Number 599, July 24, 2002
by Phil Schewe, James Riordon, and Ben Stein
An Ultra Low-Density Liquid
An ultra low-density liquid, some 1013 times thinner than
water, might form inside Bose-Einstein condensates under the action
of the "Efimov effect," a quantum phenomenon in which the
atoms in the cloud attract each other when considered two at a time
but repel each other when considered three at a time. In such an Efimov
cloud the atoms would be some 20 times farther apart that in a BEC,
which is itself pretty sparse---a million times thinner than air. And
yet this new type of condensate would not be a gas but a liquid!
According to Aurel Bulgac of the University of Washington (bulgac@phys.washington.edu,
206-685-2988), the exquisite coordination of atoms in an Efimov condensation
would allow it to be self-bound (the constraining magnetic fields used
to keep a BEC from drifting apart would be unnecessary); moreover, it
would be neither compressible nor dilutable. This extraordinary quantum
liquid---the smallest density condensed matter system yet proposed---could
probably only be formed at much colder temperatures than are now available
in BEC experiments. Bulgac proposes that Efimov droplets made from boson
atoms be called "boselets." The fermion version would be "fermilets."
(Aurel Bulgac,
Physical Review Letters, 29 July 2002.)
Ice Ages and Spiral Arms
New research suggests that ice age epochs on the earth may result from
our solar system's trek through the spiral arms of the Milky Way. Nir
Shaviv (shaviv@phys.huji.ac.il,
+972-54-738555), of the University of Toronto and Jerusalem's Hebrew
University bases this hypothesis on correlations he has found between
apparent changes in the flux of cosmic rays reaching the earth and geological
evidence for major ice ages in the past billion years. Galactic spiral
arms are not permanent, rigid fixtures; rather they are transient and
result from density ripples traveling around the galaxy. Many massive
stars form in the wake of the density waves and later explode as supernovae,
which are a primary source of cosmic rays. It seems reasonable to expect
our planet to receive more cosmic rays when it is near the supernovae
in a major spiral arm. If there is a connection between cosmic ray flux
and climate (see
Update 401), past ice ages should correlate with the solar system's
location relative to the traveling spiral arms.
One of the challenges in making the climatic connection is finding
records of cosmic ray flux over past eons. Shaviv deduced the earth's
exposure to cosmic rays by considering the cosmic ray exposure of 42
iron meteorites. The meteorite record seems to indicate that the cosmic
ray flux varies with a period of about 143 million years, which correlates
well with both the geological records of ice age epochs and the solar
system's location relative to the spiral arms. Our current position
in the minor Orion spiral arm should lead to cosmic ray fluxes about
half of what we would receive in a major spiral arm. Shaviv's model
places us in the wake of a major ice age epoch, and is consistent with
the global temperatures that we are now experiencing. Shaviv points
out that the weakest link in his proposal is uncertainties in the extent
and timing of glacial periods indicative of ice age epochs, and that
further geological research is necessary to confirm that galactic spiral
arms affect our climate. (Nir J. Shaviv, Physical Review Letters,
29 July 2002.)
Sonoluminescence is Chemical in Nature
Sonoluminescence is chemical in nature, not nuclear. A new experiment
at the University of Illinois relieves some of the mystery previously
hanging around sonoluminescence, the conversion of ultrasonic waves
into picosecond light pulses via the rapid oscillations (cavitation)
of bubbles in a liquid. Yuri Didenko and Kenneth Suslick assert that
the intense sound compresses the bubble, increasing temperatures to
such a level (10-20,000 K) that many gas molecules in the bubble would
be ionized and a furious session of chemical reactions initiated. Studying
the ultrasound effects on a single bubble of air in a bath of water,
the researchers carefully monitored the reactant products, mostly nitrite
ions (NO2), hydroxyl radicals (OH), and light. How then is the incoming
sonic energy allocated? The larger part seems to go into chemical reactions
with a much smaller portion being converted to light, leaving very little
for the kind of nuclear fusion reactions reported earlier this year
by scientists at Oak Ridge. (Didenko and Suslick, Nature, 25
July 2002.)
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