Text 354, 87 rader
Skriven 2005-02-26 09:50:00 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 721
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 721 February 24, 2005
by Phillip F. Schewe, Ben Stein
        
THE BIGGEST SPLASH OF LIGHT FROM OUTSIDE THE SOLAR SYSTEM to be recorded here
at Earth occurred on December 27, 2004.  The light came from an object called
SGR 1806-20, about 50,000 light years away in our own galaxy.  SGR stands for
"soft gamma repeater," a class of neutron star possessing a gigantic magnetic
field.  Such "magnetars" can erupt violently, sending out immense bolts of
energy in the form of light at gamma rays and other wavelength regions of the
electromagnetic spectrum.  The eruption was first seen with orbiting telescopes
at the upper end of the spectrum over a period of minutes and then by more and
more telescopes; at radio wavelengths emissions were monitored for months.  For
an instant the flare was brighter than the full moon. (NASA press conference,
18 February; www.nrao.edu/pr/2005/sgrburst/;
www.ras.org.uk/html/press/pn0505ras.html; many telescopes participated in the
observations and results will appear in a forthcoming issue of Nature.)

FRACTAL JAMMING OF NANOTUBES.  Carbon nanotubes, those tiny hollow carbon
whiskers nanometers wide but microns or longer in length, have intriguing
optical, electrical, thermal, and mechanical properties.  Perhaps the earliest
big practical use for nanotubes will be as an additive in many composite
materials, both liquid and
solid.   NIST physicist Erik Hobbie gauges nanotube flow properties
by suspending them in a liquid polymer solvent between two parallel plates and
then subjecting the fluid to shear force by moving one of the plates.  In
general getting the long nanotubes lined up is like herding cats; they get
tangled very easily.  But at low concentration and high enough shear, the tubes
do line up, as if the mixture were a "nematic" liquid crystal, a liquid in
which rod-shaped polymer molecules are aligned with each other.  Lower the
amount of shear or raise the nanotube concentration and the tangles begin. 
Increase the concentration further and the tangling gets more elaborate; the
nanotubes form bands (visible to the human eye) parallel to the plates and
perpendicular to the flow direction.  At even higher concentrations (around 3%)
the aggregation becomes so great that fluid flow comes to a halt.  In this
tangled state the web of interconnections between nanotubes takes on a
fractal-like geometry.  Knowing this geometry well will be of use in numerous
upcoming industrial processes
involving carbon nanotubes.  Hobbie reported his results at last week's meeting
of the Society of Rheology in Lubbock, Texas.  (Paper MF9,
www.rheology.org/sor/annual_meeting/2005Feb/default.htm )
                                                                               

"OPTICAL VORTICES" MIGHT EXTRACT ABUNDANT INFORMATION FROM MATTER, providing a
new and potentially wide-ranging optical tool, a Spain-US team has proposed
theoretically.  An ordinary light beam, when viewed head-on, looks like a
bright circle. But a special light beam called an "optical vortex," when viewed
head-on, looks like a bright ring surrounding a dark central core (see
www.aip.org/png/2001/133.htm).  Optical vortices are the simplest kind of beam
carrying a property called "orbital angular momentum" (see Update 639).
Extensively studied since the early 1990s, such light beams, when viewed from
the side, trace out a three-dimensional corkscrew pattern (see figure at
www.aip.org/png/2005/229.htm); the pattern represents regions of constant phase
(for example, regions of maximum electric field). This spiraling of light
represents an extra "degree of freedom" that researchers can use as a new
handle to optically encode information and subsequently to retrieve information
from objects the beam strikes.
  In conventional laser beams, the energy flows parallel to the beam axis, like
water in a jet.  However, for light with orbital angular momentum (OAM), the
energy spirals around the beam axis. Ordinary beams carry only "spin angular
momentum," encoded in the polarization of light.  All possible spin states can
be constructed with just two polarization states (vertical and horizontal, or
clockwise and counterclockwise).  For light with nonzero OAM, however, many
states are possible, with higher states denoting tighter corkscrews (and
consequently, a faster spiraling of energy; see figure at
www.aip.org/png/2005/229.htm). For this reason, one can encode a huge amount of
information in an OAM beam by creating light made of a superposition of many
OAM states. The researchers call the different OAM components "spiral spectra."
In the "digital spiral imaging" concept now put forward by Lluis Torner at the
new Institute for Photonic Sciences (ICFO) in Barcelona and his colleagues, a
light beam of a convenient shape illuminates a sample to be probed.  The sample
scatters the beam and alters its spiral components. Breaking down the altered
beam into its individual orbital-angular momentum components (and thereby
analyzing the "spiral spectrum" of the scattered beam) can yield a wealth of
information from the object. The spiral spectra would, for example, be
sensitive to nonuniformities in geometrical and structural properties of
objects, and could be potentially useful for detecting biological and chemical
agents, for probing biological specimens sensitive to OAM light, and might even
aide recent proposals to increase the amount of data that can be imprinted on a
compact disk using OAM. (Torner, Torres, Carrasco, Optics Express, Feb. 7,
2005; text at www.aip.org/physnews/select ; contact Lluis Torner,
http://www.icfo.es ; for more background on OAM light, see Physics Today, May
2004, and New Scientist, 12 June 2004).

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