A Perfect Invisibility Cloak Is Impossible, Proves New Research

Invisibility cloaks can't hide a wearer from all observers. Wiki Commons

Technology is constantly proving that the magical concoctions dreamed up in fiction can often be transformed into fact with enough ingenuity, but there's at least one fanciful invention that we'll never see in reality: perfect invisibility cloaks.

Physicists Jad Halimeh from the Ludwig Maximilian University of Munich, and Robert Thompson from the University of Otago, New Zealand, have published a paper that proves that invisibility cloaks like those seen in the "Harry Potter" films are fundamentally impossible to design, reports Phys.org.

The researchers demonstrate that even the best invisibility cloaks can only hide an object from some viewers. There will always be at least some observer that can detect light distortions that reveal the object's presence, however.

To put things in terms of the sorts of invisibility cloaks that have been presented in fiction, this means that the best invisibility cloaks will only meet the standards of movie "Predator," which features translucent but visible creatures. Cloaks that perfectly hide you underneath them, such as in "Harry Potter," can never be.

"In principle, what this paper shows is that invisibility cloaking is not possible for all observers," said Halimeh. "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."

The reason that a perfect invisibility cloak is intractable is because of special relativity. Basically, because the straight path directly through a region of space is always shorter than the path that curves around the region, light must travel a longer distance around a cloaked object than it would if the cloaked object were not there. This time delay, however minute, leads to visible distortions. Even when the naked eye can't detect the distortions, they're still there and they'd be detectable with instruments.

A further related problem is something called Fresnel-Fizeau drag. As light travels through a moving medium, it is dragged along by that medium. This means that if the wearer of an invisibility cloak moves, it must drag light along with it, which would lead to distortions.

Scientists have long recognized these problems, but it took Halimeh and Thompson to calculate that the problem is fundamentally unsolvable.

The good news is that this doesn't mean the technology is useless. Invisibility cloaks can still conceal an object that is not in relative motion compared to its viewer, for instance. It may even be possible to create an invisibility cloak that reduces these distortions to render a wearer invisible enough for the cloak to serve its intended purpose.

Also, as seen in the world of "Predator," even an imperfect invisibility cloak can be deadly useful.

"Although our results may be disappointing for would-be wizards, understanding the limitations of cloaking devices is actually important in real life," explained Thompson. "New technologies are beginning to emerge from cloaking research, and we're looking for effects that could either compromise the functionality of these technologies, or which could be exploited for some new practical purpose in the future."