PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institut

PHYSICS NEWS UPDATE
A digest of physics news items by Phillip F. Schewe, American Institute of
Physics
Number 172  April 7, 1994

THE COLD DARK MATTER (cdm) theory attempts to account for the observed
rotation of spiral galaxies and interactions between galaxies by invoking
the presence of considerable amounts of nonluminous, slowly-moving
particles.  Some experiments have sought to observe cdm particles (lurking
in a halo around our galaxy) either directly in detectors or indirectly by
measuring the presumed gravitational microlensing effect of such dark matter
(at least that part of it consisting of baryons) on the starlight coming to
us from the Large Magellanic Cloud, the nearby satellite galaxy.  Evalyn
Gates and Michael Turner of the University of Chicago work with a model in
which the dark matter near the Milky Way resides in a roughly spherical
halo.  They calculate that the cdm halo has a density of at least 10**-25 g
cm**-3.  They also assert that current microlensing experiments cannot
sensitively probe the local cdm density.  (Upcoming article: Physical Review
Letters, 18 April 1994.)

POROUS SILICON, unlike bulk silicon, can emit light, a fact that may lead to
new opto-electronic devices. Studies over the past few years have determined
that the luminescence arises not from any surface residue left behind by the
silicon electrochemical etching process but rather from some quantum effect
relating to the small size of the silicon structures responsible for light
emission.  A new experiment at AT&T Bell Labs (Louis Brus, 908-582-3119)
shows that these structures are not wire-like, as was previously thought,
but more particle-like with a typical size of only 2 or 3 nm.  (Upcoming
article: S. Schuppler et al., Physical Review Letters, 18 April.)

THE SURVEY OF GALAXY REDSHIFTS published by Harvard-Smithsonian astronomers
(Margaret Geller, 617-495-7409) a decade ago, a survey which revealed that
the universe is filled with immense sheets and clumps of galaxies, as well
as regions nearly devoid of galaxies, has now been extended to cover parts
of the southern celestial hemisphere.  The new survey, supplementing
northern observations at the Whipple Observatory in Arizona with southern-
hemisphere observations in Chile, Argentina, and South Africa, shows that
southern skies are also clumpy on a scale of 100 million light years.
(Astrophysical Journal Letters, 20 Mar. 1994).

FEMTOSECOND LASERS keep getting better. In just the past three years typical
laser power has increased by a factor of ten and pulse size has shrunk by a
factor of three.  Practical devices can now generate 10-20 fsec pulses.
Light chopped up in parcels this small has a variety of uses such as for
observing chemical reactions or melting in real time, or for studying
photosynthesis.  Henry Kapteyn and Margaret Murnane of Washington State
University believe that the pulse-size limit for titanium-doped sapphire
lasers, commonly used in this research, will be 3 fsec, which constitutes
about 1 cycle of light at a wavelength of 800 nm. To produce shorter pulses,
they say, will require the use of shorter-wavelength light such as x rays.
(Optics and Photonics News, March 1994.)

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