Scientific Method / Science & Exploration

Peregrine soliton may explain ocean’s rogue waves

By combining a series of small light packets, a group of scientists have …

Researchers have finally observed a special type of wave that has eluded experiments for almost 25 years. The Peregrine soliton, a special type of large wave that can retain its size and shape while traveling at a constant speed, has finally been demonstrated using light pulses traveling through fiber optics. Studies of the Peregrine soliton could help us model the rogue waves that can cause sudden disasters in the ocean, and give definite limits for a large class of solutions to the non-linear Schrodinger equation.

In waves and optics parlance, a soliton is a single wave that retains its shape while traveling at a constant speed for significant distances. This type of wave can only happen in certain media, like water, where movement is unrestricted. For example, as a water wave moves, it tends to break and curl forward. But sometimes its forward motion is sufficient that the wave will continually catch itself and can't break, resulting in a soliton.

A Peregrine soliton is a special type of soliton that is very large and isolated compared to its surroundings. Researchers have long thought of the Peregrine soliton as, among other things, a model for rogue waves in the ocean, huge towers of water that come seemingly out of nowhere (though often during storms) and knock over things like cruise ships.

While rogue waves haven't been witnessed too often, they are suspected to be the cause of several freak accidents on the ocean. The sinking of the MS München with all hands during a storm in 1978 is most often attributed to a rogue wave. Another rogue wave 100 feet high hit the Aleutian Ballad during an episode of Deadliest Catch, and a fictionalized rogue wave capsized an ocean liner in the movie Poseidon.

How an enormous wave could arise in often chaotic media like light and water confounded scientists for some time. They worked out the theory that the large wave must be formed as a combination of smaller waves, but it had never been experimentally demonstrated.

To make an artificial Peregrine soliton happen, researchers took a nonlinear fiber optic channel and sent through light waves called "breathers." Breathers are nonlinear waves that have concentrated energy and are either localized in space and oscillate in time, or vice versa.

By timing the size and spacing of the breathers just right, researchers were able to get them to combine into a large, solitary wave—a Peregrine soliton. The scientists also found that waves that were more localized in space and time came together into a Peregrine soliton more easily. This may be the reason that rogue waves are relatively rare and seem to happen more often during storms.

Now that they have proved a Peregrine soliton can be created in the lab, the authors hope that meteorologists will be able to use this information to search for and forecast oceanic rogue waves. As a nice side benefit for mathematicians, many implications of the Peregrine soliton extend to nonlinear math in general. The nature of its formation and dynamics should place limits on a set of solutions to the nonlinear Schrodinger equation.

Nature Physics, 2010. DOI: 10.1038/NPHYS1740  (About DOIs).

Listing image by NOAA

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