There have long been certain tropes in science fiction that seem to be forever over the horizon and beyond our grasp. Interstellar travel, teleportation, time travel, living on other worlds, these have always remained fantasies that seem forever unacheivable and relegated to fiction. Another such technology that can claim to join these ranks is that of invisibility, with a person able to just vanish and move about unfettered and undetected. The idea goes pretty far back, but while it has always seemed to be just another fantastical piece of unattainable tech, there have been many who are working to make it a reality.
In H.G. Wells’ classic 1897 novel The Invisible Man, a scientist named Griffin has devoted his life to researching a way to make himself invisible by using chemicals capable of making it so that the body neither absorbs nor reflects light. He successfully does so and must then deal with the consequences of not being able to change himself back, all while his mind slips and he becomes ever more dangerous and violent as he takes advantage of his unstable new powers. At the time the novel was published it was a new and exciting idea, that there could be this invisible person, essentially a living ghost, and this launched the novel’s popularity and helped to establish Wells as the "father of science fiction." Over the years The Invisible Man has been made into several films, most recently in Leigh Whannell’s 2020 film of the same name, but is any of this really possible, and could there ever be a real invisible man or woman menacing us? The answer to this question is that it is seemingly indeed quite possible.
In the original story by H.G. Wells, Griffin achieves his invisibility through using a chemical concoction to change his body’s refraction index, that is, a measure of how much the path of light is bent, or refracted when entering a material, as well as how fast light travels through a material and the amount of light that is reflected. A simple way to see this effect is to halfway submerge an object in water and see the way the object seems bent, displaced, or distorted. All objects that we see have a positive refractive index, but if a material had a negative index then it would theoretically bend light completely around itself to render it invisible. Although using exotic chemicals in some sort of injection or potion is a pretty magical way to do it and reflects the lack of understanding of science of the time period in which H.G. Wells’ novel came out, he actually had the basic concept pretty right, as refracting and bending light is one of the key ways real invisibility has been pursued.
One way to achieve a real, functioning invisibility suit would be to do it through the use of metamaterials, that is, materials possessing properties not found in nature, with a negative refraction index, which was first hypothesized all the way back in 1967 by Soviet physicist Victor Viselago. Such a material could theoretically create a metamaterial cloak that would bend light completely around the wearer to make them appear to vanish. One scientist who has been trying to pursue this angle is physicist and professor of electrical and computing engineering at Duke University, David R. Smith, who along with associates R.A. Shelby and S. Schultz was able to come up with the first known negative refractive material. The material itself is described as a “a lattice of copper wire strips on circuit board that demonstrated inverted optical properties in response to a microwave beam,” and the team has continued to develop and test what they call “transformation optics.”
One of their achievements has been to bend microwave frequencies completely around an object to the other side, as well as to develop metamaterials that can distort or redirect the waves of electromagnetic fields. The problem with their research is that, while it does provide a real, achievable theoretical approach to the possible "invisibility cloak,” and they have been able to bend microwaves, infrared, X-rays, ultraviolet, and radio, they have not yet managed to bend light in the visible spectrum, which of course is what we actually see and is what is necessary for achieving true invisibility. The difficulty lies in the fact that our visible spectrum is made up of many wavelengths of all of the different colors we perceive, which would require the metamaterial to bend a huge number of different wavelengths simultaneously rather than the one at a time method the team has been using. José Azaña, a fiber optics researcher with Canada’s Institut National de la Recherche Scientifique has said of this:.
In reality, what we are dealing with when we see an object is white light, so it has all the possible colors together over the whole spectrum. Thus, an “invisible man” cloaked in a garment of metamaterials wouldn’t make it very far out of the lab.
Basically, the more wavelengths you are trying to bend around you, the more difficult it gets, and with the visible spectrum you are dealing with a whole lot of them. However, Smith thinks it is very possible to eventually overcome this obstacle, and other researchers have been pursuing similar metamaterials as well, using the same concept of bending light around objects to make them invisible. Canada-based company Hyperstealth Biotechnology has developed a metamaterial called Quantum Stealth that manages to create invisibility with all wavelengths of visible light by using a slightly different spin on the basic concept of bending light. In the case of Quantum Stealth, light coming from the background on both sides of the sheet of material toward the viewer is bent and distorted to essentially create a blind spot in the middle. Guy Clark, Hyperstealth’s CEO and Quantum Stealth inventor, says of it:
You get the background that is to the left of me appearing on the right of the material and the background that’s to the right of me appears on the left of the material. And in that overlap zone in between those two you can actually hide a target in the middle.
