• This 'invisibility cloak' could conceal satellites — or hide your kids

    Paul and James Howell demonstrate how mirrors can be used to create a simple invisibility cloak.

    By Alan Boyle, Science Editor, NBC News


    Scientists have been slaving over the development of exotic materials that can serve as invisibility cloaks for small objects in specific wavelengths, but father-and-son researchers are showing off a type of invisibility that's done with simple sets of mirrors, lenses or tanks of water.

    In a paper submitted to the American Journal of Physics, University of Rochester physicist John Howell and his 14-year-old son, J. Benjamin Howell, say such cloaking devices can conceal high-flying satellites. Or Harry Potter. For real.

    MIT Technology Review's Physics ArXiv Blog calls the approach "head-slappingly simple." There are caveats, however. These methods are basically funhouse-mirror tricks, which rely on precise placement of the apparatus to make objects disappear when seen from a specific vantage point. Howell & Howell admit as much. "Invisibility with mirrors has been done and are YouTube hits," they say. "The point we wish to emphasize is not the novelty but the ease of scaling to nearly arbitrary size."

    You can watch an example in videos above and below, in which one boy's body appears to be missing below the head, even when another boy walks around behind him. The trick is that two sets of mirrors have been placed in such a way as to reflect the scene behind the "cloaking region" in which the seemingly disembodied kid is standing. (The actors in the movie are all John Howell's sons — Paul and James above, Benjamin and Isaac below.)

    The cloaking effect isn't perfect. A wider-angle view would show you the retro-reflecting mirror, standing off to the side. Another major drawback is the fact that the effect is uni-directional: If you move around to a different position, the effect is spoiled.

    Wanna see it again? Benjamin and Isaac Howell show how the mirror-based cloaking device works.

    John Howell / Univ. of Rochester

    This is the setup for a "cloaking device" that can hide a child inside the angle created by the mirrors at right.

    The uni-directional drawback also applies to two other methods laid out by the Howells. In one, L-shaped tanks of water are used to refract light around an object in the middle. In another, Fresnel lenses reroute light around the "invisible" object. The effect is way cool when you see it from the correct angle. These images show how the setups can hide a toy copter in front of a toy truck:

    John Howell / Univ. of Rochester

    L-shaped water tanks have been arranged to show the top of a toy helicopter, while making the bottom of the helicopter "invisible." The bottom half of a toy truck sitting behind the helicopter remains visible, however.

    John Howell / Univ. of Rochester

    This shows how water tanks are arranged to hide the copter in front while revealing the truck behind it.

    John Howell / University of Rochester

    An arrangement of Fresnel lenses can also conceal the helicopter in the foreground while showing the toy truck in the background - as long as you view the scene from the correct angle.

    Andrea Alu, a researcher at the University of Texas at Austin who is working on more exotic cloaking devices, says he's familiar with these effects. "Overall, I believe that this is interesting engineering work, as a smart composition of mirrors or materials may realize an optical trick that hides an object for certain observers," he told NBC News in an email. "I don’t necessarily find the concept scientifically too exciting. It is more of an optical engineering challenge."

    Alu stressed that his efforts are focused on metamaterials that would suppress the overall scattering of light from an object, independent of the illumination or the observer's position. "Still, this and other solutions may have some merit in specific applications, in which we know the illumination position and the object does not move," he said.

    Mirrors, lenses and water tanks might make for a cool set of invisibility art installations, for example, or for some more serious cloaking devices. "The devices may have value, for example, in cloaking satellites in mid- to high-Earth orbit, or for any low field-of-view cloaking," the Howells wrote.

    John Howell provided a more down-to-Earth example: A police car could theoretically be hidden within a cloaking device, some distance from the side of the highway. The setup could be arranged so that the patrol officer is "invisible" while your speed is being clocked. The apparatus would become visible as you came closer, of course. But by then, "it'd be too late," Howell told NBC News.

