Mini-hole made of metamaterials ensnares microwave light
BY Geoff Brumfiel  /  15 October 2009

Physicists have created a black hole for light that can fit in your coat pocket. Their device, which measures just 22 centimetres across, can suck up microwave light and convert it into heat. The hole is the latest clever device to use ‘metamaterials’, specially engineered materials that can bend light in unusual ways. Previously, scientists have used such metamaterials to build ‘invisibility carpets’ and super-clear lenses. This latest black hole was made by Qiang Chen and Tie Jun Cui of Southeast University in Nanjing, China, and is described in a paper on the preprint server ArXiv.

Black holes are normally too massive to be carried around. The black hole at the centre of the Milky Way, for example, has a mass around 3.6 million times that of the Sun and warps the very space around it. Light that travels too close to it can become trapped forever. The new meta-black hole also bends light, but in a very different way. Rather than relying on gravity, the black hole uses a series of metallic ‘resonators’ arranged in 60 concentric circles. The resonators affect the electric and magnetic fields of a passing light wave, causing it to bend towards the centre of the hole. It spirals closer and closer to the black hole’s ‘core’ until it reaches the 20 innermost layers. Those layers are made of another set of resonators that convert light into heat. The result: what goes in cannot come out. “The light into the core is totally absorbed,” Cui says.

“I am very impressed,” says John Pendry, a theoretical physicist at Imperial College London. Pendry says that the black hole is yet the latest example of the many strange devices that can be built with metamaterials. But, he adds, it is not a perfect black-hole analogy. The enormous gravity of real black holes causes them to emit an eerie quantum glow, known as Hawking radiation. “The optical device reported in this paper has no internal source of energy and therefore cannot emit Hawking radiation,” Pendry says. Nevertheless, Cui says that the hole could prove useful. By the end of the year his team hopes to have a version of the device that will suck up light of optical frequencies. If it works, it could be used in applications such as solar cells.

{References : Cheng, Q. & Cui, T. J.}
http://arxiv.org/abs/0910.2159v1 (2009)


Device mimics black hole event horizon
BY Roger Highfield  /  13 Feb 2008

A team of researchers in Scotland has been able to boldly go where science fiction writers have only dreamt of visiting – inside the maw of a black hole, to crack some of the deepest mysteries of the cosmos. Black holes, the remains of collapsed stars, are the most extraordinary objects in the universe, where the pull of gravity is so intense that light is sucked in if it strays beyond a boundary called the event horizon. Now it seems these horizons can be mimicked using a table-top device that harnesses lasers to create an artificial black hole, according to a study by Prof Ulf Leonhardt of the University of St Andrews that could help win a Nobel prize for the world’s best known physicist, Prof Stephen Hawking.

At St Andrews, Prof Leonhardt works on what are called quantum catastrophes, where so-called “singularities” can be created where the laws of wave physics are in danger of breaking down. Black holes are also singularities, where the pull of gravity is so intense that even light is sucked in. The professor told the recent the Cosmology Meets Condensed Matter meeting in London that his team accomplished the feat of simulating key features of a black hole by firing lasers down an optical fibre, exploiting how different wavelengths of light move at different speeds within the fibre.

His team first shot a relatively slow moving laser pulse through the fibre, and then sent a faster “probe wave” chasing after it. The slower light pulse distorts the optical properties of the fibre, forcing the speedy probe wave to slow down dramatically when it catches up so it becomes trapped and can never overtake the pulse’s leading edge, so that it acts in just the same way as a black hole event horizon, beyond which light cannot escape.

And the measurements by Prof Leonhardt, Dr Chris Kuklewicz and Dr Friedrich Koenig at St Andrews, with Dr Thomas Philbin of the University of Erlangen, agree with the predictions of cosmologists, who have already worked out exactly how light should change frequency as it approaches an event horizon – from both the outside or the inside of a black hole.

Prof Hawking’s chance of winning the Nobel prize has improved markedly because this device makes it possible to test his theories, which make specific predictions about the event horizon – the rim of a black hole. “We show by theoretical calculations that such a system is capable of probing the quantum effects of horizons, in particular Hawking radiation,” say the St Andrews team in a preprint of their paper.

