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University of Mississippi

A Sonic Zoom: UM Physicist Verifies that Sound can Travel Faster Than Light

Using sound pulses lasting less than a fraction of a second, a University of Mississippi physicist has verified that sound waves in water can exceed the speed of light.

Dr. Joel Mobley

Dr. Joel Mobley, assistant professor of physics and astronomy and research scientist at UM’s National Center for Physical Acoustics, embarked on studying the acoustics of microscopic spheres in 1994 as a graduate student at Washington University in St. Louis. In 2005, his research indicated that sound can travel at superluminal velocities when the spheres are randomly mixed in water.

Mobley’s latest research confirms that indication, and his internally funded work was published in the Sept. 21 issue of the peer-reviewed physics journal Physical Review Letters.

“This is an interesting observation in the propagation of ultrasound in water,” said the journal’s editor, Jack Sandeweiss of Yale University. “This certainly yields knowledge about dispersive systems.”

“The idea that acoustic wave groups could move faster than light is not a new idea, but no one had seen it happen until recently,” Mobley said. “This phenomenon is naturally limited in a way that makes it compatible with Einstein’s theory of relativity, which says that no information can be transmitted faster than light. Still, this work provides a novel look at the effect usually observed in experiments with light, not sound.”

In earlier work, Mobley argued that water enhanced with microscopic-size plastic beads, which are as small as the thickness of a human hair, could support ultrasonic pulses with speeds faster than light. A report on the simulations was published in the July 2007 Journal of the Acoustical Society of America.

“The research originated as an effort to standardize biomedical measurements,” Mobley said. “Some aspects of this current work could be applied to the study of new types of pharmaceuticals that employ microscopic particles in the circulatory system.”

The laboratory experiment is conducted in a water tank by firing pulses of ultrasound less than one-millionth of a second long between two transducers, one acting as a loudspeaker and the other as a microphone. A sample chamber, which resembles a floatation buoy found in swimming pools, holding approximately 40,000 plastic microspheres in less than a teaspoon of water is placed in the path of the acoustic pulses. The sample chamber is agitated manually to keep the spheres randomly mixed while the sound waves pass through.

“Many researchers might expect this effect would require an ordered medium, like a crystal that has a regular repeating pattern,” Mobley said. “So to get this to happen in a random cloud of spheres is surprising.”

Assisting Mobley was UM Sally McDonnell Barksdale Honors College student Evans Heithaus, a 20-year-old senior physics major from Hattiesburg. Scheduled to graduate in May 2008, Heithaus said the opportunity to conduct laboratory research with Mobley is invaluable to him as both an undergraduate and future medical school student.

“Dr. Mobley is an incredible professor and a great person,” Heithaus said. “He’s not only a professor, but also a colleague and great mentor. I’m thankful he has taken me under his wing.”

With proof that the spheres can conduct sound at such speeds, Mobley is hopeful his research receives future funding: “There are some general features common to microwave transmission and ultrasound propagation, so that discoveries made in our ultrasound lab could potentially be applied to microwave systems, which are used for cell phones, wireless internet, satellite TV just about anything you can think of outside of traditional TV and radio,” Mobley said.

The experiments were carried out at NCPA, where Mobley is a part of the Ultrasonics Research and Engineering Group: “The NCPA was established to conduct cutting-edge research in the field of acoustics, and Dr. Mobley’s work is the latest in a line of such discoveries,” said Hank Bass, NCPA director.

Mobley joined the UM faculty in 2005, after working at both the U.S. Army Research Laboratory in Maryland and the Oak Ridge National Laboratory in Tennessee.

For more information about UM’s National Center for Physical Acoustics, visit