativity theory. These rotating massive astrophysical objects drag and intermix their surrounding space and time, deflecting and phase-modifying light emitted nearby them. We have found that this leads to a new relativistic effect that imposes orbital angular momentum onto such light. Numerical experiments, based on the integration of the null geodesic equations of light from orbiting point-like sources in the Kerr black hole equatorial plane to an asymptotic observer, indeed identify the phase change and wavefront warping and predict the associated light-beam orbital angular momentum spectra. Setting up the best existing telescopes properly, it should be possible to detect and measure this twisted light, thus allowing a direct observational demonstration of the existence of rotating black holes. Since non-rotating objects are more an exception than a rule in the Universe, our findings are of fundamental importance.
Physics arXiv link: Twisting of light around rotating black holes
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