Text 332, 73 rader
Skriven 2005-02-04 14:03:05 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 718
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 718 February 2, 2005
by Phillip F. Schewe, Ben Stein

COMPLEX HYBRID STRUCTURES, part vortex ring and part soliton, have been
observed in a Bose-Einstein condensate (BEC) at the Harvard lab of Lene
Vestergaard Hau.  Hau previously pioneered the technique of slowing and then
stopping a light pulse in a BEC consisting of a few million atoms chilled into
a cigar shape about 100 microns long.  In the new experiment, for the first
time, two such light pulses are sent into the BEC and stopped.  The entry of
these pulses into the BEC set in motion tornado-like vortices.  These swirls
are further modulated by solitons, waves which can propagate in the condensate
without losing their shape.  The resultant envelope can act to isolate a tiny
island of superfluid BEC from the rest of the sample.  The dynamic behavior of
the structures can be imaged with a CCD camera by shining a laser beam at the
sample (see figure at www.aip.org/png ).  Never seen before, these bizarre BEC
excitations sometimes open up like an umbrella.  Two of the excitations can
collide and form a spherical shell (the vortex rings taking up the position of
constant latitudes).  Two such rings, circulating in opposite directions, will
co-exist for a while, but after some period of pushing and pulling, they can
annihilate each other as if they had been a particle-antiparticle pair.  Hau
(hau@physics.harvard.edu, 617-496-5967) and her colleagues, graduate student
Naomi Ginsberg (ginsber@fas.harvard.edu) and theorist Joachim Brand (at the Max
Planck Institute for the Physics of Complex Systems, Dresden), have devised a
theory to explain the strange BEC excitations and believe their new work will
help physicists gain new insights into the superfluid phenomenon and into the
breakdown of superconductivity. (Ginsberg, Brand, Hau, Physical Review Letters,
4 February; lab website http://www.deas.harvard.edu/haulab/mainframe.htm )

ROD-SHAPED NUCLEI, even slablike nuclei, might occur amid the cataclysm of a
supernova.  This is when nuclear matter---normally hard, spherical, and dense
(3 x 10^14 g/cm^3)---can thin out, to an average density only half that of
normal nuclear matter.  The nuclear "rods" would still be densely packed in the
star (like a liquid crystal) and the rods might coalesce into slabs, says
Gentaro Watanabe, temporarily at the NORDITA lab in Denmark.  He and his
colleagues at the Japan Atomic Energy Research Institute, the University of
Tokyo, the RIKEN lab, and Keio University, have modeled alternative nuclear
shapes in an effort to address the subtle problems in simulating supernovae. 
One of these problems is that shock waves stall in the stellar core.  The
Japanese researchers expect that incorporating effects of "pasta" phases (the
collective name for rod or slab nuclei) in core collapse simulations would help
them to model the explosion more realistically.  The "pasta" phases would be
formed in the central region of the collapsing core, while the region where the
shock waves propagate and stall is much further out.  Neutrinos from central
region contribute "neutrino heating" and would help the shock waves to revive. 
This scenario is more tenable if the pasta phases are present, and not just
uniform nuclear matter.  (Watanabe et al., Physical Review Letters, 28 January
2005; contact, gentaro#nordita.dk )
                                
CONTROLLING BRAIN WAVES.  A new study conducted at George Mason University
confirms predictions that electrical fields can be used to modify waves
traveling through brain tissue.  This is perhaps the first example of electric
modification of neuronal thresholds to control wave movement. Indeed, it is one
of the first times waves have been controlled in an excitable medium through
changing thresholds.  The researchers begin with a section of rat brain; the
tissue consists of 6 layers of 2-dimensional sheets of neurons.  A neural wave
is initiated at one end of the network and the signal is observed at the other
end.  By using electrical fields, the excitability of individual neurons can be
modified.  Doing this can slow down, speed up, or stop any wave propagating
through the sample.  Previously neural waves had only been modified by
pharmacological means.  This action can be negated only by washing out the drug
used, which takes seconds, whereas the electric method takes only microseconds
to have an effect.  One potential application for modifying brain waves would
be in mitigating epileptic seizures. (Richardson et al., Physical Review
Letters, 21 January 2005; lab website, www.neuraldynamics.org; contact Bruce
Gluckman, bgluckma@gmu.edu, 703-993-4384 or Steven Schiff, sschiff@gmu.edu) 
Part of the George Mason contingent also was involved in the recent discovery
of true spiral waves in the sensory cortex of the brain (Huang et al J Neurosci
24: 9897-9902, 2004).

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