Number 280, July 22, 1996 by Phillip F. Schewe and Ben Stein
EARTH'S CORE SPINS INDEPENDENTLY of the rest of the planet. Columbia
scientists Xiadong Song and Paul Richards have combined computer simulations
with measurements of seismic waves traveling through the Earth to deduce
the rotational behavior of the planet's deep interior. What they find is
that the solid inner core---which might consist of a single immense crystal
of iron 2400 km across---rotates slightly faster than the rest of the world.
The tiny differential spin amounts to about one degree a year. This means
that the equator of the inner core swivels past the inside of the outer
core at a rate of tens of km per year, 100,000 times faster than the fastest
tectonic plates move past each other up near the Earth's surface. A better
understanding of how the core gimbals about would in turn provide insights
into the nature of Earth's magnetic field, which has reversed itself many
times over geological time. (Nature, 18 July 1996.)
ICE CHANGES FROM A MOLECULAR SOLID TO AN IONIC SOLID when squeezed hard
enough. Scientists at the Carnegie Institution of Washington used a diamond
anvil cell to crush ice until the asymmetric hydrogen bonds, which usually
keep water molecules in their proper places in the ice crystal, become
symmetrical. In other words, instead of owing primary allegiance to one
particular oxygen, the hydrogens became egalitarian in their alignment
with the surrounding oxygens. It took 60 gigapascals of pressure (about
6 x 10**5 atm) to produce the transformation in H2O and 70 gigapascals
in D2O. (A.F. Goncharov et al., Science, 12 July 1996.)
LASER-INDUCED TRANSPARENCY FOR ISOTOPE DISCRIMINATION. Shining lasers
on a gas containing two isotopes of the same element, Athos Kasapi of Stanford
University (athos@loki.stanford.edu) has made one isotope highly transparent
to the laser light while causing the other to become highly opaque, enabling
a trace isotope to be detected amidst a far more abundant isotope. Isotopes
of the same element will absorb light at very slightly different energies
because the electron-nucleus interaction inside an atom depends subtly
on the spin of the nucleus, which in turn is different for each isotope.
Demonstrating the isotope discrimination technique in a gas containing
99.97% lead-208 and 0.03% lead-207, Kasapi employs a "probe"
laser which ordinarily sends the abundant lead-208 atoms from a low-energy
ground state to a high-energy excited state, and a "coupling"
laser which normally sends the lead-208 from an intermediate "metastable"
state to an excited state. When the probe and coupling lasers are both
turned on, quantum interference between the two pathways to the excited
state prevents the atom from absorbing any light. Meanwhile, Kasapi made
the other isotope, lead-207, highly absorbing by adjusting the coupling
laser's intensity. Although this technique cannot currently compete with
conventional methods of isotope separation, it does represent another interesting
application of "laser-induced transparency." A new tool in quantum
optics, laser-induced transparency has previously been used to create "lasing
without inversion" (Update #240), which allows laser action to be
initiated without the normal requirement of reversing the populations of
low-energy and high-energy atoms. (A. Kasapi, upcoming paper in Physical
Review Letters.)
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