You Will Never Have a Harry Potter Invisibility Cloak: University Of Texas Researchers

Austin, TX — Harry Potter fans and others longing for a fool-proof invisibility cloak: Don't hold your breath.

Researchers at the Cockrell School of Engineering at the University of Texas at Austin have discerned the fundamental physical limitations of such a cloaking device. First, a recap: Notwithstanding its presence in fantasy narratives, a la Harry Potter, invisibility cloaking devices are really a thing — albeit not a perfected thing.

Such devices harness the technology at play that renders objects invisible or undetectable to electromagnetic waves — including microwaves, infrared and visible light — researchers explain. But here's the bummer: While researchers confirm it's possible to use cloaks to "...perfectly hide an object for a specific wavelength," hiding something from illumination containing different wavelengths "...becomes more challenging as the size of the object increases," researchers said, inadvertently bursting the collective balloon of all those who've waited for a fail-safe invisibility cloak.

Because, really, what's the point of having an invisibility cloak if it's unreliable in hiding something when exposed to multi-wavelength illumination — not to mention one's entire body? I mean, really.

"Andrea Alù, an electrical and computer engineering professor and a leading researcher in the area of cloaking technology, along with graduate student Francesco Monticone, created a quantitative framework that now establishes boundaries on the bandwidth capabilities of electromagnetic cloaks for objects of different sizes and composition," university officials added, pouring more salt into the wound.

Their findings have been published in the journal Optica. The research yields ways to calculate the likely optimal performance of invisibility devices before taking the trouble of designing and developing specific cloaks for a targeted object.

Because few things in life are worse than spending hours developing an invisibility cloak only to find it's not impervious to illumination containing various wavelengths. It would be very frustrating.

But seriously, here's how researchers explain their plausibility: "Cloaks are made from artificial materials, called metamaterials, that have special properties enabling a better control of the incoming wave, and can make an object invisible or transparent. The newly established boundaries apply to cloaks made of passive metamaterials — those that do not draw energy from an external power source."

Researchers explain that having a grasp on bandwidth and size limitations of cloaking is key in assessing the potential of cloaking devices for real-world applications — communication antennas, biomedical devices and military radars, Alù said in ticking off some examples.

"The researchers’ framework shows that the performance of a passive cloak is largely determined by the size of the object to be hidden compared with the wavelength of the incoming wave, and it quantifies how, for shorter wavelengths, cloaking gets drastically more difficult," officials said.

Harry Potter fans, it gets worse: "For example, it is possible to cloak a medium-size antenna from radio waves over relatively broad bandwidths for clearer communications, but it is essentially impossible to cloak large objects, such as a human body or a military tank, from visible light waves, which are much shorter than radio waves."

So, sadly, you won't be able to walk around undetected. But if you still want to hide some stuff, it'll only work with an object of similar size to the wavelength you would harness to make the thing invisible.

“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,” Monticone said. “But for objects comparable in size to the wavelength that excites them (a typical radio-wave antenna, for example, or the tip of some optical microscopy tools), the derived bounds show that you can do something useful, the restrictions become looser, and we can quantify them.”

Alù is more blunt than his colleague, explaining the impossibility of any of us ever having an invisibility cloak given the stubbornly unforgiving tactics of science. What's more, both researchers were candid in explaining that part of the reason they undertook the research was to yield a framework to "...help dispel some of the myths that have been developed around cloaking and its potential to make large objects invisible."

Alù twists the knife further, with a measure of finality. “The question is, can we make a passive cloak that makes human-scale objects invisible?” he asks. “It turns out that there are stringent constraints in coating an object with a passive material and making it look as if the object were not there, for an arbitrary incoming wave and observation point.”

So you're saying there's hope?

Well, no, not really. Armed with the knowledge on that whole bandwidth limits thing, "....researchers can focus on developing practical applications with this technology that get close to these limits," they write, inadvertently sounding a little mean about the whole matter.

Monticone attempts a positive spin on this for the invisible-cloak wishers among us: “If we want to go beyond the performance of passive cloaks, there are other options,” he said. “Our group and others have been exploring active and nonlinear cloaking techniques, for which these limits do not apply."

Offering a glimmer of hope in the difficulty of harnessing light and wavelengths to create a cloaking effect, Monticone adds: "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.”

Alù, meanwhile, will focus on designing active cloaks using metamaterials plugged to an external energy source to achieve broader transparency bandwidths, officials explained.

“Even with active cloaks, Einstein’s theory of relativity fundamentally limits the ultimate performance for invisibility,” Alù said. “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.”

But no invisibility cloaks, kids. Sorry.

>>> Photos via University of Texas of Alù and Monticone, the two men to whom you should direct letters of complaint (not to Patch, please) for having all but definitively dispelled the notion you will one day get your hands on an invisibility cloak.