In 2001, US physicists predicted that solitary waves, bundles of energy that travel without dispersing, could break into smaller, "baby", or secondary, solitary waves. At the time, experts suggested that the research was "great work" but could never be confirmed experimentally. Now, the team has proved the "critics" wrong.

"A central theme behind the physics of any system is how its particles share and transmit energy," explains Surajit Sen of the State University of New York at Buffalo, "This work goes to the heart of nonlinear systems because it provides insights into how such systems propagate energy." Solitary waves are, by definition, energy bundles, that propagate intact and do not transform. However, Sen, Adam Sokolow and his colleagues used computer simulations to show that solitary waves could, under certain conditions, break apart, forming secondary or "baby" solitary waves. The size of the predicted baby waves was much less than one percent of the energy carried by the entire solitary wave; which was way below experimental detection limits of the time. "When we published our papers, I too believed these phenomena would be undetectable," admits Sen.

Nevertheless, experimental techniques have advanced apace since the publication of his original computer simulations. Now, colleague Francisco Melo of the University of Santiago, Chile, Sen and their co-workers have produced baby waves that were as large as 15-20% of the energy propagated through the entire system. "My assumption was that when two solitary waves in a granular system collide head-on, the physics would be similar to bouncing a solitary wave off an infinitely hard wall," Sen explained. The breakthrough came when Melo realised that colliding solitary waves travelling through a chain of twenty identical stainless beads against walls made of soft materials might amplify the effect.

Melo and colleague Stéphane Job (now at the Superior Institute of Mechanics in Paris) embedded non-intrusive force sensors into one bead and the reflecting wall. The beads were bounced against the wall and the sensor recorded the amount of force with which the last bead hit the soft wall. "The idea is that by introducing a large mismatch of mechanical properties at the wall, the reflected solitary wave needs to adapt more dramatically, thus producing such large baby solitary waves," explained Melo. This set-up resulted in sufficiently amplifying the effect Sen had predicted to provide the first experimental evidence of the original predictions. "This work proves that these solitary waves can be made to 'leak,' in a sense, producing these secondary or baby waves," says Sen.

The new results may have significant implications in the understanding of how energy propagates through strongly nonlinear systems, where almost every detail of the system is relevant, and that comprise many disparate physical systems from weather systems to earthquakes. The findings could completely overturn the generally accepted idea that equilibrium states - or at least a similar type of state - cannot easily occur in nonlinear systems.

An unexpected spin-off of the research undertaken by colleague Francisco Melo, a leading experimental physicist from the University of Santiago, in Chile, is that the team was able to determine accurately and non-invasively the Young's modulus of a material, i.e. its resistance to deformation, simply by bouncing a solitary wave against the surface of the material. Sen told us that, "Typically, measuring Y for small samples is a challenge." Obtaining the Young's modulus of a material at some point is critical in designing and testing almost all engineering structures. A quick and easy approach could be used more effectively and correlated more readily with spectroscopic and analytical data on engineering materials.

Sen told Spectral Lines that he is "Very excited!" about the results. He points out that the earlier "critics" said that the baby waves, if they existed, would be too small to detect experimentally. "Basically, their fear was that the signatures in sensors would be 'below' ambient noise levels," he said, "Our prediction was such that the size was to be exceedingly small anyway...Francisco Melo is a true genius as he was quick to figure out how to magnify these babies and he was right!"