A technique, which a University of Manchester press release describes quite incorrectly as a “Harry Potter style ‘cloaking’ device”, could protect buildings from earthquakes. Dr William Parnell and his team have shown that by cloaking components of structures with pressurised rubber, powerful waves such as those produced by an earthquake would not ‘see’ the building – they would simply pass around the structure and thus prevent serious damage or destruction. The building, or important components within it, could theoretically be ‘cloaked’.
The abstract for the paper in the February 2012 issue of Proceedings of the Royal Society A, “Nonlinear pre-stress for cloaking from antiplane elastic waves“, says:
A theory is presented showing that cloaking of objects from antiplane elastic waves can be achieved by employing nonlinear elastic pre-stress in a neo-Hookean elastomeric material. This approach would appear to eliminate the requirement of metamaterials with inhomogeneous anisotropic shear moduli and density. Waves in the pre-stressed medium are bent around the cloaked (cavity) region by inducing inhomogeneous stress fields via pre-stress. The equation governing antiplane waves in the pre-stressed medium is equivalent to the antiplane equation in an unstressed medium with inhomogeneous and anisotropic shear modulus and isotropic scalar mass density. Note however that these properties are induced naturally by the pre-stress. As the magnitude of pre-stress can be altered at will, this enables objects of varying size and shape to be cloaked by placing them inside the fluid-filled deformed cavity region.
This comes as one of a series of announcements in recent years on various aspects of invisibility but the production of this sort of invisibility without the requirement for metamaterials is significant. Dr Parnell said:
Five or six years ago scientists started with light waves, and in the last few years we have started to consider other wave-types, most importantly perhaps sound and elastic waves. The real problem with the latter is that it is normally impossible to use naturally available materials as cloaks.
We showed theoretically that pre-stressing a naturally available material – rubber – leads to a cloaking effect from a specific type of elastic wave. Our team is now working hard on more general theories and to understand how this theory can be realised in practice.
This research has shown that we really do have the potential to control the direction and speed of elastic waves. This is important because we want to guide such waves in many contexts, especially in nano-applications such as in electronics for example.
If the theory can be scaled up to larger objects then it could be used to create cloaks to protect buildings and structures, or perhaps more realistically to protect very important specific parts of those structures.