Magazine article American Scientist

A Protective Cloak against Earthquakes and Storms

Magazine article American Scientist

A Protective Cloak against Earthquakes and Storms

Article excerpt

Almost a decade ago, the first scientific research on optical invisibility cloaking was unveiled, but in spite of a furious amount of theoretical and experimental effort, there are still numerous obstacles to overcome before a Klingon or Harry Potter-style cloaking device comes even close to reality. Indeed, serious theoretical limitations indicate that such devices may not ever be possible, even in principle.

An ideal cloak.guides waves around a central region, much like water flowing around a boulder in a stream, and objects in this region will not "see" the waves at all. This property actually is a disadvantage for an invisibility cloak-the wearer of the cloak won't be seen, but also won't be able to see anything. But there are other types of waves than light, and attempts to apply the ideas of cloaking to protect objects or people from the damaging effects of water, sound, magnetic, or even seismic waves suggest that the technology may play an important practical role in science and engineering after all. In fact, the use of cloaking devices for protection, rather than hiding, may be the greatest application that comes out of the still quite young discovery.

Invisible Roots

The roots of invisibility in physics start with a very different sort of protecfive technology altogether. In the early 1970s, British engineer Sir Godfrey Hounsfield developed the first threedimensional medical imaging technique, the now-ubiquitous computed axial tomography (CAT) scan, and demonstrated its safe and effective use in imaging the brain. In CAT, the object to be studied is exposed to X-rays from multiple directions, producing a set of individual images that are combined via computer to form a complete three-dimensional picture of the object.

X-rays, however, are relatively ineffective at resolving soft tissue. Researchers thus began to develop new medical imaging techniques using other types of waves, but an immediate concern arose: What if some objects can't be seen using these new techniques? In other words, is it possible for some objects to be invisible to an imaging technology? If "invisible tumors," for instance, are possible, then any technique that uses waves to probe the human body could have huge, literally fatal, flaws.

Fortunately, theoretical work in the late 1980s demonstrated that, in general, invisible objects do not exist. It turns out that it is possible for an object to be perfectly invisible when illuminated from a single direction-for instance, many people have walked into unseen glass doors when the lighting is just right-but it was seemingly impossible to have an object invisible for all directions of illumination.

This result seemed to settle the matter until 2006, when a pair of theoretical papers appeared in the journal Science. Both papers produced theoretical designs for invisibility cloaks using a new technique called transformation optics. When light passes from one material to another, such as from water to air, it changes direction in a process called refraction. This effect is evident when looking at a straw in a glass of water: The straw appears to have a kink in it at the interface between the water's surface and the air above. The key to transformation optics was the observation that bending light waves, in many cases, is mathematically analogous to warping space. By first designing the desired warping, in this case the bending of light around a cloaked region, one can reverseengineer the type of materials needed to make the cloaking device.

How could cloaking devices possibly exist, when earlier research had seemingly proven invisibility to be impossible? The authors of the papers came up with different resolutions of this seeming contradiction. Ulf Leonhardt, now at the Weizmann Institute of Science in Israel, simply noted that the nonexistence of "perfect" invisibility does not exclude the possibility of near-perfect invisibility: An object that is 99 percent invisible, for instance, would likely be acceptable for most practical applications. …

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