Active Cloaking Could Counter Radar, Earthquakes, and Tsunamis

Today's stealth fighters, such as the F-22 Raptor, may do pretty well in concealing their radar signature, but mathematicians say that a new active cloaking technique could someday generate electromagnetic fields to hide submarines from sonar, or even protect buildings from earthquakes.

Active cloaking differs from cloaking technologies that rely on special materials to bend light or other electromagnetic waves around an object. Scientists have focused on next-generation metamaterials that can only shield very small objects from visible light

"The problem with metamaterials is that their behavior depends strongly on the frequency you are trying to cloak from," said Graeme Milton, a mathematician at the University of Utah. "So it is difficult to obtain broadband cloaking. Maybe you'd be invisible to red light, but people would see you in blue light."

By contrast, active cloaking could generate electromagnetic waves that adapt to match the frequency and amplitude of incoming waves. That creates the phenomenon known as destructive interference, where waves cancel each other out, much like how noise cancellation headsets work on sound waves.

Milton and two other mathematicians have described their theoretical work in the journal Optics Express and Physical Review Letters, with funding from the National Science Foundation and the University of Utah.

A video created by mathematician Guevara Vasquez shows how the concept might work. The first part of the video shows a kite-shaped object getting struck by an incoming wave, which then creates an expanding ripple effect similar to how radar or sonar detects objects. The second part of the video shows three point-like cloaking devices which emit waves that cancel out incoming waves.

Previous research has only allowed for cloaking very small particles. But Milton and his colleagues ran new calculations that showed how the active cloaking method could hide objects up to 10 times wider than the wavelengths in question. For instance, the method could cloak an object 40 inches wide from radar microwaves that have wavelengths of about four inches.

The new calculations suggest that cloaking objects from the smaller end of the electromagnetic spectrum, such as light, remains extremely difficult. But they still leave open the easier possibility of protecting oil rigs from incoming tsunami waves, or using vibrations to cancel incoming seismic waves from earthquakes. Such waves are much larger than those of visible light, which means that scientists would have an easier time creating cloaking devices for the tasks at hand.

PopSci previously covered another seismic invisibility cloak concept, which would use concentric rings in building foundations to vibrate at the frequencies of earthquakes and minimize damage.

"It would be wonderful if you could cloak buildings against earthquakes," Milton said. "That's on the borderline of what's possible."

Keep in mind that none of this has been demonstrated yet in actual experiments. And there's a big downside in that scientists must somehow know when the pulse begins, as well as the frequencies and amplitudes of waves they wish to cancel out. Milton proposed that an advance sensor network could perhaps pick up such information for earthquakes or tsunamis.