Researchers in Nature Physics say a property of light called orbital angular momentum may be detectable because of a 'twist' in this momentum caused by black holes. And we could detect it, if we just know what to look for.
As Bo Thide, a co-author of the paper, explained to Jason Palmer at the BBC, photons carry two kinds of momentum. The spin angular momentum of photons manifests itself in the familiar phenomenon of polarization - an effect that some sunglasses and even 3D glasses exploit but orbital angular momentum is trickier. It was predicted by James Clerk Maxwell but it wasn't until 1992 that experimentalists found a way to manipulate it.
"If we take the Earth as an example, it spins around its own axis in about 24 hours - that's its spin angular momentum, and it also moves in orbit around the Sun - that's its orbital angular momentum," said Thide. "Photons can carry both. Around a spining black hole, space and time behave in such an odd way; space becomes time, time becomes space, and the whole space-time is acutally dragged around the black hole, becomes twisted around the black hole.
"If you have radiation source... it will then sense this twisting of spacetime itself. The light ray may think that 'Wahey, I'm propagating in a straight line', but if you look at it from the outside, you see it's propagating along a spiral line. That's relativity for you."
Total phase variation of light generated in a region of size 100RS×100RS in the equatorial xy plane of a quasi-extremal rotating black hole (a=0.99) as seen by an asymptotic observer. This region of the sky shows what would be observed with a telescope if the black hole rotation axis is inclined an angle i=45° relative to the observer.
Read the BBC story for the full scoop on what it means or Nature Physics if you have a subscription.
Citation: Fabrizio Tamburini, Bo Thidé, Gabriel Molina-Terriza,Gabriele Anzolin, 'Twisting of light around rotating black holes', Nature Physics (2011) doi:10.1038/nphys1907
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