When it comes to invisibility cloaks, nobody has done more to advance the field than John Pendry, a theoretical physicist at Imperial College, London. It was he who suggested the idea in the first place and mapped out how one could be built in theory. He even got his hands dirty by collaborating with the team of engineers who first built a working cloak.
So when he pronounces on the subject, we sit up and listen.
Pendry has clearly been worrying about the limitations of invisibility cloaks. For a start, they work only in the microwave part of the spectrum and at a single specific freqeuncy. (Optical invisibility cloaks seem as far away as ever because of problems with light absorption.)
The cloaks must be made of exotic materials with properties that vary throughout their structure and are in any case unobtainable in nature and so have to be designed and made by hand.
The resulting cloaks are not perfect and probably never will be. To hide an object completely, the permittivity and permeability of these metamaterials must take infinite values at some points.
So what to do? Pendry argues in a paper on the arxiv that instead of making objects invisible, you can hide them just as well by making them look like a flat conducting sheet. An eminently sensible suggestion.
The advantage of this approach, he calculates, is that it readily works for visible light and over a wide range of frequencies. What’s more, it can be done with ordinary materials that are available today.
All that’s needed is to hide your object under a material that he calls an isotropic dielectric. He’s even done a number of simulations to show how such a material would make anything it covers look like a flat conducting sheet.
Pendry doesn’t bother with the practical details of how to make an isotropic dielectric material. But maybe he doesn’t need to. He wouldn’t by any chance be referring to water, would he?
Ref: arxiv.org/abs/0806.4396: Hiding Under the Carpet: a New Strategy for Cloaking