Bad news for anyone who’s dreamed of pretending to be Harry Potter with his Cloak of Invisibility: Making a perfect invisibility cloak in real life might be physically impossible, at least according to a new paper in Physical Review A.
The authors—Jad Halimeh at the Ludwig Maximilian University of Munich, Germany and Robert Thompson at the University of Otago, New Zealand—came to this conclusion after studying the limitations currently imposed on invisibility technology.
For example, special relativity—a theory proposed by Albert Einstein that describes how light and matter moves at high speeds—limits cloaks’ invisibility. As the wearer of the cloak moves, obvious visual distortions would appear, making them only partially transparent.
“In principle, what this paper shows is that invisibility cloaking is not possible for all observers,” Halimeh told Phys.org. “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 the Predator’s, giving away what it hides via distortions when you move relative to it.”
Or, for a visual:
Light’s role in invisibility
The general idea behind invisibility cloaks is that the cloak diverts light around an object—making it appear that nothing is there. But of course, this has one major problem: The fact that light must move around the object and not through it means it takes longer for the light to reach behind it—which also can cause noticeable distortions.
There is a way to get around this, which has been explored by other research. Namely, if you don’t block the full spectrum of light, but instead focus on just one frequency—like the frequency for red—it is possible to create a cloak without the time delay issue.
Of course, then you have a cloak that only blocks a single frequency of light—which has obvious limitations—but besides, that, previous research has shown that even those cloaks don’t work if the object underneath is moving too fast. This is because of the relativistic Doppler Effect: The frequency of incoming light is changed, meaning it no longer matches the frequency the cloak is designed for—rendering it useless.
There is another option the researchers investigated: amplitude cloaks. These cloaks preserve the amplitude of light (the “power” of the waves), but not the phase (roughly, the relationship of different waves to each other). Phase is the part that is affected by the time delay, so by nixing this, one could create an invisibility cloak that blocks light across the spectrum without distortions.
Unfortunately, though, the researchers found that once again movement would render this cloak partially visible, as distortions would appear thanks to something known as the Fresnel-Fizeau drag. As it turns out, light travelling through a medium—like a cloak—would drag the light along with it, making the invisibility become distorted.
“Although our results may be disappointing for would-be wizards, understanding the limitations of cloaking devices is actually important in real life,” said 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.”
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Image credit: Thinkstock
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