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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 751  October 26, 2005  by Phillip F. Schewe, Ben Stein
                                                                
SUPERLUMINAL ULTRASOUND? The speed of light waves in vacuum, 300,000 kilometers
per second (186,000 miles per second), and denoted as c, remains the absolute
speed limit for transferring matter, energy, and usable signals (information). 
However, a wave property known as group velocity can surpass c while still
complying fully with the theory of special relativity, since it is not involved
in transferring information, matter, or energy. Superluminal group velocity has
been experimentally demonstrated in light (see Updates
495 and 536, for example).  At last week's meeting of the Acoustical Society of
America in Minneapolis, Joel Mobley of the University of Mississippi
(jmobley@olemiss.edu) argued that even the sound waves (which normally travel
about one mile per second in water) could take on superluminal properties. 
Ultrasound's group velocity, he said, could jump by five orders of magnitude over
its ordinary values and exceed c, when pulses of high-frequency sound strike a
mixture of water and tiny (approximately 0.1-mm diameter) plastic spheres.  While
Mobley has not yet demonstrated this feat experimentally, his preliminary
experiments on ultrasound in a water-sphere mixture have shown close agreement
with theory and indicate that very large group velocities are possible.  If
experimentally confirmed, superluminal group velocity in sound waves could
potentially be exploited for useful applications, such as making electronic
filters and high-frequency ultrasound oscillators.
    At this point, it is worth remembering that sound waves--like all
waves--are made of two main parts: (1) the underlying wave oscillations, in this
case pressure oscillations in a medium such as air or water, which travel at the
normal speeds of sound, and (2) the "envelope" that gives the wave its shape.  In
Mobley's setup, the envelope has the shape of a bell curve.  The speed at which
the envelope moves is called the group velocity.  One measures the group velocity
by following the envelope's peak (its maximum height, or amplitude). In a mixture
of water and beads, an ultrasound pulse experiences severe dispersion, meaning
that different frequencies in the pulse travel at very different speeds.  The
components of the wave add up so that the peak of the wave can move faster than
c.
With even greater degrees of dispersion, the peak can actually start traveling
backwards, so that a detector deeper in the mixture detects the peak earlier than
a shallower detector. This would result in a negative group velocity. None of
this violates the principles of causality, since the leading edge of the
ultrasound wave still arrives at the shallower detector first and the deeper
detector next.  It's just the peak of the envelope (which determines the group
velocity) which would move around in weird ways.
    In the late 1990s, Mobley and experimental colleagues at Washington
University in St. Louis and Mallinckrodt performed initial experimental
measurements of ultrasound waves moving through a volume of approximately 100
parts water to one part plastic spheres (Mobley et al., Journal of the Acoustical
Society of America, August
1999; and Hall et al., JASA, February 1997).    Mobley estimates
that superluminal group velocity would be achieved in a denser collection of
beads, namely a mixture of 20 parts water to one part plastic beads.  The catch?
The severe dispersion required for superluminal group velocity would so weaken
the wave that it would become very hard to detect.  Still, Mobley has shown
mathematically how such behavior can occur, in what may be considered a mixture
of 19th-century wave physics and 21st-century ultrasonics with some granular
science thrown in. (Movies and much additional explanation in Mobley's
lay-language paper: http://www.acoustics.org/press/150th/Mobley.html  )

WALKING MOLECULES.  A single molecule has been made to walk on two legs. 
Ludwig Bartels and his colleagues at the University of California at Riverside,
guided by theorist Talat Rahman of Kansas State University, created a
molecule---called 9,10-dithioanthracene (DTA)---with two "feet" configured in
such a way that only one foot at a time can rest on the substrate. Activated by
heat or the nudge of a scanning tunneling microscope tip, DTA will pull up one
foot, put down the other, and thus walk in a straight line across a flat surface.
The planted foot not only supplies support but also keeps the body of the
molecule from veering or stumbling off course.  In tests on a standard copper
surface, such as the kind used to manufacture microchips, the molecule has taken
10,000 steps without faltering.  According to Bartels (ludwig.bartels@ucr.edu,
951-827-2041), possible uses of an atomic-sized walker include guidance of
molecular motion for molecule-based information storage or even computation.  DTA
moves along a straight line as if placed onto railroad tracks without the need to
fabricate any nano-tracks; the naturally occurring copper surface is sufficient. 
The researchers now aim at developing a DTA-based molecule that can convert
thermal energy into directed motion like a molecular-sized ratchet.  (Kwon et
al., Physical Review Letters, upcoming article; text at
www.aip.org/physnews/select; see movie at
www.chem.ucr.edu/groups/bartels/)