We would all love to own an invisibility cloak. But new research suggests that if they do ever exist they won’t be perfect: They will be more like Predator, less like Harry Potter.
A pair of researchers from the Ludwig Maximilian University of Munich and the University of Otago have carried out theoretical modelling in an attempt to understand how an invisibility cloak might actually work in real life. There are different ways to make invisibility cloaks, all of which have to contend with the fact that diverting light around something introduces time delay problems — because the light can’t pass straight through the object that’s being cloaked.
The most realistic way to do it using an “amplitude cloak”. This approach discards information about the phase of light, instead recreating its colour and attempting to introduce a time delay to make up for the differences in path length along which the light travels. The good news is that the cloaks create a compelling representation of reality. In fact, labs have already been able to show off amplitude cloaks that kind of work.
But the new research brings some bad news, too. Sadly, it seems this kind of cloak would only work when the hidden object and its observer are stationary. The reason is to do with something known as Fresnel-Fizeau drag. When light propagates through a moving medium — like, say, a raindrop — it is dragged along with the medium. So a moving invisibility cloak — or a moving observer — would drag light with it. Jad Halimeh, one of the researchers, explains what that actually means for invisibility cloaks:
Real invisibility cloaks will have to stay in the realm of fiction. Your cloak, if it is to be pragmatically broadband, will pretty much look like that of Predator, giving away what it hides via distortions when you move relative to it.
In other words, object could only ever be made partially transparent using amplitude cloaks: There would always be a whisper of what lurks beneath floating in the air. And certainly enough to reveal its presence when it or the observer was moving.
Image by Tobias Hone