Number 753, November 9, 2005
by Phil Schewe and Ben Stein
Guided Slow Light
Guided, slow light in an ultracold medium has been demonstrated by
Mukund Vengalattore and Mara Prentiss at Harvard.
pulses in a sample of atoms had been accomplished before (see for
example PNU 521)
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
Physics News Graphics).
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-Kelvin) 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,
email@example.com), 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).
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.
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 degrees Celsius,
even on a freezing day (see Physics
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 plantís central spike-like flowering stalk through
chemical reactions in the cellsí mitochondria.
According to one of
the authors of the new study, Takanori Ito (firstname.lastname@example.org),
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 plantís 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 plantís 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
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.