Invisibility is one of those science fiction superpowers, along with flight or super speed, that most everyone has at least dreamt of having. In our current age, with the emerging science of metamaterials, invisibility seems closer than ever to becoming a reality. But there is still one problem: if a person achieved true, perfect invisibility, they would probably be blind.
When it comes to invisibility and cloaking, it’s all about light, and metamaterials look like our best way to mess with it. Metamaterials are specially designed materials that have been created to produce a specific effect that wouldn’t naturally occur. More specifically, Dr. Stephane Larouche, an Assistant Research Professor at Duke University’s Meta Group explained, “Metamaterials are structured materials to which it’s possible to assign effective properties.” To create these futuristic materials, researchers are taking small pieces of metal or plastic and shaping them at almost unimaginably small scales, in order to create a specific response.
In terms of invisibility and cloaking, developing metamaterials science is providing us with our best hope yet of one day disappearing entirely, by bending light around us. “The way cloaking works, when a beam of light enters a glass of water it shifts a little bit, so it appears crooked,” says Larouche. He says that the cloak would work in a similar way, in that when light enters it, the beams would be directed in the opposite direction as it went in. “So if you were to look at it, you wouldn’t see anything there, just because all the beams of light have been moved around the center region," he says.
To get light to bend around a space or object and seamlessly come back together on the other end is no easy feat, and it requires that the metamaterial be shaped in nanometers. The ability to bend wavelengths of energy was demonstrated by Duke researchers in 2006 using microwaves, but creating the effect on visible light would mean working on a much smaller scale than currently possible.
But even if we could use the meta material cloak in a bigger way, there's another problem. You'd be invisible to others, sure, but you couldn't see anything.
Since your vision is based on the light rays that enters your eyes, if all of these rays were diverted around someone under an invisibility cloak, the effect would be like being covered in a thick blanket. Total darkness. As Larouche points out, this is due to a physics principle known as reciprocity.
As it pertains to optics, reciprocity means that if you can see in one direction, you need to be able to see in the other direction. Larouche used the example of a two-way mirror, which is in its way a kind of invisibility trick. With a two-way mirror, the subject on the hidden side must be in lower lighting than the subject on the observed side for the trick to work, but the light must still pass through both sides. So if we were to have an invisibility cloak tomorrow, it would likely not make you 100 percent invisible, like Harry Potter’s magic cloak does. “[The cloaking device seen in Predator] is probably closer to the truth than, let’s say, the one in Star Trek that makes you totally invisible, but you can still navigate through space,” said Larouche. “Because of this reciprocity principle, no perfect invisibility cloak would allow you to see outside.” (He points out that this is only true of a potential form of true invisibility, but that we might one day create something that makes us hard to see, if not completely invisible.)
How do we plan on overcoming this invisible visibility problem? Larouche thinks that it might be a bit early to be asking. “This is a problem that we’re conscious of,” he says.
It seems like the larger issue for the moment is to even create an effective invisibility cloak that could blind you in the first place. In 2015, Berkeley scientists published a paper about a new “wrap-around invisibility cloak” made of a super-thin metamaterial. Although this, too, was only achieved on the small scale. Hopefully one day in the not-too distant future, true invisibility can blind us all.