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| Science Forum Index » Physics Forum » PHYSICS NEWS UPDATE -- Number 718 February 2, 2005 by Phill |
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| Sam Wormley |
 Posted: Wed Feb 02, 2005 3:52 pm |
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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
http://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, http://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).
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week. |
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