An ‘invisibility cloak’ for tsunamis?

‘Invisible’ ears and vanishing subs
Costas Synolakis, director of the Tsunami Research Center at the University of Southern California in Los Angeles, hailed the creativity of Guenneau and his colleagues. But Synolakis likewise wondered whether their experiment would eventually prove useful for protecting either offshore platforms or coastal installations. “From the point of view of sheer brilliance I would give it an A,” he said. “But from the point of applications I would give it a low C.”

He likened the team’s aluminum structure to a strainer that creates interference between waves — with the peak of one canceling out the trough of another — and thus dissipating the wave energy while still allowing the water through. “In this case, they make it perfectly,” he said.

The general notion, he said, is akin to making someone invisible by scattering light waves (not yet possible despite growing research efforts), or by canceling out sound by disrupting acoustic waves (a feat demonstrated in the 1980s).

“It is very similar in some ways to what people call ‘anti-sound’ or the sound isolation headphones like the ones from BOSE,” Synolakis said. Because sound-isolating headphones cancel the incoming noise, he noted, “they make your ear ‘invisible’ to the noise coming in.”

The newly demonstrated concept, he said, likewise works well for waves that move in a well-defined, linear manner. The physical realities of tsunamis, however, could interfere with translating the concept into an effective full-scale cloak.

Off-shore platforms, for example, are already essentially immune to tsunamis because most are positioned at depths of 30 meters (roughly 100 yards) or more, where the effects of tsunami waves are negligible.

Closer to shore, he said, a protective checkerboard pattern of pillars could be breached by non-linear effects caused by interactions with the seabed, like the shoaling and breaking of waves.

“It’s quite clear that the application for tsunamis is very, very far-stretched,” he said. Even so, Synolakis said he is impressed by the study’s underlying calculations and pointed out that science has to start somewhere. “This is how science works — this is a provocative idea. The basis of it is not bunk.”

And who knows? Perhaps the idea will prove useful for other applications that haven’t yet been considered, he said.

Guenneau already has one in mind. The physicist said he’s begun working on a three-dimensional cloak that could, in theory, hide objects from waves propagating underwater.

“I’m interested in effectively making something invisible underwater, and for this I need to have a three-dimensional cloak,” he said. The same checkerboard concept would apply, but Guenneau would add another dimension and use cubes instead of squares.

Invisibility cloaks for submarines? If it works, he said, why not?

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