Number 602, August 30, 2002
by Phil Schewe, James Riordon, and Ben Stein
A DNA Laser
A DNA laser has been demonstrated by scientists at the Chitose Institute
of Science and Technology in Japan who have expanded on the idea of
dye lasers, in which light-emitting dye molecules (of which there are
many, offering a welcome versatility in tuning the output laser wavelength)
are embedded in a surrounding matrix material. Putting too many dye
molecules close together, however, can lead to the quenching of the
fluorescence. In DNA this problem is greatly decreased since the dye
molecules can be lodged (intercalated) in the double-strand scaffold
of the DNA structure. It is too early to talk of such optimal properties
as emittance or energy efficiency, but the researchers believe that
because they have achieved thin-film lasing with such a high dye concentration
then the engineering of compact, tunable lasers will follow. (Kawabe
et al., Applied Physics Letters, 19 August; contact
Yutaka Kawabe, y-kawabe@photon.chitose.ac.jp; also see figure.)
Distinguishing Between Pointlike and "Extended"
Dark Matter
Distinguishing between pointlike and "extended" dark matter
will be possible in new experiments. Detecting the elusive dark matter
that seems to pervade the universe is difficult enough, but it's an
even bigger challenge to distinguish between the different kinds of
the invisible matter.
Candidates for dark matter include pointlike particles such as neutralinos
(predicted by supersymmetry theory) and extended objects such as "Q-balls,"
large bags of particles such as squarks and sleptons which are also
predicted in supersymmetry. Numerous planned and ongoing experiments
aim to detect directly dark matter, which is believed to possess energies
in the minuscule range of 0.001-0.01 MeV.
To catch such novel forms of matter researchers place their detector
deep underground, to shield their equipment from cosmic ray particles
such as muons and electrons. A dark matter detector typically contains
some transparent medium, such as a clear crystal or liquid made of specific
atoms or molecules. A dark matter particle usually passes easily through
ordinary matter, but occasionally it collides with an atomic or molecular
nucleus in the medium. As a result, the nucleus recoils, radiating a
flash of light with a specific energy that can provide information on
the identity of the dark matter.
Pointlike and extended dark matter would strike their target in different
ways, according to researchers (Alexander Kusenko, UCLA, 310-825-4814,
kusenko@ucla.edu). Hitting a car with a hammer, they point out, produces
different results from hitting a car with a pillow, even though the
two may carry the same amount of energy. The pointlike (hammer-like)
matter transfers its momentum instantaneously, while the extended (pillow-like)
matter transfers its momentum more slowly. Such a difference can be
detected in a plot of the number of collision events versus momentum.
Extended dark matter would produce a greater amount of "softer" collisions
in the lower-momentum range.
Current dark matter experiments are good enough in certain cases to
tell the difference between the two types of dark matter, though a welcome
improvement would be to detect even lower-momentum collisions than presently
possible. (Gelmini
et al., Physical Review Letters, 2 Sept 2002)
A New Kind of Ocean Wave Has Been Discovered
A new kind of ocean wave has been discovered by geophysicists in the
US and Mexico (Rhett Butler, IRIS Consortium, rhett@iris.edu and Cinna
Lomnitz, UNAM, cinna@prodigy.net.mx).
At the Hawaii-2 Observatory, an unmanned research laboratory sitting
on the seafloor between Hawaii and California, ocean waves of many varieties
are observed.
Some are acoustic waves, underwater cousins of sound waves in the air,
and consisting of pressure waves that alternately expand and compress
water as they propagate through the ocean at the speed of sound in water.
Others are Rayleigh waves, seismic waves that propagates near the surface
of the earth. Triggered by earthquakes, Rayleigh waves propagate as
horizontal and vertical motions in the sediments and underlying crust.
Researchers have now detected a new kind of wave created by seismic
events, for example, a 6.2-magnitude earthquake 10 km below the Pacific
Ocean in June 2000.
The newly discovered wave, the researchers have concluded, is a "coupled"
acoustic and Rayleigh wave that swaps energy above and below the seafloor.
Propagating at the sound velocity of water, the wave both induces horizontal
and vertical motions in the seafloor sediments and creates regions of
expansion and compression in the water. This coupled wave, the researchers
found, carries more energy than conventional deep-Earth waves observed
at the Hawaii-2 Observatory. (Butler and Lomnitz, Geophysical
Research Letters, 24 May 2002.)
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