PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 708 November 10, 2004
by Phillip F. Schewe, Ben Stein
                        
OUR SIXTH SENSE IS AS FINE TUNED AS IT CAN BE says Todd Squires, a
physicist at Caltech.  He has investigated why the natural selection
process, operating over evolutionary time, settled upon specific dimensions
for the vestibular semicircular canals (SCC), the set of three mutually
perpendicular, fluid-filled  tubes housed in the inner ear of vertebrates
that give an organism its sense of balance.  Scientists sometimes recognize
the perception of balance and motion as being a sixth sense, in addition to
the usual five---smell, touch, sight, hearing, and taste. The balance sense
organ, the SCC structures, are essentially donut-shaped, with a major
radius of 3 mm and minor radius of 0.2 mm. Furthermore, the torus is
interrupted by a membrane called a cupula impregnated with tiny sensory
hairs for sensing the sloshing of the fluid through the canals.  Sensing an
acceleration or rotation involves the fluid being momentarily left behind
while the head (and the SCCs) rotate in a new direction.  The fluid
displaces the cupula, deflecting the sensory hairs and triggering a neural
signal to the brain and muscles controlling the eye, and this is what gives
us the sense of motion, and sometimes dizziness. Squires addressed himself
to the question of why the SCC should be roughly the same size (to within a
factor of three) in mice as it is in whales.  In humans, for instance, the
SCC reaches its full adult size in about the 14th week of pregnancy.  Why
should SCCs be all of this one size, as if evolutionary pressures had
"converged" on an optimal solution?  In performing studies of
optimal design, Squires varied four different key physical parameters---SCC
major radius, minor radius, cupula thickness and height---and discovered
that the greatest canal sensitivity occurred for those parameter values
manifested in actual vertebrates.  Knowing how the canals work is important
for understanding various forms of dizziness (such as "top-shelf
vertigo," the light-headedness experienced by some when they tilt
their heads back in looking at a top shelf) and for understanding
peculiarities of some ordinary visual experiences. For example, since the
SCC output is wired into eye-control muscles, some motions can be
compensated: you can read a fixed page while swiveling your head, but with
your head fixed you can't read a page swivelled by a friend.  The SCC-eye
feedback effect also explains why some home video, recorded while the
filmer is in motion, doesn't look so good afterwards in the editing stage,
when the neuro-feedback mechanism isn't at work.  (Todd Squires, Physical
Review Letters, 5 Nov 2004; tsquires{at}acm.caltech.edu, 626-395-4640; for
further background, see Scientific American, 243, p118, 1980)

CHEMICAL "DEFECT ENGINEERING."  At next week's AVS Science &
Technology meeting in Anaheim, University of Illinois researchers (Edmund
Seebauer, eseebaue{at}uiuc.edu) will report an approach to reliably make
small-scale versions of a pn junction, the crucial region of a
semiconductor that changes from electron-rich (the "n" zone) to
electron-poor (the "p" zone).  Today, pn junctions are only 25
nanometers (100 atoms) deep.  But to make increasingly smaller (and faster)
silicon chips, the International Technology Roadmap for Semiconductors
dictates that by 2010 the pn junctions must have depths of 10 nanometers,
or just 40 atoms.   The conventional method for making the junctions is
called "ion implantation," in which charged versions of a foreign
atom ("dopant") are accelerated into a silicon wafer to create
electrically active regions that are either electron-rich or electron-poor.
 Unfortunately, current ion-implantation methods cannot make 10-nm-deep pn
junctions without inadvertently moving silicon atoms into some of the spots
intended for dopants.   But the Illinois researchers  are using surface
chemistry to come to the rescue of this conventional technology.  In
computer simulations, they showed how removing surface layers such as
silicon dioxide frees up dangling bonds. Silicon atoms then preferentially
rise to the surface while tending to leave the dopant atoms in place. 
Verified in subsequent experiments, this idea for "defect
engineering" has been shown to be a feasible solution for using
traditional ion-implantation technology to make smaller-scale silicon-based
electronic devices. (Meeting Paper EM-TuA7; see also UIUC news release at
http://www.news.uiuc.edu/news/04/0927seebauer.html and meeting lay-language
paper at
http://www2.avs.org/symposium/anaheim/pressroom/seebauer.pdf.)

100TH ANNIVERSARY OF ELECTRONICS. Researchers are marking November 16, 2004
as the 100th birthday of electronics, which began with British scientist
John Ambrose Fleming's 1904 invention of the first practical electronic
device. Known as the thermionic diode, this first simple vacuum tube,
containing only two electrodes, could be used to convert an alternating
current (ac) to a direct current (dc). A special AVS meeting session,
taking place exactly 100 years after the day that Fleming applied for a
British patent on the diode, will celebrate this seminal invention and the
subsequent evolution of electronic components based on vacuum devices.
(Contact Fred Dylla, Jefferson Lab in Virginia, dylla{at}jlab.gov, and Paul
Redhead of the National Research Council in Canada, redhead{at}magma.ca; more
information on this and other AVS meeting stories at
http://www2.avs.org/symposium/anaheim/pressroom/news.pdf)

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