The problem is that the blind spot only exists if the viewer is looking at it from a certain angle and from a certain position and distance. If they or the invisible person or object move too much to the left or right the spell is broken and they become visible again. Another problem is that Quantum Stealth comes in the form of a screen of thin material that the object or person is hidden behind, meaning it is not currently wearable like a true invisibility cloak, and the screen is quite obviously still there, although what is behind it still completely disappears. Nevertheless, Clark believes that future versions of Quantum Stealth will fix these issues and allow for a true invisibility cloak. You can see a video of Quantum Stealth in action here.
A similar effect has been found through more low-tech methods by researchers at the University of Rochester, who discovered that by simply placing four lenses spaced just right, they form a “blind spot” where light bends around an object to cause it to disappear. However, once again it only works from a certain angle or distance on an unmoving object or the illusion is shattered. You can see the four lenses effect in action here. Another different spin on bending wavelengths of light is called "spectral cloaking," in which the metamaterial only allows certain electromagnetic wavelengths to pass through, after which they are modulated and changed back to the original wavelengths afterward. This would theoretically create invisibility because the incoming light would never actually interact with whatever it passes through and would therefore not reflect off of it, creating a negative refraction index. However, again it only works from certain angles on static objects. Yet another method for making objects invisible is called “plasmonic cloaking,” and entails modulating the interaction of light and metal nanostructures to create electrical currents that render objects invisible. In 2012, Stanford University engineers demonstrated this effect by painting the silicon nanowires of a semiconductor with gold, after which the light from the metal and silicon canceled each other out, making the device effectively invisible. The problem with this method is that so far only very small objects have been made to disappear this way, and it does not seem feasible for larger objects or humans. So, any mad scientists out there reading this might be asking right about now, what other options do I have for getting my invisibility suit to run amok in? Well, let’s take a look at another option.
One way is what is called “active camouflage” or “optical camouflage,” and the basic concept is simple, in that you merely use cameras and optics to project what is behind you onto the front and vice versa. It is similar to what cuttlefish and chameleons do, projecting background colors onto the front through chromatophore cells. You could achieve this effect at home using two tablets or smartphones, one facing forward and the other facing behind. The one behind takes images on the camera that it shows on the tablet facing forward and vice versa and voila, anything in between them would be effectively invisible when viewed from a certain angle. The idea of active camouflage is to create a suit covered with OLED pixels and an array of 360-degree cameras, allowing real-time video of the world around the wearer to be displayed on the surface of the suit. This would then make everything around the wearer be projected to the opposite side and make them invisible from all angles and even while moving, but there are still hurdles to this kind of technology.
The main problems here would be resolution, power source, and computing power. The array would not only need copious amounts of power to function for any appreciable length of time, but the computer system would need to process a huge amount of information in real-time simultaneously with no lag to be believable, and there is also the fact that the pixels would have to have incredibly sharp resolution to truly trick the eye of an observer and make them think nothing was there. The result would likely be that the suit would be less like a perfect invisibility cloak and appear more like the flickering, blurry shimmering invisibility effect produced by the alien hunter in the movie Predator, especially when moving. It might still be useful in low light conditions or in chaotic backgrounds like a jungle, but still far from allowing one to just waltz through a room full of people without being seen, and it would likely run out of batteries very quickly. Nevertheless, active camouflage suit prototypes are currently being developed and tested by the military and other researchers, so there may come a time when these issues are resolved.
So the question remains, are we ever going to achieve perfect, fully functional invisibility suits like the one in the 2020 Invisible Man film? Has it already been secretly developed somewhere? Although as far as we know it is currently out of our grasp, it certainly seems possible that the technology is feasible and well within the realm of scientific possibility. Yet some researchers have disagreed that the technology is possible with what we have now and that it could very well forever remain a sci-fi fantasy without some major new strides in technology. Andrea Alù, an electrical and computer engineering professor from the Cockrell School of Engineering at the University of Texas at Austin has done a study on the feasibility of cloaking large objects and people, and has said of the fundamental restrictions they have uncovered:
The question is, ‘Can we make a passive cloak that makes human-scale objects invisible?’ Unfortunately, the answer appears to be ‘no.’ 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. If we want to go beyond the performance of passive cloaks, there are other options. Our group and others have been exploring active and nonlinear cloaking techniques, for which these limits do not apply. 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.
Whatever the case may be, the promise of true invisibility to too good to pass up, and it continues to be actively pursued, especially for its obvious possible military applications. As new concepts and metamaterials are developed, who knows how far we might come along with such technology, or how far someone might have already come? It is a rather strange, exciting, and not a little scary avenue of research and technology that seems inevitable, and whether it is ultimately possible or not there will likely be those who continue to research and pursue it.