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.
Nature’s request required an equipment upgrade. For the next 48 hours, Gaeta’s team scoured the Cornell physics building for the amplifiers they needed to boost the power of the time lens. “We did it on the weekend so nobody would be mad at us for disturbing their experiments,” says Moti Fridman, the project’s lead researcher. After borrowing equipment from six labs, they finally had the necessary wattage.
After three more weeks of tweaking the apparatus, Gaeta’s team was ready to send another beam into the fiber. This time the souped-up time lens opened a gap as wide as 40 trillionths of a second, and the light sensor confirmed that the laser had been cloaked. Nature promptly approved the team’s paper, which made it to print in January.
Gaeta’s next goal is increasing the gap to billionths of a second, which will require about 20 times more power. Brief as it is, a gap on that timescale could have immediate implications for the tech industry, which relies on streams of data traveling at light speed over fiber-optic cables. For instance, people streaming video on an iPad could potentially use a time cloak to create a fissure in the data stream and download a file without interrupting the video. A time cloak could also allow antiterrorism agents to monitor enemy communications undetected. The agents could slow down the flow of data, record the information, and then speed it back up so that neither the sender nor recipient could possibly know it was intercepted.
Human-scale time cloaks would have far more profound implications. One can’t help but wonder whether bank robbers and terrorists could use the technology to conceal their activities (after all, the U.S. Defense Advanced Research Projects Agency partially funded this project). They’d be able to hide not only what they are doing but that they are doing anything at all.
Gaeta says these nefarious characters will have to wait a bit longer. “This is just the first step. There needs to be another 100,000 steps before you can create a full spatial time cloak that can hide people,” he says. “Then again, when the transistor was invented 50 years ago, I don’t think anybody imagined something like the iPhone. We don’t know where this will take us.”
Invisibility CLOAK timeline
Researchers have just begun developing the technology for concealing events, but the art of masking objects is significantly more advanced. Below, some milestones in the development of the invisibility cloak.
2006 Physicists at Duke University build a small cloak of synthetic metal that steers microwaves around a cylinder, rendering it “invisible” in the microwave spectrum. (Microwaves are far easier to manipulate than beams of light.) It is the first successful cloaking experiment, but it works only in two dimensions—the cylinder is detectable when viewed from above.
2008 Researchers at the University of California, Berkeley, create a fabric that bends light in ways impossible with natural materials. While it isn’t an invisibility cloak, the fishnet-like structure demonstrates that light could be bent around an object to hide it from detection by the human eye.
2010 German researchers take a different route to invisibility by constructing a cloak that can hide a small bump on an otherwise flat surface. When infrared light strikes the cloak, it bounces back as if the bump were not there.
January 2012 Physicists at the University of Texas at Austin cloak a 7-inch-high cylinder so that it is hidden from microwaves at every angle. Rather than causing waves to bend around the cylinder, the cloak cancels out the microwaves bouncing off it.