Prof Hawking predicts that radiation would be given off at the horizon of black holes so that they would evaporate. In his book, The Universe in a Nutshell, the Cambridge University physicist said that only smaller black holes give off enough “Hawking radiation” to be detectable and there do not seem to be many of them around. “That is a pity. If one were discovered, I would get a Nobel prize.” Prof Ray Rivers at Imperial College London tells New Scientist: “They’ve done some clever stuff to give us a chance of seeing Hawking radiation for the first time.”

Ulf Leonhardt
email : ulf [at] st-andrews.ac [dot] uk

Scientists Make Blackest Material Ever
By Brandon Keim  /  March 31, 2009

Scientists have fashioned what may be the blackest material in the universe: a sheet of carbon nanotubes that captures nearly every last photon of every wavelength of light. The substance absorbs between 97 percent and 99 percent of wavelengths that can be directly measured or extrapolated. It’s the closest that scientists have yet come to a black body, a theorized state of perfect absorption whose closest analogue is believed to be the opening of a deep hole. The material, described Monday by Japanese nanotechnologists in the Proceedings of the National Academy of Sciences, is made from a flat array of vertically-aligned, single-walled carbon nanotubes. Photons that aren’t immediately absorbed by a single nanotube deflect off and are absorbed by its neighbors. “This interaction,” write the researchers, “repeats until the attenuated light is completely absorbed by the forest.” To the naked eye, the substance appears perfectly flat; in effect, it’s a sheet of deep holes. By comparison, the blackest paints and coatings absorb between 84 and 95 percent of all light. Researchers say the material would be useful in solar panels or to collect heat in the frigid vacuum of space.


Black holes: The ultimate quantum computers?
BY Maggie McKee  /  13 March 2006

Nearly all of the information that falls into a black hole escapes back out, a controversial new study argues. The work suggests that black holes could one day be used as incredibly accurate quantum computers – if enormous theoretical and practical hurdles can first be overcome. Black holes are thought to destroy anything that crosses a point of no return around them called an “event horizon”. But in the 1970s, Stephen Hawking used quantum mechanics to show black holes do emit radiation, which eventually evaporates them away completely.

Originally, he argued that this “Hawking radiation” is so random that it could carry no information out about what had fallen into the black hole. But this conflicted with quantum mechanics, which states that quantum information can never be lost. Eventually, Hawking changed his mind and in 2004 famously conceded a bet, admitting that black holes do not destroy information. But the issue is far from settled, says Daniel Gottesman of the Perimeter Institute in Waterloo, Canada. “Hawking has changed his mind, but a lot of other people haven’t,” he told New Scientist. “There are still a lot of questions about what’s really going on.”

Quantum entanglement
Now, Seth Lloyd of the Massachusetts Institute of Technology in the US, has used a controversial quantum model called final-state projection to try to solve the paradox. The model holds that under certain extreme circumstances – such as the intense gravitational field of a black hole, objects that would ordinarily have several options for their behaviour have only one. For example, a black hole could cause a coin thrown into it to always come up “heads”.

This allows information to escape from a black hole without any ambiguity about how to interpret it. The information escapes through a quantum process called entanglement, in which objects are not independent if they have interacted with each other or come into being through the same process. They become linked, or entangled, such that changing one invariably affects the other, no matter how far apart they are.

In black holes, Hawking radiation arises just inside the event horizon and has two components – one that leaves the black hole and another that falls towards the point-like singularity that is the black hole itself. These components are entangled, so when matter that has been sucked into the black hole interacts with the infalling Hawking radiation at the singularity, the interaction instantaneously produces a change in the Hawking radiation that has escaped the black hole. Because the final-state projection model forces this interaction to behave in only one way, this radiation therefore carries information about material inside the black hole.

Smooshed up
Gottesman and colleague John Preskill of the California Institute of Technology in Pasadena, US, found that previous calculations by other researchers using this model allowed information to escape for only certain interactions between the infalling matter and the infalling Hawking radiation. Now, Lloyd has calculated that the process is quite robust – the random nature of these interactions means the system is almost perfectly entangled.

