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Shades of Blackness

February 28, 2008


Occasionally a story about something far afield from your normal range of activities and interests catches your eye and draws you in. I found an article about scientists developing their own little black hole such a story [“Their Deepest, Darkest Discovery,” by Rick Weiss, Washington Post, 20 February 2008]. Technically, of course, what scientists have done is not create a black hole but a material that absorbs light — almost all of it.

“Researchers in New York reported this month that they have created a paper-thin material that absorbs 99.955 percent of the light that hits it, making it by far the darkest substance ever made — about 30 times as dark as the government’s current standard for blackest black. The material, made of hollow fibers, is a Roach Motel for photons — light checks in, but it never checks out. By voraciously sucking up all surrounding illumination, it can give those who gaze on it a dizzying sensation of nothingness. ‘It’s very deep, like in a forest on the darkest night,” said Shawn-Yu Lin, a scientist who helped create the material at Rensselaer Polytechnic Institute in Troy, N.Y. ‘Nothing comes back to you. It’s very, very, very dark.'”

I guess you could say that the scientists have created the antithesis of what the British band Procol Harum called “A Whiter Shade of Pale.” The scientists are now ready, they believe, to help move science fiction into reality. Are we ready for a Klingon cloaking device?

“Using other new materials, some are trying to manufacture rudimentary Harry Potter-like cloaks that make objects inside of them literally invisible under the right conditions — the pinnacle of stealthy technology. Both advances reflect researchers’ growing ability to manipulate light, the fleetest and most evanescent of nature’s offerings. The nascent invisibility cloak now being tested, for example, is made of a material that bends light rays ‘backward,’ a weird phenomenon thought to be impossible just a few years ago. Known as transformation optics, the phenomenon compels some wavelengths of light to flow around an object like water around a stone. As a result, things behind the object become visible while the object itself disappears from view.”

That’s cool and scary. It doesn’t take much imagination to realize how such a device could be used for both good and ill. Weiss reports we have a few years yet to ponder such a dilemma.

“‘Cloaking is just the tip of the iceberg,’ said Vladimir Shalaev, a professor of electrical and computer engineering at Purdue University and an expert in the fledgling field. ‘With transformation optics you can do many other tricks,’ perhaps including making things appear to be located where they are not and focusing massive amounts of energy on microscopic spots. U.S. military and intelligence agencies have funded the cloaking research ‘for obvious reasons,’ said David Schurig, a physicist and electrical engineer at North Carolina State University who recently designed and helped test a cloaking device. In that experiment, a shielded object a little smaller than a hockey puck was made invisible to a detector that uses microwaves to ‘see.’ The first working cloaks will be limited that way, he said — able to steer just a limited part of the light spectrum around objects — and it could be years before scientists make cloaks that work for all wavelengths, including the visible spectrum used by the human eye. But even cloaks that work on just a few key wavelengths could offer huge benefits, making objects invisible to laser beams used for weapons targeting, for example, or rendering an enemy’s night goggles useless because objects would be invisible to the infrared rays those devices use.”

Weiss reports that the military didn’t fund Lin’s research into light absorbing material, but it was one of the first groups to knock on his door after its existence was made known. Such a material could be added to the military’s other stealth coatings to help protect its assets. What immediately sprang to my mind, however, and obviously to others, was a more peaceful use for the material.

“Solar panels coated with it would be much more efficient than those coated with conventional black paint, which reflects 5 percent or more of incoming light. Telescopes lined with it would sop up random flecks of incidental light, providing a blacker background to detect faint stars. And a wide array of heat detectors and energy-measuring devices, including climate-tracking equipment on satellites, would become far more accurate than they are today if they were coated with energy-grabbing superblack. That helps explain why Lin has been fielding queries from solar-energy companies such as SolFocus of Mountain View, Calif., and the European Space Agency.”

Lin’s material is made out of nanotubes; a substance that has received a lot of attention the past few years.

