SPUN OUT: Light emanating from the vicinity of a rotating black hole would bear the imprint of its twisting origins. Above, an artistic representation of twisted light.
Image: Courtesy Miles Padgett, University of Glasgow
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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."
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13 Comments
Add CommentWouldn't a (spinning?) coil of light (of some unspecified diameter) be detected by even a large telescope as simply a random scattering of photons? As I understand, object are identified by the the arrival rate/density of photons at a specific location on the telescope lens. How would astronomers be able to identify a helical coil of light?
Reply | Report Abuse | Link to thisI will be interesting to see if they can pull this kind of experiment off. Assuming so, whether light can be twisted depends on whether or not space/time frame-dragging really happens in the manner predicted by GR. Gravity Probe B scientists have been filtering the data for years and claim the effect has been seen. However there is still a 14% statistical uncertainty and about a 10% systematic uncertainty within the experimental results. I think it's fair to say the question is still less than certain.
Reply | Report Abuse | Link to thisAre they really sure they know what they are getting themselves into with the so called black holes?
Reply | Report Abuse | Link to thisIf life is sustained/derived from structure, then it would be interesting to see what effect is generated by nanotube induced photon spiraling in conjunction with special spiraling photons from a black hole. A resonance effect may be achieved that could render useful data from beyond the visible horizon of a black hole. Special photon 'links' could create a kind of information 'ladder' up from the pit of the gravity well. WHAT is down there really...?
Reply | Report Abuse | Link to thisI am sure lots of people would like to know...
If life is sustained/derived from structure, then it would be interesting to see what effect is generated by nanotube induced photon spiraling in conjunction with special spiraling photons from a black hole. A resonance effect may be achieved that could render useful data from beyond the visible horizon of a black hole. Special photon 'links' could create a kind of information 'ladder' up from the pit of the gravity well. WHAT is down there really...?
Reply | Report Abuse | Link to thisI am sure lots of people would like to know...
oops! I don't know how I posted twice when I only clicked the post button once... No span intended! :)
Reply | Report Abuse | Link to this"Twisted" light = circular polarization, n'est-ce pas?
Reply | Report Abuse | Link to thisLots of things can circularly polarize light, not just black holes.
I'm not sure what you mean Didonai. If you are thinking that your suggested resonance effect could somehow get info from behind the event horizon of a black hole ... it isn't possible. Firstly your nanotube induced spiraling must be done at the detector here or around earth. It would pollute the incoming signal. Secondly BH twisted light could only result from interactions outside of BH event horizons. This depends on whether space/time framing dragging occurs as per general relativity's solution/prediction.
Reply | Report Abuse | Link to thisAs an active black hole devours matter/energy some unknown or poorly understood process kicks energy back away from the BH. Any future twisted light experiment could only seek to capitalize on the space/time framing dragging effect on this backed up light/energy. At our detectors it should appear as a BH twisted light signature which would have to be theoretically modeled and compared to incoming detections. If we get a hit ... then we will have a new method (in addition to gravitational studies) to detect the presence of BHs. The beauty of a BH twisted light detection is it would enable us to speed up the process of BH detection. If we see the twisted light signature it would be enough to know that at least we are detecting an active & spinning BH.
However BH twisted light experiments, even if successful cannot tell us anything about what is behind BH event horizons. It is black, unknown and this territory is currently only being theoretically modeled. Perhaps some other kind of future experiment such as BH gravitational/anti-gravitational waves (if they exist) might succeed in giving us a detection-based glimpse of what is going on within BHs.
No, orbital angular momentum of light is distinctively different than spin angular momentum of light, a.k.a. polarization. Read http://www.scientificamerican.com/article.cfm?id=all-screwed-up or visit http://en.wikipedia.org/wiki/Optical_vortex to learn more.
Reply | Report Abuse | Link to thisBo Thidé, co-author of the Nature Physics article
You are up to date bothide but the new suggestive research is theoretical. Their hope is that space/time itself spins like a top around spinning BHs and thereby light emitted back from the backlash of a black hole feast takes on an unusual twist. If so, this unique signature would be tell tale. I personally (for what it's worth) don't buy the black hole spin story as told via the common extension of the simple GR tale. I expect a lessor effect, but in the end detections will eventually trump speculative alternatives. Time like always, given enough research dollars could solve the issue.
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I'm pretty sure I recently read an article that stated that black hole eject very high energy particles in the upper frequency ranges and that this would cause a black hole that had no new matter input to gradually dissipate. Did the article just gloss over this or is that not true?
Reply | Report Abuse | Link to thisThis idea is nice but I see a clear flaw: you don't conserve the total angular momentum with this process !
Reply | Report Abuse | Link to thisIf you impart orbital angular momentum to a photon, then a particle at infinity that absorbs that photon would rotate with the same orbital angular momentum as the photon. Now, if the photon started with zero orbital angular momentum whereas at inifinity it has non-zero angular momentum, you violate the simple law of conservation of angular momentum. The problem is that the photon had initially zero angular momentum, so this must be true also at infinity. So what the paper describes cannot be true. Am I wrong ? I'm surprised nobody noticed this inconsistency...