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{at}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{at}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/)

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