Earthquake-proofingDream of ideal “invisibility” cloaks for stress waves dashed

Published 11 June 2019

Whether Harry Potter’s invisibility cloak, which perfectly steers light waves around objects to make them invisible, will ever become reality remains to be seen, but perfecting a more crucial cloak is impossible, a new study says. It would have perfectly steered stress waves in the ground, like those emanating from a blast, around objects like buildings to make them “untouchable.”

Whether Harry Potter’s invisibility cloak, which perfectly steers light waves around objects to make them invisible, will ever become reality remains to be seen, but perfecting a more crucial cloak is impossible, a new study says. It would have perfectly steered stress waves in the ground, like those emanating from a blast, around objects like buildings to make them “untouchable.”

Despite casting serious doubt on dozens of theoretical papers that have pursued “elastodynamic” cloaking, the new study’s authors from the Georgia Institute of Technology don’t think civil engineers should completely give up on it, just on the idea of an ideal cloak. Limited cloaking could still add a degree of protection to structures, particularly against some stress waves common in earthquakes.

“With cloaking, there is this expectation that if you get any kind of stress wave from any kind of direction, a cloak should be able to hide the object from it. We now see that it is not possible,” said principal investigator Arash Yavari, a professor in Georgia Tech’s School of Civil and Environmental Engineering and in the George W. Woodruff School of Mechanical Engineering. “But for a large class of disturbances, namely the in-plane disturbances, you could probably design a good cloak.”

In an earthquake, in-plane disturbances are seismic waves that track along flatter and broader — or planar — paths through the surface of the Earth.

Yavari and coauthor Ashkan Golgoon, a graduate research assistant studying with Yavari, published their study the journal Archive for Rational Mechanics and Analysis. The research was funded by the Army Research Office.

Here’s what the dream of cloaking for stress waves looks like, some theoretical errors the researchers say have errantly perpetuated that dream and what to do now that the bubble has burst.

The dream cloak
Georgia Tech says that the theoretical dream of elastodynamic cloaking to steer stress waves past a structure like it isn’t even there has a lot in common with the dream of an invisibility cloak, which would bend light — electromagnetic waves — around an object then point it out the other side.