Emory Report

November 15, 1999

Volume 52, No. 12

Family tries to decipher how nature thinks On Sunday, Sept. 12, the National Hurricane Center began issuing warnings to the citizens of Florida. Favorable low winds had begun gathering heat and energy from warm Caribbean waters, directing moisture and air into the heart of a growing storm. As this swirling moisture reached the storm's core, it began spiraling up into the atmosphere where it met light upper-level winds. These are the conditions of a hurricane, and this one--Floyd--was a monster, almost as large as Florida itself. With hurricane Andrew fresh in their minds, Floridians could only wait in fear as Floyd continued expanding. The next day the governor declared a state of emergency, and the state began the largest peacetime evacuation in American history. By the following morning, President Bill Clinton issued a preemptive disaster declaration for Florida. Two days later, Floyd slammed in the eastern seaboard hundreds of miles north, in Cape Fear, N.C.; Florida was only grazed. "The atmosphere is a highly complex system," explained Fereydoon Family, Dobbs Professor of Condensed Matter Physics. "Even though air molecules obey physical laws, their collective behavior is extremely complex. And physicists, until recently, didn't know how to deal with such complex systems." Family does not study hurricanes specifically, but he is fascinated with the turbulence that causes hurricanes. Air turbulence interests him because-like traffic flow, biological growth and snow-flake formation-it is far from a state of equilibrium. Consequently, it exhibits complex, nonlinear behavior. Nonlinear phenomena do not follow conventional physical laws. Push a ball under normal conditions, and it will move. Push it harder, and it will move farther. Apply a brake to the ball while it is moving, and it will slow. But in nonlinear systems, pushing the ball hard may cause it to slow down, while a slight push may send it zipping off to the right. Likewise, applying a brake may actually cause the ball to spin in place. The effect of force on nonlinear phenomena is unpredictable and often chaotic. But why? Since the birth of modern physics in Renaissance Europe, scientists believed that discovering basic laws of physics would lead to an understanding of all natural phenomena, but this has proven not to be true. Beginning in the late 1980s, physicists realized that many phenomena, especially those involving a large number of particles, exhibit collective behavior that is highly complex and unpredictable. Family studies a process related to this in the electronics industry. Molecular beam epitaxy (MBE) is a technique for making thin films used in a variety of electronics devices, such as the lasers in CD players. Because MBE is a very expensive procedure, it has been given another name: mega-bucks epitaxy. "Epitaxy means perfect order," Family said. "Thin films grown by MBE are absolutely the most 'perfect' materials that exist." The MBE process begins with a smooth crystal surface, onto which are dropped atoms that create the film. As the atoms fall onto the surface, they roll around and spread, coming to the edge of the film sheet and dropping down to a lower level of the crystal. The process occurs multiple times, and the film eventually builds up layered sheets, each a single atom in thickness. But this is just a dream. "Experiments have shown that this is not what is happening," Family said. "The deposited materials give undulations to the surface that we call mounds, little hills. There was a big question as to why the mounds develop." This is not surprising, since these mounds generally make the film defective. Supported by the National Science Foundation and the Office of Naval Research, Family began looking at this problem by making computer simulations of epitaxial thin film formation. The computer simulations found that atoms coming to the edge of a growing film are often repulsed. Energetically, the atoms are unable to drop down a step, and the small hills begin to form. Thanks to Family's research, engineers can now begin searching for solutions to this problem. However, physicists like Family have much loftier goals than correcting problems with electronics or figuring out how to predict the path of a hurricane. Currently, there exists no single unifying theory for understanding complex systems. If one is discovered, this will not only change physics, it will literally alter our comprehension of the universe. Family stated the goal of physicists with a mixture of grandiosity and guarded optimism: "We're trying to understand nature's algorithms, how nature works. If you think of nature as a computer, the question is, 'What is the program nature is using to carry out its calculations?' By computer simulation, I try to mimic what nature does. Basically, I'm trying to understand how nature thinks." -Paul Thacker Return to November 15, 1999 contents page