Text 650, 87 rader
Skriven 2005-11-11 08:47:45 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 753
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 753   November 9, 2005  by Phillip F. Schewe, Ben Stein

GUIDED, SLOW LIGHT IN AN ULTRACOLD MEDIUM has been demonstrated by Mukund
Vengalattore and Mara Prentiss at Harvard.  Slowing light pulses in a sample of
atoms had been accomplished before (see for example
http://www.aip.org/pnu/2001/split/521-1.html) by sending light pulses into a
highly dispersive medium, that is, a medium in which the index of refraction
varies greatly with frequency.
    Previously this dispersive quality had come about by tailoring the internal
states of the atoms in the medium.  In the present Harvard experiment, by
contrast, the dispersive qualities come about by tailoring the external
qualities of the atoms, namely their motion inside an elongated magnetic trap
(see figure at www.aip.org/png/2005/238.htm).  In the lab setup two pump laser
beams can be aimed at the atoms in the trap; depending on the frequency and
direction of the pump light, the atomic cloud (at a temperature of about 10
micro-K) can be made more or less dispersive in a process called recoil-induced
resonance, or RIR.  If now a separate probe laser beam is sent along the atom
trap central axis, it can be slowed by varying degrees by adjusting the pump
laser beam.  Furthermore, the probe beam can be amplified (the intensity of the
light can be increased by a factor of up to 50) or attenuated depending on the
degree of dispersiveness in the atoms.  This process can be used as a switch
for light or as a waveguide.
    According to Mukund (now working at UC Berkeley,
mukundv@calmail.berkeley.edu), slowing light with the recoil induced resonance
approach may be a great thing for nonlinear-optics research.  Normally
nonlinear effects come into play only when the light intensities are quite
high.  But in the RIR approach, nonlinear effects arise more from the strong
interaction of the two laser beams (pump and probe) and the fact that the slow
light spends more time in the nonlinear medium (the trap full of atoms).  All
of these effects are enhanced when the atoms are very cold.  Moreover, because
the slow light remains tightly focused over the length of the waveguide region,
intensity remains high; it might be possible to study slowed single-photon
light pulses, which could enhance the chances of making an all-optical
transistor.  The light in this setup has been slowed to speeds as low as 1500
m/sec but much slower speeds are expected when the atoms are chilled further.
(Vengalattore and Prentiss, Physical Review Letters, upcoming article;
MIT-Harvard Center for Ultracold Atoms at atomsun.harvard.edu)

ZEN AND THE ART OF TEMPERATURE MAINTENANCE.  Scientists at the Iwate University
in Japan have shown that the skunk cabbage---a species of arum lily and whose
Japanese name, Zazen-sou, means Zen meditation plant---can maintain its own
internal temperature at about 20 C, even on a freezing day (picture at
www.aip.org/png/2005/239.htm).
    The plant occurs in East Asia and northeastern North America, where its
English name comes from its bad smell and from the fact that its leaves are
like those of cabbage.  Unlike the case of mammals, which maintain their body
temperature by constant metabolism in cells all over the body, heat in the
skunk cabbage is produced chiefly in the spadix, the plant's central spike-like
flowering stalk through chemical reactions in the cells' mitochondria. 
According to one of the authors of the new study, Takanori Ito
(taka1@iwate-u.ac.jp), only one other plant species, the Asian sacred lotus, is
homeothermic, that is, able to maintain its own body temperature at a certain
level.  Most other plants do not produce heat in this way because they seem to
lack the thermogenic genes (the technical name for which, in abbreviated form,
is SfUCPb).  Moreover, the researchers, studying subtle oscillations in the
plant's internal temperature, claim that the thermo-regulation process is
chaotic and that this represents the first evidence for deterministic chaos
among the higher plants.  The resultant trajectory in the abstract phase space
(where, typically, one plots the plant's temperature at one time versus the
temperature at another time) is a strange attractor, which the authors refer to
as a Zazen attractor, a "Zen meditation" attractor.  (Physical Review E,
November 2005)

DROWNING IN QUICKSAND IS IMPOSSIBLE, according to a new study, relegating this
popular plot device in adventure stories to the category of pure folklore. 
Consisting of a mixture of sand, salt water, and clay, quicksand captured the
attention of University of Amsterdam physicist Daniel Bonn when he went on a
family trip to Iran, the birthplace of his wife.  Collecting a sample of
quicksand near a body of water in Iran, and bringing it to his laboratory for
study, Bonn and his colleagues showed that shaking aluminum beads, designed to
have the same density as human beings, would partially, but never fully,
submerge them.  Since quicksand is twice as dense as water, the beads (and
humans) only sink about halfway.  Shaking or otherwise disturbing the quicksand
liquefies it, increasing the downward flow of the beads by a factor of a
million. This is how humans can get stuck in it.  Since quicksand is often
located near bodies of water, Bonn speculates that high tidal floods passing
over individuals stuck in quicksand may have caused casualties incorrectly
ascribed to sinking fully in it.  Bonn says his conclusions apply to all kinds
of quicksand. Nonetheless, the force required to lift a foot out of quicksand
can be equal to that required to raise a car. His solution: wiggling the stuck
foot will cause water to trickle down, allowing the hapless adventurer to get
out of it. (Khaldoun et al., Nature, September 29, 2005)

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