Text 574, 86 rader
Skriven 2005-08-06 09:40:23 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 740
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 740   August 5, 2005  by Phillip F. Schewe, Ben Stein

TRACKING FLUID FLOW INSIDE A POROUS MATERIAL can now be performed with remote
MRI viewing.  MRI is an important means for sub-surface viewing of soft objects
like biological tissue or moist in solid things like rice grains.  In a new
approach, scientists at Lawrence Berkeley National Laboratory and UC Berkeley
in collaboration with Schlumberger-Doll Research have developed a style of MRI
that can be used to see how a gas flows through a porous rock, an experimental
tool with possible applications in oil exploration, in situ monitoring of
natural and manmade structures, and industrial processes where the flow of a
fluid through an opaque material is important. To accomplish this, Josef
Granwehr (joga@waugh.cchem.berkeley.edu) and Yi-Qiao Song (ysong@SLB.com) and
their colleagues use not one radio coil but two, separated in space.  In MRI it
is customary to cause atomic nuclei in a sample (given an orientation by an
external magnet) to be disturbed by magnetic waves induced by the coil.  The
same coil is used a moment later to detect the radio waves given back out by
the target nuclei, thus providing information about their whereabouts.  In the
Berkeley setup, one coil surrounds the porous sample and can, in combination
with magnetic field gradients, selectively disturb nuclei of the fluid in a
voxel (a tiny volume element) anywhere in the sample, while a second
independent coil, positioned at the exit of the sample, can detect the emerging
material. The first coil is therefore used to tag certain nuclei at a given
point in time, while the second coil is used to record the time of flight of
the affected nuclei as they leave the sample. Possessing location and velocity
of any portion of the gas allows researchers, in effect, to look inside the
rock and watch its flowing and unfolding.  One can trade off the minimum
detectable partial pressure of the target nuclei (tens of millibar up to one
bar) for time resolution (tens of microseconds to
milliseconds) or vice versa. (Granwehr et al., Physical Review Letters,
upcoming article)

A NEW KIND OF NANOPHOTONIC WAVEGUIDE has been created at MIT, overcoming 
several long-standing design obstacles.  The resultant device might lead to
single-photon, broadband and more compact optical transistors, switches,
memories, and time-delay devices needed for optical computing and
telecommunications.
If photonics is to keep up with electronics in the effort to produce smaller,
faster, less-power-hungry circuitry, then photon manipulation will have to be
carried out over scales of space, time, and energy hundreds or thousands of
times smaller than is possible now.  One or two of these parameters (space,
time, energy) at a time have been reduced, but until now it has been hard to
achieve all three simultaneously.  John Joannopoulos and his MIT colleagues
have succeeded in the following way.  To process a photonic signal, they
encrypt it into light waves supported on the interface between a metal 
substrate and a layer of insulating material. These waves, called surface
plasmons, can have a propagation wavelength much smaller than the free-space
optical wavelength. This achieves one of the desired reductions: with a shorter
wavelength the spatial dimension of the device can be smaller.  Furthermore, a
subwavelength plasmon is also a very slow electromagnetic wave. Such a
slower-moving wave spends more ti
me "feeling" the nonlinear properties of the device materials, and is therefore
typified by a lower device-operational-energy scale, thus achieving another of
the desired reductions. Finally, by stacking up several insulator layers, the
slow plasmon waves occupy a surprisingly large frequency bandwidth.  Since the
superposition of waves at a variety of frequencies can add up to a pulse that
is very short in the time domain, the third of the desired scale reductions is
thereby achieved. Reducing energy loss is another great virtue of the MIT
device.  The plasmons are guided around on the photonic chip by corrugations on
the nano-scale.  In plasmonic devices the corrugations have usually been in the
metal layer; this has always led to intractable propagation losses.  However,
in the MIT device they reside in the insulator layer; this, it turns out,
allows for a drastic reduction of the losses by cooling.    (Karalis et al.,
Physical Review Letters, 5 August; contact Aristeidis Karalis,
aristos@mit.edu)        
                        
POSSIBLE NEW PLANETS in our solar system have been spotted recently. 
Reservations about claiming new planets arises not from anything to do with the
observations, but with semantics; there is no universally accepted definition
for planet.  Even Pluto is not a planet according to some scientists.  The two
newest planet candidates are the latest residents to be discerned in the Kuiper
Belt, the zone of debris material beyond the orbit of Neptune.  Two earlier
specimens go by the name of Sedna
(www.aip.org/pnu/2004/split/677-1.html) and Quaoar
(www.aip.org/pnu/2002/split/608-3.html).  One of the new objects, discovered by
astronomers at the Sierra Nevada Observatory in Spain, is called EL61, with an
orbital radius of about 51 AU (1 AU, or astronomical unit, is equal to the
Earth-sun distance) and a size about 2/3 that of Pluto (which itself orbits at
a distance of about
32 AU).  The other object is called UB313 and was spotted by astronomers at the
Palomar observatory in California and the Gemini telescope in Hawaii.  It
orbits at a distance of 97 AU and has an estimated size larger than Pluto).

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