Chink found in armor of perfect cloak

Tiny charged particles could reveal the location of a perfect invisibility cloak. Such a cloak — which exists only in theory at the moment — would render an object invisible by gently deflecting photons around it. But charged particles wouldn’t be fooled: they would interact with the cloak in a telltale way, giving up the cloak’s location, researchers report in a paper to appear in an upcoming Physical Review Letters.  

Cloaking research is in its early days (SN: 11/21/09, p. 18). Special materials that trick specific wavelengths of electromagnetic radiation into bending around an object, rather than bouncing off of it, are at the heart of the new field. New devices can partly disguise tiny objects at specific wavelengths, but a cloak that perfectly hides objects at all wavelengths of radiation — including AM radio waves, visible light and X-rays — would be extremely difficult to create.

One of the easiest ways to expose a less-than-perfect cloak would be to bombard it with radiation in a wavelength outside of the cloak’s range. For instance, if a cloak only guards against a specific shade of green, then blue or red light would reveal the object. Only a perfect cloak could keep an object hidden at all wavelengths.

In the new study, Baile Zhang and Bae-Ian Wu, both of MIT, propose a way to beat a perfect cloak by showering the hidden object with fast-moving, charged particles, such as electrons. Lke hundreds of pingpong balls dropping from a ceiling onto a covered object, electrons would act differently when they encountered the cloaked object. “You would know that something is wrong with that part of space, even though you don’t see anything,” Wu says.

Unlike pingpong balls, charged particles can move through an object, leaving telltale radiation in their wake. Wu and Zhang mathematically simulated a charged particle moving through a spherical cloak. The researchers identified radiation patterns emitted as electrons entered the cloak, crossed, and exited. “There will be radiation that corresponds specifically to the entrance point and the exit point,” Wu says.

The research is too preliminary to know whether such a system could be built. “It’s a very theoretical paper, basically,” Wu says. “We’re not saying how to make it practical.” In its simplest form, such a cloak-detection system would require both a source of fast-moving charged particles and also detectors at intervals around the space to be monitored.

Physicist Nikolay Zheludev of the University of Southampton in England points out that an electromagnetic cloak can also be detected by throwing a stone at it. “You may say that neither throwing a stone, nor throwing a charged particle through the cloak, is a big deal,” he says. “It is not, but the paper is interesting as it again proves a universal truism that nothing is perfect.”