Number 170, March 28, 1994 by Phillip F. Schewe and Ben Stein
CONTROLLING CHAOS through the use of small perturbations has been possible
in a number of systems, such as erratically vibrating metal strips, certain
electrical circuits, and mixing in chemical reactions. At the APS Meeting
last week in Pittsburgh, Rajarshi Roy of Georgia Tech reported that by
using a subtle feedback mechanism he has been able to synchronize two separate
chaotic lasers, a development which might be applicable to schemes for
encrypting data. Understanding and controlling fibrillating heart tissue
may also be possible with chaos-control methods. Mark Spano of the Naval
Surface Warfare Center in Whiteoak, Maryland (also speaking at the meeting)
has determined that human atrial fibrillation, the massively irregular
beating of the atrial chambers of the heart, is chaotic in nature. He also
has preliminary evidence that the more life-threatening ventricular fibrillation
(erratic behavior in the ventricle chambers) is also chaotic. In studies
with rabbit heart tissue, he has been able to control arrhythmia through
the application of electrical stimuli. He reported similar work done with
rat brain tissue in an effort to control (apparently chaotic) electrical
patterns characteristic of epileptic behavior.
BUILDING WHOLE INSTRUMENTS ON A CHIP with integrated-circuit technology
is a major goal in the field of microelectromechanical systems (MEMS).
At a session devoted to this subject at the APS meeting, Susanne Arney
of AT&T Bell Labs described, for example, efforts to make tiny tunneling
probe microscopes with the same lithographic, etching, undercutting (etc.)
steps used in micro-circuit fabrication. Jason Yao of Rockwell described
micro-resonators consisting of micron-sized arms of silicon which, once
excited by voltage pulses, oscillate consistently at MHz frequencies. The
purity of the tone of this "tuning fork" is such that the resonator
might serve (particularly if encased in a tiny evacuated shell) as an internal
clock for computers. At higher frequencies (100 MHz) the resonator could
serve as a generator of radio waves.
HIGH-SPEED STM can reveal the incessant motion of atoms across a silicon
surface. Not yet as fast as a movie and looking more like a jerky time-lapse
study of commuters on the go, picture sequences recorded by scanning tunneling
microscopes at a rate of several frames per second for area views, or up
to 1000 per second for single rows of atoms, show how atoms touch down
and lift off of terraced surfaces under the influence of thermal agitation.
At the meeting, Ellen Williams of the University of Maryland referred to
these images as "a stunning visualization of the ideas of statistical
mechanics."
SOFT GAMMA RAY REPEATERS (SGR's) are celestial sources of gamma bursts.
Only three are known to exist in our galaxy. Unlike the larger sample of
gamma bursts---more than a thousand discovered in recent years by the Gamma
Ray Observatory---bursts from SGR's are repeated and are at lower ("softer")
gamma energies. The first SGR was discovered 15 years ago. Its apparent
association with a supernova remnant was doubted by some, but now a counterpart
of the second SGR, an object called SGR1806, has been observed at x-ray
(with the ASCA spacecraft) and radio (with the VLA telescope) wavelengths.
The gamma-ray and x-ray observations were made while the object was in
the act of emitting bursts. The astronomers involved believe that SGR's
are indeed neutron stars. (Nature, 10 Mar.)
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