By Andrea K. Dobson
Shortlisted for the Award in Science
How we see, how light works, how it interacts with materials, is an ancient question. An obvious next question is how to avoid being seen. And is it possible to be truly invisible, in some more fundamental way than simply hiding behind a rock or turning the lights off in a windowless room? In Invisibility: the History and Science of How Not To Be Seen Gregory Gbur, science writer and professor of optical physics, tackles these questions.
Understanding how not to be seen requires understanding how we see: the majority of Invisibility is a clearly written history of optics. Gbur explains the physics in an accessible manner: Diagrams help illustrate concepts such as electromagnetic waves, refraction, and birefringence, and Gbur keeps the mathematics to a minimum. Gbur includes vignettes about the lives of many of those advancing our understanding of optics as well as those who adopted those advances in physics and put them to use in works of fiction. For instance, the discovery of “invisible rays,” i.e., wavelengths of light—infrared and ultraviolet—beyond the visible, is paired with “invisible monsters” such as “That Damned Thing,” imagined by Ambrose Bierce in 1893. The “Thing” is “a being . . . made of a substance whose color is outside the visible spectrum” and thus not visible to our eyes. Invisibility might lead to monsters; it might also advance medicine. For example, humans are relatively transparent, effectively invisible, to x-rays. Once x-rays were discovered, it became possible for physicians to “see” inside bodies, e.g., to determine the location of a break in bone without the painful manipulation that would previously have been required.
The modern physics of invisibility pushes the boundaries of optics well beyond understanding how light interacts with matter. Rather than asking “what does light do?,” today the question has become “how can we make light do whatever we want it to?” For example, the last few decades have seen a competition to develop the blackest possible paints, substances that will absorb over 99.99 percent of the incident light, at least over some reasonably broad range of wavelengths. Lenses made of metamaterials with negative indices of refraction, i.e., materials that bend light in the direction opposite to what would normally be expected, can be constructed with better resolution than lenses made of materials normally found in nature. The discussion of “invisibility cloaks,” once only a device of science fiction-authors, is now taken seriously by physicists: can we design materials that would mimic the general relativistic warping of space and guide light around an object, effectively making an object disappear, at least over some range of wavelengths? Early efforts worked with microwaves, but researchers rapidly expanded into the visible regime: Gbur describes a 2013 cloaking device capable of making a house cat disappear.
How far can invisibility go? Since we are designing structures that manipulate the direction of waves, why stop with waves of light? Could we, for instance, design cloaks on a large enough scale to protect platforms at sea from rogue waves or make cities invisible to earthquake waves? Those ideas are no longer as far-fetched as they might have sounded just a few years ago.
While you may not want to make your house cat disappear at home, Gbur recognizes that his readers may want to experiment for themselves with some of the simpler ways of demonstrating invisibility. He obligingly provides a do-it-yourself appendix—for instance, Pyrex rods in mineral oil will look as though they have disappeared because the two indices of refraction are nearly identical, and a simple cloaking device can be constructed using a set of prisms. For those interested in the imaginative uses of invisibility, a second appendix provides examples of science fiction and horror stories in which invisibility plays a role.
Our understanding of the nature of light has changed dramatically over the centuries. Invisibility eloquently tells the tale of those changes and prepares us to appreciate the astonishing advances of modern optics.
Andrea K. Dobson (ΦΒΚ Whitman College) is Chair of the Astronomy Department at Whitman College.