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Number 455, November 2, 1999 by Phillip F. Schewe and Ben Stein
ORIGIN OF RADIO JETS NEAR A BLACK HOLE. Black holes don't just sit there spiderlike swallowing stars. They also fling out vast plumes of light-emitting material; these collimated streams can stretch for hundreds of thousands of light years. One of the closest of these conspicuous jets is to be found at the heart of galaxy M87, about 50 million light years away from Earth. Presumably the jet originates at an accretion disk surrounding a supermassive black hole. Previously radio mapping of this spot in the sky did not possess sufficient resolving power to see precisely where the jet begins. But now, by pooling the extended radiowave gathering power of the Very Long Baseline Array (VLBA), the Very Large Array (VLA), and telescopes in Italy Sweden, Finland, Germany, and Spain, astronomers have nailed down the jet origin to within tenths of a light year of the black hole's location. The resulting image (see www.aip.org/png) shows that the jet's initial opening angle is 60 degrees, the widest ever seen for a jet, although the jet becomes much more focused (6 degrees) further downstream. (Junor et al., Nature, 28 Oct.; also www.nrao.edu/pr/m87.collimation.html)
GOLD CHAINS ARE PRIZED not only as jewelry but also for their atomic properties. By plunging a scanning microscope probe into a gold surface and then retracting the tip a string of several (perhaps as many as seven) gold atoms can be produced. The binding strength between atoms in the chain is at least about half that between atoms in bulk gold and so the chain is somewhat stable. Transmission electron microscope (TEM) pictures of the chains seem to indicate that the atoms are much as 4 to 5 angstroms apart, but other measurements, such as conductance tests, imply the gap was more like 3 angstroms or less. So what are the gold atoms doing? This puzzle is addressed by a group of scientists from several Spanish labs (plus a contingent at the University of Illinois - contact Daniel Sanchez-Portal, daniel@roma.physics.uiuc.edu) whose computer simulations suggest that the atoms lie not on a straight line but on a zig-zag (spaced about 2.5 angstroms apart) and that, furthermore, the chain should be spinning around its long axis (see a figure at www.aip.org/png). The TEM pictures would then be explained as capturing only a misleadingly averaged position for the gold atoms. Knowledge of where the gold atoms are and what they're doing is important to those hoping to develop circuitry using nanowires. (Sanchez-Portal et al., Physical Review Letters, 8 November 1999; Select Article.)
MACH CONES: SHOCK WAVES IN DUSTY PLASMAS. Plasmas--collections of charged particles such as ions and electrons--usually behave as a gaslike substance, with particles dancing around each other with little deflection. But under the right conditions, physicists can make plasmas act like liquids and solids, in which particles sit almost stationary, interacting almost exclusively with their nearest neighbors. This is especially true when plasmas are mixed with dust, as is the case in interstellar space. In laboratory experiments at the University of Iowa (John Goree, 319-335-1843, john-goree@uiowa.edu), the "dusty plasmas" are micron-sized spheres loaded up with approximately 10,000 electrons apiece. When illuminated by an intense sheet of light, the researchers can see the microscopic structure and movements of these particles in a way that is not possible with conventional atomic matter. For this reason, plasmas can serve as a model system for investigating condensed matter physics. By firing a particle at the dusty plasma at supersonic speeds, the researchers produced a Mach cone (figure at www.aip.org/png), similar to the V-shaped shock wave produced by a supersonic airplane. Mach cones are well known in gases (airplanes, for example), but almost unknown in solids. One of the only other known examples is in seismology: a sound wave traveling down the surface of a liquid-filled borehole moves faster than the sound speed in the surrounding rock, causing a Mach cone to be produced in the rock. (D. Samsonov et al, Phys. Rev. Letters, 1 November 1999; also see paper H12.02 in the upcoming American Physical Society Division of Plasma Physics meeting; also Select Article.)
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