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Strike the invisibility cloak off your wish list. They are impossible to build for human-sized objects, says a new study.

Invisibility cloaks have been a goal for about 10 years, since John Pendry and colleagues at Imperial College London proposed a way to guide light over or around an object using substances with exotic optical properties, called metamaterials.

Ideally, these would work passively – without you needing to pump in extra energy – and for all wavelengths of light.

But now, and at the University of Texas at Austin have showed that a passive invisibility cloak large enough to hide a human – or any macroscopic object – is impossible to build.

A practical cloak would redirect light perfectly around an object to meet the observer’s eye on the other side, and would have to be about as big as the object meant to be hidden. Borrowing from the field of electrical circuit theory, Alù and Monticone showed that the larger the cloak is, the smaller the wavelength of light it can redirect.

“What it means is that only a specific colour can be made invisible,” says Alù.

That doesn’t mean you can make an invisibility cloak that could perfectly redirect light for all shades of, say, blue, either. The wavelength of light would have to be even narrower than that emitted by the best laser that can be built. “Essentially it would be impossible,” says Alù.

Making things worse

Not only that, but trying to hide that one uber-specific shade of blue might make the object you’re cloaking even more conspicuous. To make that sliver of blue invisible, whatever energy the object would have emitted in that blue wavelength would be transferred to other colours, making it shine brighter in all other parts of the spectrum.

“Whatever scattering you suppress in one bandwidth has to be paid back in other bandwidths,” says Alù. “So cloaking may actually make things worse.”

Different kinds of invisibility cloaks could still be possible. But those have challenges, too, and there are no indications that they would be feasible for human-sized objects.

For instance, Alù is currently working on an active cloak using materials that can pump energy into the system. But that is only feasible for wavelengths much longer than that of visible light, such as radio waves – and even then the largest object you could cloak is about 30 centimetres in size, Alù says.

Another kind of cloak that can bend light would only make objects invisible for certain light intensities. If the light got stronger or weaker, the object would pop back into view.

“I’m not sure that we will get to the point of invisibility for a large object to visible frequencies,” says Alù.

“It reminds us of the sad truth that perfect invisibility is still in the realm of fairy tales,” says Jad Halimeh at the Ludwig Maximilian University of Munich in Germany.

Journal reference: Optica, DOI: 10.1364/OPTICA.3.000718