From: LARRY KLAES (ljk4_at_msn.com)
Date: Fri Apr 30 2004 - 04:26:18 PDT
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From: physnews_at_aip.org<mailto:physnews_at_aip.org>
To: ljk4_at_MSN.COM<mailto:ljk4_at_MSN.COM>
Sent: Thursday, April 29, 2004 1:47 PM
Subject: Physics News Update 683
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 683 April 29, 2004 by Phillip F. Schewe, Ben Stein
ILLUMINATING THE DARK AGES. In the very early universe the so called
MAGNESIUM-DIBORIDE SUPERCONDUCTORS can tolerate twice the usual
WHAT KIND OF FLUID IS QUARK-GLUON PLASMA (QGP)? The hot soup of
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: Fri Apr 30 2004 - 04:41:08 PDT
"dark age" comes after the time of the first atoms---a moment when
suddenly neutral atoms, mostly hydrogen, could form, allowing
photons to stream freely, photons we now see as the microwave
background---but before the first stars formed. But maybe this era
needn't be so dark. Just as numerous finds of arts and crafts from
the European dark ages have helped to enlighten us on what the sixth
to the eleventh centuries were like, so too some bits of light from
the cosmic dark ages might illuminate that epoch. Abraham Loeb and
Matias Zaldarriaga of Harvard believe that the early, cold, neutral
hydrogen can be made to speak, as it were. These atoms, in a
redshift window of about 30 to 100, would be colder than the
background radiation. The atoms would absorb photons and cause a
deficit in the microwave background at cold hydrogen's
characteristic wavelength of 21 centimeters. This absorption
wavelength, in turn, would be stretched out, courtesy of the
universal expansion of the universe, to a wavelength of 6-21 meters
or so. Because the cosmic hydrogen is not uniform, the level of
absorption varies across the sky and the microwave background would
show anisotropies at these long wavelengths. These anisotropies
could be sought using special radio interferometers. (Some efforts
are already underway to see this kind of light: see
http://www.lofar.org/
telescopes mapping the early sky see minute temperature variations,
so the primordial hydrogen could also be mapped. This map might
well show the influence of dark matter through its influence in
shepherding early hydrogen. Interest in this hydrogen has been
expressed before, but the Harvard proposal is the first to be
specific about how to search for information imprinted in the
dark-age atom distribution. (Physical Review Letters, upcoming
article; text at www.aip.org/physnews/select<http://www.aip.org/physnews/select>; contact Abraham Loeb,
aloeb_at_cfa.harvard.edu<mailto:aloeb_at_cfa.harvard.edu> )
amount of magnetic field if you spike them with some carbon atoms.
The main reason superconducting wires are used as the windings in
magnets is not
because they save energy, but because they can generate large
magnetic fields by carrying large current densities without the
resistive heating associated with ordinary copper wire, giving you a
much more intense field for the same amount of volume employed in
your MRI machine. MgB2 superconductors, which made their debut
three years ago (see
http://www.aip.org/enews/physnews/2001/split/530-2.html
superconducting at around 40 K, in a colder regime than for the
ceramic superconductors (which can be bathed in liquid nitrogen),
but much warmer than traditional metal superconductors (such as
niobium-tin) which must be cooled in liquid helium. Some consider
that the MgB2 materials (which can be chilled with refrigerators
without the use of expensive liquid helium) might be advantageous in
some applications where NbSn is presently used. For this to happen,
the MgB2 materials need to be able to stand up to high fields and
high current densities. At Iowa State, a new test of carbon-doped
MgB2 shows that the critical field can now be doubled, up to a value
of 32.5 Tesla; this is the field at which superconductivity in
unadulterated MgB2 would be undone. This is now higher than the
best value for NbSn. The researchers (contact Paul Canfield,
canfield_at_ameslab.gov<mailto:canfield_at_ameslab.gov>, 515-294-6270) would like MgB2 to tolerate even
higher fields, and to enhance the critical current too. (Wilke et
al., Physical Review Letters, upcoming article; text at
www.aip.org/physnews/select<http://www.aip.org/physnews/select> )
free quarks and gluons that existed in the very early universe, and
a state of matter that physicists have been trying to re-create amid
high-energy nuclear collisions, QGP is actually not a superfluid, as
Update 681 erroneously suggested. According to University of
Washington physicist Laurence Yaffe (206-543-3902,
lgy_at_phys.washington.edu<mailto:lgy_at_phys.washington.edu>), QGP is actually a normal, conducting
fluid. It has viscosity, eliminating it from the list of
superfluids. It is somewhat electrically resistive, precluding it
from being a superconductor. Yaffe and coworkers recently performed
calculations of several QGP fluid properties from first principles
(P. Arnold, G. D. Moore and L. G. Yaffe, Journal of High Energy
Physics, 17 June 2003 and 14 February 2003). Still, observations of
high-density quark matter produced thus far at Brookhaven's RHIC
accelerator suggest that QGP might prove to be the most ideal
regular fluid observed in nature, according to Ohio State nuclear
theorist Ulrich Heinz (614-688-5363, heinz_at_mps.ohio-state.edu<mailto:heinz_at_mps.ohio-state.edu>). The
viscosity of the RHIC matter appears to be exceedingly low, and it
redistributes its heat ("rethermalizes") extremely quickly. This
near-ideal regular fluid behavior should greatly facilitate
comparisons between theory and experimental observations of QGP,
once its presence is confirmed at RHIC.
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