That suggests the outgoing Hawking radiation carries away nearly all of the information of the matter – such as a spaceship – that falls into the black hole. According to Lloyd, the most that could be lost is half a quantum unit of information, or 0.5 qubit. “Passengers on a spaceship would like some guarantee that when they fall into this black hole and get smooshed into the singularity, they can be recreated as it evaporates,” Lloyd told New Scientist. “With a few simple precautions, the travellers would be almost exactly the same, with less than an atom of difference.” Lloyd also says the work suggests black holes could be used as quantum computers. “We might be able to figure out a way to essentially program the black hole by putting in the right collection of matter,” he says.

Mission implausible
But both applications would require an understanding of the properties of specific black holes, says Gottesman. “And you’d have to collect every little piece of Hawking radiation because the spaceship would get spread out with everything that fell into the black hole – ever,” Gottesman says. “So you’d have to sort out which bits were the spaceship and which bits were other things. It’s implausible.”

Lloyd agrees. Understanding how to decode the outgoing Hawking radiation will require researchers to weave together quantum physics and general relativity into a seamless theory of quantum gravity – a goal that has so far proved elusive. “Until we understand quantum gravity, we’re not going to be running Linux on a black hole,” he jokes. But beyond the practical difficulties, Gottesman says the work has a more serious theoretical flaw. Despite the fact that just half a qubit of information is lost, “from a fundamental point of view, there is no real difference between a little bit of information being lost and a lot being lost,” he says. “In standard quantum mechanics, no information is ever lost, so if he is right, quantum mechanics would have to be revised to allow information loss. We have no real idea of what theory could take its place.”

{Journal reference: Physical Review Letters (vol 96, no 061302)}

Seeking Objects ‘Weirder Than Black Holes’  /  September 24, 2007

Researchers’ equations suggest gravitational lensing could lead astronomers to ‘naked singularities,’ if such entities exist despite being banned by ‘cosmic censorship’

Researchers from Duke University and the University of Cambridge think there is a way to determine whether some black holes are not actually black. Finding such an unmasked form of what physicists term a singularity “would shock the foundation of general relativity,” said Arlie Petters, a Duke professor of mathematics and physics who worked with Marcus Werner, Cambridge graduate student in astrophysics, on a report posted online Monday, Sept. 24, for the research journal Physical Review D. “It would show that nature has surprises even weirder than black holes,” Petters added.

Albert Einstein originally theorized that stars bigger than the sun can collapse and compress into singularities, entities so confining and massively dense that the laws of physics break down inside them. Astronomers have since found indirect evidence for these entities, which are popularly known as black holes because of the “cosmic censorship conjecture.” This conjecture is that “realistic” singularities — meaning those that can be formed in nature — must always hide within a barrier known as an “event horizon” from which light can never escape. That makes them appear perpetually black to the rest of the universe.

But cosmic censorship is “an open conjecture that is very difficult to prove, and very difficult to disprove,” said Petters. And, despite the general support for the universality of black holes, Kip Thorne and John Preskill, two experts in the cosmology of relativity at the California Institute of Technology, have suggested for more than a decade that naked singularities could exist in certain instances. Now Petters and Werner have devised a way to test for their presence.

Astronomers cannot say for sure whether all black holes are actually black, having never fully penetrated the obscuring outward matter surrounding such objects, Petters said. As their main evidence, scientists can only point to effects that the massive gravitational pull of certain unseen entities exert on surrounding matter. Those effects include emissions of highly energetic radiation, or the extreme orbits of nearby stars.

Petters is an expert in “gravitational lensing,” another effect of relativity that permits massive sources of gravity to split light from background astronomical features into multiple images. In earlier reports in the November, 2005 and February, 2006 issues of Physical Review D, he and Charles Keeton of Rutgers University suggested a way to use gravitational lensing to show whether cosmic censorship can ever be violated.

