It is all about making the visible appear invisible, and there may soon be a way to someday shield submarines from sonar, planes from radar, buildings from earthquakes, and oil rigs and coastal structures from tsunamis.
“We have shown that it is numerically possible to cloak objects of any shape that lie outside the cloaking devices, not just from single-frequency waves, but from actual pulses generated by a multi-frequency source,” said Graeme Milton, senior author of the research and a distinguished professor of mathematics at the University of Utah.
“It’s a brand new method of cloaking,” Milton said. “It is two-dimensional, but we believe it can be extended easily to three dimensions, meaning real objects could be cloaked. It’s called active cloaking, which means it uses devices that actively generate electromagnetic fields rather than being composed of ‘metamaterials’ [exotic metallic substances] that passively shield objects from passing electromagnetic waves.”
Milton said his previous research involved “just cloaking clusters of small particles, but now we are able to cloak larger objects.”
Radar microwaves have wavelengths of about four inches, so Milton said the study shows it is possible to use the method to cloak from radar something 10 times wider, or 40 inches. That raises hope for cloaking larger objects. So far, the largest object cloaked from microwaves in actual experiments was an inch-wide copper cylinder.
Cloaking involves making an object partly or completely invisible to incoming sound waves, sea waves, and seismic waves, but usually electromagnetic waves such as visible light, microwaves, infrared light, radio, and TV waves.
In recent years, scientists devised and tested various cloaking schemes. They acknowledge practical optical cloaking for invisibility is many years away. Experiments so far have been limited to certain wavelengths such as microwaves and infrared light, and every method tried so far has limitations.
Compared with passive cloaking by metamaterials, the new method, “which involves generating waves to protect or cloak an object from other waves,” can cloak from a broader band of wavelengths, Milton said.
“The problem with metamaterials is that their behavior depends strongly on the frequency you are trying to cloak from,” he said. “So it is difficult to obtain broadband cloaking. Maybe you’d be invisible to red light, but people would see you in blue light.”
Most previous research used interior cloaking, where the cloaking device envelops the cloaked object. Milton said the new method “is the first active, exterior cloaking” technique: cloaking devices emit signals and sit outside the cloaked object.
The new studies are numerical and theoretical and show how the cloaking method can work. “The research simulates on a computer what you should see in an experiment,” Milton said. “We just do the math and hope other people do the experiments.”
Milton’s study partner, Guevara Vasquez, created short videos (http://vimeo.com/6092319) of mathematical simulations showing a pulse of electromagnetic or sound waves rolling past an object:
Milton said the cloaking devices cause “destructive interference,” which occurs when two pebbles are thrown in a pond. In places where wave crests meet, the waves add up and the crests are taller. Where troughs meet, the troughs are deeper. But where crests cross troughs, the water is still because they cancel each other out.
The principle, applied to sound waves, is “sort of like noise cancelation devices you get with headphones in airplanes if you travel first class,” Milton said.
“We proved mathematically that this method works when the wavelength of incoming electromagnetic radiation is large compared with the objects being cloaked, meaning it can cloak very small objects,” Milton said. “It also can cloak larger objects.”
Because visible light has tiny wavelengths, only microscopic objects could become invisible by the new method.
“The cloaking device would have to generate fields that have very small wavelengths,” Milton said. “It is very difficult to build antennas the size of light waves. We’re so far from cloaking real-sized objects to visible light that it’s incredible.”
But imagine incoming waves as water waves, and envision breakwater cloaking devices that would generate waves to create a quiet zone that would protect oil rigs or specific coastal structures against incoming tsunami waves. Or imagine cloaking devices around buildings to generate vibrations to neutralize incoming seismic waves.
“Our method may have application to water waves, sound, and microwaves [radar],” including shielding submarines and planes from sonar and radar, respectively, and protecting structures from seismic waves during earthquakes and water waves during tsunamis, Milton said. All those waves have wavelengths much larger than those of visible light, so the possible applications should be easier to develop.
“It would be wonderful if you could cloak buildings against earthquakes,” Milton said. “That’s on the borderline of what’s possible.”
For related information, go to www.isa.org/sensors.