You Will Never Be Invisible

The sci-fi dream of perfect passive invisibility will forever remain a dream, argue engineers.

Engineers at the University of Texas have quantified fundamental limits on the performance of electromagnetic invisibility cloaks—technologies that function to make objects undetectable to radio waves, microwaves, visible light, and all other sorts of electromagnetic radiation.

The group's work, which is described in the current issue of Optica, confirms that while it is indeed possible to perfectly mask an object from electromagnetic waves of limited wavelengths, this comes at the cost of revealing the object to other wavelengths. That's a significant limitation.

Invisibility cloak research gets a lot of hype for the obvious reason that it's sci-fi as hell, whether we're cloaking aircraft carriers or diamond thieves. The whole idea remained mostly the stuff of sci-fi and theory until the 1990s, when metamaterials began to take off.

All of a sudden, it became possible to tweak the physical properties of a material to awesome degrees, and this has led to several advances over the past couple of decades. A study published in Science last fall, for example, describes a cloaking material capable of wrapping objects of arbitrary shape and that can be manufactured at large scales. Pretty much the sci-fi invisibility dream.

The fundamental idea behind invisibility cloaking is easy enough to see. When a beam of light encounters an obstacle, its various spectral components—the different frequencies making up a beam of what may seem to be homogenous light—will naturally deflect and reflect off of it. Which components are affected and by how much depends on the material. This interference between visible light and intervening objects is how we manage to see those objects at all.

Image: Andrea Alù et al

The task of an invisibility cloak is then to make it seem as though passing light has not actually been distorted by the cloaked object. This is easier said than done, naturally. To cloak an object then requires perfectly reconstructing the optical fields around that object such that it appears that no light scattering ever occurred.

It's intuitive that the ability to cloak objects will vary wildly depending on the object being cloaked. Cloaking a human body from the whole range of visible light is a much different matter than, say, hiding a thin radio-wave antenna. The UT work quantifies the range of possibilities in between those, offering a framework for determining cloakability.

As "scatterers" become larger, light reflected/deflected away from them excites more and more surrounding frequency harmonics. Analogously, we might consider a violin string being plucked or bowed harder and harder, causing increasing resonance in the other strings and the body of the instrument itself. To make it seem as though the violin/object doesn't exist, more and more of these excitations must be accounted for.

The result is a mathematical limit on cloakability that is imposed by increasing complexity. "Our results ultimately confirm that broadband cloaking of macroscopic objects, in the sense of total scattering suppression, is impossible with linear and passive cloaks of arbitrary complexity," the UT paper concludes. (Note that this is an expansion on previous UT work demonstrating that in many cases macroscale optical invisibility cloaking will necessarily make cloaked objects even more detectable.)

The message is that we probably shouldn't be wasting our time dreaming of total human-scale invisibility using passive technologies. This doesn't mean large-scale invisibility is impossible, but that to make it work will likely require active technologies. For example, an active cloak might calculate and display a recreation of some scene in which the cloaked object has been disappeared. The most naive implementation would be just using a camera to record everything behind the object and then projecting that on the front of the object.

This is a bit less exciting, yes, and even an active cloak will be limited by Einstein's theory of relativity. The speed of light is constant—if light is forced to take a detour around some hidden object, it's not going to arrive at an observer at the same time as if it had passed through the object (as if there were no object, that is).

So, the sci-fi dream of perfect passive invisibility cloaking shall remain just that. This could be a good thing.