Space-Time Cloak Possible, Could Make Events Disappear?

Material would adjust speed of light to hide actions, physicists say.



Richard A. Lovett
for National Geographic News
Published July 11, 2011

It's no illusion: Science has found a way to make not just objects but entire events disappear, experts say.

According to new research by British physicists, it's theoretically possible to create a material that can hide an entire bank heist from human eyes and surveillance cameras.

"The concepts are basically quite simple," said Paul Kinsler, a physicist at Imperial College London, who created the idea with colleagues Martin McCall and Alberto Favaro.

Unlike invisibility cloaks—some of which have been made to work at very small scales—the event cloak would do more than bend light around an object.

(Also see "Acoustic 'Invisibility' Cloaks Possible, Study Says.")

Instead this cloak would use special materials filled with metallic arrays designed to adjust the speed of light passing through.

In theory, the cloak would slow down light coming into the robbery scene while the safecracker is at work. When the robbery is complete, the process would be reversed, with the slowed light now racing to catch back up.

If the "before" and "after" visions are seamlessly stitched together, there should be no visible trace that anything untoward has happened. One second there's a closed safe, and the next second the safe has been emptied.

Event Cloak "Fun" but Challenging

The concept of an event cloak "is definitely an interesting idea and great fun," said invisibility researcher Ulf Leonhardt, a physicist at the University of St. Andrews in the U.K. who wasn't part of the study team.

Steve Cummer, a cloaking specialist at Duke University, calls the concept "interesting and exciting," but he thinks that actually making such a cloak would be "really, really challenging."

"All of the material parameters need to be time-varying in a very specific way," Cummer said by email.

Currently, nobody knows how to do that except in fiber optics, in which the speed of a signal can be varied by a few percent by changing the intensity of the light. (Related: "Nobel Prize in Physics Goes to 'Masters of Light.'")

"You can use an intense control beam to slow a signal down," Kinsler said. In that way, an event occurring inside a fiber optic cable—such as an electrical signal moving from "on" to "off"—could be hidden from view.

"You would see the fiber, but some event occurring in the fiber could be cloaked," he said. Such a proof of concept, he added, could be possible within a few years.

(Related: "Long-Distance Quantum Message May Advance Code Making, Code Breaking.")

Powerful Laser Key to Cloaking?

There are still a few hitches to address, though, before attempting such an experiment, according to the University of St. Andrews's Leonhardt.

For instance, being able to cloak an event lasting more than a few femtoseconds—one-millionth of a nanosecond—would require light from an immensely powerful laser, he said.

"The experiment is not entirely impossible, but it is at the limit of what one can do with present technology in an ordinary university laboratory," Leonhardt said.

(Related: "Fusion Power a Step Closer After Giant Laser Blast.")

Study co-author Kinsler agrees that the fiber optics experiment is a technological stretch, but he counters that the problem can be eliminated by using a miles-long cable, as is routinely done in telecommunications.

"You can reduce the required power by using a longer fiber," he said. That's because the duration of the event you can hide depends on how long you can delay the light. A longer cable would allow you to get a longer delay from the same percentage slowdown, which would take less power.

Still, it may be a long while before police have anything to worry about—the materials needed for speeding and slowing light precisely enough for a bank heist have yet to be invented.

The event-cloak story and an accompanying article by Leonhardt appear in a special issue of Physics World magazine published June 30.