Invisibility has fascinated human imagination through time and cultures. In Greek mythology, the goddess Athena provided demigod Perseus with a cap of invisibility, which allowed him to sneak up on Medusa and slay her. In Plato’s Republic, Gyges of Lydia entered a cave to discover a magical golden ring that allowed him to become invisible at will. More recent examples include the magical ring of invisibility stolen by the hobbit Bilbo in J.R.R. Tolkien’s fiction, H. G. Wells’ The Invisible Man, Star Trek’s Romulan technology, and Harry Potter’s Cloak of Invisibility.
Theoretically, invisible objects do not reflect, scatter or absorb light, or produce a shadow. An invisibility cloak would bend light or other electromagnetic waves around the object. Such magical devices may not be constrained to fantasy scenarios for long as they can now be at least approximately realised physically using a new class of artificially structured materials, metamaterials.
In 2006, Pendry, Schuring, and Smith published an idea for a cloak that would render an object invisible to probing by electromagnetic waves at a fixed frequency by surrounding it with a metamaterial. The same technique could, in principle, be scaled to work at optical wavelengths. Green, Lassas and Uhlmann had already described essentially the same notion back in 2003, in a study of the inverse problem for electrical impedance tomography (i.e. the same technology used by the author for breast cancer screening described in the Sound Bites section). Also, a group of researchers from Tokyo University presented at the Ars Electronica Festival in 2008 an optical camouflage technology that makes anything transparent.
Even though cloaking technology is not yet ready for public consumption, recent research indicates that most of us can experience an invisibility of sorts. In a study published in 2016 in the Journal of Personality and Social Psychology, researchers from Yale University reported a phenomenon they named ‘The Invisibility Cloak Illusion’: our incorrect belief that we observe others more than they observe us.
References: K. Bryan and T. Leise, Impedance Imaging, Inverse Problems, and Harry Potter’s Cloak, Siam Review 52(2): 359-377, (2010).https://blogs.scientificamerican.com/illusion-chasers/the-invisibility-cloak-illusion/