Departments
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Week of 29 October 1999 |
Vol. III, No. 12 |
Feature
Article
Zero-gravity physics
ENG student mulls weighty matters in airborne adventure
By Hope Green
Charles Thomas is intrigued by the science of sound. Appropriately enough, he also has an ear for music. While earning a bachelor's degree in physics at the University of Rochester he studied French horn, and now he plays in BU's All-University Orchestra.
But it was his inner ear that concerned the College of Engineering graduate student -- not to mention his stomach -- when he assisted with an acoustics experiment 30,000 feet over the Gulf of Mexico.
In late August, Thomas (ENG'01) and Glynn Holt, an ENG assistant professor of aeromechanical engineering and a former astronaut, ran a portable physics laboratory aboard the Weightless Wonder, a NASA KC-135 airplane originally designed to prepare astronauts to work in zero-gravity conditions.
Pilots fly the four-engine turbojet in parabolic waves to create periods of weightlessness for people and objects on board. In rollercoaster fashion, the affectionately nicknamed "Vomit Comet" repeatedly dips in a curving free fall toward the ocean and then noses upward while accelerating.
"There were no windows on the plane," says Thomas during an interview at ENG's physical acoustics lab, where symphonies on CD enliven long hours of research. "I was kind of glad there weren't."
Thomas and Holt flew a series of two-hour missions for four straight days. They were carrying precious cargo: a three-inch Plexiglas cube filled with water. Fastened onto the cube was a sound-producing device, and the assemblage sat inside a light-tight cell wired to an array of monitoring instruments. The purpose of the experiment was to test a theory regarding the role of gravity in sonoluminescence -- the phenomenon in which short pulses of light are emitted by bubbles in a liquid excited by sound.
For the better part of a year, Thomas had puttered over the equipment with Sean Wyatt (ENG'99) in the Cummington Street laboratory. Wyatt, who based his master's thesis on the experiment and wrote most of the requisite computer programs, worked on the ground with Thomas and Holt at NASA's Johnson Space Center in Houston. (This fall, Wyatt took an engineering position with the Ford Motor Company in Detroit.)
Holt had undergone zero-gravity training as an astronaut and could tell Thomas what to expect while in flight. At each crest of the plane's undulating pathway, the force of gravity on board disappears for 25 seconds, and Thomas describes the giddy sensation of floating through the aircraft and losing all notion of floor and ceiling. He was living a lesson in neuroscience: when weightless, fluid inside the balance-sensitive inner ear stops activating nerve endings that inform the brain which end is up. "It was like hanging upside-down in a jungle gym," he says.
But as the plane comes out of a dive, passengers and payloads double in weight, and during those intervals Thomas could feel the blood slowing in his veins. After a few parabolas, adjusting to the relentless swings in gravity became nauseating.
"I had a pretty bad first day," Thomas admits, "but by the fourth day I was fine."
Much of the time Thomas was strapped in a seat, his eyes glued to a computer monitor, while Holt adjusted the controls on a video camera recording activity inside the cell.
BU is one of only a few institutions conducting zero-gravity research on sonoluminescence. Holt explains the curious phenomenon this way: when any bubble in a liquid implodes, gas molecules at the bubble's point of collapse suddenly pack more closely together. But in sonoluminescence, which can only be produced under certain laboratory conditions, sound waves make the gas molecules clash so forcefully that a flash of light is produced.
In 1990, a researcher at the University of Mississippi found a method to intensify the process by applying a sound wave to one bubble at a time. The nature of the light emission intrigues physicists because it suggests that an extremely high-energy process is achieved during that fraction of a second when the bubble caves in.
Scientists believe that in the future, sonoluminescence might be employed in thermonuclear fusion reactions. But for the moment they are simply trying to understand its laws and mechanics.
The BU experiment will need some fine-tuning, and Thomas is surprisingly eager to reboard the Weightless Wonder next year. A classmate or two might join him.
"NASA has funded us for four years to do as much as we can on this project, and I hope that will involve more than a few KC-135 flights," Holt says. "It's a level of exposure to some very practical problems that most graduate students don't get if they're stuck in a laboratory on the ground."