Science > Physics > PHYSICS NEWS UPDATE -- Number 751 October 26, 2005 by Phillip F.Schewe, Ben Stein
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PHYSICS NEWS UPDATE -- Number 751 October 26, 2005 by Phillip F.Schewe, Ben Stein |
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/)
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
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