Physicists routinely baffle reporters, but for once things went the other way. Alexander Gaeta was sitting in his Cornell University office in the fall of 2010 when a reporter called to ask his opinion of a strange new paper in the Journal of Optics: What did he think about the claim that it might be possible to create a time cloak, a device that would render events undetectable?

Gaeta was caught off guard. He was still grappling with the invisibility cloak, a wild idea that turned into reality in 2006, when physicists demonstrated that a class of synthetic materials could bend light completely around an object. (Think of water in a stream flowing around a rock.) Without light bouncing off the object, it would essentially disappear.

But creating a time cloak—something that could hide not just an object but an event—is even more ambitious. Rather than just rerouting the rays of light striking an object, a time cloak would have to deflect all the light beams influenced by the object as it moves through space. The time cloak would, in essence, create an interval during which all information about what an object is doing disappears.

Although Gaeta had not heard of the time-cloak study until that phone call, he dove into it as soon as the reporter sent it over. The author, theoretical physicist Martin McCall of Imperial College London, proposed splitting a light beam into two segments moving at different speeds. As one fragment built a lead on the other, a gap of complete darkness would open up between them. Anything happening within that gap, McCall reasoned, would be impossible to detect, since there would be no light to scatter. Then, to complete the trick, McCall proposed bringing those two segments back together so that by the time the beam of light reached an observer, there would be no way to detect that the gap ever existed.

McCall left it to his experimentalist colleagues to figure out how to build such a device, which he estimated would require 5 to 10 years to complete. Gaeta immediately knew he could tackle the task much more quickly. Since 2007 he had been developing a device called a time lens, which alters the speed of a beam of light. In a vacuum, the speed of light is constant. But that speed changes when light passes through a material, like glass or water, or when it runs into another light beam. The time lens combined the two techniques: It involved hitting a beam of light with a laser just as it passed through a glass fiber, allowing considerable control over the beam’s speed.

Gaeta envisioned using the time lens to slow down light so he could measure it in fleeting phenomena like controlled explosions, which usually occur too quickly for accurate readings. But after reading McCall’s paper, he realized he could also use his lens to speed up one section of a light beam and slow down another, thus opening and closing the gap of darkness described by McCall.

Over the next three months, Gaeta and his team assembled a jumble of optical fiber resembling a giant bowl of spaghetti, with lasers and time lenses plugged in along the route. Then one day in April 2011, Gaeta sent a beam of light into one end of the fiber and through a time lens, splitting the beam into two parts. As the leading segment of the beam surged ahead, the time gap widened. By the time the fragmented beam had traveled a kilometer, the gap of darkness had reached 15 trillionths of a second. At that point, the team introduced a marker event by shooting a laser across the fiber.

Ordinarily, the laser would noticeably alter the color of the original beam of light. But the cloak worked to perfection: Because the laser passed through the unlit gap, the color of the beam didn’t change. After passing through another time lens that sped up the slower fragment and slowed down the faster one, the reunified beam reached its endpoint, in one piece, with the same properties as when it started. An independent observer would have no way of knowing the laser had ever been fired.

It was an intellectually exhilarating achievement, but the 15-
trillionths-of-a-second gap of darkness was so small that the editors at Nature, the journal where Gaeta submitted his findings for publication, were not sold on his claim. They requested that he create a gap about three times larger so it would be detectable by a sensitive light sensor.