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.
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.
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.
Tuned rings
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.
Unlike some of the optical invisibility cloaks that have been studied in physics labs in recent years, the new cloak is "broadband", meaning that it can divert waves across a range of frequencies.
This is made possible by tuning different rings of the cloak to incoming waves of different frequencies. Waves pass largely unaffected through rings not tuned to their frequency.
"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.
Sound foundations
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.
Work remains to be done to replicate the theoretical results experimentally, says physicist Ulf Leonhardt at the University of St Andrews, UK, who was not involved in the study. But it is possible that invisibility physics may see its first real world applications of in guiding seismic or ocean waves rather than to manipulate light, he told New Scientist.
"I think this is fantastic – I really like taking ideas that have emerged from optics and using them in other applications."
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