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Sunday, February 13, 2011

CORKSCREW LIGHT: Measuring Orbital Angular Momentum Will Give Us Extra Information About Black Holes and Frame Dragging

Frame Dragging, an effect spinning black holes have on spacetime and on the light in its vicinity, is causing a measurable corkscrew effect on photons, newly discovered and published in this issue of Nature Physics.  "Twisting of light around rotating black holes"

Simplified, frame dragging occurs when massive objects such as a black hole drags spacetime around with it while moving.  Imagine gently stirring a tiny circle within a big bowl of elmers glue.  Translate that into a graph and overlay this information on top of your familiar image of curved spacetime, and you have a dynamic and realistic scenario of general relativity in motion. When you aim your telescope at a spinning black hole, the light you're receiving is distorted, and this requires scientists to dig deep into the meat of general relativity.  This effect is different than the smooth curvature of spacetime as seen in the picture below.

Photons are massless, but they can interact with matter by a quantitative effect measured as momentum.  The angular momentum of photons expressed as Spin has been studied at great lengths, but a new measurement of orbital angular momentum has just been discovered by Fabrizio Tamburini and his team at the University of Padua in Italy.  

In the current issue of Nature Physics, we measure photons exhibiting a corkscrew spin, a technique that Martin Bojowald of Pennsylvania State University says can be incorporated into our telescopic equipment in the future.

So what exactly is this corkscrew effect?  The orbital angular momentum is explained in Universe Today: "The authors suggest visualizing this as non-planar wavefronts of this twisted light like a cylindrical spiral staircase, centered around the light beam. The intensity pattern of twisted light transverse to the beam shows a dark spot in the middle — where no one would walk on the staircase — surrounded by concentric circles

This new information gives us another way to measure the size of black holes.  New Scientist explains, "Currently, astronomers infer the spin by measuring the distance between the black hole and the nearest matter around it, a technique that requires high-resolution observations. Using twisted light would require less spatial resolution and therefore "should make it possible to measure the spin of black holes farther away", Bojowald says."

Nature Physics requires an 18$ fee to read the entire article, but the abstract can be seen here, as posted below:

ABSTRACT: "Twisting of light around rotating black holes"
"Kerr black holes are among the most intriguing predictions of Einstein’s general relativity theory1, 2. These rotating massive astrophysical objects drag and intermix their surrounding space and time, deflecting and phase-modifying light emitted near them. We have found that this leads to a new relativistic effect that imprints orbital angular momentum on 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 observer3, indeed identify the phase change and wavefront warping and predict the associated light-beam orbital angular momentum spectra4. 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. As non-rotating objects are more an exception than a rule in the Universe, our findings are of fundamental importance."

Tamburini, F., Thidé, B., Molina-Terriza, G., & Anzolin, G. (2011). Twisting of light around rotating black holes Nature Physics DOI: 10.1038/nphys1907

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