OXFORD, Miss. – 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.
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