Text 577, 55 rader
Skriven 2005-08-19 12:36:53 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 742
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 742   August 19, 2005
by Phillip F. Schewe, Ben Stein, and Davide Castelvecchi        

ROOM TEMPERATURE ICE is possible if the water molecules you're freezing are
submitted to a high enough electric field.  Some physicists had predicted that
water could be coaxed into freezing at fields around 10^9 V/m.  The fields are
thought to trigger the formation of ordered hydrogen bonding needed for
crystallization.
Now, for the first time, such freezing has been observed, in the lab of Heon
Kang at Seoul National University in Korea, at room temperature and at a much
lower field than was expected, only 10^6 V/m. Exploring a new freezing
mechanism should lead to additional insights about ice formation in various
natural settings, Kang believes (surfion@snu.ac.kr).  The field-assisted 
room-temperature freezing took place in cramped quarters: the water molecules
were constrained to the essentially 2-dimensional enclosure between two
surfaces: a gold substrate and the gold tip of a scanning tunneling microscope
(STM).  Nevertheless, the experimental conditions in this case, modest electric
field and narrow spatial gap, might occur in nature.  Fields of the size of
10^6 V/m are, for example, are thought to exist in thunderclouds, in some tiny
rock crevices, and in certain nanometer electrical devices.   (Choi et al.,
Physical
Review Letters, 19 August 2005; for another example of seemingly
room-temperature ice, see http://www.aip.org/pnu/1995/split/pnu225-1.htm )
        
NETWORKING CAN BE CRITICAL, LITERALLY.  The theory of "small-world"
networks yields insight into innumerable real-world situations, from the
Internet to the power grid, from epidemics to opinion making. A small-world
network is one where certain nodes, called hubs, have an unusually large number
of connections, so that going through hubs one can reach any other node in just
a few steps.  In real-life small-world networks, researchers have observed
"critical" thresholds -- for example, epidemics that spread uncontrollably or
spontaneously die out, depending on thresholds in the disease's degree of
infectivity or in the number of social contacts individuals have. But network
theory has so far been poor at modeling critical thresholds.  Now, Joseph
Indekeu of Katholieke Universiteit Leuven in Belgium
(joseph.indekeu@fys.kuleuven.be) and his colleagues have shown that small-world
networks can model critical thresholds if one tunes the hubs to be less
influential on their neighbors than the ordinary nodes. For example, a friend's
opinion could be more influential in shaping your voting preferences than the
opinion of a prominent TV commentator, whose wide audience makes him a hub in
the network.  The tuning idea, the paper shows, is mathematically equivalent to
cutting off most of a hub's
connections.   The authors also say their results could shed light on, and
perhaps help prevent, phenomena such as electrical blackouts and epidemics. 
The new model even suggests a  parallel between networks and general relativity
since trading in the interactions between nodes for changes in the network's
structure is reminiscent of the gravitational interactions between
bodies---gravitational attraction---which can be mimicked by changes in the
structure of spacetime---that is, the curvature created by the presence of
mass.  (Giuraniuc et al., Physical Review Letters, upcoming article)

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 * Origin: Big Bang (1:106/2000.7)