Earthquakes"Active cloak" protects buildings from earthquakes
Researchers say real objects could be cloaked by active cloaking — which means the technology uses devices that actively generate electromagnetic fields rather than being composed of “metamaterials” (exotic metallic substances) that passively shield objects from passing electromagnetic waves
The seismic waves of an earthquake fall into two main groups: body waves that propagate through the Earth, and surface waves that travel only across the surface. Invisibility cloaks could be used to make buildings invisible to surface waves — yes, physicists in France and the United Kingdom say that borrowing from the physics of invisibility cloaks could make it possible to hide buildings from the devastating effects of earthquakes. Two months ago we wrote that the “earthquake cloak” idea comes from the team led by Stefan Enoch at the Fresnel Institute in Marseille, France. Colin Barras wrote that they were the first to show that the physics of invisibility cloaks could have other applications — designing a cloak that could render objects “invisible” to destructive storm waves or tsunamis (see 28 June 2009 HSNW).
Note that scientists also say that rather than fortifying sea platforms and coastal towns to withstand tsunamis, it may be possible to use invisibility cloaks to make off-shore platforms, islands, and even cities “invisible” to waves (29 September 2008 HSNW).
Lewis Page offers an update on the cloaks protecting buildings from earthquakes. Researchers now say that their “active cloaking” research offer even better hope of protecting buildings from earthquakes. “It’s a brand new method of cloaking,” says Utah University maths prof Graeme Milton. “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.”
University spokespersons insist that “This is not a ‘Star Trek’ or ‘Harry Potter’ story.”
According to Milton, the method is not applicable to visible light as it requires the use of unfeasibly small active emitters. Even if these could be built, it would only be possible to cloak very very small things. “It is very difficult to build antennas the size of light waves,” says the prof. “We’re so far from cloaking real-sized objects to visible light that it’s incredible.”
He says, however, that with longer wavelengths the technique becomes practical. Microwave active cloaks could hide things from radar; sonic ones could make submarines disappear from active sonar. Destructive ocean waves could perhaps be nullified, even — Milton thinks — to the extent of squelching tsunamis before they hit the beach.
“It would be wonderful if you could cloak buildings against earthquakes,” Milton adds. “That’s on the borderline of what’s possible … even if it’s not going to result in a ‘Harry Potter’ cloak, it will have spinoffs in other directions.”
Page writes that, as ever, there are a few little issues to sort out before the kit goes mainstream. For one, the cloaking machinery would need to know all about the incoming waves before it could nullify them, probably requiring some kind of remote sensors in the case of the tsunamis, earthquakes and so on.
-read more in Fernando Guevara Vasquez, Graeme W. Milton, and Daniel Onofrei, “Broadband Exterior Cloaking,” Optics Express 17, no. 17 (2009): 14800-05 (doi:10.1364/OE.17.014800); and Fernando Guevara Vasquez, Graeme W. Milton, and Daniel Onofrei, “Active Exterior Cloaking for the 2D Laplace and Helmholtz Equations,” Physical Review Letters 103, 073901 (11 August 2009)
