The physics community let out a small gasp six years ago when researchers reported the first successful “spatial cloaking,” in which light is bent around an object in a way that makes it disappear from view. The new report in the journal Nature shows how they can play with something that would seem to be even harder to manipulate: the perception of time.
“We think of time in the way that other people think of space. What other people are doing in space, we can do it in time,” said Moti Fridman, a researcher at the School of Applied and Engineering Physics at Cornell University .
“I think it’s a big step forward,” said Vladimir M. Shalaev, a professor of electrical and computer engineering at Purdue University, who has worked on spatial cloaking. “It’s another example of the beauty of ‘transformational optics,’ which is behind all these ideas.”
Temporal cloaking, like spatial cloaking, is not magic. It follows all the ironclad laws of physics and is to some extent a parlor trick, albeit performed in a highly unusual parlor. Whether it will have a use isn’t known, as the hole in time created by the Cornell team lasts only 50 trillionths of a second.
“It is not enough time to steal a painting from a museum,” Fridman said, joking. He added that there might nevertheless be practical uses of the technology.
Cloaking things, either in space or in time, requires the manipulation of light. Light carries information; it bounces off objects, defining their shape and sending those details to detectors such as our eyes. If an object can prevent light from doing that, it will become invisible.
But because light travels and has speed, it also defines when something happens. The lightening and darkening that occurs when a beam of light illuminates an event marks the event in time. If something can happen and yet leave the light unperturbed, which is the essence of temporal cloaking, then the event can become as invisible as a cloaked object.
Those manipulations are possible because of man-made substances that behave in ways that natural substances don’t. The extreme bending of light that makes spatial cloaking possible requires so-called optical metamaterials made through nanotechnology. Temporal cloaking depends on special lasers and optical fibers that disperse or undisperse light in predictable ways.
In their experiment, previewed this summer in an online archive and reported in detail in Nature on Tuesday, Fridman and his collaborators sent a laser beam of light down a fiber-optic cable. At the starting end of the cable, they pulsed the beam with a second laser that changed the light from a single wavelength to a range of wavelengths, essentially different colors.
The beam then entered a section of cable that had the property of carrying light of different wavelengths at different speeds, specifically blue light faster than red. As a consequence, the two colors separated until there was a space between them where there was no light at all. This blip of total darkness — one centimeter long and lasting 50 picoseconds — is what the researchers called a “time gap” or “time hole.” The beam was then reassembled by reversing the steps, sending it through glass with the opposite effect on blue and red light, and then through another laser that restored the light to the original single wavelength.
To show that an event occurring in the “time gap” was undetected, the researchers pulsed a ray of light through it. Normally, that would perturb the first beam in a way that was obvious when the light came out the far end of the cable. But when the ray went through the time gap and then the beam was reassembled successfully, the detector at the end of the cable perceived no change.
Here is a rough analogy:
Imagine you are watching a train of 40 cars coming toward you head-on. You notice a man on a motorcycle stopped at a crossing. If somehow the train uncoupled between the 20th and 21st cars and the front half of the train sped up a little and the back half slowed down, a gap would open. If the gap opened at the crossing and the motorcyclist was fast enough, he could pass through the train to the other side of the track. If the cars then recoupled and the train regained its constant speed, it would appear to you that nothing had occurred — except, of course, the position of the motorcyclist changed. (The analogy isn’t perfect. In the Cornell experiment, the gap is opened by compressing light, not speeding up or slowing down the whole front and rear of the beam as would be the case with the train.)
In both the experiment and the analogy, it appears an impossible event has transpired. But of course that’s not the case. All that’s happened is that detection of an event through ordinary means has been made impossible.
Could this apparatus be configured in a way that makes time gaps appreciable on a human time-scale, say lasting seconds?
“I would say the laws of physics don’t preclude it but known materials don’t allow it,” said Robert W. Boyd, a physicist at the University of Rochester who wrote a commentary in Nature about the experiment.
Fridman, an Israeli postdoctoral researcher in the laboratory of Alexander L. Gaeta, imagines there might be a use for a time gap lasting fractions of a second in routing competing streams of data to a processor.
Shalaev, of Purdue, thinks there are more practical applications of transformational optics, such as more efficient solar collectors and “super-resolution” of images. But he doesn’t dismiss the idea that cloaking might have its uses.
“Normally when you have such a beautiful physics, it does come eventually to an interesting application,” he said.