Borrowing from the physics of invisibility cloaks could make it possible to hide buildings from the devastating effects of earthquakes, say physicists in France and the UK.
The "earthquake cloak" idea comes from the team led by Stefan Enoch at the Fresnel Institute in Marseille, France. 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.
Although Enoch's team have calculated that controlling body waves would be too complex, controlling surface waves is within the ability of conventional engineering, they say. Fortunately, it is surface waves that are more destructive, says team member Sebastien Guenneau at the University of Liverpool in the UK.
The new theoretical cloak comprises a number of large, concentric rings made of plastic fixed to the Earth's surface. The stiffness and elasticity of the rings must be precisely controlled to ensure that any surface waves pass smoothly into the material, rather than reflecting or scattering at the material's surface.
When waves travel through the cloak they are compressed into tiny fluctuations in pressure and density that travel along the fastest path available. By tuning the cloak's properties, that path can be made to be an arc that directs surface waves away from an area inside the cloak. When the waves exit the cloak, they return to their previous, larger size.[...]
"The outer rings remain nearly still, but the pair of rings tuned to the frequency of the wave move like crazy, bending up and down and twisting," says Guenneau. "For each small frequency range, there's one pair of rings that does most of the work." The team has simulated cloaks containing as many as 100 rings, says Guenneau, although fewer would be needed to protect against the most common kinds of earthquake surface waves.
When it comes to installing them into buildings, they could be built into the foundations, Guenneau suggests. It should be possible to make concrete structures with the right properties. To protect a building 10 metres across, each ring would have to be about 1 to 10 metres in diameter and 10 centimetres thick.
The concentric ring design can also be scaled down, and could offer a way to control vibration in cars or other machinery, he adds.