From: LARRY KLAES (ljk4@msn.com)
Date: Sat Mar 02 2002 - 10:57:02 PST
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Sent: Saturday, March 02, 2002 1:10 AM
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 578 February 27, 2002 by Phillip F. Schewe, Ben Stein,
and James Riordon
FRACTAL CARBON NANOPORE NETWORK. Activated
carbon, porous materials not unlike the charcoal used for
barbecuing, performs important industrial functions such as
filtering air, removing toxic vapors, and purifying our food and
beverages (e.g., sugar, molasses, vodka). For that reason, a far-
flung collaboration of scientists (the Universities of Missouri and
New Mexico, the CNRS lab in France, the Universidad de Alicante
in Spain, the Air Force Research Lab, and Los Alamos) set out to
learn more about the internal structure of the material. To their
surprise they discovered a fractal network of uniform channels,
what is perhaps the first documented pore fractal.
The researchers (contact Peter Pfeifer, pfeiferp@missouri.edu,
573-882-2335) take simple olive pits, "char" them (burn them into
charcoal), and then treat them in steam at 750 C. How ironic that
in this case water, normally used to put out fire, here sustains
combustion by providing oxygen to burn with surface carbon.
What happens is not the removal of layer after layer or the carving
of holes of various sizes but instead the local etching and collapse
of pore walls to form channels of uniform size, about 2 nm wide.
This oxidation process will then abruptly branch in a new
direction. When it's all over the solid is riddled with a maze
governed by a fractal geometry. Scattering x rays from the
material establishes a "fractal dimension" of nearly 3, meaning that
surface of the internal pore network practically fills all the inside
space.
The fractal nature of solid shapes has been measured many
times, but this might be the first time a fractal mapping has been
performed for the empty space inside a void, namely the nanopore
network. (For comparison of pore, surface, and solid fractals, see
the figure at www.aip.org/mgr/png.) The surface area of this great
inland realm works out to about 1000 square meters (or one
football field) per gram. The researchers expect that methane and
other fuels could be stored in this kind of structure (the molecules
are readily taken up into the branching alleyways by the weak
attraction of induced electric dipole "van der Waals" forces), and at
pressures much less than the 200 atm needed to store methane in
steel cylinders. Gas separation can also be accomplished because
the narrow channels are negotiated more easily by some molecular
species than others. Electricity storage might be accomplished by
building capacitors enhanced by intermediate layers of activated
carbon networks filled with an ionic conducting fluid. (Pfeifer et
al., Physical Review Letters, 18 March 2002)
USING CLOCKS IN SPACE TO SEARCH FOR NEW
PHYSICS. Einstein's theory of relativity holds several things
sacred. One is the idea that if you rotate a particle or object, or
boost it up to a high velocity, the laws of physics affecting the
object should stay the same. This is called Lorentz invariance.
But in some "extensions" of the standard model of particle physics,
interactions of particles with certain hypothetical universal fields
(very roughly analogous to the way in which Higgs bosons are
supposed to make some particles massive) might lead to subtle
violations of Lorentz invariance. In a new paper Alan Kostelecky
of Indiana University and his colleagues show how this can
happen, and how such a violation could be detected in clock-
comparison experiments now being readied for the International
Space Station (ISS).
In general an atomic clock works by shooting microwaves into a
sample of cooled cesium atoms and reading out the microwave-
absorption frequency which corresponds to a specific quantum
transition for electrons in the cesium atoms. The microwave
frequency setting is used to define the "second." If one can cool
the atoms to lower temperatures (thus reducing the blurring caused
by their movement) or observe them for longer periods, the
precision of the whole readout process (and the standardization of
the second) would improve. The world's best clock, NIST F-1,
currently has an uncertainty of one part in 10^15. It achieves this
by chilling Cs atoms in a trap and then gently boosting them
upwards. Where they reach the top of their trajectory (subject
always to the attraction of gravity) and are at their slowest is where
they are subjected to the microwave bath. A related apparatus
mounted on the ISS could gain in precision because the atoms
would never fall (at least not relative to the atom trap setup) and
could be sampled for longer periods. The goal is to have several
such "space clocks" in orbit within a few years (see, for example,
www.boulder.nist.gov/timefreq/cesium/parcs.htm).
According to Kostelecky (kostelec@indiana.edu, 812-855-1485)
certain Lorentz-violation effects, expected to show up as a tiny
shifting of an atom's energy level, would be more readily
accessible in space thanks to the speeds, rotation rates, and clock
orientations available on space platforms (see animations at
physics.indiana.edu/~kostelec/mov.html). With sensitivities in
space comparable to those in Earth-based experiments, the
expected tests of Lorentz-violating effects would be measured with
uncertainties at the level of parts in 10^27. (Bluhm et al., Physical
Review Letters, 4 March 2002)
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