The great unappreciated weakness of invisibility cloaks is that they only make things invisible to human eyes. Or x-ray imagers. Or ultraviolet sensors, infrared image analyzers, echo-location audio signal receivers or, in one case, a poking finger.
Invisibility cloaks, which are very successful in movies and so close to being successful in labs that the million or so physicist working on them can taste it, have a crucial weakness: They make things invisible to the perception of the audience their creators had in mind.
Invisibility cloaks are supposed to hide things by bending light around them in a that would not allow any to be reflected back at a potential viewer, which would allow the "invisible" thing to be seen.
A cloak made to hide things form humans would be able to bend all the colors of visible light, but might not be able to do the same with waves of heat or sound. It is possible, even likely, that a Harry Potter who was invisible to witches, wizards and all manner of magical creatures could be seen very clearly using an infrared night-vision camera, or by a bat that "sees" by echolocation.
It was never clear whether it was even theoretically possible to make an invisibility cloak that could hide the same object from both man and bat.
Not only is it possible to make a cloak hide things within two wavelengths, a team of researchers at Zhejiang University in Hangzhou, China claim to have done it, in principle at least, while discovering why such a thing is so much more difficult than a run of the mill invisibility cloak in the process, according to a paper the team published in the Nov. 12 Physical Review Letters.
The problem isn't just that the cloak material has to bend light, but that it has to have an effect on the light powerful enough to change its behavior in very specific ways. A mirror can change the behavior direction of a beam of light dramatically, and can hide a person standing behind it, for example, but doesn't do those things with enough subtlety to convince anyone it's not there.
An advanced metamaterial from the category of metamaterials often used in attempts at invisibility might have a powerful, innate ability to scatter light, but could melt when exposed to much heat, or catch fire on contact with electricity.
Researchers from Zhejiang University in Hangzhou, China addressed the need for a material that could manipulate more than one type of material by making a cloak out of two different layers of material with different properties, but each had to be able to shield the other from view within its own slice of the energy spectrum.
The team built a double layer of material as a "cloak" that can keep objects from being seen using either heat or electricity as a viewing medium., using a layer of silicon that attracts and concentrates both electrical current and heat flow and an inner layer that is actually an empty cavity that scatters both current and heat away from itself.
So, when an object is placed within the invisibility zone, heat and electricity are diverted around it and scattered around the inside of the cavity. The outers hell then pulls both heat and electricity toward itself, removing the means of being seen from the neighborhood of the object being hidden. Holes drilled in the outer layer and filled with a material called polydimethylsiloxane, which conducts heat and electricity little or not at all compared to silicon.
The combination of the two materials scatters heat and electricity randomly enough to make the accumulated energy difficult to notice.
Student wizards aren't likely to use this "cloak" to sneak out of any castles, but its combined effect on heat and electricity could be used to help eliminate unwanted electrical and heat energy from sensitive electronic components, which could eliminate much of the source of static that make radio reception less clear or sap the performance of the device itself, according to Yungui Ma, a professor at Zhejiang University and lead author of the paper.
The technique – which boils down to protecting an object from heat and electricity by scattering it away – might also be useful in overcoming the performance barriers that have made it difficult to increase the efficiency of thermophotovoltaic cells in solar power systems.
A dual-purpose cloak that scatters electricity and magnetism could make low-frequency signals clearer, Ma said in a release announcing publication of the paper. A powered device that accomplishes the same thing could make the technique useful at a more macro level as well, he said.
"Our goal is to find an efficient way to solve the fundamental problems on the undesired electrical-thermal entanglement, e.g., turning the dissipated heat into useful source energy," Ma said.