Nature News | Edwin Cartlidge | 2011 Feb 13
Twisted Light Could Enable Black Hole DetectionTwists in space-time caused by rotating black holes should be visible from Earth.
An international group of astronomers and physicists has found that rotating black holes leave an imprint on passing radiation that should be detectable using today's most sensitive radio telescopes. Observing this signature, they say, could tell us more about how galaxies evolve and provide a test of Albert Einstein's general theory of relativity.
General relativity says that very massive objects such as black holes warp space-time, bending the path of light that passes them — an effect known as gravitational lensing. The theory also predicts that a rotating black hole will drag space-time around with it, creating a vortex that constrains all nearby objects, including photons, to follow that rotation.
Astronomers already have indirect evidence that the supermassive black holes believed to lie at the core of many galaxies rotate. The rotation of the Milky Way's black hole, for example, is suggested by the velocity distribution of stars within the galaxy, but this provides only an inexact measurement, because it is not known exactly how much matter the galaxy contains. Some astronomers believe that the black hole is rotating very quickly, whereas others maintain that its rotation is slow.
In a paper published today by Nature Physics1, Fabrizio Tamburini, an astronomer at the University of Padua in Italy, and his colleagues show how the rotation can be detected more directly, by measuring changes to the light that passes close to a black hole.
The team says that a wavefront of radiation travelling in a plane perpendicular to the black hole's axis of spin will get twisted as it passes close to the black hole, because half of the wavefront will be moving in the direction of advancing space-time and the other half in the direction of receding space-time. This will give the phase of the radiation — that is, the precise position of the waves' peaks and troughs — a distinctive distribution in space. This will make it possible to determine the speed at which the black holes are spinning much more accurately.
The researchers used a computer simulation to model the phase distribution resulting from the rotation of the Milky Way's black hole, and found that the pattern ought to be visible from the ground. They say it could be measured by pointing an array of radio telescopes at the centre of the galaxy, using different telescopes to observe different segments of the approaching wavefront, and then superimposing these segments on each other to calculate their relative phase. This procedure would be repeated many times, with the telescopes pointing to a different section of the sky surrounding the black hole each time.
Galactic origins
Tamburini describes his group's findings as "fundamentally important", given that most massive objects in the universe rotate. In particular, he says, studying the rotation of black holes in active galactic nuclei will improve astronomers' understanding of these active galaxies, given that the rotation of these black holes would heat the galaxies considerably and so potentially alter their evolution .
The researchers say that, assuming they receive funding, they could carry out measurements of the phase distribution of photons around black holes within the next two years using an existing array of radio telescopes, such as the Very Long Baseline Array of ten radio telescopes in Socorro, New Mexico. The planned Square Kilometre Array, an international project consisting of thousands of antennae set to be 50 times as powerful as any radio instrument in use today and scheduled to be in operation from 2024, would be even more useful for the task.
Richard Matzner, an astrophysicist at the University of Texas at Austin, agrees that the proposed measurements would give us a much better idea of what is happening near black holes. He also points out that observation of the phase-distribution pattern found by Tamburini and his colleagues would provide extra experimental proof of the theory of general relativity. Conversely, if the pattern isn't found, it might indicate that an alternative theory of gravity should be sought, or at least that previously unknown astrophysical processes are in play.
But Matzner does not believe that current radio telescopes are sensitive enough to make such demanding observations. The measurements involve not only imaging an extremely small portion of the sky, but also measuring the phase variation across it, which will tax the capabilities of the Very Long Baseline Array.
Matzner says that because the radiation emitted from the vicinity of a black hole tends to be brightest at high frequencies, such as those of X-rays or gamma-rays rather than light or radio waves, it would make more sense to use instruments operating at these frequencies. Matzner points out, that would mean launching new space-based observatories, given that X-rays and gamma-rays are absorbed by the atmosphere.
Scientific American | John Matson | 2011 Feb 14
Pasta-Shaped Light From Spinning Black Holes Could Challenge EinsteinRotating black holes should put a spin on the light passing by them, potentially allowing astronomers a new way to gauge their properties
Black holes, as their name suggests, are dark. Perfectly dark. A black hole's gravity is so intense that beyond a certain boundary in its vicinity, known as the event horizon, nothing can escape. Not a rocket with its boosters on full blast nor a photon of light. Nothing.
Despite the fact that astronomers cannot peer at what goes on inside the event horizon, a black hole's gravitational effects on its neighborhood allow for a number of indirect observations. Swirls of infalling gas heat up and give off radiation to illuminate a black hole's vicinity, and the orbits of stars around a black hole allow astronomers to estimate its mass. Now researchers have proposed a new optical technique to observe and study black holes by measuring the imprint they should leave on the light that passes near an event horizon.
