From: LARRY KLAES (ljk4_at_msn.com)
Date: Thu Jul 22 2004 - 11:16:02 PDT
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From: physnews_at_aip.org<mailto:physnews_at_aip.org>
To: ljk4_at_MSN.COM<mailto:ljk4_at_MSN.COM>
Sent: Thursday, July 22, 2004 1:58 PM
Subject: Physics News Update 693
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 693 July 22, 2004 by Phillip F. Schewe, Ben Stein
OPTICAL HALL EFFECT. Physicists in Japan have theoretically shown
that an optical equivalent of the Hall effect exists, and that this
hypothesis could be borne out with experiments with polarized light.
In the classic Hall effect, an electric current, pulled along a
conductor by an electric field, will be deflected sideways somewhat
if in addition a magnetic field (perpendicular to the electric field
and to the plane of the conductor) is applied. One can attribute to
the sideways motion a "Hall voltage" and a "Hall resistance." If the
experimental conditions are even more stringent---extremely cold
temperatures and high magnetic field---a quantum equivalent of the
Hall effect manifests itself. In this case the electrons execute
trajectories that are quantized; that is, the Hall resistance can
take only certain discrete values. Something like this might be
happening when a light ray moves from one medium into another. The
amount of the shift sideways at the deflection will depend on the
change in the index of refraction from the one medium into the
other. Masaru Onoda (m.onoda_at_aist.go.jp<mailto:m.onoda_at_aist.go.jp>, 81-29-861-2985) at the
National Institute of Advanced Industrial Science and Technology
(Tsukuba, Japan) and his colleagues at the University of Tokyo
believe that the topological aspects of light refraction in
materials can be explored in upcoming
experiments using photonic crystals. In effect, they are predicting
a correction to Snell's law for spin-polarized light. (Onoda et
al., Physical Review Letters, upcoming article)
SYNCHRONIZED SWIMMING IN BACTERIA creates dramatic, previously
DEFENDING NETWORKS AGAINST CASCADING FAILURE. Just as foresters can
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: Thu Jul 22 2004 - 11:28:30 PDT
unknown fluid patterns, researchers have discovered. With the
Summer Olympics a few weeks away, physicists are showcasing some
remarkable water action in aerobic bacteria, those that require
oxygen to survive. Bacteria swim through fluids by quickly rotating
corkscrew-shaped "flagella," hair-like appendages that can be up to
five times greater than the length of their main body (generally a
few microns in size). It's not a routine feat for a bacterium to
stay above water: a typical organism is about 10 percent denser than
H2O, so gravity tends to sink the creatures. Nonetheless, aerobic
bacteria often swim up to the oxygen-rich surface in order to find
and consume the million O2 molecules per second that they need to
survive. Conventional wisdom has been that such swimming does little
to stir up the fluid itself. Now, studying concentrated populations
of the common aerobic bacterium Bacillus subtilis in small,
half-inch-diameter fluid drops, a group of physicists at the
University of Arizona (Raymond Goldstein, gold_at_physics.arizona.edu<mailto:gold_at_physics.arizona.edu>
and John Kessler, kessler_at_physics.arizona.edu<mailto:kessler_at_physics.arizona.edu>) has found that the
combination of upward swimming and downward sinking in the
suspension can produce striking flows that strongly mix the fluid
(see pictures at www.aip.org/png<http://www.aip.org/png>) and concentrate the bacteria. The
crowd of swimming bacteria creates arrays of circulating vortices
whose size is orders of magnitude larger than an individual
bacterium. Jets and surges of fluid that straddle the vortices can
move 100 microns per second and be as large as 100 microns. These
speeds and lengths greatly exceed the swimming speeds
and sizes of the organisms themselves, which move only tens of
microns per second. The new results provide, possibly for the first
time, information on the way in which concentrated swimming bacteria
order themselves. Such accumulations can have many important
consequences. For example, they may greatly aid in the formation of
biofilms, and can even be micromixers in tiny quantities of fluid.
In addition, the way the fluid currents concentrate bacteria into
small spaces may be crucial for triggering the phenomenon of "quorum
sensing," whereby congregated bacteria sense sufficiently high
amounts of each other's secreted chemicals to turn on specific
capabilities, such as the emission of light in bioluminescent
bacteria. Quorum sensing is found in many important bacteria,
including those that create gum disease. (Dombrowski et al.,
Physical Review Letters, upcoming article; also see Univ. Arizona
press release at http://uanews.org/cgi
bin/WebObjects/UANews.woa/3/wa/SRStoryDetails?ArticleID=8615.)
often halt a forest fire from burning out of control by deliberately
setting firebreaks, it might be possible to reduce the size or
spread of outages in a network in the wake of an attack or
overload. The Internet and the electrical grid are just two such
networks that might benefit from a new model devised by Adilson
Motter of the Max Planck Institute for the Physics of Complex
Systems in Dresden. Several previous network models have shown how
an attack on key nodes of a system can cascade into a catastrophic
failure. Motter's model shows how such a failure can be mitigated
by shutting down selected peripheral nodes that handle only small
amounts of the network's total load. Simulating attacks on networks
showed that answering the original attack with several successive
rounds of precautionary node shut-down drastically reduced the size
of the overall cascade. (Physical Review Letters, upcoming article;
motter_at_mpipks-dresden.mpg.de<mailto:motter_at_mpipks-dresden.mpg.de>)
PHYSICS NEWS UPDATE is a digest of physics news items arising
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