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Physics News Update
Number 693, July 22, 2004 by Phil Schewe and 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@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

Synchronized swimming in bacteria creates dramatic, previously 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@physics.arizona.edu and John Kessler, kessler@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 Physics News Graphics) 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 University of Arizona press release.)

Defending Networks Against Cascading Failure

Just as foresters can 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; motter@mpipks-dresden.mpg.de)

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