Time-Space Cloak Seen Within Grasp

Detail from "Harry Potter and the Deathly Hallows" book cover, published by Scholastic.
First came the formulation of an invisibility cloak that could bend light around an object. Then, this spring, German scientists took that idea and made it three-dimensional. Is the invisibility cloak now ready to go 4D? For a study in the Journal of Optics, British researcher Martin McCall's team adds the dimension of time to the invisibility cloak idea, creating a theoretical "space-time cloak."

The key feature of the proposed space-time cloak is that its refractive index - the optical property that governs the speed of light within a material - is continually changed, pulling light rays apart in time. When the leading edge of a light wave hits the cloak, the material is manipulated to speed up the light, but when the trailing edge hits, the light is slowed down and delayed. "Between these two parts of the light, there will be a temporal void - a space in which there will be no illuminating light for a brief period of time," explains McCall.

Taking advantage of these differences, he says, it is theoretically possible to imagine a cloak that allows you-at least from my point of view-to transport instantaneously across space.

"By manipulating the way the light illuminating an event reaches the viewer, it is possible to hide the passage of time," [McCall] told AFP. "Not only can specific events be obscured, but it is possible for me to be watching you, and for you to suddenly disappear and reappear in a different location." Like previous invisibility research, McCall's relies on metamaterials: materials specifically designed to control electromagnetic waves (see a more in-depth description here). Unfortunately for people longing to disappear, he says, those materials today are efficient enough for only a nanoscale demonstration.

"Light travels at 100 million meters per second and in order to cloak it you'd need that many meters (of metamaterial), roughly speaking," he said. But McCall said current optical-fibre technology could be used to construct a "poor man's cloak" capable of demonstrating "proof of concept" by imperfectly hiding events taking place over a few nanoseconds. So McCall's space-time cloaking requires some major breakthroughs to take it to the sci-fi kind of invisibility. But, that's not his problem: "We've provided a theoretical recipe and suggested how the experiment can be done. We believe the proof of principle experiment is available with current technology that experimentalist groups could achieve. It's up to the experimentalists to rise to the challenge," he said on CNN.

By Andrew Moseman
Reprinted with permission from Discover