As children, we obsess over our favorite superheroes and the superpowers that we wish we could have. We dress up as Spiderman for Halloween and pretend that we’re invisible like Violet from the Incredibles. Some powers, like telekinesis, are out of the reach of science. But what if some are not? In the past decade, researchers have begun developing technologies that render objects invisible to the human eye. Maybe in the near future, we’ll be able to make ourselves invisible too!
“Invisibility,” in its use in this article, means that an object is not being seen by an observer. There are two main paths that scientists are taking to achieve this effect. The first is creating physical materials, like cloaks, that can be put over 3D objects and will render them invisible. The second is creating ‘digital’ invisibility, where objects look invisible through a filter. The scientific basis of both techniques harnesses the wave nature of light. Light is made of electromagnetic waves that have properties like wavelength (distance from one crest of the wave to another), frequency (how often a wave front passes an arbitrary point), and phase.
Two waves traveling next to each other are “in phase” if their crests line up. If one of the waves hits an object, it will be reflected, or refracted, or generally changed so that the two waves are no longer in phase. When your eyes detect this change in phase, it indicates that there is an object where the light waves changed.
In the past ten years, many research institutions have developed physical cloaks that make objects invisible. Through a collaboration between the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California Berkeley, scientists created one of these prototypes. They created a microscopic ‘cloak’ that will conform to the shape of the object it is placed on. The cloak works by ‘reconstructing the phase’ of the waves. When light hits the cloak and the waves go out of phase with each other, the particles of the cloak interact with the light waves and make them go back into phase with each other. Thus, your eyes never process the change in phase, and they do not know that there is an object there. The object is now invisible to the human eye. Cloaks like these are being made from materials called ‘meta-materials’ that can reconstruct the phase of waves, making the light essentially ‘pass-over’ the surfaced unchanged. The researchers at Berkeley used gold nanoparticles to make their cloak.
The two major limitations of this method of creating invisibility are that the cloak can only make one specific wavelength of light invisible (so an object will only be fully invisible if it is all the same exact color) and that the cloak is currently microscopic. However, the researchers at Berkeley say that their design for an invisibility cloak has the best chance for being scalable to macroscopic objects. Other institutions that have developed these microscopic ‘cloaks’ include Queen Mary University in London and Ben-Gurion University in Israel.
Another way to render objects invisible is digitally (through a lens). The Canadian National Institute of Scientific Research (INRS) in Montreal has developed a device that uses lenses to filter out specific colors. The device works by changing the frequency of the light that passes through the lens. Each color corresponds to a specific frequency of light. The reason that objects have color is because objects absorb some frequencies of light (the ones that you don’t see) and reflect other frequencies of the light (the ones you do see). The lenses in the device will ‘bump’ the light it encounters up or down to another frequency that the object will absorb, so when the object is placed behind the lens, the viewer will not see that color on the object anymore. According to one of the researchers, Jose Anzana, when the object is viewed through the lens, you are able to see behind the object as if it were not there.
Similarly to the physical cloak, a limitation of this kind of device is that currently, the device can only filter specific programmed frequencies of light at one time, rather than all visible light. However, the researchers say that theoretically the device can be developed to filter out whole spectrums of light.
Researchers at the University of Rochester are developing another type of digital invisibility device that uses basic optics to direct light around 3D objects and cloak them. To learn more about this technology, you can watch these videos from the University of Rochester: The Rochester Digital Cloak: A New Age of Invisibility and The Rochester Cloak.
So invisibility is cool and all, but why is it practical to pursue this technology? Well, invisibility can be used for stealth technology, which has military applications. Doctors performing surgeries would be able to see through their hands and see their work better. Wave phase reconstruction technology can reduce the scattering of waves in telecommunications and improve the quality of terrestrial and aerospace communications. By making parts of computers or other electronic hardware invisible, invisibility technology can also strengthen the encryption of secure devices. And maybe one day, when you’re shopping for your Halloween costume at Spirit, on the display table will be a Harry Potter costume, wand, glasses, invisibility cloak and all.
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