Physics News Update, Number 468

Number 468, January 28, 2000 by Phillip F. Schewe and Ben Stein

OPTICAL BLACK HOLES, objects that attract and trap specific colors of light, can be made in earthly laboratories, two researchers have shown theoretically, offering possibilities for lab-based analogs of general relativity and potentially even quantum gravity phenomena. According to researchers at the Royal Institute of Technology in Sweden and at the University of St Andrews in Scotland (Ulf Leonhardt, leonhardt@quantopt.kth.se, 011-46-8-791-1324), the trick is to create a vortex of fluid that whirls at velocities comparable to the speed of light inside the fluid. Such a feat is now possible, with the advent of techniques for slowing down light to just a few meters per second through such substances as a Bose-Einstein condensate (Update 415) or a rubidium gas (Phys. Rev. Focus, 29 June 1999). If a sufficiently fast-spinning vortex of these or similar materials could be created, light inside the fluid could lose maneuverability and become trapped in the vortex. Since light in an optical black hole would behave analogously to matter in a real black hole, these light-trapping whirlpools would permit laboratory study of Hawking radiation, the hypothetical emissions from evaporating black holes; this radiation, which consists of particles made near the hole's boundary, is next-to-impossible to observe directly since it is obscured by the cosmic microwave background. In addition, the researchers speculate that studying quanta of light interacting with the quantum-mechanical matter waves in BECs could even help establish "a testable prototype model of quantum gravity." In the meantime, physicists are also pursuing the idea of creating "acoustical black holes" (dumb holes), regions that capture and trap sound waves. (Leonhardt and Piwnicki, Physical Review Letters, 31 January 2000; Physical Review A, December 1999; Select Articles; also see http://www.st-and.ac.uk/~www_pa/group/quantumoptics/media.html)

"THE FORMATION AND EVOLUTION OF GALAXIES are intimately connected to the presence of a central massive black hole," asserts Douglas Richstone of the University of Michigan. Richstone was at the recent meeting of the American Astronomical Society in Atlanta to report the new identification of supermassive black holes at the cores of three nearby elliptical galaxies, adding to an already substantial association between galaxies possessing centralized, high-density spheroidal clumps or bulges of stars and nearby heavy black holes (star concentration correlating closely with black hole mass). Richstone pointed to the growing consensus that these massive black holes are the remnants of quasars (a notion underscored at the meeting by the report given by Andrew Wilson of the University of Maryland--of many "dying quasars" in nearby galaxies, objects whose radio spectra resemble a quieter version of quasar spectra) and to the historical fact that the age of quasar formation occurred before the time when most stars were forming in galaxies (to judge from high redshift observations). Richstone concluded that "Radiation and high-energy particles released by the formation and growth of black holes are the dominant sources of heat and kinetic energy for star-forming gas in protogalaxies."

SNOW SCREECHING ON WATER. With its ability to create muffled winter landscapes, snow is usually associated with quiet. When the white stuff falls on a body of water, one would expect it to be just as silent, since it doesn*t make much of an impact. But as researchers have discovered, it unexpectedly creates high-pitched screeching sounds that can sometimes disrupt underwater sonar experiments. Investigating these sounds, which last for roughly a ten-thousandth of a second, Larry Crum of the University of Washington (206-685-8622) and his colleagues implicate air bubbles as the source of snowflake noise. According to their explanation, the snowflake's presence on a water surface creates capillary action (the attraction between a liquid and solid surface), causing water to rush upwards. The upward flow of water either generates air bubbles on its own, or unleashes air bubbles in the snowflake as it melts. The bubbles oscillate as they reach equilibrium with their environment, creating sound waves of up to 200 kilohertz--out of the range of human hearing (which stops at 20 kHz) but potentially audible to dolphins. Researchers have been known to shut down sonar surveys of salmon population during snowfall because of these sounds. (Select Article, Journal of the Acoustical Society of America, October 1999; see also New Scientist, 25 December 1999.)