From The Editor | September 27, 2013

Disappearing Skyscraper: The Latest In Invisibility Cloaking

By Jim Pomager

Infinity Tower Disappearing

First pets, then children, and now… skyscrapers??

Invisibility cloaking has long been a topic of great interest among Photonics Online readers. Articles about advances in cloaking research — which we publish to the site every other month or so — are always heavily viewed, and one inevitably ends up in our annual top 10 stories of the year list. I’ve enjoyed watching the technology mature over the past 6 or 7 years, and who wouldn’t? Just hearing the words “invisibility” and “cloaking” call to mind Romulan warships, Harry Potter hijinks, and other sci-fi fantasies.

So you can imagine my excitement when I read about what is expected to be the world’s first invisible high rise.

The South Korean government recently approved plans to build Tower Infinity on the Cheongna City waterfront, some 20 miles west of Seoul. When it is completed, in the next 3 to 4 years, the tower will stand nearly 1500 feet high, making it the sixth-tallest tower in the world. Tower Infinity will be home to the world’s third-highest observation deck, a roller coaster, a movie theater, a water park, and numerous restaurants. As impressive as these entertainment offerings are, they pale in comparison to the tower’s most prominent feature: its outward (dis)appearance.

While Tower Infinity’s crystalline form is quite elegant, the architects (GDS Architects) don’t want its steel and glass structure to dominate the surrounding landscape, at least not all the time. So, they designed the building with the ability to remove itself from view every now and then.

Here’s how the vanishing act will work: 18 weatherproof cameras positioned at different heights around the building’s exterior will constantly capture images of the scenes around it. Digital processing software will scale, rotate, and merge the images into a single, panoramic view, which will then be projected in real time onto an LED “skin” on the opposite side of the tower. Under certain daylight conditions (lights inside the building will foil the trick at night), and from particular viewing distances and angles, the system will make the tower seem to be transparent. (Somewhat paradoxically, it will also be able to transform the tower into a 1500-foot-high billboard.) At the time of this writing, the architects were unable to provide more specific detail on the system’s technical specifications.

Proposed location of Tower Infinity’s 18 cameras, at three different heights

Digital image processing will scale, rotate, and merge images from 18 different cameras into a single, panoramic view.

I’ll allow that Tower Infinity’s approach to invisibility isn’t the most technologically sophisticated, at least not from a physics point of view. However, it is representative of an emerging trend in invisibility cloaking research: using common materials and comparatively straightforward methods to hide larger objects in the visible spectrum.

Up until very recently, most research into invisibility cloaking was more theoretical than practical. The most common approach involved using exotic metamaterials to bend the path of very specific frequencies of electromagnetic waves (typically microwaves) around very small objects. While research in this area has been impressive, and may eventually yield functional devices, it still seems a long way off from real-world use.

While the more recent trend involves the use of unexceptional materials and techniques that border on mere optical illusion, it is nonetheless more practical (today) and scalable. Here are a few examples, from actual researchers at reputable institutions:

  • Cloaking children: John Howell, a professor of quantum optics and quantum physics at the University of Rochester recently built three simple, unidirectional optical cloaking devices using only $150 worth of off-the-shelf materials (plexiglass, $3 lenses, mirrors, etc.) – and the help of his 14-year-old son. Download their research paper to learn more, and view the video below to see one of the devices cloak a child.

  • Cloaking pets: Adapting earlier work by John Pendry (Imperial College London) on the cloaking of microwaves, researchers at Nanyang Technological University (Singapore) have demonstrated multidirectional optical cloaking of a goldfish and a cat using a hexagonal device made of thin layers of glass. For more, check out the group’s research paper or watch the video below.

  • Cloaking time: Miguel Lerma of Northwestern University has figured out a way to arrange switchable transreflective mirrors to achieve a “temporal cloak,” which hides objects in time by creating a temporary gap of obscurity and then closing that gap afterward. As a mathematician, Lerma hasn’t yet demonstrated the technique in a lab, but the mirrors needed to generate his temporal cloak are readily available now and could be implemented in the near future.

More than just clever parlor tricks, such rudimentary approaches to invisibility cloaking could have meaningful and practical applications — and much sooner than cloaks based on metamaterials. The Rochester researchers believe that small field-of-view objects such as mid- to high-earth orbit satellites, for instance, could be cloaked using their techniques. It’s not hard to imagine soldiers or military vehicles hidden from sight behind inexpensive, mirror-based cloaks. For those working in the optics field, the possibilities are almost endless. After all, if you can make a skyscraper disappear….

All images courtesy of GDS Architects

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