“It is made of carbon nanotubes: microscopic, hollow fibers whose walls are just one atom thick. Importantly, the fibers are widely spaced, providing plenty of space to allow light in and almost no surfaces to bounce it back out. ‘There are a lot of materials that are very absorbing of light so that once the light gets in, very little is reflected. That is not the big issue,’ said John Pendry, a physics professor at Imperial College London. ‘The big issue is persuading the light to go in there in the first place’ — something the New York team accomplished by spacing the nanotubes so widely.”

As a side note, there are fears being raised about the safety of nanotubes such as those expressed in an article the Economist [“A Little Risky Business,” 22 November 2007].

“Waving a packet of carbon nanotubes accusingly at the assembled American politicians during [an October 2007] hearing … in Congress, Andrew Maynard was determined to make a point. The nanotechnology expert at the Woodrow Wilson International Centre for Scholars in Washington, DC, had bought the tiny tubes on the internet. They had arrived in the post along with a safety sheet describing them as graphite and thus requiring no special precautions beyond those needed for a nuisance dust. Dr Maynard’s theatrics were designed to draw attention to a growing concern about the safety of nanotechnology. The advice he had received was at best uncertain, and at worst breathtakingly negligent. For a start, describing carbon nanotubes as graphite was rather like describing a lump of coal as a diamond. Graphite is made of carbon, just like the nanotubes, although the tubes themselves are about 1m times smaller than the graphite that makes up the ‘lead’ in a pencil. Carbon nanotubes may be perfectly safe, but then again, they may have asbestos-like properties. Nobody knows. Indeed, industry, regulators and governments know little about the general safety of all manner of materials that are made into fantastically small sizes.”

Back to the subject at hand — invisibility!

“Pendry and others are hoping to [perfect] complete invisibility. … A superblack object, even if invisible to the eye, still casts a shadow behind it, while an object shielded by an invisibility cloak does not. Pendry pioneered much of modern thinking about how to attain full invisibility using ‘metamaterials’ — substances engineered to manhandle light. Ordinary matter, such as glass or water, slows and bends light as it passes through. Metamaterials contain bits of metal or other substances embedded in precise patterns to make the light bend in an opposite direction from normal paths. … The first generation, metamaterial ‘cloaks’ are not thin and flexible like Harry Potter’s imagined version but are inches thick and solid, resembling canisters, making them able to hide a stationary object but not a moving person. But the science is progressing quickly, physicist Schurig said. To make a thin, flexible metamaterial cloak, Schurig said, ‘is technically challenging but not fundamentally impossible.’ And although no cloak can yet make objects fully invisible to the human eye, he added, it may not be long before scientists can bend the visible spectrum enough to make an object hard to see. … Pendry added a cautionary note about invisible cloaks, making a real-life distinction from the stuff of fiction: People inside them will not be able to see out. By definition, if no light is bouncing off them, none can reach their eyes, either.”

That removes a bit of the creepiness about invisibility cloaks. When people think about invisibility, they undoubtedly remember the Invisible Man shows and imagine all of the unseemly things an invisible man could do. Weiss concludes his article by mentioning another potential use of light bending technologies.

“There is a flip side to the emerging ability to manipulate light, scientists say. ‘Think anti-cloaking,’ said Shalaev, the engineering professor. ‘Instead of excluding light from an object, you can concentrate light in a small area.’ Normally, light cannot be squeezed into a space smaller than its own wavelength, he said, but transformation optics create the possibility of accomplishing just that — packing loads of energy into a vanishingly small space. Such beams could pack a destructive punch, or could be tamed to serve as ultrasensitive needlelike probes, able to detect even a single molecule of some substance of interest.”

All of this work with light is fascinating. You might have read about other researchers who have managed to capture and stop light then release it [see “Researchers now able to stop, restart light,” by William J. Cromie, Harvard University Gazette, 24 January 2001]. As that article notes, nobody has ever disproved Albert Einstein’s theory that light cannot travel faster than 186,282 miles per second; but everything else about the properties of light appear to be up for grabs.

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