A black hole's gravitational pull is so strong that it warps the spacetime around it. And if a black hole rotates, as would be the case for a hole that forms from the collapse of a spinning star, it drags spacetime along with it, a phenomenon known as frame dragging. (Less massive bodies also cause frame dragging on a smaller scale; NASA's Gravity Probe B launched in 2004 to measure the frame-dragging effects of Earth's rotation with sensitive gyroscopes.) According to a new analysis, the frame dragging of a black hole should put a detectable twist on nearby photons by imparting a trait known as orbital angular momentum. A light beam with orbital angular momentum looks a bit like a helix or coil when its component waves are mapped out. Whether any point along the beam is a wave peak, a trough or something in between depends on where that point lies with respect to the helix's central axis.
"It is a strange, rotating type of light," says Bo Thidé of the Swedish Institute of Space Physics in Uppsala. "We call it twisted light, spiraling light—there's no good name for it." The orbital angular momentum is distinct from polarization, which relates to the orientation of a light wave. Thidé and his colleagues from the University of Padua in Italy, Macquarie University in Australia and the Institute of Photonic Sciences in Spain reported their finding in a paper published online February 13 in Nature Physics. (Scientific American is part of Nature Publishing Group.)
Twisted light has not been exploited much for astronomy; it was not until relatively recently that physicists in the lab developed the ability to create and detect it. "Even for experimental physicists it takes some time to understand what it's doing," Thidé says. But in a 2003 paper, astronomer Martin Harwit noted that observing the orbital angular momentum from astrophysical sources could have numerous useful applications, including detecting and characterizing black holes.
Thidé and his colleagues have now calculated that a black hole's dragging of spacetime should indeed impart a twist to photons flying out from the vicinity of an event horizon. And what is more, the current generation of world-class telescopes might be able to detect and measure that twisted light. "The trick is not that it's difficult to observe, but you must look for different things than you have done," Thidé says. What is needed is a special instrument called a holographic detector, he notes, which would distort the phase structure of an incoming light beam to weed out light without the proper twist. "It's very analogous to polarized glasses," he adds. Thidé says the group is in discussions with "major telescopes" to explore the possibility of studying black holes by the new method.
Picking out twisted photons from a black hole would provide new information about the objects themselves and provide important tests of general relativity, says Martin Bojowald, a theoretical physicist at Pennsylvania State University who wrote a commentary on Thidé and his colleagues' work for Nature Physics. "I think it's very promising," he says. "Thus far we haven't gotten a lot of information about black holes."
"For astrophysics itself it gives us a new means to measure the spins and see how they are distributed," Bojowald says. But the bigger-picture implications may come from gaining more information of how matter and light behave in extremely powerful gravitational fields. Some modifications to relativity, Bojowald says, might even be ruled out by measurements of twisted light from black holes. At the very least, he notes, it is worth a try, since black holes are such important physical objects and yet so frustratingly difficult to observe. "It hasn't been done yet, so it's not clear how strongly one can constrain the parameters, but it's at least something you can try," he says. "And there's not much else you can do."
Wired Science | Lisa Grossman | 2011 Feb 14
Message in a Wobble: Black Holes Send Memos in LightRotating black holes could leave a twisty signature on light escaping their gravitational maws. If this screwy light can be detected from Earth, it would give astronomers a new way to detect exotic black holes and a new test of Einstein’s theory of general relativity, says a team of physicists.
“For relativity, it’s very important,” said physicist Martin Bojowald at Penn State University, who was not involved in the new work. “There are very few classic tests of relativity. It now seems that we are pretty close to actually using this.”
Black holes are greedy beasts. Not only do they attract matter so strongly that even light can get trapped in their great gravitational bellies, they also grab hold of the fabric of space-time in their vicinity. When a black hole spins — and astronomers expect that most do, although none have been definitively observed — it swirls its surrounding space-time around with it like water spiraling around a drain.
This phenomenon, called frame-dragging, has been proven to work even around bodies as small as Earth. Observations of two Earth-orbiting satellites over the last few decades show that the satellites drag by several feet per year as Earth’s spin tows the fabric of space and time in circles.
“If you can see it, such a tiny little effect from this minute mass that the Earth has compared to a black hole, how much easier would it be to see it around a black hole?” said space physicist Bo Thidé of the Swedish Institute of Space Physics, coauthor of a paper published online February 13 in Nature Physics. “That’s how we started.”
From other researchers’ experiments using lasers and lenses, Thidé and colleagues knew that light traveling in a straight line can be forced into a spiral if sent through the right kind of lens. The twisted beams come out looking like corkscrew-shaped fusilli pasta, Thidé says.
