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.)