    In the future, these funhouse tricks just might lead to optical invisibility cloaks that would keep Harry Potter (or the Highway Patrol) hidden from observers looking around from any vantage point. "A cloak with spherical symmetry (much like retro-reflecting spheres achieve multi-directional reflection) may achieve this end," the Howells suggest.

    For other approaches to seeming invisibility, check out these videos:

    Mercedes-Benz put a camera and LED displays on an "invisible" car to highlight its hydrogen fuel-cell technology. For more about the project, check out http://livetothehype.tumblr.com/

    This Halloween costume uses two iPads to make it look as if you have a huge hole in your torso.

    Researchers in Japan show off a retro-reflective invisibility cloak.

    More about invisibility:

    Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

  • This invisibility cloak is thinner than anything in Harry Potter's closet

    These microwave images show how an object looks in normal view (top row) and oblique view (bottom row) when it's uncloaked, and when it's cloaked by a metascreen. A free-space view of the scene is included as well.

    By Alan Boyle, Science Editor, NBC News


    If you liked last year's bulky invisibility cloaks, you'll love this year's fashionable ultra-thin invisibility wrap — which is just a tenth of a millimeter thick but can still make the objects inside undetectable to microwave scans.

    "This is the first time an ultra-thin cloak has been realized, much thinner than the wavelength," Andrea Alu, a materials-science researcher at the University of Texas at Austin, told NBC News in an email. "The approach is unique."

    Invisibility cloaks have been the stuff of science-fiction stories ranging from the "Star Trek" TV series to the Harry Potter sorcery saga, but they're also becoming the stuff of science fact. The first real-life invisibility cloak was created in 2006, and they've gotten a lot better since then.

    Alu and his colleagues describe what they call a "3-D stand-alone mantle cloak" this week in the New Journal of Physics. The research builds on past work with bulkier kinds of cloaking devices. The first invisibility cloaks guided light waves around hidden objects. Last year, Alu's group showed how a shell of plasmonic materials could cancel out the scattering of light waves by an object, rendering it invisible. This week's research paper features a new kind of wave-canceling cloak that's much thinner than the shell.

    The University of Texas researchers took a 18-centimeter-long cylindrical ceramic rod and wrapped it in what they call a "metascreen," a layer of flexible plastic film overlaid with a fishnet pattern of copper tape. In the visible spectrum, the wrapped-up object looked like a tube of kitchen plastic wrap. But when the researchers beamed microwaves at the object, their microwave imagers couldn't pick up the object's signature.

    "The wave can pass through the object, if it is penetrable," Alu explained.

    Alu et al. via New Journal of Physics

    This image shows the experimental set-up for far-field microwave observations. The cylinder at the center of the scene is a ceramic rod wrapped in an invisibility cloak that's just a tenth of a millimeter thick.

    Alu et al. via New Journal of Physics

    A near-field experiment demonstrated that the rod wrapped in a copper-and-plastic metascreen was invisible to microwaves, even when the rod was inclined at an angle.

    The researchers reported that invisibility effect was present over a moderately broad bandwidth, with optimal performance at a wavelength of 3.6 gigahertz. The same technique could be used to produce invisibility in different wavelengths.

    "In terms of applications, radar camouflaging is one," Alu said. He said the technique could defeat advanced countermeasures for stealth radar detection, such as looking for the radar "shadow" of a stealth-concealed object. Alu and a colleague also have proposed a method for terahertz-wave invisibility, which could theoretically make objects invisible to airport security scanners.

    Alu said the potential applications aren't limited to stealth and spycraft. "The main civil applications we have suggested for this technology are in the area of non-invasive sensing, biomedical and optical nanodevices for computing, and energy harvesting," he said.

    Harry Potter might not want to give his old cloak of invisibility cloak to Goodwill just yet, though. The metascreen constructed by Alu and his colleagues will work only for microwaves, and not for the visible-light wavelengths that our eyes can see.

