Text 238, 83 rader
Skriven 2004-11-13 04:01:47 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 708
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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@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@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@jlab.gov, and Paul Redhead of the National Research
Council in Canada, redhead@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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 * Origin: Big Bang (1:106/2000.7)