Frame-dragged space-time can produce twisted light in exactly the same way, the physicists argue. A photon fleeing the warped region near a black hole’s event horizon will pick up a wiggliness that could be visible to telescopes on Earth.
“If we have empty space but the space itself has this strange behavior, you don’t need a lens,” Thidé said. “The space itself is already twisted.”
The twist would show up in a property of light called orbital angular momentum, which describes how a light particle revolves around a fixed point, similar to the way the Earth revolves around the sun. Orbital angular momentum is invisible to human eyes, but it’s as fundamental as color, Thidé says. In principle, there’s no reason why an array of telescopes working together couldn’t see light do the twist.
“Light can have color, light can be polarized, and light can have twists,” he said. “There are many qualities of light that we are unfamiliar with because our eyes are so stupid.”
Thidé and colleagues generated simulation data describing light emitted from near the black hole at the center of the galaxy. They then combined traditional techniques for computing the paths light waves take near a black hole with new ways of determining the twisting.
They found that the amount of twisting depends on how fast the black hole is rotating, a result that could allow astronomers to directly measure the rotation rate of a black hole for the first time. Previous estimates of black holes’ spinning speeds were based on the way stars moved in the black holes’ vicinity, but they were not very precise.
“If we can see this twisting, it would be a much more sensitive way to detect the rotation and compare different black holes,” Bojowald said. “To me it was surprising, the sensitivity that can be achieved.”
Getting precise measurements of the spins of lots of black holes could help figure out how black holes form in the first place. The twisted-light signature could also help detect the faint glow black holes may emit as they evaporate, called Hawking radiation, which was predicted in 1974 but has yet to be observed in space.
But Thidé is most excited about the possibility of knocking over Einstein. His computer experiments were based on the predictions of Einstein’s theory of general relativity, which describes how gravity warps time and space. Since Einstein’s 1915 paper describing the theory, only about five real-world tests have been completed.
If a real telescope detects fusilli-shaped light, as Thidé and colleagues predict, it’s another feather in Einstein’s relativistic cap. But if not, space-time may be even more warped than Einstein thought.
“The nice thing is when you find there is a contradiction between existing theories and reality,” Thidé. “That is what everybody is hoping for, including myself.”
Universe Today | Anne Minard | 2011 Feb 13
Twisting of light around rotating black holes - F Tamburini et alImagine a spinning black hole so colossal and so powerful that it kicks photons, the basic units of light, and sends them careening thousands of light years through space. Some of the photons make it to Earth. Scientists are announcing in the journal Nature Physics today that those well-traveled photons still carry the signature of that colossal jolt, as a distortion in the way they move. The disruption is like a long-distance missive from the black hole itself, containing information about its size and the speed of its spin.
The researchers say the jostled photons are key to unraveling the theory that predicts black holes in the first place.
“It is rare in general-relativity research that a new phenomenon is discovered that allows us to test the theory further,” says Martin Bojowald, a Penn State physics professor and author of a News & Views article that accompanies the study.
Black holes are so gravitationally powerful that they distort nearby matter and even space and time. Called framedragging, the phenomenon can be detected by sensitive gyroscopes on satellites, Bojowald notes.
Lead study author Fabrizio Tamburini, an astronomer at the University of Padova (Padua) in Italy, and his colleagues have calculated that rotating spacetime can impart to light an intrinsic form of orbital angular momentum distinct from its spin. 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,” they write. “The twisting of a pure [orbital angular momentum] mode can be seen in interference patterns.” They say researchers need between 10,000 and 100,000 photons to piece a black hole’s story together.
And telescopes need some kind of 3D (or holographic) vision in order to see the corkscrews in the light waves they receive, Bojowald said: “If a telescope can zoom in sufficiently closely, one can be sure that all 10,000-100,000 photons come from the accretion disk rather than from other stars farther away. So the magnification of the telescope will be a crucial factor.”
He believes, based on a rough calculation, that “a star like the sun as far away as the center of the Milky Way would have to be observed for less than a year. So it is not going to be a direct image, but one would not have to wait very long.”
Study co-author Bo Thidé, a professor and program director at the Swedish Institute of Space Physics, said a year may be conservative, even in the case of a small rotation and a need for up to 100,000 photons.
“But who knows,” he said. “We will know more after we have made further detailed modelling – and observations, of course. At this time we emphasize the discovery of a new general relativity phenomenon that allows us to make observations, leaving precise quantitative predictions aside.”
- Nature Physics (online 13 Feb 2011) DOI: 10.1038/nphys1907