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Friday, 25 July 2014

The Quest for Invisibility

Since long before Harry Potter, scientists have been searching for a way which can allow things to pass us by unnoticed. The invisibility cloak which features in J.K. Rowling’s books may seem magical and otherworldly, but in fact devices which have the effect of making objects completely disappear are much more tangible than you’d think. While they may not look like a silky blanket, cloaking devices are very effective at manipulating signals and jamming detectors so as to obscure the truth about their location.

So there it is, we’ve done it. We have successfully created magic and are able to hide enormous ships or helicopters from being spotted by the enemy – haven’t we?

Well, not exactly. The perfect cloaking device is still just a theoretical concept. Camouflage paint is often applied to try and confuse the eye, “stealth” coatings are used to hide from radar, while cooling techniques are employed to reduce the amount of infrared emission coming from the object trying to stay hidden. However, while these techniques are effective at helping to disguise ships and aeroplanes, we can hardly call them invisible. It is hoped the answer lies in the development of metamaterials – materials which possess properties not found in nature.

Image demonstrating variety of wavelengths of the electromagnetic spectrum
The electromagnetic spectrum covers all wavelengths of radiation, from radar to visible light to x-rays and gamma-rays. Until last year we could only hide things from very specific parts of the electromagnetic spectrum, in some cases by making the object more visible in other parts of the spectrum. Image credit: NASA (public domain)
The development of such materials has huge implications for lens and invisibility devices. The idea of cloaking devices is to create a material which can take an incoming signal, say visible light, and then send it on its way without any interruption from the cloaked object. If you could create a material which can do this effectively enough, it will trick any detectors into thinking there is no object to be seen, since there is no radiation signal to be detected. In theory it’s possible, but there are many obstacles blocking the way.

In 2011 carbon nanotubes were a big candidate for invisibility cloaks. Carbon nanotubes have incredible superhero-like properties. They are lightweight and easy to handle, whilst also able to withstand extreme temperatures and forces. When heated to very high temperatures, the tubes appear to begin to shimmer and become hazy. This effect is similar to the one which makes mirages appear in the desert – the huge temperature difference between the surface of the nanotubes and the air just next to the nanotubes actually bends the light away from them. Unfortunately, this device isn’t perfect. The effect is most prominent when underwater, difficult for tanks and helicopters. Moreover, when heated to the temperature necessary, sheets of these nanotubes actually become incandescent in polarized light[1].

2014 has been huge for advancements in cloaking devices, with the creation of the world’s first acoustic invisibility device[2]. Created by researchers at Duke University in North Carolina, USA, this device is just one example of metamaterial technologies. The design is believed to work just as well in water as in air (great news if you’re hiding a submarine), and could pave the way for other technologies which will work for other types of waves – such as light.

Photograph of acoustic cloaking device
The strange pyramidal stack of perforated plastic sheets might not look like much, but it is actually capable of shaping sound waves to reflect as if they had bounced off the floor, making the device and whatever it might be placed upon disappear from the face of the earth – as far as sonar is concerned. Image Credit: Duke University

Photograph of "LEGO firewalk" in South Portland
Metamaterials could make challenges such as the “LEGO firewalk” a lot easier. Photo by Mick Pratt, store manager of the Mill Creek Bull Moose in South Portland.
Even more recently in June, researchers from Karlsruhe Institute of Technology, Germany, have created a material by carefully arranging microscopic polymer cones such that the material has ‘unfeelability’ [3]. While we can see this material, the idea is that in fact our brains do not register it as being there when we touch the stuff. It’s known as a “mechanical invisibility cloak”. This bizarre metamaterial works in a similar vein to the sound cloak, redirecting not waves, but force in such a way that there is no resultant pressure to your finger if you press down on it. Interestingly, this kind of material could be commercially available in the not-too-distant future, perhaps (hopefully) leading to LEGO-proof socks.

Going even further into the realm of wizardry, one cloaking device has actually been proposed to work not by bending light, but effectively stopping time. In theory by placing such a device in front of an object we’re intending to use it on, any passers-by would see that object as it was when the device was placed in front of it. For example, you could place this temporal cloaking device in front of your housemate’s box of chocolates, then eat all the chocolates, and to them it would appear as if nothing has gone amiss. What’s more, this particular magic box is made up from materials that most of us have in our homes – mirrors. The idea is that by delaying how long it takes for the light to get to the object and back, you delay the appearance of any event happening. Unfortunately you’d need to place mirrors in such a way so as to send light on a journey millions of miles long before you’d get a delay long enough for you to scoff those choccies unnoticed. The applications for a device such as this could change the way we view reality, but as it stands no one has built one yet.

Cloaks which hide us from light are still a long way from being perfect. The combination of metamaterial research with nanotechnologies is allowing materials to become lighter and more effective in what they do. The University of Texas, Austin, claims the secret to widening the band in which we can hide things lies in using materials which are ‘active’ [4]. In other words, the material needs to be able to shift and change its structure by using electricity so as to account for different frequencies of light. This way, the device can actually cover a larger frequency range and therefore is better at hiding from different types of radiation, rather than just visible light or just x-rays. Active cloaks can also be thinner, more flexible and easier to handle, making them a much better choice for everyday use. Who knows, maybe one day we can all have our own invisibility cloak.

References
why don't all references have links?

[1] Aliev, A. E., Gartstein, Y. N., & Baughman, R. H. (2011). Mirage effect from thermally modulated transparent carbon nanotube sheets. Nanotechnology, 22(43), 435704. DOI: 10.1088/0957-4484/22/43/435704
[2] Zigoneanu, L., Popa, B. I., & Cummer, S. A. (2014). Three-dimensional broadband omnidirectional acoustic ground cloak. Nature materials, 13(4), 352-355. DOI: 10.1038/nmat3901
[3] B├╝ckmann, T., Thiel, M., Kadic, M., Schittny, R., & Wegener, M. (2014). An elasto-mechanical unfeelability cloak made of pentamode metamaterials. Nature Communications, 5. DOI: 10.1038/ncomms5130
[4] Monticone, F., & Alu, A. (2013). Do Cloaked Objects Really Scatter Less? Physical Review X, 3(4), 041005. DOI: 10.1103/PhysRevX.3.041005