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# Invisibility Cloak May Be Mathematically Impossible

The question is, ‘Can we make a passive cloak that makes human-scale objects invisible?’

Good question, whether you’re a Harry Potter fan or just someone who needs to disappear for a little while. What’s the answer? According to a new study conducted at the University of Texas … it depends on how big or small you are.

Andrea Alù, an electrical and computer engineering professor in the Cockrell School of Engineering at The University of Texas at Austin, attempted to answer his own question. When discussing cloaking devices, “invisible” means unable to be detected by radio waves, microwaves, infrared and visible light. “Passive” means operating without an external power source – in other words, like an actual wearable cloak or cape. “Human-scale” means the size of your average man, woman or child wizard.

Size is the problem, says Alù in his report in the journal Optica. Specifically, the size of the object and the size of the wave trying to detect it.

We have shown that it will not be possible to drastically suppress the light scattering of a tank or an airplane for visible frequencies with currently available techniques based on passive materials.

It’s easier to deflect a single broadband wave than a wide variety of  shorter waves

Alù and his team members were able to develop a simple mathematical equation to model this. It showed that, as an object gets bigger relative to the wavelength of light, more harmonics are excited at each frequency, all of which must be minimized for an object to be rendered invisible. As the wavelengths get shorter, difficulty increases. It won’t work, says Alù.

[Cloaking] is impossible with linear and passive cloaks of arbitrary complexity.

You’re too big

Broadband wavelengths are easier to cloak. A medium-size radio antenna can be cloaked from radio waves because the antenna and the waves are of comparable size. Big deal … radio antennas aren’t humans. Is there any hope for another form of invisibility cloak? Graduate student graduate student Francesco Monticone (obviously younger and a bigger Potter fan than Alù, offers a little.

Alternatively, we can aim for looser forms of invisibility, as in cloaking devices that introduce phase delays as light is transmitted through, camouflaging techniques, or other optical tricks that give the impression of transparency, without actually reducing the overall scattering of light.

Unfortunately, these forms require external power sources. Alù says don’t blame him, blame Einstein.

Even with active cloaks, Einstein’s theory of relativity fundamentally limits the ultimate performance for invisibility. Yet, with new concepts and designs, such as active and nonlinear metamaterials, it is possible to move forward in the quest for transparency and invisibility.

Curse you, Einstein!

Paul Seaburn is the editor at Mysterious Universe and its most prolific writer. He’s written for TV shows such as "The Tonight Show", "Politically Incorrect" and an award-winning children’s program. He's been published in “The New York Times" and "Huffington Post” and has co-authored numerous collections of trivia, puzzles and humor. His “What in the World!” podcast is a fun look at the latest weird and paranormal news, strange sports stories and odd trivia. Paul likes to add a bit of humor to each MU post he crafts. After all, the mysterious doesn't always have to be serious.
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