    "In principle, this technique could also be used to cloak light," Alu said in a news release. "In fact, metascreens are easier to realize at visible frequencies than bulk metamaterials, and this concept could put us closer to a practical realization. However, the size of the objects that can be efficiently cloaked with this method scales with the wavelength of operation, so when applied to optical frequencies, we may be able to efficiently stop the scattering of micrometer-sized objects."

    That means Harry will still have to keep the bulky old cloak in his closet — unless he can use the "Decresplitudo" spell to shrink himself to a millionth of a meter in size. And if he can do that, who needs a cloak?

    More about invisibility:

    In addition to Alu, the authors of "Demonstration of an Ultralow Profile Cloak for Scattering Suppression of a Finite-Length Rod in Free Space" include J.C. Soric, P.Y. Chen, A. Kerkhoff, D. Rainwater and K. Melin.

    Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

  • A different kind of 'invisibility cloak' can serve as heat shield

    Sebastien Guenneau / Institut Fresnel, CNRS/AMU

    This schematic shows that the object in the center of a thermal invisibility cloak stays cold while the heat diffuses elsewheree. The source of the heat is on the left side, maintaining a temperature of 100 degrees Celsius (212 degrees Fahrenheit).

    By Alan Boyle, Science Editor, NBC News

    By now, you've heard of "invisibility cloaks" that can hide objects from prying eyes, or military scouts, or sonar scopes — but how about a cloaking device that can keep your computer circuits cool?

    That's just the kind of thermal cloaking device that French researchers are proposing in the journal Optics Express, and it might not be too long before it becomes a reality.

    "We expect to have the first prototype ready in a few months, since as usual there are a few fabrication constraints which need to be fixed, but nothing really serious," lead researcher Sebastien Guenneau told me in an email. "All seems to be under control."

    Most of the invisibility cloaks under development work by using metamaterials to bend light waves or sound waves around a shielded object, making the object undetectable in those wavelengths. Guenneau, who is affiliated with the University of Aix-Marseille and France's Centre National de la Recherche Scientifique, decided to work with his CNRS colleagues to adapt the wave-bending approach to thermal diffusion.

    "Our key goal with this research was to control the way heat diffuses in a manner similar to those that have already been achieved for waves ... by using the tools of transformation optics," Guenneau said in a news release issued by the Optical Society, which publishes the open-access Optics Express.

    Instead of controlling wave propagation, the thermal cloak would control the flow of heat. "The mathematics and the physics at play are much different," Guenneau explained. "For instance, a wave can travel long distances with little attenuation, whereas temperature usually diffuses over smaller distances."

    The basic design of the thermal invisibility shield is similar, however: Rings of specially shaped material guide the heat flow along the desired path.

    "We can design a cloak so that heat diffuses around an invisibility region, which is then protected from heat. Or we can force heat to concentrate in a small volume, which will then heat up very rapidly," Guenneau said.

    The thermal protection arrangement could be used to channel the heat created by microelectronics away from sensitive areas — an issue that's familiar to owners of the new iPad, for instance. The heat concentrator arrangement, meanwhile, could increase the efficiency of thermal photovoltaic cells or solar thermal power generators.

    There are already lots of other methods available for thermal protection — ranging from the plastic-foam insulation used in a cheap beer cooler, to the high-tech aerogel used on NASA's Mars rovers, to the reinforced carbon-carbon panels and protective tiles that were used on the space shuttles. But Guenneau told me that the system he and his colleagues have proposed is "much different" from any existing thermal protection method.

    "The flow of heat follows the direction of highest diffusivity, which in our case is around the invisibility zone," he wrote. "Earlier thermal protections require you to basically surround the region to protect with a coating with low diffusivity (e.g., air or polymer, just like your double-glazed windows). To use an analogy with optics, it's just like putting Harry Potter in a box and saying, 'Look, you cannot see Harry anymore, he has been made invisible.' Our approach is to really make Harry invisible, so we should not see the box either."

