Monday, December 14, 2009

Electromagnetic Detection of a Perfect Invisibility Cloak

One of the areas of physics that had been getting a lot of publicity is the area of metametrials/left-handed materials that produces negative index of refraction. The "sexy" aspect of this study is the possibility of "cloaking" an object having such left-handed properties from electromagnetic radiation, even if it is only within a limit bandwidth. I've highlighted several of these cloaking reports previously (read here, here, and here).

Now comes something that throws cold water over everything. A paper that appeared in PRL[1] recently pointed out that while any cloaking device, even a perfect one, cannot be detected via EM radiation, shooting charged particles at them allows them to be detected!

Abstract: A perfect invisibility cloak is commonly believed to be undetectable from electromagnetic (EM) detection because it is equivalent to a curved but empty EM space created from coordinate transformation. Based on the intrinsic asymmetry of coordinate transformation applied to motions of photons and charges, we propose a method to detect this curved EM space by shooting a fast-moving charged particle through it. A broadband radiation generated in this process makes a cloak visible. Our method is the only known EM mechanism so far to detect an ideal perfect cloak (curved EM space) within its working band.

So all you need is to shoot electrons and voila, you see these things being cloaked. Of course, this doesn't work to well in air since, depending on the electrons' energy, the mean free path of the electrons in air can be quite limited. But in the vacuum of outer space, that's a different story. Just think, we could have told Capt. Kirk how he could have detected those cloaked Klingon warbirds! It would have been so easy!


[1] B. Zhang and B.-I. Wu, Phys. Rev. Lett. v.103, p.243901 (2009).

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