February 14, 2011

‘Twisted Light’ Observed In Black Holes

The fabric of space-time around spinning black holes should impart a "twist" in the property of light called orbital angular momentum. This momentum which could be detected here on Earth, the researchers say, may provide the first unique signature of black holes, reports the journal Nature Physics.

Light particles , or photons, carry two kinds of momentum but only one of them is familiar. "If we take the Earth as an example, it spins around its own axis in about 24 hours - that is its spin angular momentum, and it also moves in orbit around the Sun - that's its orbital angular momentum," explained Bo Thide, a co-author of the paper. "Photons can carry both," he told BBC News.

Spin angular momentum of photons reveals itself in the phenomenon of polarization - an effect that some sunglasses and even 3D glasses exploit. But orbital angular momentum is trickier, and though it was predicted by the father of electromagnetic theory James Clerk Maxwell, it took until 1992 to find a way to manipulate it.

It has only recently been explained by Professor Thide, of the Swedish Institute of Space Physics, and his colleagues, how the unique conditions around a spinning black hole could also impart orbital angular momentum to passing light.

"Around a spinning black hole, space and time behave in such an odd way; space becomes time, time becomes space, and the whole space-time is actually dragged around the black hole, becomes twisted around the black hole," Professor Thide explained to BBC News.

"If you have radiation source... it will then sense this twisting of space time itself. The light ray may think that "ËœWahey, 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."

The idea that spinning black holes could leave their imprint on passing light was first put forward by Martin Harwit of Cornell University in 2003 in an article in the Astrophysical Journal. But The light's "phase", showing a characteristic pattern, astronomers have never thought to look for.

Presuming the light would be coming from some point beyond the black hole and simply passing nearby, Professor Harwit's calculations showed the effect would be minimal.

The massive centers of galaxies, including our own Milky Way, are presumed to be black holes, and they are likely to be rotating. So a signature of light that can reveal spinning black holes is a potentially powerful tool in discovering how our galaxy is constructed.

"While this effect has been known to theorists for some time, it has been regarded as obscure and of no practical importance," said Saul Teukolsky, an astrophysicist and black hole expert at Cornell University in Ithaca, US.

"What is exciting here is the prospect it might be used to measure the spin of the black hole at the center of our galaxy - the measurement will be difficult, though," he told BBC News.

Only the very largest telescope arrays can be used says Professor Thide, and that the Very Large Array in New Mexico, and the coming Square Kilometer Array and Atacama Large Millimeter Array telescopes could be the proving grounds for the idea.

"You would have to modify (the telescope arrays) slightly - probably only through software, and then you combine the signals in a way that's never been done before. If we are lucky, we should see the twisted light stand out, and then we open a bottle of champagne or two," he concludes.

Image 2 Credit Fabrizio Tamburini (University of Padua, Italy)


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