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An ‘invisibility cloak’ for tsunamis?

A team of French and British physicists has devised an ‘invisibility cloak’ that could, in theory, hide susceptible platforms or coastlines from ocean waves such as tsunamis.

A tsunami is headed right for a vulnerable shallow-water gas platform. The next minute, the first wave passes by harmlessly as if the structure had completely disappeared. Impossible? Perhaps not, according to a team of French and British physicists that has devised an ‘invisibility cloak’ that could, in theory, hide susceptible platforms or coastlines from ocean waves such as tsunamis.

The solution published last month in the journal Physical Review Letters is earning high marks from some experts for its creativity. However, others are skeptical as to whether the small-scale lab experiment could ever be worked up to ward off a full-scale disaster — such as the Indian Ocean tsunami in that killed more than 200,000 on Dec. 26, 2004.

Sebastian Guenneau, a study co-author, said the experiment’s main focus was to use liquid waves as stand-ins for microwaves to visualize the movement of linear waves that radiate out from a source in a fairly stable pattern — and how they might be disrupted. Initially, Guenneau and physicists from the Centre National de la Recherche Scientifique and Aix-Marseille Universite in France did not consider the potential application for redirecting tsunami waves. But Guenneau said their experimental results suggest an anti-tsunami cloak is well within the realm of possibility.

The cloaking concept is based on positioning multiple rows of pillars at specific intervals within a cylindrical pattern, so that the pillars and intervening spaces resemble a round checkerboard from above. In the lab, a small aluminum cylinder was subdivided into 50 precisely spaced rows of pillars radiating out from a flat center. The columns essentially dissipate oncoming waves so that anything behind the structure is hidden from them.

Maybe even from tsunami waves.

“It’s some kind of crazy idea,” conceded Guenneau, also a lecturer in applied mathematics at the University of Liverpool.

The cloak might work for a smallish island or a structure, such as a coastal nuclear facility, by surrounding it with a semicircular checkerboard pattern of columns, though surrounding much larger land masses would be implausible. More practically, he said, the concept might be used to protect offshore oil and gas platforms.

“You can coat the legs of the platform with this kind of cloak; this will make the legs of the platform, in some sense, invisible to ocean waves,” Guenneau said. “This is an application that is not too crazy.”

Research aimed at bending acoustic, optical and other waves away from specific objects in order to shield them has taken off within the past few years.

Vladimir Shalaev, a professor of electrical and computer engineering at Purdue University, wrote in a review published earlier this month in the journal Science that a new research field known as transformation optics is applying mathematical principles like those in Einstein’s theory of general relativity to produce electromagnetic cloaks that bend light — meaning actual invisibility cloaks may not be far off.

Hiding from waves in plain sight
Whether anti-tsunami cloaks will follow suit remains debatable. Nevertheless, the experiment used several straightforward physics concepts that Guenneau said could be demonstrated even in a high school lab. For their wave generator, the experimenters blew air into a small cube positioned just above the waterline, creating acoustic disturbances that propagated out in concentric rings of small surface waves.

“The acoustic signal is myopic,” Guenneau said. Translation: it doesn’t know that the cloak has multiple rows of pillars and space, like black and white cells in a checkerboard pattern. “It can only see that you have some uniform gray area.”

The concept of adding black and white and ending up with gray, he said, is known in physics as homogenization: “replacing the individual cells with something that behaves the same way, on average.”

To ‘fool’ the acoustic signal into seeing nothing but gray, the researchers cut out their checkerboard-patterned structure from an aluminum cylinder measuring 10 centimeters in diameter (about three inches). Guenneau said any non-porous material, whether rock, wood or cement, could work just as well. The only thing that matters is the periodicity of the pillars, he said.

Water can still flow through the cylinder, unlike the solid wall of a dike designed to stop all water. “You don’t break the wave, you let the wave flow through into your dike,” Guenneau said. “Instead of reflecting the wave, you want to guide the wave to the left and right so the middle zone is protected.”

Incoming waves, Guenneau said, may have many possible points of entry, but the way forward becomes smaller and smaller as they move toward the center of the checkerboard cloak. Consequently, waves lose most of their oomph and much of the water is diverted around the cloak instead, as if it was a solid, imperturbable mass of gray.

In their small-scale laboratory experiment, the physicists found that water was too viscous and unable to flow freely through the cylindrical cloak due to the same phenomenon that sucks up the edges of water in a full drinking glass. As a stand-in, they used a much less viscous liquid known as methoxy-nonafluorobutane, originally formulated as a more environmentally friendly replacement for ozone layer-depleting industrial solvents.

Dan Cox, director of the Hinsdale Wave Research Lab at Oregon State University, expressed doubt over the utility of a scaled-up experiment.

“I don’t see how this work could ever be practically implemented to reduce the threat of tsunamis,” he said in an e-mail. “If you read the article carefully, the authors state ‘we were unable to produce similar results with water.’ ”

But that limitation only exists at the small-scale level, Guenneau argues. For larger-scale applications, he said, the problematic viscosity issue with water is no longer be a constraint.

‘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?