Skulls in the Stars

About a week ago, I reported on another ‘teaser’ in the media about ‘optical cloaks’, hypothetical devices which would in principle make objects contained in their core completely invisible. Such devices have gotten a lot of attention, both scientifically and in the press, since the publication of two fascinating theoretical papers in 2006. I recently wrote a post, which can be found here, summarizing those original two papers.

The press reports a week ago suggested another major breakthrough in cloaking research, with headlines such as “Science close to unveiling invisible man” and “Invisibility cloak closer than you think.” They were a little confused about what exactly had been accomplished, however: had the researchers made a three-dimensional invisibility cloak, a cloak that works at visible frequencies, or both?

Well, it turns out that they’ve done neither! This is another example of the press hunting for the best ‘hook’ for the story, no matter how tangentially related to the actual research. What has been accomplished, however, is the development of low-loss, three-dimensional negative refractive index materials which work for visible wavelengths, which is an important and interesting accomplishment in and of itself. I give a brief answer to the question, “What is a metamaterial?” below the fold, followed by an explanation of the actual results of the recent Berkeley papers and an analysis of how the press got themselves confused again.

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As I noted a couple of days ago, apparently there has been another significant experimental breakthrough in the development of dielectric cloaking devices. Researchers at UC Berkeley were responsible, though it is a little unclear what exactly the breakthrough is. The results will appear this week in Science and Nature. In the meantime, it seemed like a good time to review the two articles that started the whole cloaking craze.

As I’ve noted in a pair of previous posts (here and here), the search for objects which can be considered in some sense invisible goes back nearly a hundred years. For the most part, however, the idea that one could make a truly invisible object was considered impossible — and theory backed up that view.

This changed with the publication of two back-to-back theoretical papers in Science in 2006. The first, by U. Leonhardt, was titled “Optical conformal mapping,” and the second, by J.B. Pendry, D. Schurig and D.R. Smith, was titled “Controlling electromagnetic fields”. Both papers mapped out strategies — in a nearly literal sense — for creating what could be called a dielectric invisibility device. How would such a device work? Let’s recall a little basic optics that will help us understand the process…

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