    So what's this cloak going to be made of? In the paper, Guenneau and his colleagues say the materials that go into a concentric multilayered cloak could range from PVC-type polymers to metals such as silver and gold. Production of the prototype cloak is currently under way at the University of Lille, Guenneau said, "but I cannot reveal exactly what it is made of at this stage."

    In the next few months, we should be hearing a lot more about the thermal invisibility cloak from Guenneau and his French colleagues ... provided they don't disappear.

    More about invisibility:

    In addition to Guenneau, the authors of "Transformation Thermodynamics: Cloaking and Concentrating Heat Flux" include Claude Amra and Denis Veynante.

    Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

  • Invisibility gets a reality check

    Warner Bros.

    Harry Potter (Daniel Radcliffe) and the goblin banker Griphook (Warwick Davis) are concealed beneath an invisibility cloak in "Harry Potter and the Deathly Hallows: Part 2."

    By Alan Boyle, Science Editor, NBC News

    Harry Potter's invisibility cloak comes in handy for the final installment of the boy wizard's film saga, but real-life invisibility technologies might well be at least as useful — even if they aren't as cool as Harry's cloak.

    For the foreseeable future, the benefits provided by the real-life gizmos that have come to be called "invisibility cloaks" or "cloaking devices" really won't have much to do with the kind of tricks you'll see in "Harry Potter and the Deathly Hallows: Part 2." Sorry to disappoint, but you won't be able to throw a high-tech fabric over your head and disappear from the scene when the bad guys come looking for you.

    However, it is conceivable that scientists could look at viruses and protein molecules directly, using new breeds of ultra-high-resolution "superlenses." Physicians might have more accurate ultrasound scanners at their disposal. Acoustical cloaks could hide ships or underwater assets from sonar detection. And offshore facilities could be engineered to soften the effects of wave erosion on the shore ... or amplify ocean waves for generating renewable power.

    All this magic could come about through the power of metamaterials. These are materials that are specially structured and shaped to bend electromagnetic waves or acoustical waves in weird ways. Real-life invisibility cloaks, for example, are actually devices or layers of material designed to divert light around the object that's concealed.

    This month's issue of Physics World delves into the past, present and future of invisibility — and the best part about this particular  issue is that you can download it for free as a PDF file. 

    One of the limiting factors for invisibility cloaks has to do with wavelength. Shorter wavelengths require smaller structures in order to produce the bending effect. That's why bending sound waves is easier than bending electromagnetic waves, and why bending microwaves is easier than bending visible light. Scientists have been able to develop "invisibility carpets" that can render bumps in the carpet undetectable — but the bumps have to be so tiny that you couldn't see them with the naked eye anyway (on the order of a millionth of a meter), and the invisibility effect only works for near-infrared wavelengths.

    A different research team came up with a way to hide objects in a region about three-quarters of an inch wide, using calcite crystals, but the invisibility effect is produced only with respect to light of a specific polarization.

    "While what has been so far achieved in invisibility science has been a tour de force of physics and engineering, our children will probably still have to wait some time for that real Harry Potter cloak," Stanford University's Wenshan Cai and Purdue's Vladimir Shalaev write in Physics World.

    Metamaterials aren't the only way to achieve invisibility, however. There are also active-camouflage techniques, ranging from video projection to a high-tech light-emitting matrix that's inspired by a squid's skin.

    The latest scientific buzz focuses on space-time cloaking, which involves using "time lenses" to compress and then decompress light. This would result in an apparent time gap during which an event could go unobserved. Scientists have discussed time cloaking as a theoretical possibility for quite some time (so to speak), but researchers led by Cornell University physicist Moti Fridman say they actually created a "time hole" that lasted 15 trillionths of a second. The Guardian, the Physics arXiv Blog and Science News delve into the details.

    For much, much more about metamaterials, event cloaking, active camouflage and other real-life magic spells, check out Physics World's special issue as well as these links:

    Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. You can also add me to your Google+ circle, and check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.