Imagine that something is happening before your eyes, but you can’t see it — not because you have eye trouble or because it’s a microscopic event, but because of something like Harry Potter’s invisibility cloak. Two years ago, researchers from Cornell University made that fantasy come true, on a tiny scale. They not only hid the existence of a brief event — the movement of a light ray from one point to another — but also the fact that it had been hidden.
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One of the lead researchers on that study, Moti Friedman, is now setting up his own physics lab at Bar-Ilan University and looking for students. He seems unlikely to have trouble finding them. The study he published in the leading journal “Nature” in January 2012 roused great interest in the scientific community, while also setting the popular imagination on fire — at least among fans of physics and science fiction. Friedman, 35, managed to manipulate light to create a “hole in time” that hid the ray’s movement from point to point, a phenomenon known as time cloaking. Events taking place within this hole can’t be sensed in any way.
"When we do spatial cloaking, we cause the light to circumvent an object instead of striking it,” he said. “But that isn’t enough. You also have to put everything back as it was to create the cloaking ... It’s the same thing with time. When I open a ‘hole’ in time, I push part of the light forward and part of the light backward in time, and then close the hole,” he explained.
Now, in his new lab, he hopes to increase the duration of the hole by a factor of 10,000.
You can pause for a moment to give your imagination free rein, to recall science fiction stories about leaping over time and space, to indulge in philosophical reflections about the relationship between human consciousness and the real world, or to once again doubt the reliability of your senses. But in the real world, science offers excitement only in very small portions — in this case, microscopic. Friedman’s time cloak lasted for only 50 picoseconds. A picosecond is a trillionth of a second, or a thousandth of a nanosecond — much shorter than the blink of an eye.
Seeing occurs when a light rays hits the eye and the signal is transmitted to the brains. The Cornell researchers interrupted this light ray by making part of it speed up and part of it slow down. They thereby created a hole in which something could happen without being sensed.
“When you’re dealing with things moving at the speed of light, time and space are connected,” Friedman explained. “For instance, when you turn on a flashlight, the ray widens as you move farther from the flashlight. The same thing happens with time ... mainly because the equations that describe the progress of light through space are completely identical to those that describe the changes in light over time.” What this means is that just as something can be concealed in space, it can also be concealed in time.
Friedman’s experiment dealt with the basic question that preoccupies every art thief: how to pass through a laser sensor without the sensor noticing. The researchers achieved this by means of a “time lens,” a device based on a special optical fiber that allows one light ray to interact with another light ray.
First, they sent one ray, the “probe beam,” through an optical fiber. They then introduced additional rays through the time lens and used them to split the probe beam so that the front section was blue and the rear section was red. The probe beam was then sent through another optical fiber in which blue light travels faster than red, thereby opening up a gap in the beam. Within this gap, an event can occur without the beam detecting it.
To close the hole, the probe beam was sent through another optical fiber in which blue light travels more slowly than red, enabling the rear section to catch up to the front. Finally, the beam was sent through another time lens to turn it back into a uniform ray with the same color and wavelength as the original. In this way, the very fact that the hole in time occurred was also concealed.
This hole is both spatial and temporal, with a size of about 1.5 centimeters and a duration of 50 picoseconds. The optical fibers used in the experiment are thinner than human hair; the tangle of them on Friedman’s long work bench looked like a pile of spaghetti.
Friedman earned his bachelor’s degree in physics and computers at Bar-Ilan, then did his master’s and doctorate in physics at the Weizmann Institute. In 2010, he did a two-year post-doctoral fellowship at Cornell.
The theory of cloaking has made considerable progress over the past decade, due in part to the development of new optical materials that enable theories to be tested. In 2006, professor Ulf Leonhardt of Weizmann surprised the scientific world with an article describing how light rays could be split in two and then bent so that the top and bottom parts travel in semicircles rather than a straight line, thereby producing a circular hole in space where an event could occur without being detected. Friedman has focused on creating a hole in time, but the principle is similar.
The study Friedman published two years ago is just the beginning. In his new lab, he hopes to increase the duration of the time cloak 10,000 times as well as the diameter of the hole in space. He already has ideas about how to do this using stronger lasers, cleaner light and higher-quality optical fibers.
His dream is to achieve time and spatial cloaking simultaneously. The biggest obstacle is actually the progress of spatial cloaking, which will depend in part on the development of new materials. Until now, Friedman says, spatial cloaking has been achieved only on rays with very long wavelengths, like radio waves or microwaves; nobody has yet managed to do it with visible light. Time cloaking, in contrast, works even at wavelengths a million times shorter — as narrow as 10 nanometers.
“So time cloaking has advantages over spatial,” he said. “But on the other hand, in space there are more dimensions, which enables more sophisticated manipulations, and that’s more interesting. Therefore, I want to combine the two and use the advantages of both together.”
Concealing events doesn’t sound good in the real world, but Friedman says the ability to manipulate light in time and space could actually have some useful applications — for instance, enabling information to be transmitted at higher speeds. Fantasies about invisibility cloaks, in contrast, are still far in the future.
“We’re light years away from that,” he said. “It’s roughly like the inventor of the wheel, who had to wait a bit to build a Ferrari.”
Nevertheless, he’s pleased by all the attention his research has attracted. “There was a lot of interest, and that’s very good, because it makes people who aren’t doing science get interested in optics,” he said. “And if this whole story ultimately makes a single child become a scientist instead of a lawyer, then I will have done my bit.”