Researchers in the US have shown that there's a fundamental limit to how well cloaking devices can really work, thanks to the laws of physics.
This means that, based on today's technology, while we might be able to develop invisibility cloaks that hide objects for a specific electromagnetic wavelength - such as visible light or radio waves - it's highly unlikely that we'll ever be able to cloak large objects across multiple wavelengths at once.
"The question is, 'Can we make a passive cloak that makes human-scale objects invisible?'" said lead researcher Andrea Alù from the University of Texas at Austin."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."
In other words… probably not. And if you listen closely enough, you can almost hear the sound of 1 million nerd dreams being crushed. You lied to us, J. K. Rowling.
The passive materials Alù's referring to are materials that have special properties that allow them to cloak items without drawing energy from an external power source, such as metamaterials that are inherently capable of bending or absorbing light.
To figure out the limits of their performance, the engineers used something called the Bode-Fano theory of broadband matching. In very simple terms, this means they looked at how big an object is, how long the electromagnetic waves we're trying to cloak them from are, and based on existing passive materials, figured out if it could ever work.
According to the results, it's not all bad news. They showed that while larger objects are hard to cloak, particularly from shorter wavelengths, there's hope yet to totally hide smaller objects.
"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," said one of the researchers, Francesco Monticone.
"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."
They're not trying to be Debbie downers, either. The reason the team decided to try to figure out these limits in the first place is that they want cloaking devices to improve as quickly as possible, and knowing real limits to work towards is a better way to get there than researchers unrealistically attempting to, sigh, make a human invisbility cloak out of passive materials.
The good news is that the University of Texas team is now looking into other ways to exceed these limits, now that they've set them.
"Our group and others have been exploring active and nonlinear cloaking techniques, for which these limits do not apply," said Monticone.
"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."
While progress has been slow, researchers have already shown it's already possible to completely cloak microscopic objects, and use lenses to bend light around slightly bigger ones.
Impressively, a team of scientists from Columbia University has also designed a radio wave cloaking device that could hypothetically shield Earth from alien lifeforms.
So while things are looking pretty improbable right now, never say never, because you never know what kind of technology we might come up with in the future.
"Even with active cloaks, Einstein's theory of relativity fundamentally limits the ultimate performance for invisibility," said Alù. "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."
The research has been published in the journal Optica.