However, that evaluation was limited to non-spinning singularities that are considered only theoretically possible. The suspected singularities astronomers have found in space so far all appear to be rapidly spinning, sometimes at more than 1,000 times a second. So Petters and Werner teamed up to see if they could generalize such an application of gravitational lensing to all realistic spinning singularities. Their surprising result was yes, Petters said.

In work supported by the National Science Foundation in the United States and the Science and Technology Facilities Council in the United Kingdom, the pair employed a finding that a black hole could be shed of its event horizon and become a naked singularity if its angular momentum  — an effect of its spin —  is greater than its mass.

That would translate into a spin of a few thousand rotations a second in the case of a black hole weighing about 10 times more than our Sun, said Werner. In the event that the required conditions were met, Petters’ and Werner’s calculations show that a naked singularity’s massive gravitation would split the light of background stars or galaxies in telltale ways that are potentially detectable by astronomers using existing or soon-to-be instruments.

Those possible ways are outlined by six different equations in their study that connect a singularity’s spin to  the separations, angular alignments and brightness of the two split images. “If you ask me whether I believe that naked singularities exist, I will tell you that I’m sitting on the fence,” said Petters. “In a sense, I hope they are not there. I would prefer to have covered-up black holes. But I’m still open-minded enough to entertain the ‘otherwise’ possibility.”

Werner and Petters first began interacting in the Duke professor’s native Belize, where Petters has established an institute for math and science education and the Cambridge graduate student had come to help excavate a Mayan ruin. Their collaboration then moved to the Duke campus. Petters is currently serving as one of Werner’s thesis advisors.

Arlie Petters
email : petters [at] math.duke [dot] edu


The smallest black hole ever
BY Noel McKeegan / April 6, 2008

Using measurements taken by the Rossi X-ray Timing Explorer satellite, NASA scientists have identified the smallest known black hole in the universe. At 3.8 times the mass of our Sun and estimated at only 15 miles in diameter, the black hole known as XTE J1650 is also close to the smallest size thought to be theoretically possible for such an object.

A star is basically a huge nuclear reactor at core, and a black hole is formed when it runs out of fuel and collapses upon itself to form an incredibly small but incredibly massive “singularity”, a point in space with so much gravitational force that even light cannot escape. Stars need to be several times more massive than the Sun for a this to occur, if not a Neutron Star, rather than a Black Hole, will be the result. Astronomers believe that the minimum mass required for the creation of a Black Hole is around 3 times the weight of our Sun, meaning that the newly discovered Black Hole in the Milky Way Galaxy binary system named XTE J1650-500 is almost as small as they come.

The research was conducted by Nikolai Shaposhnikov and Lev Titarchuk at NASA’s Goddard Space Flight Center in Greenbelt, Md. Scientist have been aware of the presence of a lightweight black hole in the system since not long after its discovery in 2001 but its mass has never been measured with any accuracy. This information has only been discovered using a new technique that links the mass of the Black Hole to the X-rays being radiated from the surrounding discs of hot gases that are being dragged inwards by the object’s massive gravitational forces. By measuring the regular pattern created by these X-rays (known as quasi-periodic oscillation, or QPO), the mass of the black hole can be established with X-rays are emitted on a shorter timescale for smaller black holes.

Shaposhnikov and Titarchuk verified their method by applying it to black holes whose masses had been measured by other techniques. “In every case, our measurement agrees with the other methods,” says Titarchuk. “We know our technique works because it has passed every test with flying colors.” The measurement of the black hole’s mass is due to high-precision timing observations made by NASA’s Rossi X-ray Timing Explorer, a small satellite that launched in late 1995 and made the first observations of the XTE J1650-500 in 2001. Einstein’s equations predict that a black hole with 3.8 times the mass of our Sun would be only 15 miles across — the size of a city. “This makes the black hole one of the smallest objects ever discovered outside our solar system,” says Shaposhnikov. Previously, the smallest known black hole would weigh about 6.3 Suns. The black hole that center of the Milky way by comparison, is thought to have a mass somewhere around three million times that of our Sun and others are estimated to be billions of times more massive.

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