Text 133, 81 rader
Skriven 2004-08-19 12:02:06 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 697
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News Number 697 August
19, 2004  by Phillip F. Schewe, Ben Stein

NEWLY CREATED ANTIHYDROGEN ATOMS have been caught speeding for the first time. 
Owing to the vast preponderance of ordinary matter over antimatter in the
visible universe, and the propensity of any antimatter around to annihilate
hastily with any conventional particulate matter in the vicinity, the only
place anti-atoms exist on Earth for more than a microsecond is in a chambered
vault at the CERN Antiproton Decelerator (AD) lab in Geneva.  There,
antiprotons created artificially in high-energy proton collisions and
anti-electrons (positrons) from a radioactive source are cooled and brought
together in a bratwurst-sized vessel filled with electrodes at various
voltages.  By careful husbandry (first of all, the antiprotons have to be
slowed by a factor of 10 billion, from an energy of 5 MeV to .3 meV)
anti-hydrogen (or H-bar) atoms are made from antiprotons and positrons. 
Although the anti-h's haven't yet been definitely fixed in space or produced in
their lowest quantum state (which is what you need to do laser  spectroscopy),
there are still other studies that can be made on these very rare atoms as they
mill about. (For some previous CERN anti-H results see
www.aip.org/pnu/2002/split/605-1.html and
www.aip.org/pnu/2002/split/611-1.html.)  One thing that can be done is to
measure the speeds of the anti-atoms by seeing how many of them emerge from a
region of oscillating electric fields without being ionized.  The ATRAP
collaboration, one of the CERN H-bar groups, has done exactly this.  They have
determined that the anti-atoms are moving with an average energy of 200 meV,
which corresponds to a velocity only about 20 times that of the thermal speed
of an equivalent sample of atoms kept at a temperature of 4.2 K.  This is still
too warm for the purpose of holding the anti-atoms in a trap, but the
researchers suspect that their current crop of anti-atoms contains some with
much lower velocities and that there will be a way to cull an ever colder
allotment in the future now that there is a speedometer for antihydrogen atoms.
 (Gabrielse et al., Physical Review Letters, 13 August;
gabrielse@physics.harvard.edu, 33-450-28-38-95)
        
WHY ARE SEACOASTS FRACTAL? In a famous paper written decades ago, Benoit
Mandelbrot asked how long the coastline of Britain really was. The answer
depends on what kind of meter stick you use. The closer one looks at any scale
of a rocky coast map, from well above the 100 kilometer level to the kilometer
level, and so on to the meter level, the more indented and lengthy the
"coastline" becomes. Not only that, but the coast's underlying geometry seems
be fractal, meaning that it is extremely fractured and also self-similar: the
shape looks, in a statistical sense, the same at all levels of magnification.
Now, scientists in France have inquired into the physical processes that
actually could carve out a fractal coast. Their simulation of a rocky coast
evolution depends on an iteration of erosion action. First, waves are allowed
to erode the weak points in a smooth shoreline. This makes the shore
irregularly indented and longer. This erosion exposes new weak points, but at
the same time mitigates the force of the sea by increasing the wave damping.
These steps are then repeated over and over. The resultant coast is fractal,
with an effective dimension of 4/3. According to Bernard Sapoval and A.
Baldassarri of the Ecole Polytechnique (Palaiseau, France) and their colleague
A. Gabrielli of the "Enrico Fermi" Center (Rome), this new study provides the
first suggestion of how a fractal shoreline comes about. (Sapoval et al.,
Physical Review Letters, 27 August 2004 bernard.sapoval@polytechnique.fr,
33-169334172)

NANOTUBE DYNAMOS.  Two scientists in India have produced a tiny voltage in a
small electrical circuit by blowing gas across a mat of carbon nanotubes and
doped semiconductors. This result arises from two physical effects.  First, in
the Bernoulli effect, gas rushing past a surface produces pressure differences
along streamlines, which in turn can produce a temperature gradient along a
material sample.  Second, in the Seebeck effect, a temperature gradient (the
far ends of the material being at different temperatures) can generate a
voltage difference across the sample.  In the experiment of Professor Ajay.K.
Sood and his graduate student Shankar Ghosh at the Indian Institute of Science
(Bangalore) gas is blown over a mat of carbon nanotubes as well as doped
silicon and germanium.  With a small sliver of germanium as a sample,  a
voltage difference of 650 micro-volts was generated.  The power flow amounted
to 43 nano-watts.  This doesn't sound like much power, and the researchers have
not yet determined whether the effect could be scaled up (a no-moving-parts
carbon nanotube/doped-semiconductor generator of electricity), but one definite
near-term application would be in a new type of gas flow velocity sensor for
research in problems of turbulence or aerodynamics.   Compressed air was used
to produce the tiny amount of electricity, but even human breath blown at the
inclined sample produced a measurable result of several micro-volts.  (Physical
Review Letters, 20 August 2004; asood@physics.iisc.ernet.in,
shankar@physics.iisc.ernet.in)

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