There’s a well-known story in Plato’s Republic in which a humble shepherd named Gyges finds a magical gold ring that renders whoever wears it invisible. Gyges proceeds to use his newfound power to murder a king and take over the throne. Plato intended it as a cautionary tale about whether a man could act justly even if the fear of consequence was removed. (The fictional Gyges clearly failed that moral test.) The parable famously inspired J.R.R. Tolkien’s Lord of the Rings trilogy, among other works. And it’s one of the earliest examples of the longstanding human fascination with invisibility in both fiction and scientific pursuits.
“Invisibility represents the perfect merger of not being seen while being able to see others, which would be great if you were a primitive hunter-gatherer,” Greg Gbur, a physicist at the University of North Carolina, Chapel Hill, told Ars. “But more purely, it represents power. You see that in the story of the Ring of Gyges, where the ability to make yourself unseen gives you a tremendous advantage over others. So it’s fascinating as a symbol of pure power and how people might use and abuse it.”
Gbur is the author of a new book from Yale University Press, Invisibility: The History and Science of How Not to Be Seen, covering the earliest discoveries in optical physics through to the present, along with how invisibility has been portrayed in science fiction (a longstanding passion for Gbur). He’s also the author of 2019’s fascinating Falling Felines and Fundamental Physics, which explored the surprisingly complicated physics of why cats always seem to land on their feet, ferreting out several obscure scientific papers spanning decades of research in the process. His interest in invisibility science dates back to his graduate school days when his advisor assigned him a project on the topic.
“At first I thought, well, he’s given me a project that no one’s going to care about,” said Gbur. “But it turned out that for a few years, by default, I was the world expert on the subject.” Starting his blog, Skulls in the Stars, gave him a platform to write regularly about invisibility, particularly after two seminal papers appeared in 2006, demonstrating that invisibility was at least theoretically possible. “I started thinking that there was a nice story to be told about the origins of invisibility and how it’s progressed through the years from something that was purely fantasy to something that’s almost plausible,” said Gbur.
Most so-called “invisibility cloaks” (cloaking devices) created thus far work in the electromagnetic regime and rely on metamaterials. A “metamaterial” is any material whose microscopic structure can bend light in ways light doesn’t normally bend—a property called “the index of refraction.” Natural materials have a positive index of refraction; certain manmade metamaterials—first synthesized in the lab in 2000—have a negative index of refraction, meaning they interact with light in such a way as to bend light around even very sharp angles.
Metamaterials typically involve a highly conductive metal like gold or copper arranged in carefully layered periodic lattice structures. When light passes through the material, it bends around the cloaked object, rendering it “invisible.” You can see an object directly behind it but can’t see the cloaked object itself. However, the effect is typically limited to specific wavelengths: microwaves, infrared light, or certain frequencies of sound or heat waves.
There have also been novel designs for hydrodynamic “invisibility cloaks,” where instead of shielding objects from light, the cloaks would shield them from fluid flows. These kinds of cloaking structures could one day help reduce drag on ships or submarines or protect ships at a port or wharf from potential damage from strong waves. That’s just a few of the ingenious breakthroughs over the last 15 years or so in the burgeoning field of invisibility research.
Ars spoke with Gbur to learn more.